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Keywords:

  • protein intake;
  • bone density;
  • elderly;
  • osteoporosis;
  • longitudinal study

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Few studies have evaluated protein intake and bone loss in elders. Excess protein may be associated with negative calcium balance, whereas low protein intake has been associated with fracture. We examined the relation between baseline dietary protein and subsequent 4-year change in bone mineral density (BMD) for 391 women and 224 men from the population-based Framingham Osteoporosis Study. BMD (g/cm2) was assessed in 1988-1989 and in 1992-1993 at the femur, spine, and radius. Usual dietary protein intake was determined using a semiquantitative food frequency questionnaire (FFQ) and expressed as percent of energy from protein intake. BMD loss over 4 years was regressed on percent protein intake, simultaneously adjusting for other baseline factors: age, weight, height, weight change, total energy intake, smoking, alcohol intake, caffeine, physical activity, calcium intake, and, for women, current estrogen use. Effects of animal protein on bone loss also were examined. Mean age at baseline (±SD) of 615 participants was 75 years (±4.4; range, 68-91 years). Mean protein intake was 68 g/day (±24.0; range, 14-175 g/day), and mean percent of energy from protein was 16% (±3.4; range, 7-30%). Proportional protein intakes were similar for men and women. Lower protein intake was significantly related to bone loss at femoral and spine sites (p ≤ 0.04) with effects similar to 10 lb of weight. Persons in the lowest quartile of protein intake showed the greatest bone loss. Similar to the overall protein effect, lower percent animal protein also was significantly related to bone loss at femoral and spine BMD sites (all p < 0.01) but not the radial shaft (p = 0.23). Even after controlling for known confounders including weight loss, women and men with relatively lower protein intake had increased bone loss, suggesting that protein intake is important in maintaining bone or minimizing bone loss in elderly persons. Further, higher intake of animal protein does not appear to affect the skeleton adversely in this elderly population.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

OSTEOPOROSIS IS an important public health problem, affecting 20-25 million Americans. (1–3) Although major risk factors have been identified, dietary factors, specifically macronutrients, represent an important understudied area in osteoporosis research.(4) Few studies have evaluated diet and bone loss in free-living elders. Protein is of particular interest and concern because biochemical and nutritional studies from as early as 1920,(5,6) and again more recently, (7–10) have shown that high protein intake is a powerful determinant of urinary calcium loss, which could potentially upset calcium balance and lead to bone loss. Despite the fact that several studies suggest a role for protein in bone health, none has examined the effect of levels of dietary protein on bone loss.

Dietary protein can cause an increased acid load, which may be buffered by bone calcium. It is thought that the high sulfur content of meat may determine an endogenous acid load that contributes to bone loss. Sebastian et al. showed that bone loss in 18 postmenopausal women resulted from mobilization of skeletal calcium salts used to balance acid from dietary protein metabolism, and that increased acidity stimulated bone resorption and inhibited bone formation through suppression of osteoblast function.(11) The increase in urinary calcium may depend on the amount and type of protein intake because dietary acid load may be somewhat influenced by the ratio of meat to vegetable protein intake,(4) although the effect of type of protein intake on bone remains unclear. (12–14)

Evidence also suggests that protein undernutrition is associated with osteoporosis. Low protein intake or insufficiency, perhaps a marker of total dietary insufficiency, has been associated with frailty and fracture in the elderly. (15–17) Evidence from a nursing home study of fractures and protein intake, although not controlling for confounders, suggested that protein undernutrition may be associated with osteoporotic fracture.(18) Thus, extreme levels of dietary protein intake may pose a problem in elderly persons.

To our knowledge, no previous studies have addressed the possible contributions of dietary protein to bone loss or considered the effect of animal protein specifically in population-based groups. Elderly persons are of particular interest because they often suffer from chronic diseases and may have a range of protein intakes below amounts previously linked to negative calcium balance. Thus, the purpose of our study was to examine the association between dietary protein and the skeletal health of elderly men and women. We examined the relation between baseline dietary protein and subsequent 4-year change in femoral, radial shaft, and spine bone mineral density (BMD) for the elderly members of the population-based Framingham Study. Further, our study evaluated the effect of animal protein and nonanimal protein intake for a possible effect on bone loss in older individuals.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

