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Objective: Previous results from this laboratory suggest that a 1-year dairy intake intervention in young women does not alter fat mass. The objective of this study was to determine the impact of the 1-year dairy intervention 6 months after completion of the intervention.
Research Methods and Procedures: Previously, normal-weight young women (n = 154) were randomized to one of three calcium intake groups: control (<800 mg/d), medium dairy (1000 to 1100 mg/d), or high dairy (1300 to 1400 mg/d) for a 1-year trial (n = 135 completed). In the current study, 51 women were assessed 6 months after completion of the intervention trial. Body compositions (body fat, lean mass) were measured using DXA. Self-report questionnaires were utilized to measure activity and dietary intake (kilocalories, calcium).
Results: The high-dairy group (n = 19) maintained an elevated calcium intake (1027 ± 380 mg/d) at 18 months compared with the control group (n = 18, 818 ± 292; p = 0.02). Mean calcium intake over the 18 months predicted a negative change in fat mass (p = 0.04) when baseline BMI was controlled in regression analysis (model R2 = 0.11). 25-Hydroxyvitamin D levels were correlated with fat mass at each time-point (baseline, r = −0.41, p = 0.003; 12 months, r = −0.42, p = 0.002; 18 months, r = −0.32, p = 0.02) but did not predict changes in fat mass.
Discussion: Dietary calcium intake over 18 months predicted a negative change in body fat mass. Thus, increased dietary calcium intakes through dairy products may prevent fat mass accumulation in young, healthy, normal-weight women.
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Obesity is a rapidly growing epidemic world wide. Recent estimates suggest that 30% of adults age 20 years or older are obese (1). In 2003, more than 25% of the adult population in four states were obese (1). Furthermore, 16% of the children and teens in the U.S. were overweight, and this percentage has tripled since 1980 (1). Although genetics play an important role, the rapid rise in the development of obesity supports that lifestyle factors are likely to contribute substantially to this condition (1).
Results from epidemiological research suggest that calcium from dairy products is negatively associated with body weight (2, 3, 4). However, results of intervention trials have been variable. Increased dairy product intake incorporated into weight loss diets leads to decreased fat mass and waist circumference in several randomized intervention trials (6, 7). The results of weight loss trials that include calcium supplementation are more variable (7, 9). Little information is available on the effects of dairy products or calcium intake on prevention of fat mass or weight gain. A 2-year prospective trial (n = 54) showed that calcium intakes, when corrected for energy intakes, were negatively associated with the 2-year change in weight and fat mass (9). Of the 17 calcium supplementation intervention studies reviewed by Barr (10), only one showed less weight gain in the calcium-supplemented group compared with the controls (11, 12). Therefore, the impact of calcium or dairy products on body fat remains controversial.
There are several proposed mechanisms for the negative impact of dietary calcium on body weight regulation. There are data to suggest that high concentrations of calcium intake can decrease absorption of dietary fatty acids through the formation of indigestible calcium soaps in the gastrointestinal tract (13, 14). Dietary calcium regulation of parathyroid hormone (PTH)1 and 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] has also been proposed to mediate the effects of calcium on fat mass. Higher intakes of calcium suppress PTH and subsequently 1,25(OH)2D. Both PTH and 1,25 dihydroxyvitamin D3 increase levels of intracellular calcium in adipocytes, which can lead to a decrease in lipolysis and an increase in lipogenesis through increases in fatty acid synthase levels in the cell (15). This shift in lipid use may lead to an accumulation of fat. In addition, 25-hydroxyvitamin D (25OHD), the marker for vitamin D status, is negatively associated with serum PTH (16, 17). Thus, vitamin D status may be associated with fat mass through regulation of PTH as well.
Gunther et al. (18) reported results from a 1-year dairy product intervention trial in healthy, normal-weight young women. In this trial, women with low calcium intakes were randomized to one of three calcium intake groups: control (<800 mg/d), medium dairy (1000 to 1100 mg/d), or high dairy (1300 to 1400 mg/d), where dairy products were substituted for other diet components to maintain an isocaloric diet. There was no effect on body fat mass or weight during increased dairy product intake in the 1-year intervention trial. However, it is likely that an effect related to prevention of increases in fat mass may be small, albeit significant, over a period of time (3, 4). For example, the difference between the placebo control and the calcium-supplemented groups in the study of Davies et al. (12) was 0.346 kg/yr. This demonstrates a small but significant effect; thus, the length of this trial (∼4 years) may have enhanced the ability to measure these small changes. Trials of shorter duration, small sample sizes, and/or lack of control for individual energy intake may not yield sufficient power to detect small changes in body fat that dietary calcium may elicit.
