AbstractFour main hypotheses have been presented in the last four decades to explain the mode of action of lactose on calcium metabolism, but not one of these has been widely accepted. Three of them relate the effect of lactose to enhanced calcium absorption and one to improved calcium deposition in bone. These hypotheses listed in chronological order state that the ingested lactose improves calcium metabolism as a result: A, of providing an acidic pH in the intestinal tract which increases the solubility and the availability of calcium for absorption; B, of chelating calcium in the lumen of the small intestine as a soluble chelate which increases calcium absorption; C, of inhibiting a metabolic system which is responsible for controlling the uptake of divalent cations at the absorption sites in the small intestine and results in an increased calcium absorption; and D, of stimulating certain metabolic steps at the ossification sites which increase the deposition of calcium and improve the efficiency of calcium utilization. Findings by the authors (1,2,3,4,18) agree with hypothesis A and suggest that diet composition affects the mode of action of lactose on calcium absorption. Dietary lactose produced a more acid pH in the intestine, increased calcium retention, raised serum calcium and enhanced bone calcification. The effect on calcium retention and osteogenesis appears to be indirect as lactose-fed rats maintain a relatively younger body composition, i.e., a lower body fat with higher water, protein, ash and calcium, compared to rats receiving no lactose in their diet.
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