The effect of dietary protein on intestinal calcium absorption in rats

Abstract

Increasing dietary protein intake in humans acutely increases urinary calcium. Isotopic absorption studies have indicated that, at least in the short term, this is primarily due to increased intestinal Ca absorption. To explore the mechanisms underlying dietary protein's effect on intestinal Ca absorption, female Sprague Dawley rats were fed a control (20%), low (5%), or high (40%) protein diet for 7 d, and Ca balance was measured during d 4-7. On d 7, duodenal mucosa was harvested and brush border membrane vesicles (BBMVs) were prepared to evaluate Ca uptake. By d 7, urinary calcium was more than 2-fold higher in the 40% protein group compared with control (4.2 mg/d vs. 1.7 mg/d; P < 0.05). Rats consuming the 40% protein diet both absorbed and retained more Ca compared with the 5% protein group (absorption: 48.5% vs. 34.1% and retention: 45.8% vs. 33.7%, respectively; P < 0.01). Ca uptake was increased in BBMVs prepared from rats consuming the high-protein diet. Maximum velocity (V(max)) was higher in the BBMVs prepared from the high-protein group compared with those from the low-protein group (90 vs. 36 nmol Ca/mg protein x min, P < 0.001; 95% CI: 46-2486 and 14-55, respectively). The Michaelis Menten constant (K(m)) was unchanged (2.2 mm vs. 1.8 mm, respectively; P = 0.19). We conclude that in rats, as in humans, acute increases in protein intake result in hypercalciuria due to augmented intestinal Ca absorption. BBMV Ca uptake studies suggest that higher protein intake improves Ca absorption, at least in part, by increasing transcellular Ca uptake.

Figure 1

UCa excretion in response to dietary protein. Data are shown as means ± sem. Mean value is significantly different from d 7 of the control group (*, P < 0.05; **, P < 0.01 by one-way ANOVA).

Figure 2

A, Time course of calcium uptake into duodenal BBMVs isolated from rats consuming a 5% or 40% protein diet for 1 wk. Data are means ± sem (n = 10–11). B, Lineweaver-Burke plot of 1-min calcium uptake into duodenal BBMVs isolated from rats consuming a 5% or 40% protein diet for 1 wk. Data are means ± sem (n = 3).