Therefore, the current study examined the cohort used in the study by Gunther et al. (18) at 6 months after the completion of the 1-year dairy product intervention trial. The current study tested whether differences in fat mass may be detectable when higher calcium intakes are maintained for a longer period of time. The purpose of the current study was to determine whether participation in a 1-year dairy calcium intervention results in changes in calcium intake and changes in body composition 6 months after the completion of the original study.
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Of the 154 participants who were enrolled in the parent study, 135 completed the 12-month intervention, and 51 participants returned at 6 months after completion of the intervention (18 months). The participants who did not complete the 18-month follow-up (n = 83) were older (20.6 ± 2.6 years) at baseline compared with the participants who completed the follow-up (n = 51, 19.4 ± 1.6 years, p < 0.001). There were no other differences between the completers and non-completers. There were no significant differences in baseline characteristics (0 months) by intervention dietary group assignment of the 51 subjects who completed the 18-month follow-up (Table 1). There were also no significant differences in age, weight, fat mass, lean mass, or energy intake at 18 months by intervention dietary group assignment (Table 1).
Table 1. Baseline and 18-month characteristics of subjects (mean ± standard deviation)*
| ||Control n = 18||Medium dairy n = 14||High dairy n = 19|
|Parameter||Baseline||18 Months||Baseline||18 Months||Baseline||18 Months|
|Age (y)||19.6 ± 1.7||21.2 ± 1.8||19.1 ± 0.9||20.9 ± 0.83||19.5 ± 2.0||20.9 ± 1.9|
|Weight (kg)||60.8 ± 11.6||61.2 ± 10.9||66.1 ± 13.7||66.3 ± 14.8||61.2 ± 7.6||61.5 ± 7.4|
|BMI (kg/m2)||21.9 ± 3.4||22.0 ± 3.0||23.4 ± 4.7||23.6 ± 5.1||21.9 ± 2.6||22.1 ± 2.5|
|Fat mass (kg)||17.0 ± 8.3||16.6 ± 7.2||21.3 ± 10.4||21.1 ± 11.8||17.4 ± 5.6||17.0 ± 5.4|
|Lean mass (kg)||40.1 ± 4.7||40.9 ± 5.0||40.1 ± 3.4||41.0 ± 3.4||39.8 ± 4.5||40.6 ± 3.7|
|Calcium intake (mg/d)||765 ± 248||818 ± 293||721 ± 265||799 ± 308||686 ± 239||1028 ± 380*|
|Energy intake (kcal/d)||1786 ± 523||1732 ± 343||1670 ± 507||1605 ± 519||1838 ± 377||1699 ± 473|
|25OHD (ng/mL)||24.1 ± 7.2||23.8 ± 7.2||23.2 ± 8.0||21.5 ± 8.9||25.9 + 11.4||24.9 ± 9.0|
The impact of higher dairy product intervention for 12 months on 18-month dietary calcium intakes was assessed. There was a trend toward group assignment predicting 18-month calcium intake (r = 0.07; p = 0.06) in ANOVA. In these analyses, 18-month calcium intake of the high-dairy product intake intervention group was greater than the control (1028 ± 380 vs. 818 ± 293, respectively; p = 0.04) and trending to being greater than the medium-dairy intake intervention group (799 ± 308; p = 0.06) (Table 1). Furthermore, the high-dairy group, but not the control or medium-dairy groups, increased calcium intake from baseline (Table 2). When corrected for 18-month calorie intake in a GLM, intervention group assignment predicted 18-month calcium intake (p = 0.03), indicating that the high-dairy intake group maintained a higher calcium intake. Thus, high-dairy product intake during 1-year intervention trial increased 18-month calcium intakes.
