Dietary protein and calcium interact to influence calcium retention: a controlled feeding study

BACKGROUND: The effect of meat protein on calcium retention at different calcium intakes is unresolved. OBJECTIVE: The objective was to test the effect of dietary protein on calcium retention at low and high intakes of calcium. DESIGN: In a randomized controlled feeding study with a 2 x 2 factorial crossover design, healthy postmenopausal women (n = 27) consumed either [almost equal to]675 or [almost equal to]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 calcium and bone metabolism were measured. RESULTS: High compared with low dietary protein significantly increased calcium retention from the low-calcium (29.5% compared with 26.0% absorbed) but not the high-calcium diet (18% absorbed). For the low-calcium diet, this effect nearly balanced a protein-related 0.5-mmol/d greater urinary calcium excretion. Protein-related calciuretic effects were independent of dietary calcium. Testing at 1, 2, 3, 5, and 7 wk showed no long-term adaptation in urinary acidity or urinary calcium excretion. High compared with low dietary protein decreased urinary deoxypyridinoline and increased serum insulin-like growth factor I without affecting parathyroid hormone, 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 calcium absorption from a low-calcium diet, nearly compensating for a slight increase in urinary calcium excretion. Under practical dietary conditions, increased dietary protein from animal sources was not detrimental to calcium balance or short-term indicators of bone health.