III. Low-Fat and VLF Diets (<10% to 19% Fat)

VLF diets are defined as containing <10% fat, and low-fat diets contain between 11% and 19% fat. Both are very high in CHO, and moderate in protein. Representative VLF diets are those promoted by Dr. Dean Ornish, and Nathan and Robert Pritikin. There are no commercial diets that fit in the category of low-fat diets, although there is research on these diets meriting their inclusion in this paper.

The American consumer has traditionally ignored the VLF diet for weight loss. It seems they would rather restrict CHO intake to less than 10% of calories than to restrict intake of their favorite fat (or sweet-fat combination) to the same degree. In addition, these diets were not historically promoted as weight loss diets, but rather, diets to prevent or reverse heart disease. VLF diets recommended reduction of dietary fat and cholesterol based on the (now) well-known association between saturated fat consumption and cardiovascular disease.

However, as Americans became fatter, and their hunger for diet books remained unsatiated, proponents of VLF diets (e.g., Ornish and Pritikin) capitalized on their program's apparent effect on body weight. They changed the focus (and title) of their books from heart disease to weight loss. Neither trial (e.g., The Lifestyle Heart Trial or The Pritikin Program was originally designed to assess the effect of diet on weight loss. Thus, the scientific information on the effect of these diets on body weight and body composition is limited. In addition, caution in the interpretation of results is necessary because diet is but one component of these total lifestyle-modification plans.

A. Underlying Philosophy, Claims, and Proposed Solutions

Proponents of VLF diets support reducing caloric intake and increasing energy expenditure to achieve weight loss. Rather than counting calories, per se, dietary recommendations focus on “type of calories” (13, p. 31) and “caloric density” (17, p. 5). “Eat more, weigh less” (13) does not mean, “eat more calories.” It means, “consume more high complex CHO, and high-fiber foods whenever you feel hungry and until you feel full, but not stuffed” to lose weight (13, p. 32). The “calorie density solution” enables individuals to eat as much as they want—six to seven times daily—and lose weight safely, gradually, and without hunger (17, p. 17). These VLF diets are based primarily on vegetables, fruits, whole grains, and beans, with moderate quantities of egg whites, nonfat dairy or soy products, and small amounts of sugar and white flour. Ornish's diet is vegetarian; Pritikin allows a limited amount of low-fat animal protein daily (no more than 3.5 ounces of lean beef, fowl or fish).

Dr. Dean Ornish's Program for Reversing Heart Disease (148) and The Pritikin Program (15,16) promotes lifelong changes in diet, exercise, and lifestyle. Each plan includes a nutrition and exercise component; Ornish's plan includes stress reduction and emotional support as well. Current claims range from weight loss, to overcoming or reversing heart disease, reducing symptoms of type 2 diabetes, high blood pressure, cancer, arthritis, stress, and smoking, in addition to general wellness, sometimes in as little as 2 weeks (13,15,16,148). Pritikin also claims medications for heart disease, diabetes, and high blood pressure may be reduced or completely eliminated by following these plans.

B. Scientific Evaluation of Claims

1. Caloric Intake, Body Weight, and Body Composition

  • Do low-fat and VLF diets result in decreased caloric intake?
  • What is the effect of these diets on body weight and body composition?

Overweight subjects who consume low-fat, and VLF, high-CHO diets eat fewer calories and lose weight (20,34,38,145,149,150–152) (Table 15). Again, total caloric intake is more important than diet composition, in this case fat, for weight loss (28,33,45,154). In the context of reduced caloric intake (1200 kcal/d) percent calories derived from fat (15% to 35%) does not influence weight loss (20). Havel et al. (34) reports a family history of diabetes is predictive of weight loss (and fat loss) in women consuming LFAL diets for 6 months.

Subjects who lose weight on low-fat diets lose body fat (120,34,45,151) and lean body mass (145). However, in the context of a reduced calorie diet, the amount of dietary fat (10% to 40%) does not affect losses of body fat or lean body mass over 12 weeks (20).

Evidence Statement: Overweight subjects consuming low-fat, high-CHO diets eat fewer calories, lose weight, and lose body fat. Evidence Category A.

Alford et al. (45) fed adult, sedentary, overweight women reduced calorie diets (1,200 kcal/d) containing 10%, 35%, or 45% calories from fat for 12 weeks. The 10% fat diet was 70% to 80% CHO. Weight loss was the same on each diet. Noakes and Clifton (28) fed 62 overweight subjects (mean BMI ∼31) one of three test diets for 12 weeks. One was VLF (10% fat), high-CHO (71.6%), whereas the other two were moderate-fat (31.8%), and moderate-CHO (∼50%). Caloric intake on all diets was limited to 1533 kcal/d. Overall weight loss was 8.6 ± 0.4 kg (9.7%) with a reduction in waist circumference of 8%. There were no significant differences in weight loss between diet groups, although weight loss was least on the VLF as compared with the moderate-fat diet.

Surwit et al. (153) conducted a 6-week weight-loss trial that compared the efficacy of two hypoenergetic (1100 kcal/d), VLF (<11%), high-CHO diets (71%) varying in sucrose content. The high-sucrose diet contained 43% total energy from sucrose; the low-sucrose diet only 4%. Subjects in both groups lost comparable amounts of body weight and body fat. CHO source had no effect on weight loss as long as energy was restricted. These data clearly show that high sucrose or complex CHO consumption does not cause obesity, hyperglycemia or insulin resistance in the absence of dietary fat. Although it is quite possible that sucrose or complex CHOs may produce different effects when total energy intake is greater, the use of sucrose or other CHOs in a low-fat, weight-reduction program seems both safe and effective (providing a good refutation to proponents of low-CHO diets).

