High protein intake raises apparent but not true magnesium absorption in rats

Earlier studies with rats have shown that greater protein intake raises apparent magnesium absorption (ingested magnesium minus fecal magnesium). We addressed the question of whether high protein intake affects true magnesium absorption. Rats were fed either a normal (175 g casein/kg) or a high (525 g casein/kg) protein diet. Extra protein was added at the expense of the glucose component of the diet; the diets were balanced for magnesium, calcium and phosphorus. The high protein diet enhanced apparent magnesium absorption. True magnesium absorption, as measured with the use of oral and intraperitoneal administration of tracer doses of 28Mg, was found to be unaffected by high protein intake. Endogenous magnesium excretion in feces was significantly depressed in rats fed the high protein diet, which was associated with elevated urinary magnesium excretion. High protein intake reduced the amount of magnesium in whole ileal digesta but raised the amount and concentration of magnesium in the liquid phase. We conclude that improvement of apparent magnesium absorption in rats fed high protein diets is due to depressed fecal excretion of endogenous magnesium.