Study subjects

The population-based Framingham Cohort was established in 1948 to examine risk factors for heart disease in 5209 men and women of ages 28-62 years. (19–21) Subjects are seen biennially for a physical examination and a battery of questionnaires and tests. Since the inception of the primarily white cohort 50 years ago, nearly two-thirds of the 5209 cohort members have died. The surviving, now elderly, cohort subjects follow the same age- and sex-specific population proportions found in the general Framingham population.(22) At biennial examination 20 (1988-1989), 855 cohort members participated in the Framingham Osteoporosis Study in which they had femoral BMD measurements and proximal radial scans and completed a semiquantitative Food Frequency Questionnaire (FFQ). Because of the length of the routine biennial Framingham Study clinic examination, subjects were asked to return for a callback examination to obtain BMDs at the lumbar spine. Results from the baseline Framingham Osteoporosis Study have been reported previously.(22)

As part of their regular clinic visit 4 years later (1992-1993), 615 subjects (72%) who had baseline BMD and the FFQ assessed had repeat BMD measures. All longitudinal scan pairs were evaluated for consistency in anatomic site and quality of analysis by the original technician, and those scans showing inconsistencies were reanalyzed by two experienced investigators (M.T.H. and D.P.K.). Details on the longitudinal follow-up osteoporosis examination have been reported.(23) In brief, the mean 4-year BMD losses were much greater for women than the losses for men. Annualized rates of mean percent BMD loss are as follows: for women, femoral neck loss was 0.87%, trochanter was 0.86%, Ward's area was 1.06%, radial shaft was 1.21%, and spine was 1.12%; for men, femoral neck loss was 0.38%, trochanter was 0.04%, Ward's area was 0.16%, radial shaft was 0.90%, and spine was 0.09%.

Our study was approved by the Boston University Institutional Review Board, and written informed consent was obtained for all study subjects at both examinations.

Bone mineral density

BMDs of the proximal right femur (femoral neck, greater trochanter, and Ward's area) as well as the lumbar spine (average BMD of L2-L4) were measured in grams per square centimeter, using a Lunar dual photon absorptiometer (DP3) at baseline and a dual X-ray absorptiometry (DPX-L) densitometer (Lunar Radiation Corporation, Madison WI, U.S.A.) at the 4-year follow-up exam. We have previously shown high correlations between dual photon and dual X-ray absorptiometry.(24) However, because of a small but consistent shift in BMD values between the two technologies, femoral BMDs were adjusted for the change in equipment from DP3 to DPX-L technology, using published corrections.(24) Bone density at the proximal radial shaft site was measured in grams per square centimeter using a Lunar SP2 single photon absorptiometer (Lunar Radiation Corporation) at both examinations. The right side was scanned at each exam unless there was a history of previous fracture or hip joint replacement. For these individuals, the left side was scanned. We used standard positioning as recommended by the manufacturer. Monthly measurements of a bone phantom over the follow-up period showed no machine drift across time. At the baseline examination, the coefficients of variation in normals measured twice with repositioning for the DP3 were 2.6% (femoral neck), 2.8% (trochanter), 4.1% (Wards area), 2% (radial shaft), and 2.2% (lumbar spine). At examination 22, the coefficients of variation for the DPX-L measurements were 1.7% (femoral neck), 2.5% (trochanter), 4.1% (Ward's area), and 0.9% (lumbar spine). We examined percent change in BMD from baseline to 4-year follow-up. Percent change in BMD was calculated as the difference between exam 20 BMD and exam 22 BMD, divided by exam 20 BMD, and multiplied by 100.

Food Frequency Questionnaire

Dietary intake was assessed using the 126-item Willett FFQ(25,26) at the baseline examination in 1988-1989 and these data were converted to food and nutrient intake data. Individuals who reported intakes of less than 600 or greater than 4000 kcal or with data missing for more than 12 food items were excluded. The Willett FFQ has been validated, and nutrient intakes have been shown to correlate well with those obtained by multiple food records and with blood measures for several nutrients.(26,27) However, it is only semiquantitative in nature, and translation of results to quantitative recommendations remains to be validated for a number of nutrients. Nevertheless, the Willett FFQ has been shown to rank individuals well in relation to their actual intake. In general, usual intake patterns examined for the purpose of ranking individuals for association with an outcome measure, typically are best measured with an FFQ. The Willett FFQ has been validated extensively and has been shown to be reproducible in the long term for both cohorts of men(25) and women,(26) showing that the FFQ is a stable and reproducible measure of ranking for food intakes. As Willett points out,(28) the underlying principle of an FFQ is that average long-term diet intake is the conceptually important exposure rather than short-term intake across a few specific days. FFQ methodology therefore estimates usual intakes over the past year indicating typical long-term intakes rather than providing precise measurements of short-term intake. To evaluate whether protein intakes may change over time, we examined the change in rank from the FFQ measured at baseline in our study to another measurement of FFQ 2 years later. There were only minor changes reported in intakes, such that no one in our study changed quartile of protein intake.

