The time sequence of adaptive changes to dietary phosphorus deficiency in the chick

Ten-day old chicks were divided into 6 groups which were fed a low phosphorus (P) diet for periods of 0, 0.5, 1, 3, 7 or 11 days before killing at 3 weeks old. Labelled calcium (47Ca) was injected intraperitoneally into some birds 18 hours before killing. A marked fall in growth rate, plasma phosphorus level, plasma growth hormone level and renal 24-hydroxylase activity levels had occurred by 12 hours after the experimental diet had started. After one day on the diet, the rate of duodenal Ca absorption had risen and continued to rise up to the 11th day. During this period, the renal 25-hydroxyvitamin D-1-hydroxylase activity rose slightly while the 24-hydroxylase activity rose towards the control level. At 24 hours, the 47Ca level in the bone was markedly lower than in the control group and remained low. It was concluded that the first adaptive response of the chick to dietary P insufficiency was to suppress growth. Subsequent adaptive responses were to increase the rate of Ca and P absorption from the gut and mobilisation from the bone but despite these measures, the growth rate only recovered slightly and the plasma P level continued to fall.