Heilbronn et al. (33) studied 35 obese patients with type 2 diabetes assigned to one of three 1600 kcal/d diets for 12 weeks. The diets were VLF (10%), high CHO (72%), or high- monosaturated or high-saturated fat (32%), lower CHO (50%). Diet composition did not affect the magnitude of weight loss, with subjects losing an average of 6.6 ± 0.9 kg.

VLF diets

Do VLF diets, when consumed ad libitum, decrease caloric intake? The answer is a qualified yes because most studies of individuals consuming these types of diets were not designed to assess the effect of diet on weight loss, but rather the effects of lifestyle change (e.g., low-fat diet, exercise, stress reduction) on disease risk or reversal. One exception is the study of Shintani et al. (157) who fed 20 obese native Hawaiians a pre-Western contact traditional Hawaiian diet low in fat (7%), high in complex CHOs (78%), and moderate in protein (15%) for 21 days. Participants were encouraged to eat to satiety. Average energy intake decreased from 2594 to 1569 kcal/d and average weight loss was 7.8 kg. Although interesting, this study is not relevant outside Hawaii.

Other studies allowing ad libitum intake of VLF diets were published by Barnard (using the Pritikin diet) and Ornish. In the Barnard articles (155,158–166), the subjects were 3-week residents of the Pritikin Longevity Center who engaged in medically supervised daily aerobic exercise, primarily walking on a treadmill, and consumed the Pritikin high-complex CHO, high-fiber, low-fat, low-cholesterol, and low-salt diet. All meals and food were provided onsite. Barnard (155) reports 2643 males and 1897 females consuming VLF, high-fiber diets for 3 weeks lost 5.1 and 3.3 kg, respectively, representing a 5.5% and 4.4% decrease in body weight (men and women, respectively.) Although BMI of patients is not provided, average weights of 91.9 kg and 74.8 kg (men and women) indicate program participants were overweight.

Another 3-week study (162) assessed the role of diet and exercise in management of hyperinsulinemia. Seventy-two patients were divided into three groups based on fasting serum glucose and insulin measurements. Thirteen type 2 diabetic and 29 insulin-resistant subjects had a BMI >30. Normal subjects (n = 30) had a BMI ∼27. (Data collected before BMI levels were lowered for overweight.) An overall body weight reduction of 4% was reported for all three groups.

One drawback to the Barnard studies is the omission of information regarding actual caloric intake (or energy expenditure). It can be assumed that participants ate low-calorie diets because these diets are very high in dietary fiber (35 to 40 g per 1000 kcal), and The New Pritikin Program (15) recommends a daily caloric intake of 1000 to 1200 kcal/d. An analysis of 7 days of menus from The Pritikin Principle (17)§ indicates an average intake of 1467 kcal/d. Amazingly, this is nearly identical to intakes reported by Larosa et al. (57; 1461 kcal/d) for subjects consuming high-fat, low-CHO diets.

Ornish et al. (156) collected data on 28 patients who followed The Lifestyle Heart Trial for 1 year (experimental group), and 20 patients who made more moderate changes (control group). At 5 years (14), data were available from 20 experimental and 15 control patients (starting BMI 28.4 and 25.4, experimental vs. control, respectively). All completed a 3-day diet diary at baseline, and after 1 and 5 years. Although energy intake at baseline was slightly higher than 5 years later, these changes were not significantly different between groups, or over time (Table 16). Intake of fat, both as percent calories, and absolute amount (g/d) significantly decreased in the experimental group from the baseline high of 29.7% (63.7 g/d) to 6.22% (12.7 g/d) at 1 year, and 8.5% (17.3 g/d) at 5 years. The control group also decreased fat intake over time, from 30.5% (57.4 g/d) at baseline, to 28.8% (52.4 g) at 1 year, and 25% (44.1 g/d) at 5 years. Fat intake was significantly different between the two groups at 1 and 5 years. Patients in the experimental group lost 10.9 kg (23.9 lb.) at 1 year, and sustained a weight loss of 5.8 kg (12.8 lb.) at 5 years. Weight loss was significantly different from the control group, whose body weight changed little from baseline. The Ornish Multicenter Lifestyle Demonstration Project (167) was conducted in 333 patients (194 experimental, 139 control) at eight sites throughout the United States. Data from this project show that mean weight significantly decreased in the experimental group from baseline to 3 months (4.2 kg), 1 year (4.7 kg), 2 years (4.9 kg), and 3 years (3.3 kg). However, dietary intake data and BMI was not provided.

Table 16. . Energy intake from 0–5 years (The Lifestyle Heart Trial)
 Baseline (B)1 year5 yearsBaseline1 year5 yearsB–1 yearB–5 years
  • *

    All p values are two-tailed and each is the result of a test of the null hypothesis that the change between two particular visits (e.g. baseline and 1 year) does not differ between the experimental and control groups (14). Adapted from Ornish (14,156).

