The Effect of a Whey Protein Supplement on Bone Mass in Older Caucasian Adults Free

Context:

It has been assumed that the increase in urine calcium (Ca) that accompanies an increase in dietary protein was due to increased bone resorption. However, studies using stable Ca isotopes have found that dietary protein increases Ca absorption without increasing bone resorption.

Objective:

The objective of the study was to investigate the impact of a moderately high protein diet on bone mineral density (BMD).

Design:

This was a randomized, double-blind, placebo-controlled trial of protein supplementation daily for 18 months.

Setting:

The study was conducted at two institutional research centers.

Participants:

Two hundred eight older women and men with a body mass index between 19 and 32 kg/m² and a self-reported protein intake between 0.6 and 1.0 g/kg participated in the study.

Intervention:

Subjects were asked to incorporate either a 45-g whey protein or isocaloric maltodextrin supplement into their usual diet for 18 months.

Main Outcome Measure:

BMD by dual-energy x-ray absorptiometry, body composition, and markers of skeletal and mineral metabolism were measured at baseline and at 9 and 18 months.

Results:

There were no significant differences between groups for changes in L-spine BMD (primary outcome) or the other skeletal sites of interest. Truncal lean mass was significantly higher in the protein group at 18 months (P = .048). C-terminal telopeptide (P = .0414), IGF-1 (P = .0054), and urinary urea (P < .001) were also higher in the protein group at the end of the study period. There was no difference in estimated glomerular filtration rate at 18 months.

Conclusion:

Our data suggest that protein supplementation above the recommended dietary allowance (0.8 g/kg) may preserve fat-free mass without adversely affecting skeletal health or renal function in healthy older adults.

It has been recognized for more than a century that increasing dietary protein increases urinary calcium (UCa), but the source of this additional UCa remains unclear. Until recently it was hypothesized that the increment in UCa induced by increases in dietary protein was due to an increase in bone resorption. In particular it was assumed that bone catabolism was required to liberate buffer required to compensate for the fixed metabolic acid load imposed by the metabolism of sulfur-containing amino acids (1–4). Early balance studies (5, 6) failed to show a change in intestinal Ca absorption with increased dietary protein, making the skeleton the only plausible source of the additional UCa. However, more recent evidence from short-term studies using dual-stable Ca isotopes has demonstrated that increasing dietary protein improves intestinal Ca absorption and that the increment in UCa can be quantitatively explained by an increase in Ca absorption (7, 8). Furthermore, in one study calcium (Ca) retention was improved as dietary protein increased, although this effect was observed only when dietary Ca was relatively low, 675 mg/d (9).

To address the longer-term impact of dietary protein on skeletal metabolism, we undertook an 18-month study in older adults who were selected on the basis of a reported low-moderate protein intake (0.6–1.0 g/kg). Subjects were randomized to receive either a 45-g whey protein isolate (representing 40 g of protein) or an isocaloric maltodextrin control supplement (carbohydrate group). In addition to measuring bone mineral density (BMD) by dual-energy x-ray absorptiometry (DXA) and quantitative computed tomography (QCT), body composition, circulating levels of IGF-1, and biochemical markers of skeletal and mineral metabolism were evaluated.

Materials and Methods

Study design

This was a multicenter, double-blind, randomized controlled trial to determine the effect of 18 months of dietary supplementation with either whey protein (protein group; Provon 290; Glambia Nutritionals) or maltodextrin (carbohydrate group; Maltrin M100; Grain Processing Corp) on bone mass in older men and women. The supplements were closely matched for composition, color, and the following nutrients: kilocalories, sodium, potassium, phosphorus, fiber, and Ca (Supplemental Table 1). BMD was assessed by DXA in all subjects at baseline and at 9 and 18 months. QCT was also used to assess bone mass in half of the study subjects at baseline and after the 18 month intervention. Blood samples were analyzed for measures of intact PTH, IGF-1, creatinine, and markers of bone turnover. Twenty-four hour urine samples were collected for urea and Ca. Serum 25-hydroxyvitamin D [25(OH)D] was measured as an entry criteria. If a participant's level was less than 50 nmol/L (20 ng/mL) at randomization, they received a one-time dose of 50 000 IU of vitamin D₂ per os, and their level was rechecked at visit 3. All participants were vitamin D sufficient at baseline (Table 1). Physical activity, assessed by the Baecke Questionnaire (10), tobacco use, and alcohol consumption were evaluated at baseline and 18 months. The Baecke Questionnaire is a validated instrument (11) used to assess frequency, intensity, and duration of three components of physical activity: work, sport, and leisure. Scores can range between 3 (minimum level) and 15 (maximum level). Height and weight were obtained at each study visit using a balance beam scale and used to calculate body mass index (BMI).

