The effect of certain dietary factors on the apparent absorption of magnesium by the rat.

Numerous workers have established a connexion between the metabolism of magnesium and various dietary factors. Thus Colby & Frye (1951 b) working with young rats on a low-Mg, low-calcium diet observed reduction in blood Mg levels and increase in mortality rate when the casein content of the diet was increased from 24 to 50 yo. Increased intake of Ca salts or of phosphates has been reported to affect Mg metabolism in rats (Tufts & Greenberg, 1937-8; Colby & Frye, 195 I a; Hegsted, Vitale & McGrath, 1956; O’Dell, Morris & Regan, 1960; McAleese & Forbes, 1961), guinea-pigs ,(O’Dell et al. 1960) and young dogs (Bunce, Chiemchaisri & Phillips, 1962) but no information is available from these experiments about the absorption of Mg. The results of Heller & Haddad (1936) indicate that the addition of calcium chloride to the drinking water of rats increased faecal Mg, and recently Alcock & MacIntyre (1960) found a decrease in absorption when rats receiving normal amounts of Mg and insufficient Ca had their Ca intake brought to normal. The experimental data of Malcolm (1905) are insufficient for any conclusions to be drawn concerning the effect of increased dietary Ca on Mg absorption in the dog. An increase in dietary phosphorus level reduced Mg absorption in the guinea-pig (O’Dell, Morris, Pickett & Hogan, 1957) and chick (Nugara & Edwards, 1961). Thus, although it is evident that Ca and phosphate impair Mg absorption in certain animals, the relative and combined effects of these two factors have not been studied to any great extent. Mg absorption in healthy human subjects given a mixed diet low in protein and moderately low in Ca and supplemented with vegetables has been shown to be improved when protein replaced an equi-caloric amount of sugar or fat (McCance, Widdowson & Lehmann, 1942). The effect of increased dietary Ca is less clear; thus De & Basu (1949) found that it impaired Mg absorption, but no definite effect is apparent from the results of Leichsenring, Norris & Lamison (1951) either with increased dietary Ca alone, or with increased Ca and phosphate. In the ruminant, Head & Rook (1955, 1957) have suggested that the high levels of ammonia that occur in the rumen of cows grazing spring grass may interfere with Mg absorption from the small intestine or may reflect a reduction in the concentration there of amino acids, which could also result in poorer absorption. In the experiments now reported the effect was studied of protein level, ammonium salts, Ca carbonate and sodium orthophosphate on the apparent absorption of Mg. The main object was to study factors affecting the absorption of Mg, and retention