Energy intake1950182118461711167315720.640.86
Energy change −129−104 −38−139  
Body weight (kg)91.480.6485.6475.7477.1877.090.0010.001

That weight loss resulted from decreased fat intake is not controversial (42,43,121). One would expect that a 46.33 to 51 g/d (417 to 459 kcal) fat decrease would result in weight loss. What is curious is that the significant reduction in fat intake did not apparently result in a significant reduction in total caloric intake, and yet subjects still lost weight. Can subjects consume less fat, the same number of calories, and still lose weight? If so, these data would disagree with all studies (but two) which show that as dietary fat decreases so does caloric intake in both normal weight (168–173) and overweight subjects (20,39,40,134,137,138,140–142,145,157 [see references 27,42,43,117 for reviews of the effect of dietary fat intake and weight change]).

Two studies show that consumption of a low-fat (but not a VLF) diet results in increased caloric intake but decreased body weight (174,139). Raben et al. (174) reports that over a 12-week period, normal weight individuals consuming LFAL diets (25.6% fat) lost weight (1.3 kg) despite decreased fat intake (37.4% to 25.6%) but increased total caloric intake (3,059 to 3,203 kcal). Dietary fiber was significantly increased as well. The authors were puzzled by these results, noting subjects did not change physical activity levels while under study. They attribute the “paradoxical” finding to an underestimation of daily intake (based on 7-day food records) before the study (by 11%) and an overestimation of energy content of the experimental diet.

The study by Prewitt et al. (139) was not designed to study weight loss but rather weight maintenance. Eighteen subjects (12 with BMI 22.9 and 6 with BMI 38.4) were studied in an outpatient metabolic setting for 24 weeks to determine the effects of diets with different compositions as part of a weight-maintenance regimen. All subjects were fed a 37% (high-fat) control diet for 4 weeks, the caloric intake estimated by energy requirements (basal energy expenditure × 1.4). For the next 20 weeks, they received a low-fat (20%), low-fiber diet (3g/d) containing ∼1800 kcal. All meals were provided onsite. Throughout the study, energy adjustments were made as needed in an attempt to maintain body weight of subjects within ±1 kg of initial weight at study entry. If a subject's weight varied beyond ±1 kg over 3 days, she was switched to a higher calorie level until weight returned within 1 kg of initial weight. Over the course of the 20-week low-fat diet, despite adjustments in energy to maintain body weight, subjects lost body weight (2.8%) and body fat (11.3%). Prewitt concludes, “a higher energy intake was needed to maintain body weight on a low-fat than a high-fat diet,” especially in subjects with a BMI of >30.

How can these disparate results be explained? In examining possible reasons for weight loss in the face of increased energy (and decreased fat) intake, changes in physical activity, metabolic rate, and thermic effect of food are considered. Prewitt et al. (139) examined each of these factors and concluded that because physical activity had not changed, most, but not all, of the observed energy intake was accounted for by increased metabolic rate and increased thermogenesis. Dietary adherence may have also been a problem.

In The Lifestyle Heart Trial (14,156), weight loss from baseline to 1 year could be due to changes in physical activity. Data indicate that frequency (F) (times per week) and duration (D) (h/wk) doubled in the experimental group (from 135 min/wk to 298 min/wk) from baseline to year 1 (Table 17). Intensity was not reported in this study. The increase in frequency and duration, however, could account for almost all the first-year weight loss**. Interestingly, from 1 to 5 years, exercise duration and frequency decreased in the experimental group (although there was no significant difference between groups, or over time). Even with a possible (and probable††) increase in exercise intensity, the decrease in duration and frequency, coupled with the same energy intake is not enough to prevent some weight regain, which is exactly what happened between years one and five‡‡. It is unclear from the data if weight gain from year 1 to 5 was significant in the experimental group because analysis was conducted to determine differences between groups over time. Furthermore, that the experimental group was overweight, and the control group normal weight may confound the analysis. The data show that the normal weight controls maintained a consistent level of physical activity (1 hour, 3 times per week, e.g., 20 min/d), and energy intake (1652 kcal/d) over the 5 years, resulting in maintenance of a stable weight. This occurred in the context of a diet that contained <30% fat.

Table 17. . Energy expenditure from 0–5 years (The Lifestyle Heart Trial)
 Baseline (B)1 year5 yearsBaseline1 year5 yearsB–1 yearB–5 years
  • *

    All p values are two-tailed and each is the result of a test of the null hypothesis that the change between two particular visits (e.g. baseline and 1 year) does not differ between the experimental and control groups (14). Adapted from Ornish et al. (14,156).

  • p = 0.0008 for frequency, baseline to year one in the experimental group (156).

  • p = 0.0004 for duration, baseline to year one in the experimental group (156).

Times/week (F)2.664.974.342.382.873.570.060.64
Hours per week (D)
F× D (minutes/week)135298130145152174  

In addition to physical activity, other factors to consider include changes in metabolic rate, the thermic effect of a high-CHO diet (although this could not account for a significant portion of the weight loss) (54), and possible inaccuracies of the diet diaries. Food intake was assessed using 3-day diaries, collected at baseline, year 1 and year 5. It is likely that subjects consumed fewer than the reported 1800 kcal/d from baseline to year 1, and greater than 1800 kcal/d from years 1 to 5§§. Finally, the sample size at 5 years (experimental n = 20; control n = 15) may not have been large enough to detect differences (175).