Subject recruitment

Subjects were recruited at two study centers (The University of Connecticut Health Center, Farmington, CT, and Yale University School of Medicine, New Haven, Connecticut). Potential volunteers were screened by dietitians using a health history and diet questionnaire in an effort to recruit patients whose self-reported protein intake ranged between 1.0 and 0.6 g/kg · d. Women over the age of 60 years and men over the age of 70 years were recruited to the study. Men and women with self-reported protein intakes less than 0.6 g/kg or greater than 1.0 g/kg, or a BMI greater than 32 kg/m² or less than 19 kg/m² were excluded. Additional exclusion criteria are listed in Supplemental Table 2. Supplemental Figure 1 summarizes the number of subjects who were screened and randomized. Of the subjects screened, 208 met final inclusion criteria and were randomized. Treatment assignment was performed using a random numbers generator. All participants and members of the research team were blinded to treatment allocation except for the research pharmacists. The study was approved by investigational review boards at the two study sites. All participants gave their written informed consent.

Measurements of bone mass and body composition

BMD and body composition were measured by DXA at baseline and at 9 and 18 months using either a Hologic 4500W machine (Yale University School of Medicine) or a Lunar Prodigy DPX-IQ (University of Connecticut Health Center). The coefficient of variation for lumbar spine BMD, the primary outcome variable, was 1.3% at site 1 and 1.2% at site 2. All subjects were scanned only at the study site to which they were initially recruited. A single phantom was used to cross-calibrate both machines every 9–12 months to ensure that there was no discordant drift in densitometric measures over the course of the study. BMD by QCT was performed at the Women's Imaging Center (South Windsor, Connecticut) using a Toshiba Asteon 4 detector Helical scanner and Mindways QCTPro Software (Mindways, Inc). Because of expense, the QCT testing was performed on half of the study subjects chosen randomly from each group. Measurements were made in the lumbar spine and hip at both baseline and month 18.

Measures of skeletal and mineral metabolism

Serum and 24-hour urine samples were collected at baseline and at 9 and 18 months. Intact PTH and serum 25(OH)D were measured using commercially available RIAs. UCa and serum creatinine were measured using an ACE Wasserman autoanalyzer. Serum osteocalcin was measured in the laboratory of Dr Caren Gundberg (Department of Orthopaedics, Yale School of Medicine) using a double-antibody RIA (12). Serum amino-terminal propeptide of type I procollagen (P1NP), C-terminal telopeptide of type 1 collagen (CTX), and IGF-1 were measured using commercially available ELISAs. Urinary urea was measured using a commercially available kit. Commercial assay manufacturers normal ranges and interassays coefficients of variation for all assays are provided in Supplemental Table 3. Estimated glomerular filtration rate (eGFR) was calculated from serum creatinine, age, sex, and race using the National Kidney Foundation on-line calculator (13).

Dietary monitoring

At study entry all participants met with a registered dietitian and were instructed in estimating portion sizes using food models and measuring cups. All subjects completed 3-day food records prior to all study visits, which were reviewed by dietitians to encourage adherence to the intervention and to monitor their energy, protein, Ca, and vitamin D intake. All subjects received a daily multivitamin mineral supplement that contained 400 IU of vitamin D (One-A-Day; Bayer). The multivitamin supplement, Ca carbonate supplement (Tums, 300 mg tablets), and Ca-containing foods were used to ensure a minimum Ca intake of 1200 mg/d. Subjects who wanted to continue their own multivitamin and Ca supplements instead of those provided by the study were asked to bring all supplement bottles to their randomization visit. Total vitamin D and Ca from personal nutritional supplement labels were included in the analyses presented in Table 2. Study dietitians counseled subjects to ensure that their weight remained stable during the study. Food records collected at baseline and 6 and 18 months were analyzed using the ESHA Food Processor software program (ESHA Research; version 10.1.0).