Protein intake

Dietary protein intake was expressed as percent of energy from protein (percent protein). Further, components of protein were divided into animal and nonanimal protein intake and expressed as percent of energy from animal protein and percent from nonanimal protein.

Potential confounding variables:

We controlled for the influence of the following factors at baseline on the relation between dietary protein intake and bone loss at each skeletal site: total energy intake, age, sex, weight, height, smoking, caffeine, alcohol use, physical activity, calcium intake, and for women current estrogen use. We also included a well-known risk factor for bone loss: weight change over the 4-year follow-up. Weight in pounds (converted to kilograms) was measured at baseline using a standardized balance beam scale and weight at follow-up was measured similarly using the same calibrated scale. In addition to baseline weight, we included percent weight change over the follow-up as a continuous variable. Height (without shoes) was measured in inches using a stadiometer and recorded to the nearest ¼ inch. A participant's smoking status was assessed via questionnaire at baseline and preceding examinations as current cigarette smoker at baseline (smoked regularly in the past year), former smoker, or never smoked.

Caffeine use, incorporating coffee and tea intake at baseline, was defined as the sum of daily coffee (1 cup equals 1 caffeine unit) and daily tea (1 cup equals 0.5 caffeine units) and grouped into 0-2 caffeine units consumed per day or over 2 U/day.(29,30) Weekly use of beer, wine, or hard liquor at baseline was grouped into ounces of alcohol consumed per week based on Framingham calculations of intake.(31,32) Physical activity at baseline was defined using the Framingham Physical Activity Index, a weighted 24-h score of typical daily activity, based on hours spent doing heavy, moderate, light, or sedentary activity as well as sleeping.(33,34) Baseline dietary calcium intake, including calcium supplements, was assessed from the FFQ and evaluated as milligrams per day of calcium. For women, current use of oral conjugated estrogen, patch, or cream at or within a year of the baseline examination was determined from the baseline Framingham Study questionnaire. Estrogen use was defined as current users versus noncurrent users (including both former and never users), based on prior work showing effects only for current estrogen use in elderly women.(35) Too few of the elderly women were current users to adjust further for dose of estrogen use.

Statistical analyses

Baseline characteristics were compared using Student's t-tests or χ2 tests as appropriate. We evaluated each BMD site separately, using linear regression to examine the relation of change in BMD with percent protein intake with simultaneous adjustment for potential confounding variables. Because total energy intake is correlated with most nutrients, we adjusted for total energy intake to assess the independent effect contributed by protein. By doing so, possible differences in intake caused by body size or activity levels also are taken into account.(28,36) The model evaluated BMD change over the 4 years as a function of percent protein intake, total energy intake, age, sex, and the major known risk factors for bone loss: weight, weight change, height, smoking, and alcohol use. We further adjusted for the potential additional effects of physical activity, calcium intake, and for women, current estrogen use (yes/no).

We examined the effect of protein intake in the BMD models using several well-established statistical methods(28):(1) the residual method, regressing protein grams of intake on total energy intake and used these residuals in the BMD analyses; (2) the nutrient density model, adjusting grams of protein intake for total energy intake in standard multivariate model; (3) the energy decomposition model, adjusting protein intake for the total nonprotein energy intake; and, finally, (4) protein not adjusted for total energy. Because all these methods provided very similar results, only the results from the analyses using percent protein adjusted for total energy will be presented.