Evidence Statement: Weight loss on VLF diets may be the result of lifestyle modification, which may include decreased fat and energy intake, increased energy expenditure, or both. Evidence Category B.

Body Composition

The Ornish Multicenter Lifestyle Demonstration Project (167) reported a significant decrease in body fat from 25.7% at baseline to 21.3% at 1 year and 22.4% after 2 years. Body fat of 23.4% at 3 years was not significantly different from baseline.

2. Nutritional Analysis

  • What is the nutritional profile of low-fat and VLF diets?
  • Do diets provide adequate levels of nutrients, based on current dietary recommendations?

Nutritional analysis of a VLF diet (13, pp. 107–111) indicates that VLF diets provide adequate levels of all nutrients except vitamin E, B12, and zinc (Table 9). This 1-day analysis seems slightly high in sodium, probably the result of added seasoning (teriyaki sauce). Ornish et al. (156) report the diet to be nutritionally adequate for all nutrients except vitamin B12, as expected, which was supplemented. Scherwitz and Kesten (176) conducted the German Lifestyle Change Pilot Program (GLCPP) to gain experience applying the program to a culture other than the United States. Nutritional analysis of lifestyle and control groups show the nutritional content of the low-fat vegetarian diet was very nutrient dense, containing more vitamin and mineral content for the same caloric value than the control group's more typical German diet. However, the treatment group's intake of vitamins E, B12, D and zinc fell below the Recommended Daily Allowance because of the omission of animal food products. Addition of animal protein (e.g., Pritikin) and education to consume more diverse foods that are high in these nutrients would be beneficial and eliminate the need for supplementation¶¶. Computer analyses of menus used at the Pritikin Longevity Center show that the therapeutic plan is nutritionally adequate (161).

Evidence Statement: VLF diets are low in vitamins E, B12, and zinc. Evidence Category B.

VLF, very high-CHO diets, high in fruits, vegetables, grains, beans, and soy contain thousands of protective phytochemicals, e.g., isoflavones, carotenoids, bioflavonoids, retinols, lycopene, and genistein that have anti-aging, anti-cancer, and anti-heart disease properties. However, some VLF diets, based on poor food choices, may mean lower than recommended levels of certain nutrients such as iron, phosphorus, calcium, and zinc. Fiber intake may also be low (154). This means that specific food choices within the context of a VLF diet are critical. Data on the impact of a relatively high proportion of low-fat and fat-free alternatives to traditional foods in a free-living population in the absence of intensive dietary counseling are not yet available (29).

Nutritional questions on the use of VLF diets include uncertainty about compromised absorption of fat-soluble vitamins, and the impact of increased dietary fiber on the absorption of minerals. Twenty patients who had complied with the Pritikin diet longer than 4 years showed no signs of nutritional inadequacy in more than 50 blood tests, including those for iron status, trace minerals, and vitamins (161).

3. Metabolic and Adverse Effects

  • Do very low-fat diets affect blood lipids, blood pressure, and blood insulin levels?
  • Are adverse effects associated with these diets, or are there subgroups that should not use them?

Blood Lipids

Diets that lower serum TC, specifically LDL-cholesterol levels, are believed to lower the risk of coronary heart disease. In studies that lasted from 21 days to 1 year, reducing fat content to <10% of energy reduces total and LDL cholesterol levels in both men and women (14,28,33,154–156,15,162–165,180). Some changes were sustained for 2 to 3 years (14,159), and up to 5 years (158). Intensive diet and lifestyle modification provided additive benefit to that of cholesterol-lowering medication (165).

The Ornish Multicenter Project (167) reported significant changes in total and LDL cholesterol were sustained for 3 years (despite the program lasting for 1 year). Total cholesterol decreased from 202 mg/dL to 183.7 mg/dL, and LDL decreased from 122.9 mg/dL to 101.7 mg/dL (baseline to year 3). It is not known what affect, if any, the intervention had on use of lipid-lowering drugs, which were used by 54% of experimental group patients. In the Lifestyle Heart Trial, data collected from subjects 4 years post-treatment indicate total and LDL cholesterol were higher than at 1 year but lower than at baseline. These changes were relative to the reduction in fat intake (greatest at 1 year vs. 5 years). In this study, no significant difference between groups, or over time, was reported for total and LDL cholesterol at five years (167).

Men and women (premenopausal and postmenopausal) who participate in the 3-week Pritikin Longevity Center residential program consistently show decreased total and LDL cholesterol levels (166). When an aggressive diet and exercise program is added to cholesterol-lowering drugs, a dramatic reduction in total and LDL cholesterol is noted. For example, use of drug alone reduces cholesterol by 20%; addition of lifestyle intervention achieves an additional 19% reduction (165). In one study, patients were contacted 2 to 3 years after leaving the center. Blood samples obtained from 52 (75%) patients revealed significant increases in cholesterol. However, the follow-up values were significantly lower than the entry values (159). Similar results were reported at 5-year follow-up (158).

In addition, qualitative changes in LDL show that particle size was increased, and LDL oxidation was decreased implying a reduction in risk for atherosclerosis and its clinical sequelae (177).