Adverse events

Participants were asked about adverse events at each study visit. The relationship of adverse events to the study interventions were adjudicated by the principal investigator or subinvestigator and categorized as either definitely related or unrelated and then graded 1–5 for the degree of severity.

Sample size estimate

Sample size was estimated from a prior human study that most closely matched the current protocol (14). A sample size of at least 67 completers in each group was determined to provide 90% power to detect a 3.63% ± 6.9% (mean ± SD) difference in spinal BMD (assessed by DXA) at the two-sided 0.05 significance level. For a spinal BMD (our primary outcome variable) of 0.874 g/cm² in a postmenopausal Caucasian woman (with an associated T-score of −1.6), this would represent an absolute change of 0.032 g/cm². We targeted an enrollment of 200 volunteers to allow for an anticipated dropout rate of 30% based on a previous clinical trial from our group (15).

Statistical analysis

Normality was assessed by residual plots at each time point. None of the variables we examined demonstrated a compelling departure from normality and were therefore not transformed. Baseline characteristics were summarized by treatment group using descriptive statistics. Our primary outcome variable was change in lumbar spine BMD. Changes in bone density at the hip, trabecular and cortical skeletal envelopes, eGFR, UCa, IGF-1, and indices of body composition were considered secondary outcome variables. Exploratory end points were markers of bone turnover and mineral metabolism. Urinary urea was a compliance measure and not considered an outcome measure. The effect of treatment (protein supplementation) on the primary, secondary, and exploratory outcome variables was assessed with general linear mixed-models analysis adjusted for baseline values, an approach appropriate for longitudinal data sets with multiple missing data points (16). Outcome data are presented as least squares means from the mixed-model, repeated-measures analyses (unless otherwise noted). The models included fixed effects for treatment (two levels), time, time-by-treatment interaction, and a random effect for each participant. An unstructured covariance pattern was used to accommodate correlation from the repeated assessments.

Primary analyses were conducted according to the intent-to-treat principle using all randomized subjects. Secondary analyses using the same mixed-model approach were conducted on the following: participants categorized as having a protein intake at or below the current recommended dietary allowance (RDA; ≤0.8 g/kg) or above the RDA (>0.8 g/kg) as well as those participants who continued to take the study intervention for the entire 18 months (termed treatment completers). The proportion of adverse events per study arm was analyzed using a Pearson's χ² or a Fisher's exact test (depending on cell counts). All analyses were performed with SAS version 9.2 (SAS Institute) at the two-sided level of significance at P = .05. There was no adjustment for multiple comparisons for testing secondary outcomes.

Results

Study population

The two study groups were closely matched for gender, age, body composition, and baseline BMD (Table 1). Measures of bone turnover, except for P1NP (P = .009), as well as 25(OH)D, eGFR, and 24-hour urinary urea were also similar between the two groups. The diets in the two study groups were nearly identical in baseline total energy and macronutrient consumption as well as Ca and vitamin D intakes (Table 2). There were also no significant differences in baseline dietary fiber, phosphorous, magnesium, or sodium content (data not shown). There were no baseline differences in reported physical activity, measured by the Baecke Physical Activity Questionnaire (total physical activity score 6.7 ± 2.1 vs 6.8 ± 1.9; protein vs carbohydrate; mean ± SD), tobacco use (total number of smokers: protein 2 vs carbohydrate 1), or alcohol consumption (number of drinks per day 0.4 ± 0.6 vs 0.6 ± 0.8; protein vs carbohydrate; mean ± SD) between the two study groups.

