Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2010
Publication Date: 3/1/2011
Publication URL: http://handle.nal.usda.gov/10113/58217
Citation: Cao, J.J., Johnson, L.K., Hunt, J.R. 2011. A diet high in meat protein and potential renal acid load increases fractional Ca absorption and urinary Ca excretion, without affecting markers of bone resorption or formation in postmenopausal women. Journal of Nutrition. 141(3):391-397. Interpretive Summary: Osteoporosis affects millions of Americans. Dietary protein intake affects bone metabolism and plays a major role in affecting human calcium and bone homeostasis. However, the impact of dietary meat protein on calcium retention and bone metabolism remains controversial. For several decades, high dietary meat protein has been considered a risk factor for osteoporosis or bone fractures. We conducted a controlled study to determine how high dietary protein (mostly beef) and its associated acid load on calcium balance and markers of bone metabolism in postmenopausal women. Sixteen healthy postmenopausal women were recruited from the local community. These women consumed two diets in random order: one with low protein and low potential acid load (protein intake: 0.86 g/kg body weight) and one with high protein and high acid load (protein intake: 1.70 g/kg body weight) for 7 wk each, separated by a one-wk break. We measured calcium absorption with radio-labeled calcium-47 and we also measured other biomarkers of bone metabolism in urine and blood. We found that high meat protein intake increased calcium absorption although it also increased calcium excretion in urine. The amount calcium from increased absorption with high protein diet nearly compensated the amount of calcium excreted in urine. High protein diet also increased a growth hormone (IGF-I) in blood. Based on our data, we conclude that high meat protein intake may be beneficial to bone health.
Technical Abstract: Objective: The objective was to determine the effects of high dietary protein (mostly meat) and high potential renal acid load (PRAL) on calcium (Ca) balance and markers of bone metabolism. Methods: In a randomized crossover design, sixteen healthy postmenopausal women consumed two diets: one with low protein and low PRAL (LPLP; total protein: 61 g/d with meat protein: 12 g/d; PRAL: -48 mEq/d) and one with high protein (HPHP; total protein: 118 g/d with meat protein: 68g/d; PRAL: 33 mEq/d) for 7 wk each, separated by a one-wk break. After 3 wk dietary equilibration, the entire 2-d menu of each diet was radio-labeled with 47Ca and Ca absorption was measured by whole body scintillation counting for an additional 4 wk. Biomarkers of bone metabolism in blood and urine were measured. Results: Compared with the LPLP diet, the HPHP diet increased urinary acidity (mean ± pooled SD: pH 7.1 vs. 5.9 ± 0.2, p < 0.0001), urinary Ca excretion (156 vs. 203 ± 63 mg/d, p = 0.003), and blood IGF-I levels [geometric mean ± 1 pooled SD: 137 (114 – 164) vs. 175 (146 – 210) ng/ml, P < 0.0001], and decreased blood intact PTH levels [geometric mean ± 1 pooled SD: 59 (48 – 70) vs. 48 (40 – 57) ng/ml, P < 0.001] consistently throughout the study. In comparison to the LPLP diet, the HPHP diet increased subjects’ fractional Ca absorption (25.4 vs. 30.4 ± 5.4%, respectively, p < 0.02) and tended to increase the absolute amount of Ca absorbed (227 vs. 258 ± 47 mg/d, P < 0.08). The net difference between Ca absorbed and renal excretion was not significant between the LPLP and HPHP diets (82 vs. 59 ± 100 mg/d for the LPLP and the HPHP diets, respectively; P = 0.26). Conclusions: In healthy postmenopausal women, a diet high in both meat protein and potential renal acid load increased Ca absorption which counteracted the increase in urinary Ca excretion. The increased serum IGF-I combined with decreased serum PTH concentrations, with no change in biomarkers of bone resorption suggest a positive effect of a high protein diet on bone health. (This study was registered at ClinicalTrials.gov. number: NCT00620763.)