Schaefer et al. (38) reported consumption of a low-fat diet under weight maintenance conditions significantly lowered plasma TC, LDL and HDL-cholesterol (mean change, −12.5%, −17.1%, and −22.8%, respectively), but that this diet significantly increased plasma TG levels (47.3%) and the TC/HDL ratio (14.6%). In contrast, consumption of an LFAL diet accompanied by significant weight loss (−3.63 kg) resulted in a mean decrease in LDL cholesterol (−24.3%), and mean TG levels and TC/HDL ratios not significantly different from values obtained at baseline. They concluded that an LFAL diet when combined with weight loss is better than a low-fat diet without weight loss with respect to blood lipid levels.

In the study by Kasim-Karakas et al. (151), subjects received a controlled euenergetic diet in which dietary fat was reduced stepwise from 35% to 25% to 15% over 4 months. Thereafter, they followed an ad libitum 15% fat diet for 8 months. Two months after subjects switched to the LFAL diets, TG levels decreased. Levels remained stable for the rest of the 12 months and were not different from baseline values. During the ad libitum period, TC levels remained low. An unexpected finding was the increase in LDL cholesterol to baseline levels within 2 months of switching to the ad libitum diet (levels had previously decreased in response to decreased fat intake), although LDL levels remained stable for the rest of the study. HDL levels decreased as dietary fat decreased, and remained the same during the LFAL condition.

Blood lipid changes occurring in individuals following VLF diets may be attributed to weight loss (6), decreased intake of fat and saturated fat, and/or high fiber intake, rather than increased CHO content, per se (28,29).

However, low-fat, high-CHO diets often lower not only LDL cholesterol but also HDL cholesterol (28,33,153–155,162,164,165). Although lower HDL levels usually increase risk of coronary heart disease (178) there are no data showing that physiological reduction of HDL cholesterol levels with a low-fat diet is detrimental. In countries where VLF diets are the norm, and TC and HDL cholesterol are both very low, the incidence of heart disease is much lower than in the United States (179). In the Lifestyle Heart Trial, however, no change in HDL was reported at 1 or 5 years (14). In the Multicenter Project, HDL initially decreased from baseline to 3 months, but then showed a significant increase by 2 and 3 years (e.g., 36.7 mg/dL vs. 42.2 mg/dL, baseline vs. 3 years) (167).

Although TG levels are reported to increase in response to short-term consumption of VLF diets (20), the type of CHO consumed may play a role in determining the metabolic response. For example, diets containing 70% CHO do not lead to hypertriglyceridemia as long as leguminous, high-fiber foods are consumed (30). In addition, TG levels may be reduced by weight loss (6). These factors may be the reason why TG levels decreased (28,154,155,158,162), or did not change (180,156). Some attribute adverse metabolic effects of high-CHO diets to their sucrose content (181). However, Surwit (154) reported reduction in TG levels even after overweight women were fed a high-sucrose but reduced calorie diet (1553 kcal/d) for 12 weeks, indicating that high-sucrose is not a problem in the presence of a low-fat, low-calorie diet.

Evidence Statement: Low-fat and very-fat diets reduce LDL-cholesterol, and may also decrease plasma TG levels, depending on diet composition. Evidence Category B.

Blood Pressure

Blood pressure decreased in most subjects consuming VLF diets (28,154,160,162–164). These diets alone, or in combination with exercise, resulted in reduction or elimination of antihypertensive medication in some patients (160). Benefits may be attributed to dietary changes, physical activity, or weight loss (6).

Blood pressure did not change in individuals following The Lifestyle Heart Trial, because individuals already were being treated appropriately. Effect of lifestyle change on medication use was not addressed.

Blood Glucose, Insulin, and Leptin Levels

The very high-CHO content of VLF diets has led to concern about possible effects on blood glucose and insulin levels. Unfortunately, no study of VLF diets in the absence of caloric restriction exists, so that any effect on blood glucose and insulin could be attributed to energy restriction and weight loss rather than diet composition (33).

Nevertheless, very low-fat, high complex CHO, high-fiber, energy-restricted diets usually result in decreased blood glucose and insulin levels (28,33,154,158,162–164). In some patients with type 2 diabetes these types of diets combined with daily exercise and weight control may result in discontinuation of insulin usage (160). However, Grey and Kipnis (31) reported basal plasma insulin levels on a hypocaloric, high-CHO formula diet did not differ significantly from those observed during an ad libitum diet period, although rate of weight loss was unaffected by insulin levels.

Surwit et al. (153) reports that high sucrose or complex CHO intake does not cause hyperglycemia or insulin resistance in the absence of dietary fat, and when calories are restricted. The confusion in this area is probably due to the fact that hyperinsulinemia results from a high sucrose intake in the presence of high fat (e.g., typical American diet). VLF diets that are also high in fiber decrease blood insulin levels and improve insulin sensitivity (182) as does physical activity. Thus, the decreased blood glucose and/or insulin levels reported for VLF diets may be a consequence of caloric restriction, weight loss, dietary fiber, and/or physical activity, rather than diet composition. In the context of a low-calorie diet, consuming a very-low CHO (e.g., Atkins) or VLF diet (e.g., Pritikin) results in decreased fasting insulin levels.

Kasim-Karakas et al. (151) reported lower plasma glucose concentrations during the 10th and 12th months of the LFAL diet compared with other times. Insulin and hemoglobin A1c concentrations did not change significantly during the study. Plasma free fatty acid concentrations decreased significantly at only one time point, during the 25% fat phase of the controlled euenergetic diet.

