Relationship of Excess Calcium and Phosphorus to Magnesium Requirement and Toxicity in Guinea Pigs

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Abstract
Approximately 300 weanling male guinea pigs were used to determine the order of magnitude of the effect of excess dietary calcium and phosphorus upon the magnesium requirement. Growth rate and hemoglobin level were used as criteria of magnesium adequacy. Blood inorganic phosphorus was inversely correlated with the dietary magnesium level but was not a useful criterion. The nature of the toxicity of excess magnesium was also studied. In the presence of 0.8% phosphorus and 0.9% calcium the average magnesium requirement as measured by growth rate and hemoglobin level was 0.12%. Increasing calcium to 3.2% gave an average requirement of 0.20%. When calcium was 2.5% and phosphorus 1.7% the requirement was 0.40%. It was concluded that excess calcium and phosphorus independently increase the minimal magnesium requirement and that their effects are additive. The addition of 1.2% magnesium to a basal diet containing 0.9% calcium and 0.6% phosphorus had no detrimental effects. When the diet contained 1.7% phosphorus, excess magnesium caused high mortality and an extremely poor rate of growth. The damaging effect was alleviated completely by increasing calcium to 2.5%. Thus, excess magnesium was deleterious only when calcium was limiting.