Study retention

Of the 208 subjects randomized, 158 completed the study, and of these, 121 continued to take the study supplement for 18 months (Supplemental Figure 1). The average supplement consumption for treatment completers (ie, participants who continued to take to the supplement for 18 mo) was 31.5 and 31.0 g/d at 18 months for the protein (n = 61) and carbohydrate (n = 60) groups, respectively.

Effect of study intervention on dietary intake

Changes in nutrient intake for the study population over the 18-month intervention are presented in Table 2 and Supplemental Table 4. The prespecified study goal was to ensure that total caloric intake (diet plus supplement) remained constant in both groups. In the carbohydrate group, there were no changes in caloric intake from diet over the 18 months of the study. In the protein group, there was a decline in dietary caloric intake (from food sources), which by month 18 was significantly less than baseline. At baseline the carbohydrate group's dietary caloric intake was 1692 kcal and only marginally increased to 1761 kcal at the end of the study period with the addition of the maltodextrin supplement (1688 kcal from diet plus 73 kcal from supplement). The protein group's baseline caloric intake was 1709 kcal and declined slightly to 1661 kcal at the end of the 18-month intervention (1589 kcal from diet plus 72 kcal from supplement). Overall, the addition of either supplement to the subject's usual dietary intake did not substantially impact total energy consumption throughout the study period.

There were no significant changes in any of the macronutrients from diet or in Ca intake in the carbohydrate group. However, total vitamin D intake significantly increased in the carbohydrate group, which was due to a rise in supplemental intake rather than dietary changes (Supplemental Table 4). Fat intake declined in the protein group (18 mo vs baseline, P = .041), but there were no significant between group differences in dietary fat at 6 or 18 months. Carbohydrate intake also declined in the protein group (18 mo vs baseline, P = .024), and there was a significant between-group difference at 18 months (P = .035). Dietary protein, Ca, and vitamin D intakes remained stable in the protein group.

Changes in BMD

There were no significant between-group differences for lumbar spine BMD (primary outcome variable) or any of the other analyzed regions of interest at either 9 or 18 months (Table 3). Percentage change (least square mean ± SEM) in lumbar spine, total hip, and femoral neck BMD from baseline to 18 months was calculated for all subjects for whom there were at least two bone density determinations by DXA (protein group: n = 92; carbohydrate group: n = 79). At the end of the study, BMD had increased by 0.5% ± 0.4% in the spine; had declined by 0.7% ± 0.3% in the total hip; and had declined by 0.5% ± 0.5% in the femoral neck in the carbohydrate group. In the protein group, spinal BMD increased by 0.5% ± 0.4% and declined by 0.7% ± 0.3% in the total hip and by 1.1% ± 0.4% in the femoral neck. There were no statistically significant between-group differences in the data when expressed as a percentage change from baseline for any of these regions of interest (P > .05). When percentage change was calculated for the treatment completers (protein group: n = 57; carbohydrate group: n = 60), there was a difference between treatment groups in femoral neck BMD at 9 months (protein: −1.31 ± 0.43 vs carbohydrate: 0.14 ± 0.43, between group difference: −1.5 ± 0.6%, P = .019). However, when the data from treatment completers were analyzed at 18 months, there were no significant between-group differences in the percentage change from baseline at any of the three BMD sites (data not shown). A subanalysis of treatment completers who had good adherence (average supplement intakes: protein group: 31.5 g; carbohydrate group: 31.0 g, values that are 69% of the target 45 g intake) still demonstrated no observed differences.

Table 3 also summarizes the change from baseline in BMD in a subset of individuals who had BMD measured by QCT in the spine and hip (carbohydrate: n = 44; protein: n = 45). There were no significant between-group differences at 18 months for any of the cortical or trabecular skeletal parameters. Lastly, we performed a subanalysis in 43 individuals who had baseline protein intakes at or below the RDA. This subanalysis revealed no change in bone density when measured either by DXA or QCT (data not shown).

Biochemical measures of skeletal and mineral metabolism in treatment completers

Because of the expense in performing these analyses, they were undertaken only in subjects who continued to adhere to the supplement (ie, treatment completers; Table 4 and Supplemental Table 5). There were no significant changes in serum PTH between the two groups over the 18 months (P = .605). As expected, by 9 months UCa increased in the protein group and differed significantly from the carbohydrate group (P = .0073). However, at 18 months this difference was no longer significant.

