Dietary calcium may be more bioavailable using a weight loss diet that replaces moderate amounts of carbohydrate with protein: A calcium balance model

Weight loss causes bone mineral loss. We previously reported subjects prescribed 1.6 g/kg·d of dietary protein (PRO) consumed more dairy, excreted more calcium in urine and preserved bone mineral relative to controls consuming 0.8 g/kg·d (CARB) during weight loss. It is unclear whether attenuation of bone mineral loss was related to increases in calcium intake, availability or both. We modeled calcium balance over 120 d for 41 adult males and females (47±7 y) randomized to PRO or CARB diets using 3 d weighed food records of calcium intake, 24 h urinary calcium (UCa) and DXA‐estimated whole body bone mineral content (BMC) as a proxy for total bone calcium. We defined availability as the fraction of calcium intake explaining changes in bone mineral mass, after accounting for urinary calcium losses; this included unmeasured parameters absorbed calcium and endogenous intestinal calcium loss. Non‐bone internal calcium pools were assumed constant. Availability was solved for each subject given:Δ BMC ∼ ∑ d = 1 120 ( Ca d ) · ( availability ) ‐ UCa dEstimates of calcium availability were higher by 22.0% (95% CI: 0.2, 43.8) in PRO vs. CARB ( P =0.048) suggesting BMC change cannot be completely accounted for by increased calcium intake with a higher protein diet; however, whether benefits to BMC are related to increased calcium absorption, reduced endogenous intestinal calcium loss or a combination is unclear.