We evaluated percent protein as a continuous variable and as quartiles of intake, to evaluate the possibility of a nonlinear relation. The quartile analysis presents the protein and BMD relation across the intake ranges seen in a population setting to examine the effect of possible levels of protein intake on BMD loss, because it is unclear what intake levels may cause concern in the elderly. Men and women had similar distributions of overall protein intake and animal protein intake, as well as similar relations between protein intake and BMD change. There were no sex by protein interactions in models for any bone site. Thus, analyses for men and women were combined. For the quartile analyses, adjusted mean BMDs are presented (least squares means ± SE), for levels of protein intake resulting from the analysis of covariance (ANCOVA). All analyses used the SAS statistical analysis package (release 6.12; SAS Institute Inc., Cary, NC, U.S.A.). Models of the absolute change in BMD from baseline examination to follow-up examination showed similar results as the percent change analyses, and, thus, the absolute change analyses are not presented here. First, we evaluated the impact of dietary protein intake on bone loss. Second, we examined the influence of animal protein versus nonanimal protein intakes.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

For the 615 subjects (391 women and 224 men) with longitudinal data, the mean age (±SD) at baseline was 75 years (±4.4 years) with a range from 68 to 91 years. Cohort members without follow-up data (the majority of whom died during the follow-up period) tended to be older, of lower weight, and male, with baseline BMDs slightly lower than those followed (e.g., femoral neck BMD, 0.763 vs. 0.789 and p = 0.02); however, their baseline protein intakes did not differ from cohort members with 4-year follow-up (Table 1). Participants and nonparticipants had similar distributions for smoking, alcohol intake, and calcium intake.

Table Table 1.. Comparison of Baseline Characteristics in Framingham Cohort Members with Dietary FFQ Data Who Attended Both Baselinea and Follow-Up Examinations to Those Members Who Only Attended Baseline Examination
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Table 2 shows the distributions for mean protein intake as well as percent of energy from protein by sex. Mean protein intake for the participants was 68 ± 23.6 g/day (SD) with a range from 17 to 152 g/day. Protein comprised 16% (±3.4%; range, 7-27%) of total energy intake. Percent of energy from animal protein was 10% (±3.5%).

Table Table 2.. Distributions of Types of Dietary Protein Intake by Sex
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Baseline BMD values at the femoral neck, trochanter, Ward's area, radial shaft, and lumbar spine were not significantly associated with protein intake, with p values ranging from 0.30 (trochanter and radius) to 0.54 (Ward's area). Mean 4-year BMD losses have been reported,(23) and for women, ranged from −4.84% (radial shaft) to −3.42% (trochanter), while losses for men ranged from −3.59% (radial shaft) to −0.17% (trochanter).

 

Overall dietary protein intake:

Lower percent protein intake was significantly related to greater BMD loss at all femur and spine sites (all p < 0.02), but not at the radial shaft (p = 0.26). After adjusting for the major risk factors for osteopenia, lower percent protein intake remained significantly related to greater BMD loss at the femoral neck (β = 0.203; p = 0.02), Ward's area (β = 0.266; p = 0.04), and spine (β = 0.281; p = 0.02) with regression coefficients at all three sites comparable with 10 lb of weight or a smoking effect, both well-established risk factors for osteopenia. The p values for all the overall global ANCOVA models ranged from 0.0001 for the femoral neck and lumbar spine to 0.0628 for the radial shaft. Further adjustment for physical activity, calcium intake, and for women, current estrogen use, did not alter the effect of protein intake on change in BMD.

When quartiles of percent protein intake were evaluated, the lowest protein quartile showed the greatest bone loss. (Fig. 1). Similar results were seen at the other femur sites and the lumbar spine, with a similar trend observed for the radial shaft. When the relation between BMD loss and protein was adjusted for the major known risk factors for bone loss—weight, weight change, height, age, sex, smoking, and alcohol use—the lowest quartile of protein continued to have the greatest BMD loss (Table 3). The highest quartile, with protein intakes of 1.24-2.78 g/kg· per day, showed the least BMD loss across follow-up. Again, further adjustment for physical activity, calcium intake, and for women, current estrogen use, did not alter these results.

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Figure FIG. 1. Mean percent bone loss over 4 years (±SE) at hip, spine, and radius by quartiles of protein intake (Framingham Osteoporosis Study).

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Table Table 3.. Quartiles of Percent Protein Intake by Adjusted Least Squares Mean Percent BMD Change at Hip, Spine, and Radius Sites
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Animal protein intake:

Similar to the overall protein effect, lower percent of energy from animal protein also was significantly related to bone loss at all femoral and spine BMD sites (all p < 0.01) but not the radial shaft (p = 0.23). Percent of energy from nonanimal protein did not contribute to these BMD models (p value range, 0.79-0.98). When these models were adjusted for the major risk factors for osteopenia, the relation between BMD change and animal protein intake remained statistically significant (femoral neck, p = 0.03; lumbar spine, p = 0.02) in a manner consistent with the overall protein effect described previously. Again, further adjustment of the model for physical activity, calcium intake, and for women, current estrogen use, did not alter the results.