Table 2. Change in characteristics of subjects from baseline to 18 months (mean ± standard deviation).
|Change parameter||Control (n = 18)||Medium dairy (n = 14)||High dairy (n = 19)|
|Weight (kg)||0.4 ± 1.7||0.1 ± 6.7||0.3 ± 3.0|
|BMI (kg/m2)||0.1 ± 0.6||0.1 ± 1.3||0.2 ± 1.3|
|Fat mass (kg)||−0.39 ± 1.99||−0.32 ± 4.94||−0.43 ± 2.8|
|Lean mass (kg)||0.85 ± 1.3||0.80 ± 1.3||0.81 ± 1.3|
|Calcium intake (mg/d)*||53 ± 349||78 ± 407||341 ± 415†|
|Energy intake (kcal/d)||−53 ± 646||135 ± 521||−139 ± 407|
The impact of the higher dairy product intake intervention on 18-month body composition measures was assessed. Similar to the results of the participants who completed the 12-month dairy product intervention trial, group assignment did not predict the 12-month change from baseline in fat mass in the participants who completed the 18-month follow-up study (n = 51). In addition, group assignment did not predict change in 18-month fat mass or weight from baseline in a GLM, even when controlled for baseline BMI. There was a trend for 18-month dietary calcium intake to correlate with change in fat mass when uncontrolled (r = −0.25; p = 0.08), and even when controlled for baseline BMI (r = −0.27, p = 0.06). This trend became significant (r = −0.28; p < 0.05) when analyses were controlled for group in addition to baseline BMI. Inclusion of physical activity or mean caloric intake in the model did not alter the results. Consistent with this, the mean calcium intake for the intervention and follow-up combined (mean of 6-, 12-, and 18-month dietary records) (p = 0.04) predicted the 18-month change in fat mass when controlled for baseline BMI in a regression model (R2 = 0.11):
None of the dietary variables (group assignment nor calcium intakes) was correlated with changes in lean mass, whether or not analyses were controlled for baseline BMI. Thus, the mean dietary calcium intake over 18 months was associated with reduced fat mass.
To further explore the relationship between calcium intake over 18 months and fat mass accumulation, the cohort was subdivided by mean calcium intake of either greater than or less than 800 mg/d. The mean fat mass of each of these subgroups over the time of the intervention and the 18-month follow up are shown in Figure 1. Fat mass was significantly different between these subgroups at 18 months (p = 0.02).
Figure 1. Fat mass (kg) from baseline to 18 months by subgroups of < or >800 mg/d calcium intake. (*) p = 0.02 between groups at 18 months.
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The relationship between serum levels of 25OHD and changes in fat mass, weight, and BMI were explored. When 25OHD was added to models to predict changes in body composition, the results were similar to those reported above. Changes in 25OHD at any time-point did not predict changes in any body composition measure. However, 25OHD levels were correlated with fat mass at each time-point (baseline, r = −0.41, p = 0.003; 12 months, r = −0.42, p = 0.002; 18 months, r = −0.32, p = 0.02).
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The results of the current study indicate that increasing dairy product intake through an intervention study promotes increased calcium intakes 6 months after the end of the study period. Furthermore, increased calcium intake leads to a decreased fat mass compared with a low calcium intake over 18 months. There was no effect of group assignment on body fat mass or weight in this cohort (n = 51) at 12 months, which is similar to the total 135 who completed the 12-month study. Thus, the results of the current study support that the impact of calcium intake may be sufficiently small or that fat mass is influenced by other factors, such that it is difficult to detect an effect at 12 months in a normal-weight, healthy young female sample.
The results of this study demonstrate that higher calcium intakes were maintained once the dietary habit was established in a year-long intervention. This is similar to the concept that higher dairy product intake as a child is associated with higher calcium intakes as an adult (22). It is important to develop strategies to promote increased calcium intakes in the U.S. population because intakes are generally far below current recommendations (23). Therefore, increasing dairy product intake in either children or adults for a period of time may lead to enhanced subsequent calcium intakes. The results of this study suggest that participating in an intervention may affect a change in dietary habits for up to 6 months.
