Submitted to: American Journal of Clinical Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2009
Publication Date: 4/29/2009
Publication URL: www.ajcn.org
Citation: Hunt, J.R., Johnson, L.K., Roughead, Z.K. 2009. Dietary Protein and Calcium Interact to Influence Calcium Retention: A Controlled Feeding Study. American Journal of Clinical Nutrition. 89:1357-1365. Interpretive Summary: To test whether increases in meat protein were beneficial or detrimental to calcium retention, 27 healthy post-menopausal women were studied with 4 experimental diets differing in calcium and protein content. The participants were randomly assigned to consume either ~675 or ~1510 mg Ca/d, and both low and high protein (providing 10 and 20% energy) for 7 wk each, separated by a 3 wk washout period in which they consumed their usual diets. Calcium absorption and retention by the body was sensitively measured with a calcium isotope. High, compared with low dietary protein slightly increased calcium absorption and retention from the low, but not the high calcium diet, and increased urinary calcium excretion independent of calcium intake. For the low calcium diet, the increase in calcium absorption nearly balanced the increase in urinary calcium excretion. Compared with low protein, the high protein diets increased urinary acidity, without any adaptive change between 1 and 7 weeks of following the diets. Compared with low dietary protein, high protein increased (improved) a blood measurement of bone formation and decreased (improved) a urinary measurement of bone breakdown. These results indicated that, under practical dietary conditions, increased dietary protein from animal sources was not detrimental to calcium nutrition or bone health.
Technical Abstract: Objective: To test the effect of dietary protein on Ca (Ca) retention at low and high Ca intakes. Methods: In a randomized, controlled feeding study with a 2x2 factorial crossover design, healthy post-menopausal women (n=27), consumed either ~675 or ~1510 mg Ca/d, with both low and high protein (providing 10 and 20% energy) for 7 wk each, separated by a 3 wk washout period. After 3 wk, the entire diet was extrinsically labeled with 47Ca, and isotope retention was monitored by whole body scintillation counting. Clinical markers of Ca and bone metabolism were measured. Results: High, compared with low dietary protein significantly increased Ca retention from the low Ca diet (29.5 vs. 26.0% absorbed), but not the high Ca diet (18% absorbed). For the low Ca diet, this nearly balanced a protein-related 0.5 mmol/d greater urinary Ca excretion. Protein-related calciuretic effects were independent of dietary Ca. Testing at 1, 2, 3, 5, and 7 wk revealed no long-term adaptation in urinary acidity or urinary Ca excretion. High compared with low dietary protein decreased urinary deoxypyridinoline and increased serum IGF-1 without affecting PTH, osteocalcin, bone-specific alkaline phosphatase, or tartrate-resistant acid phosphatase. Conclusions: In healthy postmenopausal women, a moderate increase in dietary protein, from 10 to 20% of energy, slightly improved Ca absorption from a low Ca diet, nearly compensating for a slight increase in urinary Ca excretion. Under practical dietary conditions, increased dietary protein from animal sources was not detrimental to Ca balance or bone health.