When percent animal protein quartiles were evaluated, the lowest quartile showed the greatest bone loss (Fig. 2), similar to the overall protein intake results for quartiles, although no relation was seen between radial shaft bone loss and animal protein intake. Table 4 displays the least squares mean results for all BMD sites and, again, these results parallel those seen in Table 3. None of the BMD sites indicated a relation between BMD and nonanimal protein. Figure 3 shows the lack of a relation between nonanimal protein intakes (quartiles) and bone loss at the femoral neck.

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Figure FIG. 2. Mean percent bone loss over 4 years (±SE) at hip, spine, and radius by quartiles of animal protein intake (Framingham Osteoporosis Study).

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Figure FIG. 3. Mean percent femoral neck bone loss over 4 years (±SE) by quartiles of nonanimal protein intake (Framingham Osteoporosis Study).

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Table Table 4.. Quartiles of Percent Animal Protein Intake and Adjusted Least Squares Mean Percent BMD Change at Hip, Spine, and Radius Sites
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DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

The mean protein intake of 68 g/day and percent protein intake of 16% for the study participants are similar to recommendations for total protein and percent of energy from protein set by the U.S. Government and also similar to other reported values.(37,38) The current recommended daily allowance (RDA) for protein intake is 0.8 mg/kg, and roughly 32% of the Framingham Cohort have protein intake below this RDA; however, 90% have an intake at least two-thirds of the RDA (two-thirds of RDA is a common cut-off used to estimate dietary adequacy, because the RDA contains a margin of error to cover most healthy individuals). In our study, both lower protein intake and lower animal protein intake were significantly related to greater BMD loss at femur and spine BMD sites. Even after adjustment for the major risk factors for bone loss, including weight, weight loss, and smoking, lower, but not higher, protein intake remained significantly related to greater BMD loss at the femur and spine with effects comparable with 10 lb of weight or a smoking effect, both well-established risk factors for osteopenia. Those elders with higher protein intake had reduced bone loss, suggesting that protein intake is important in maintaining bone or minimizing bone loss in elderly persons.

Contrary to expectations, elders with animal protein intake up to several-fold greater than the RDA also had the least bone loss after controlling for known confounders. Nonanimal sources of protein were not related to BMD. These results suggest that typical population intakes of animal protein, within the range commonly consumed, do not result in bone loss. Rather animal protein intake appears important in maintaining bone or minimizing bone loss in elderly persons.

A number of studies (4, 39–41) reported that a doubling of protein intake increases urinary calcium loss by 50%. Parfitt also noted that the acid load from dietary protein is partially buffered by skeletal bone loss, accounting for a portion of age-related bone loss.(42) Allen reported that urinary calcium loss is correlated directly with dietary intake of protein and that high calcium diets do not prevent the negative calcium balance and bone loss induced by a diet high in protein,(43) although it is unclear what levels of protein intake would be considered high for this population. The influence of dietary protein metabolically may be not as great in the elderly as one would assume, based on estimated intake, because additional age-related changes in renal function and intestinal absorption influence calcium imbalance.(44) Although these studies examined short-term calcium loss, several cross-sectional studies of forearm BMD and protein intake reported no association between dietary protein and BMD.(14,45)

Our findings are in agreement with two papers reporting better bone health in women with greater protein intakes. Freudenheim et al.(46) showed that high protein intake protected against low radius BMD in older women. Munger et al. found an increased risk of hip fracture in elderly women consuming the lowest amounts of protein in the Iowa Women's Health Study.(47) Further, the Munger study reported that higher intakes of animal sources of dietary protein were associated with a 70% reduction in hip fracture, even after controlling for major confounding variables.