Agus et al. (152) compared the effect of high-glycemic index (67% CHO, 15% protein, 18% fat) with the effect of low-glycemic index (43% CHO, 27% protein, 30% fat) energy-restricted diets. Although weight loss was similar between the two groups, plasma insulin and serum leptin levels decreased to a greater extent with the low-glycemic index diet.

Havel (34) reported that during weight maintenance, reducing fat content from ∼30% to ∼15% of the energy content of the diet did not affect fasting plasma leptin or insulin concentrations; however, only fasting insulin and leptin concentrations were examined in this study. After the weight maintenance phase of the study, the subjects were followed during a 6-month period during which they consumed a 15% fat ad libitum high-CHO diet. In women who lost less than 7% of body mass, fasting plasma leptin and insulin levels were unchanged, despite a modest but significant average weight loss and more than 10 months on a VLF diet. However, women with a weight loss greater than 7% had larger reductions of percent body fat, and both fasting plasma leptin and insulin levels decreased by ∼35%. The decreases of fasting insulin are likely to represent an improvement of insulin sensitivity due to weight loss and the decreases of leptin are mostly due to decreases of body fat. However, they are also likely to be partially due to the decreases of insulin during the fasting period because the decreases of insulin and leptin were shown to be correlated independently of the changes of body fat (34)‖‖.

Adverse Effects

Few adverse effects of low-fat and VLF diets have been reported. Barnard et al. (162) noted an initial increase in flatus, which generally subsides. No other adverse metabolic or behavioral effects were reported.

Results of studies seem impressive but questions about long-term efficacy and risk reduction remain. Extrapolation to the general population from motivated individuals (e.g., those with coronary heart disease) is questionable. The independent effects of weight loss, physical activity and accompanying lifestyle interventions complicate interpretation (29). The American Heart Association's Science Advisory recommends persons with insulin-dependent diabetes mellitus, elevated TG levels, and CHO malabsorption illnesses avoid VLF diets (29).

4. Hunger and Appetite: Compliance

  • What is the effect of low-fat and VLF diet on hunger and appetite?
  • What data supports compliance to low-fat and VLF diets?

Low-Fat Diets

The issue of satiety following ingestion of various macronutrients (e.g., CHO, fat, and protein) has been the subject of much research and is briefly reviewed here (see also references 183–187).

Studies of early satiety (occurring within 30 minutes after a preload) found protein having the greatest effect, followed by CHO, and then fat (186,187). However, these studies did not adequately control for the differences in palatability or energy density of test foods (187). Short-term studies (2 and 12 weeks) investigating the effect of covert manipulation of the fat content of foods on total energy intake were conducted in normal weight women. Those consuming lower fat diets (15% to 20%, or 20% to 25% fat) vs. higher fat diets (30% to 35%, or 35% to 40% fat) consumed fewer calories and lost more weight (149,188). Stubbs et al. (184) provided normal weight male subjects ad libitum access to one of three covertly manipulated diets: low-fat (20% energy as fat, 67% as CHO), medium-fat (40% energy as fat, 47% as CHO) or high-fat (60% energy as fat, 27% as CHO). They reported that energy intake increased with percent fat, and that lower fat, lower-energy diets were more satiating than higher fat, higher energy diets.

When dietary fat content is drastically reduced, the weight of the food consumed is maintained or slightly increased (38,188). Thus, exposure to high-CHO foods can give rise to a marked restraining effect on the expression of appetite, the potency and time course varying with the amount consumed and chemical structure (e.g., simple vs. complex CHO) (183).

Low-fat diets (15% to 20% fat) received higher hedonic ratings compared with higher-fat diets (30% to 35% or 45% to 50% fat (150). Hunger was not a problem in subjects consuming low-fat diets. In fact, Schaefer et al. (38) reported that during the low-fat, weight maintenance phase, subjects frequently complained about the quantity of food and of abdominal fullness, making it difficult for them to consume all the food provided. The authors speculate that complaints occurred because the low-fat diet weighed more than the baseline diet. When subjects were allowed to choose their own foods during the LFAL phase, they ate less than what was provided during the low-fat weight-maintenance phase and lost weight. Decreases of leptin are related to increases of hunger in women during a prolonged, moderately energy restricted diet (120). It is therefore possible that maintaining the diurnal pattern of leptin production (induced by the insulin responses to dietary CHO) may contribute to effects of ad libitum, moderately low-fat, high-CHO diets to lower energy intake by preventing hunger from increasing during weight loss.

Schlundt et al. (145) reported that compliance to dietary advice to reduce fat or calories was best during the first 6 weeks of a 15-week study. A follow-up of 71% of subjects who completed the study, obtained 9 to 12 months later, showed average total weight losses of 2.6 kg in the low-fat group, and 5.5 kg in the low-calorie group. Of these subjects, 14% showed no weight regain from the end of the treatment to follow-up, 20% regained 1 kg or less, and 40% regained less than 3 kg. Results did not differ as a function of the treatment group. Both groups experienced compliance problems related to eating at social events, eating in the car, and emotional eating (both negative and positive emotions). Despite three treatment sessions devoted to handling social situations and three devoted to overcoming emotional eating, problems with these issues persisted. It is likely that high-risk situations that precipitate relapse are independent of diet composition.