There were significant differences between the two groups for both CTX and P1NP at month 9 (CTX: P = .0206; P1NP: P = .0007) and CTX at 18 months (P = .0414). IGF-1 levels were also higher in the protein group compared with the carbohydrate group throughout the intervention, and the difference was significant at the study end (9 mo: P = .141; 18 mo: P = .0054).

Urine urea was significantly higher in the protein group compared with the carbohydrate group both at 9 (P < .001) and 18 months (P < .001). There was no significant change in urine urea in the carbohydrate group. Mean eGFR remained above 60 mL/min per 1.73 m² in both groups throughout the study, and there was no treatment effect at 18 months.

Changes in anthropometric indices

There were no significant differences for any of the anthropometric measurements at 9 months (Table 5; P = .178 to P = .895). However, by 18 months, total lean mass (P = .011) and truncal lean mass (P = .003) had significantly declined in the carbohydrate group from baseline, whereas lean mass remained unaltered in the protein group. Total lean mass tended to be higher in the protein group (P = .069), and lean trunk mass was significantly higher with protein supplementation (P = .048) at 18 months. Lastly, total fat mass significantly increased over the course study in the carbohydrate group (P = .018), a change in body composition that was not observed with protein supplementation. These changes in body composition occurred despite the fact that there was no change in BMI between the two groups (9 mo: P = .624; 18 mo: P = .742).

Changes in lifestyle factors

There was no difference in levels of physical activity between the two study groups at 18 months. Furthermore, there were no significant changes within each group when comparing baseline scores with those at 18 months (total physical activity score: protein: 6.7 ± 2.1→6.8 ± 2.1 vs carbohydrate: 6.8 ± 1.9→7.1 ± 2.0; mean ± SD). There were two smokers in the protein group and one in the carbohydrate group. Alcohol consumption remained moderate throughout the intervention, with no differences being observed between baseline and 18 months within each group or between the groups at the end of the study (number of drinks per day: protein: 0.4 ± 0.6→0.5 ± 0.6; carbohydrate: 0.6 ± 0.8→0.5 ± 0.8; mean ± SD).

Adverse events

There were relatively equal numbers of total adverse events in each group (Supplemental Table 6).

Discussion

In this study whey protein supplementation in older men and women with adequate dietary protein intakes had no effect on BMD assessed by DXA. There were also no significant differences in QCT measurements at the lumbar spine (a site rich in trabecular bone) or at the femoral neck (a cortically enriched site). However, total lean body mass was preserved, and truncal fat-free mass was significantly higher in individuals receiving a whey protein supplement. Protein supplementation was associated with a transient increase in bone turnover markers along with an increase in UCa at the study midpoint and higher IGF-1 levels at the end of the intervention period. The eGFR remained within normal limits throughout the entire study. As expected with protein supplementation, urinary urea remained elevated in that group throughout the study (17). There were no changes in physical activity, tobacco use, or alcohol consumption that could have influenced these findings. The observed changes in body composition suggest a need to further explore the relationship between dietary protein and indices of body composition in older adults.

Our finding that truncal lean body mass was significantly higher in the protein-supplemented group may have clinical relevance. Central fat deposition tends to increase with age, especially in women after menopause, and is a known risk factor of cardiovascular disease (18). Thus, the difference in trunk lean mass we observed between our two study groups suggests a potential cardioprotective benefit of moderately high dietary protein intakes in older adults. This did not appear to be due to differences in the level of exercise between the two groups, but other potential confounding factors were not evaluated in this study because it was not a principal outcome variable. Furthermore, direct measures of cardiac health were not assessed in the current study, and thus, the long-term effect of higher protein intakes on cardiac outcomes in older adults requires investigation.