A number of studies support the adverse role of low protein intake on bone metabolism. Hirota et al.(48) reported that low protein intake in young Asian women was a risk factor for low forearm BMD in their study of 161 women ages 19-25 years. Similarly, Geinoz et al.,(49) in a study of 74 hospitalized geriatric patients (mean age, 82 years), showed that low dietary protein intake was associated with low femoral BMD. Additional studies of protein supplementation in elderly women posthip fracture clearly showed benefit in terms of BMD and muscle strength,(50,51) implying that protein insufficiency, particularly in the oldest of old, contributes to osteoporosis. In addition, two studies reported that hip fracture patients had diets particularly deficient in protein and energy.(17,18) Kerstetter et al. show that low protein intake may induce secondary hyperparathyroidism, perhaps inducing loss of bone.(52,53) Indeed, there is potential for interventions with protein intakes. Schurch et al. conducted a 6-month randomized clinical trial of protein supplementation of 20 g/day in hip fracture patients and showed 50% attenuation in femoral bone loss after 1 year of follow-up.(54)

Orwoll et al. examined vertebral and radius BMDs and reported that protein undernutrition, as assessed by serum albumen measures, was associated with osteopenia, and that low dietary protein intakes possibly influence bone metabolism.(55) They noted that although excess dietary protein has been shown to cause negative calcium balance, this occurs primarily with extremely high levels of protein, not often seen in the elderly who are at the highest risk for osteoporosis. Indeed, Chu et al., in a trial that doubled the typical protein intake in elders, reported that rather than causing negative calcium balance, the increased protein intakes improved the calcium balance in the majority of their subjects.(56) These studies raise the possibility that low protein intakes, particularly in the elderly, may adversely affect bone mineral metabolism.

These studies, when considered together, suggest that adequate protein nutriture is required for bone health. Metabolic studies showing that high intakes of protein may have an adverse consequence on calcium balance have all been of short-term duration. It remains unclear what the long-term influences of protein intake and its possible acid load have on bone. It may well be that only extreme excess protein intake or deficient protein intake may be deleterious yet uncommon problems in human populations. Our results suggest that within the normal variation in dietary protein of this population of elders low protein intake is associated with BMD loss, while higher (normal) protein intake is associated with reduced bone loss or with maintenance of BMD.

This study has several unique strengths. First, the Framingham cohort is population based, rather than a study of volunteers or those who already have disease. Second, the longitudinal design provides important information about factors responsible for bone loss in old age. Third, unlike most studies of osteoporosis, it includes a large number of men as well as women who have longitudinal BMD measurements. Fourth, this study has comprehensive dietary assessments that have shown to estimate usual nutrient intake. Finally, the finding of a consistent protein effect at multiple bone sites after control of potential confounders strongly suggests that adequate protein intake is important to skeletal health of aged persons.

Several limitations also should be noted. The data do not take into account possible longer-term effects of certain risk factors (e.g., cumulative effect of smoking or medications). Second, different technology assessed femoral BMD at baseline and follow-up, although data were “standardized.” Finally, we cannot derive precise protein intake values representing the lowest threshold, because we used an FFQ that typically best ranks individuals' dietary intakes.

As the population ages, osteoporosis will escalate in importance as a major public health problem. This study suggests an important, potentially modifiable factor that could have major implications for the diets of millions of men and women as well as affect their risk of osteoporosis. Our results indicate that low protein intake is associated with BMD loss. Campbell et al.(57) make a compelling argument that the protein RDA for older persons in the United States, established from extrapolations from healthy young men, is too low. Based on several protein requirement studies of elderly subjects, they recommend a safe protein intake to range from 1.0 to 1.25 g/kg per day. These values would correspond roughly to the third quartile in our analyses. A metabolic balance study(58) and a nutritional status survey of elders over the age of 65 years(59) also provide strong support that elders have a higher protein requirement than that currently recommended. Even so, the U.S. Department of Agriculture (USDA) data on nutrition report that 30% of all adult women are below the RDA for protein and over 25% of the Framingham cohort have protein intake below the current RDA of 0.6 g/kg. If the protein intake sufficiency threshold is even higher for elders than the current RDA, low protein intake may place many elders at risk for bone loss.

This population-based study suggests that dietary protein intake is an important component of bone health in elders, showing an effect for both women and men with age-related bone loss, even after controlling for major known confounders. Further, this study indicates that high levels of dietary protein intake, within the range commonly consumed, do not result in bone loss in elders. Ensuring adequate dietary protein intake is an important component of bone health in elders.

ACKNOWLEDGMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES

We are grateful to the Framingham cohort participants and staff, and we also thank the densitometer technicians Mimi Brodsky, Mary Hogan, and Cherlyn Mercier. This work was supported in part by a New Investigator Award from the Arthritis Foundation, National Institutes of Health (NIH) grants RO1-AR/AG41398 and AR20613, and U.S. Department of Agriculture (USDA) Agricultural Research Service (ARS) contract number 53-3K06-01.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGMENTS
  8. REFERENCES
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