Although the perception exists that dairy products will lead to increased weight, in fact, dairy product intake may prevent fat mass gain. In the original cohort (18), the control and low dairy groups did not gain weight over the year; thus, the prevention of weight gain hypothesis was not tested. Our results demonstrate that consuming higher dairy intakes over the 1-year trial promoted increased calcium intakes up to 6 additional months, although not all participants continued to consume higher levels of calcium. In our models, to predict changes in fat mass required high calcium intakes for the year of intervention (group assignment) and continued high calcium intake for another 6 months (18-month calcium intake). Thus, if one consumes higher calcium intakes for 1 year, and then reduces the intake, it may not have a measurable effect on fat mass. However, if one maintains the higher calcium intake for a longer period of time, in this case 18 months, the differences in fat mass become measurable. The current study suggests that women on low calcium intakes will gain fat mass, and higher calcium intakes will reduce fat mass accumulation progressively over 18 months (Figure 1), leading to a small but significant difference in total fat mass between these two populations.
To determine the biological impact, the regression equation described in the results section, which predicted the 18-month fat mass change by mean dietary calcium intake over 18 months when controlled by baseline BMI, was applied. Substituting various calcium intakes into Equation 1 yields, for 500 mg/d, a fat mass gain of 1.26 kg over 18 months, and for 1200 mg/d, a fat mass loss of 0.631 kg over 18 months. The difference in fat mass accumulation is ∼1.89 kg between these two dietary calcium intake exposures over 18 months in normal-weight young women. Although this is a small change, if this effect is maintained over time, it will have a significant impact on the development of obesity. These changes are slightly higher than those of Davies et al. (12), who showed a 0.346 kg/yr difference in weight change between placebo and calcium supplemented (1200 mg/d supplement) in an ∼4-year randomized control trial with post-menopausal women. Although this is a different age group than the current cohort, the estimated size effect is strikingly similar between the two studies. Using the results from specific cohorts from observational studies, Heaney (24) estimated the potential impact of doubling dietary calcium intake in these cohorts. The results suggest that, over time, doubling the dietary calcium intakes of these cohorts predicts a substantial change in overweight and obese prevalence by as much as 60% to 80% (24).
In this study, the influence of calcium on fat mass cannot be separated from other dairy product components because the increase in dietary calcium was achieved through dairy product intake. A few studies (5, 8, 12, 13), but not all (7, 11), suggest an independent effect of calcium; however, longer studies may be necessary to detect differences (3, 4, 11). There is some evidence to support the role of other dairy product components, including amount and type of protein (25, 26) and conjugated linoleic acid (27). Studies in animal models and in humans support that intake of dairy products may enhance effects of calcium intake (6). It is important to understand the mechanism of dietary calcium and other factors (physical activity, adiposity, vitamin D status, and other dairy product components) on body fat mass to predict which individuals are more likely to benefit by manipulation of these factors.
The current results that demonstrate a negative relationship between serum 25OHD levels and fat mass are consistent with previous literature (28, 29). 25OHD negatively correlated with both BMI and body fat mass in the study by Parikh et al. (29). It is proposed that adipose tissue may serve as a reserve for vitamin D storage; thus, greater adiposity reduces serum 25OHD levels. However, it is intriguing to consider that the lower vitamin D status may promote increased adiposity. It is clear that vitamin D status is a key regulator of fasting serum PTH levels (17, 18). In fact, in this cohort, the baseline 25OHD levels were negatively correlated to baseline PTH levels. Thus, it is intriguing to consider that the suppression of PTH levels by improved vitamin D status may also contribute to regulation of fat mass accumulation (3, 28, 29).
The limitations of the current study include that only 38% of the cohort that completed the original trial returned. It is possible that only those who were compliant during the study or who continued to consume dairy products volunteered to return for the follow-up study. However, the medium-dairy intake group who returned did not maintain intervention study calcium intakes and was similar to the control group at 18 months. In addition, group assignment alone did not predict the change in fat mass. Thus, the results of this study suggest that higher calcium intakes over 18 months are associated with reduced body fat, but further research is needed to confirm this association.
In summary, a 1-year dairy product intervention in young women resulted in increased calcium intakes that were sustained for 6 months after the study was stopped. In addition, maintenance of higher dietary calcium intakes over 18 months resulted in a reduced fat mass accumulation compared with low dietary calcium intakes in healthy, normal-weight, young women. Although the effect is small, 18-month maintenance of higher dairy calcium intakes may lead to prevention of slow age-related fat mass gain. Further long-term intervention studies are necessary to more fully understand the influence of calcium and/or dairy products on prevention of fat mass gain.