Our results are consistent with previous reports by Zhu et al (19) and our own group (15). The age of the study subjects were similar across the three reports. Baseline protein intake in the study by Zhu et al (19) was comparable with that of the current study, whereas subjects in our previous report (15) had slightly lower baseline protein intakes that hovered around the current RDA. In the report by Zhu et al, subjects ingested either a 30-g whey protein supplement or isocaloric carbohydrate control supplement in a 2-year randomized trial. They saw no between group differences in BMD, whether assessed by DXA or QCT. In our earlier work, subjects were asked to consume an 18-g soy protein supplement with or without isoflavones for 1 year, and we too observed no significant changes in BMD with protein supplementation. Results from these studies suggest that moderate, long-term protein intakes above the current RDA that are high in either animal or vegetable sources do not appear to be detrimental to the aging skeleton.

The findings from our current study and two prior clinical trials (15, 19) stand in contrast to recent epidemiological data (20–24) and previous work in rats (25), suggesting a beneficial effect of dietary protein on skeletal mass. A number of possibilities exist that could explain this discrepancy. The effect of any dietary intervention is likely to be modest, and to observe an effect may require longer-term studies, dietary alterations of greater magnitude, and larger numbers of individuals, or it may be that under periods of limited nutrient availability dietary protein's effect on the skeleton is more pronounced (26). Our current study was limited by a significant dropout rate. This underscores the difficulty of long-term nutrition intervention studies in older adults, which require the incorporation of a food stuff into the daily diet.

Furthermore, the level of dietary calcium was relatively high in our study population, which could represent another limitation. The estimated least square means for dietary Ca in our participants ranged between 1426 and 1553 mg/d. Hunt et al (9) found that increasing dietary protein increased total body Ca retention in individuals consuming 675 mg/d of dietary Ca. This effect was not observed when dietary Ca was increased above the RDA to 1510 mg/d.

Our study had several strengths. The study population accurately reflected the population of individuals at greatest risk for fragility fractures, namely postmenopausal women. Dietary intake and adherence to the intervention were carefully monitored by dietitians with independent validation using urinary urea as a biomarker of adherence. Dietary monitoring ensured that there were no changes in body weight throughout the study period. State-of-the-art methods were used to assess changes in body composition and bone density including analyses of cortical and trabecular skeletal envelopes. Our study also had limitations. Mean protein intake of our study participants exceeded our target protein intake. As just noted, dietary Ca in our participants was at a level that was previously reported to obscure the effects of dietary protein on bone and may have limited our ability to detect an effect of dietary protein on BMD.

In conclusion, there was no beneficial or detrimental effect on BMD of a whey protein supplement administered over 18 months to older adults who had an adequate dietary protein intake; however, protein supplementation resulted in a higher lean trunk mass. In this group with normal renal function at baseline, protein supplementation had no detrimental effect on eGFR. Based on our experience, larger and/or longer nutrition intervention trials using whole-protein supplements will be challenging. The development of amino acid-based nutriceuticals could make larger and longer-term clinical trials more feasible.

Acknowledgments

We are deeply indebted to our study volunteers who gave freely of their time and energy to complete this study. It was an enormous commitment on their part for which we are extremely grateful. We also thank the Yale-New Haven Hospital Research Unit staff, the University of Connecticut Center on Aging research staff, the bone densitometry technologists of the Yale Bone Center and the University of Connecticut Center on Aging, and the diet record analysts. We also thank Mr Peter Bihuniak for his valuable assistance in combining the supplement intake data with the dietary food log data.

This study had a clinical trial registration number of NCT00421408.

This work was supported by National Institutes of Health National Institute of Arthritis and Musculoskeletal and Skin Diseases Grant AR0537, Dairy Research Institute Project 1855, and the Yale Bone Center.

Disclosure Summary: The authors have nothing to disclose.

Abbreviations

 
• BMD

  bone mineral density

 
• BMI

  body mass index

 
• Ca

  urinary calcium

 
• CTX

  C-terminal telopeptide

 
• DXA

  dual-energy x-ray absorptiometry

 
• eGFR

  estimated glomerular filtration rate

 
• 25(OH)D

  25-hydroxyvitamin D

 
• P1NP

  amino-terminal propeptide of type I procollagen

 
• QCT

  quantitative computed tomography

 
• RDA

  recommended daily allowance

 
• UCa

  urinary Ca.

References