Secretion of Calcium and Strontium Into Milk

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Abstract
The comparative secretion of Ca45 and Sr89 into the milk of goats and dairy cows under typical farm management was studied. The effects of calcium and stable strontium levels were investigated by using four diets containing 0.25 per cent Ca, 0.50 per cent Ca, 1.6–1.7 % per cent Ca, and 0.25 per cent Ca plus 2.8–2.9 per cent stable strontium. The comparative behavior of calcium and strontium was the same on all diets except for a possible renal effect with the goats. The Sr89/Ca45 of the milk was consistently about 0.12 that of the dietary intake (ORmilk-diet = 0.121. The relative contributions to the discrimination against strontium have been estimated for the important physiological processes, i.e. absorption from the digestive tract, passage from blood to milk and renal excretion. On the mediumcalcium diet at steady state the dairy cows secreted into milk about 0.08 per cent of the ingested Sr89 per liter; the goats, primarily because of body- size, secreted much more, about 1.4 per cent of ingested Sr89 per liter of milk. Since the ORmilk-diet value did not vary with changes in dietary calcium levels it would be expected that long-term feeding of increased dietary levels of non-contaminated calcium would produce proportional decreases in the amounts of ingested radiostrontium appearing in milk; there would seem to be no advantage in the use of stable strontium for this purpose. Practical aspects are discussed.

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