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Dietary protein and calcium interact to influence calcium retention: a controlled feeding study^1,2,3,4

¹ From the US Department of Agriculture, Agricultural Research Service, Grand Forks Human Nutrition Research Center, Grand Forks, ND (JRH and ZKFR), and the University of North Dakota, Grand Forks, ND (LKJ).

² Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the US Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

³ Supported by the USDA Agricultural Research Service, the National Cattlemen's Beef Association, and the North Dakota Beef Commission.

⁴ Reprints not available. Address correspondence to JR Hunt, US Department of Agriculture, ARS, GFHNRC, 2420 2nd Avenue N, STOP 9034, Grand Forks, ND 58202-9034. E-mail: janetrhunt{at}gmail.com.

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 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 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.