Djuric et al. (150) designed an intervention trial to selectively decrease fat and/or energy intake in free-living, premenopausal somewhat overweight women, to examine the relative importance of these dietary factors on markers of cancer risk. Diets were nonintervention, low-fat (15%) maintenance of energy intake, low-energy (25% reduction), or a combination of the two (low-fat and low-energy). Meetings with a registered dietitian occurred at 2-week intervals for all diet groups. Daily records served as self-monitoring of intake and as a tool for the dietitian to verify food intake. A total of 88 women completed the 12-week program. The 25 women who dropped out did so within 6 weeks of their randomization date, with similar dropout rates in all intervention groups. Reasons for withdrawal included being too busy (n = 12), diet too hard to follow (n = 5), unhappy with diet assigned (n = 1), too stressed due to illness (n = 2), changed eligibility status (n = 2), and no longer interested/unable to contact (n = 3).

VLF Diets

Studies of ad libitum VLF diets were generally short, ranging from 3 to 12 weeks (28,153,155,157). The Lifestyle Heart Trial, originally a 1-year study (156), was extended to 5 years (14). Limited data from short- and long-term interventions indicate hunger was not a problem for subjects following these diets. Using a seven-point analog scale that ranked hunger vs. satiety, Noakes and Clifton (28) reported subjects perceived hunger more before dinner, although caloric intake at this meal was not assessed. Using a five-point analog scale, Surwit et al. (153) reported hunger decreased as diet duration increased (to 6 weeks), with all subjects reporting lower hunger levels at the end, rather than the beginning, of the study.

Because energy density has been demonstrated to have a robust and significant effect on both satiety and satiation independently of palatability and macronutrient content (187), the energy density of VLF diets must be considered when determining their effects on hunger and appetite. In addition to dietary fiber, water content of the diet must be considered, as both fiber and water decrease the caloric density of individual foods, and the overall diet.

VLF diets are often high in fiber, providing 35 to 40 g dietary fiber per 1000 kcal (13,15–17). Burton-Freeman (189) reports that women may be more sensitive to dietary manipulation with fiber than men, and obese individuals, as compared with lean, may be more likely to reduce food intake with dietary fiber inclusion. Dietary fiber promotes satiation and prolongs satiety, aids in long-term compliance to low-energy diets, and encourages healthy food choices and eating habits. Thus, the amount of fiber in the particular VLF diet is an important consideration when assessing compliance.

At present there are no long-term clinical studies of the effects of energy density independent of variations in fat content.


Although short-term effects of these diets on hunger are promising, long-term effects are more important. Do subjects continue to consume VLF diets long-term? Ornish et al. (14) reported excellent adherence to all aspects of the program during the first year, and good adherence after 5 years, as measured by percent diameter stenosis. Percentage of daily energy intake from fat was maintained at less than 10%. The average person lost 24 pounds in the first year and kept off more than half that weight 5 years later, even though they were eating more food, and more frequently, than before without hunger or deprivation. It is important to note, however, that the motivation of cardiac patients to reverse heart disease by following a lifestyle intervention plan (which includes significantly reducing fat intake) may differ from that of obese patients whose motivation to lose weight may be for reasons other than health. Thus, long-term compliance seen in the Ornish study does not necessarily translate to obese individuals.

Theusen et al. (147) studied Danish heart disease patients to assess how much dietary fat can be reduced for long-term treatment to obtain an effective cholesterol-lowering effect. For 3 months, 14 patients with severe coronary heart disease were treated with a diet containing 10% of total energy from fat. Patients (and their wives) were instructed to eliminate intake of visible fats and cholesterol-rich foodstuffs (e.g., egg yolk, liver, shellfish), limit meat to 50 mg/d, and keep sugar intake low. The consumption of rice, potatoes, vegetables and legumes was encouraged and up to five alcoholic drinks per day were allowed. After 3 months, patients were asked to maintain a diet as low in fat as possible for long-term treatment. Very few patients managed this diet for longer than 3 months; only two had a fat intake of ∼10% after 1 year. However, half had a fat intake below 20%, and a 4-day diet recall showed a mean fat intake at the end of 12 months of 21.4% (range, 7.3% to 37.8%). No explanations for increased fat (and energy intake) were provided. It is also important to note that the motivation and adherence of a patient with existing heart disease may be different from that of a patient who is overweight, but has not yet been diagnosed with a chronic disease.

Trials allowing ad libitum consumption of very low-fat (or even low-CHO diets) are equivocal in terms of efficacy due, in part, to differences in adherence to the targeted macronutrient composition. The validity of dietary information given by trial participants is not always valid (190–192). Obese subjects typically under-report energy intake, especially fat intake (190) and overestimate physical activity (191).

Lyon et al. (192) assessed compliance to dietary advice to decrease fat and increase CHO intake in eight moderately overweight Swiss women during a 2-month period. At supper, they were requested to eat a meal containing 13C-glucose, and measure 13C by self-collection of expired air. Subjects were asked not to intentionally restrain their total energy intake, but have their appetite drive their food consumption. At the end of the study, intake of fat (g), protein (g), and total calories were significantly reduced (fat: 92.5 to 52 g; protein 71 to 64 g; and calories: 1893 to 1518). Intake of CHO remained the same (182 g). When expressed as percent calories, fat intake significantly decreased (from 44% to 31%), and both CHOs and protein significantly increased (CHO: from 38% to 50% and protein from 15% to 17%). With this method, 54% of the variation in achieved weight loss was explained by differences in diet compliance (which ranged from 20% to 93%; mean 60 ± 8%)***. Patients with the greatest adherence lost the most weight. Those who fail to lose weight on any diet are likely to be those who do not adhere to the dietary composition no matter what it is (186).

