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.
Iron (Fe) deficiency is endemic worldwide. We have recently found that increasing dietary protein dose‐dependently improves Fe balance in rats. This change is associated with increased DMT1 transcript expression, a key regulator of intestinocyte Fe uptake. To confirm the Fe balance results, Fe absorption studies were performed in rats fed either a 20% (control) or 40% (high) protein diet for 1 wk. Rats on the 40% protein diet absorbed 30±9% of ingested Fe while those on the 20% protein diet absorbed 18±8% (p= 0.005). To determine if DMT1 protein expression is also increased with increasing dietary protein, Western analyses were undertaken using duodenal mucosal lysates prepared from rats on the 20% and 40% protein diets. There was a trend towards higher DMT1 protein levels on the 40% diet (1.7±1.1 fold; p=0.46). To evaluate if amino acids could directly increase DMT1 transcript expression, Caco‐2 Bbe cells were exposed to either a mixture of amino acids or amino acid‐free media for 6 hrs. Amino acid treatment induced DMT1 mRNA by 1.6±0.3 fold as determined by qPCR (p<0.02). These data indicate that increasing dietary protein increases intestinal Fe absorption in part by upregulation of DMT1. Further, there appears to be a direct effect of amino acids on DMT1 transcript expression.This work was supported by a HATCH grant from the Univ. of CT.
In humans, increasing dietary protein increases intestinal Ca absorption. To further study this effect, rats were randomly assigned to consume a control (20%), low (5%) or high (40%) protein diet for 7 days. All diets were isocaloric and contained 0.45% Ca, 0.35% P. Twenty‐four hr urines, blood and feces were collected to assess Ca metabolism, Ca balance and bone turnover. Brush border membrane vesicles (BBMV) were prepared from duodenal mucosa to measure Ca transport.Urine Ca rose during the high protein diet and fell during the low diet (p<0.01). There were no changes in serum Ca or vitamin D metabolites on any diet. Serum PTH trended higher in the low protein group (p=0.07) and serum osteocalcin was significantly higher compared to the other groups (p<0.03). Rats consuming the high protein diet absorbed and retained more Ca compared to the low protein group (48.5% vs. 34.1% and 45.8% vs. 33.7% respectively, p<0.01). Ca transport was significantly higher in the BBMV from animals consuming the high protein diet (p<0.001).This animal model recapitulates the findings in humans in that changes in Ca absorption explain the changes in UCa. The higher osteocalcin and PTH during the low protein diet suggest that bone turnover was increased during low protein intake. The BBMV studies indicate that increased transcellular Ca uptake contributes to the increase in Ca absorption during the high protein diet.
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