5. Performance and Physical Activity

• What is the effect of VLF diets on physical performance?

Physical activity and exercise is strongly recommended as part of the overall lifestyle plans recommended by both Pritikin and Ornish. Data support the use of CHO as fuel for exercising muscle (193). VLF diets have plenty of CHO (70% to 80%), supporting physical activity. No adverse affects on performance have been reported in individuals consuming VLF diets. Ornish et al. (14) report no significant difference in exercise duration or frequency in individuals following these diets for 5 years, however they do report a significant increase in exercise intensity.


  • This theory is supported by changing book titles over the past 20 years. Ornish's book titles include Stress, Diet and Your Heart (1982), Dr. Dean Ornish's Program for Reversing Heart Disease (1990), and Eat More, Weigh Less (1993). The Pritikin plan was originally popularized by Nathan Pritikin, whose books included The Pritikin Program for Diet and Exercise (1979) and The Pritikin Promise (1983). His son, Robert, head of the Pritikin Longevity Center wrote The New Pritikin Program (1990), The Pritikin Weight Loss Breakthrough (1998), and The Pritikin Principle (2000).

  • §

    It is interesting to note that The Pritikin Principle is based on caloric density, not reduction of fat. However, when one regularly consumes foods low in caloric density, a low-fat, low-calorie diet results.

  • An 11% increase in caloric intake at the beginning of the study would mean subjects consumed 3395 kcal before and 3203 after reduction in fat. These data would then make sense, and support the contention that ad libitum intake of a low-fat, high-fiber diet results in decreased caloric intake.

  • The largest single increases in caloric intake occurred when subjects went from consuming the high-fat diet to the low-fat diet (7% increase, 132 kcal) and during the last 4 weeks of the study. By the end of the 20-week low-fat diet period, individuals with a BMI >30 consumed 28% more energy (534 kcal), and those with BMI <30 consumed 14% more energy (259 kcal) as compared with their consumption during the 4-week high-fat diet period.

  • **

    For example, Joe Hartman is 5 feet, 8 inches tall and weighs 184 pounds (BMI 28). Walking 2.3 mph (26 min/mile), 298 min/wk burns 4.3 kcal/min or 1281 kcal/wk, translating into a potential weight loss of 19 pounds per year (1281 kcal × 52 weeks/3500 kcal).

  • ††

    Exercise capacity significantly increased from 9.59 METS at baseline to 11.15 after 3 months, 11.66 after 1 year, to 10.88 after 2 years, and to 11.03 after 3 years in the Multicenter Trial (167).

  • ‡‡

    After 1 year, and an ∼19-pound weight loss, Joe increases exercise intensity to 3.2 mph (18:45 min/mile), and burns 5.2 kcal/min. However, because F × D has decreased to 130 min/wk, total weekly energy expenditure is now only 676 kcal/wk, half as much as before and weight is regained.

  • §§

    This is supported by a) nutritional analysis of diets presented in Eat More, Weight Less, and Dr. Ornish's Program for Reversing Heart Disease, which indicate average 3-day caloric intake of 1315 kcal/d, not 1800, b) Ornish (148), which indicates an intake of 1400 kcal/d, and c) the question of compliance to an 1800-kcal diet containing 50 to 60 g of fiber per day.

  • ¶¶

    See Appendix for Ornish's recommendations regarding supplements (www.web.md).

  • ‖‖

    Because insulin is secreted rapidly during and in the period immediately after the consumption of meals, and because circulating leptin exhibits a diurnal pattern that is dependent and proportional to insulin responses to meals, relying on fasting levels of insulin and leptin to assess overall central nervous system exposure to changes of insulin and leptin is inadequate. Consumption of moderately fat restricted (∼20%) meals results in increased postprandial insulin secretion and higher leptin levels over a 24-hour period compared with a day during which the same subjects consumed relatively high-fat (60%), low-carbohydrate (20%) meals (36). Therefore, it is necessary to examine the time-course of insulin responses to meals and leptin concentrations over a prolonged period of time to assess the impact of dietary macronutrient content and composition on insulin secretion and leptin production adequately.Increased insulin secretion and leptin production may contribute to the effects of these diets because both insulin and leptin act as long-term signals back to the brain to regulate appetite, energy intake and energy expenditure. Regarding energy expenditure, during the weight maintenance phase of the study discussed above, subjects needed to be fed 7% more calories (+120 ± 30 kcal/d) to maintain a stable body weight when they consumed 15% energy from fat compared with when they consumed 30% energy from fat. This suggests that lowering dietary fat content also lowers regulated level of body adiposity, independent of energy intake. This change of the regulated level of body fat independent of energy intake would have to be due to an increase of energy expenditure, an effect that could potentially be mediated by increases of carbohydrate-induced postprandial insulin responses and 24-hour leptin production (36).

  • ***

    It is interesting to note the range of compliance to the advice to decrease fat intake, in this case, from 44% of total calories to 31%, an amount that is considered moderate, but not low-fat.