Fat and carbohydrate use in posthatch chicks

Metabolism of carbohydrate and fat was examined in the hatching chick by introduction of 14C labeled oleic acid, triolein, or glucose to the gastrointestinal tract or to the yolk sac. Label in plasma and exhaled air samples were determined over a 12-h period. The activity-time curves could be fitted to double exponential equations, and rate constants for appearance and disappearance of label were calculated. Following injection to the yolk, oleic acid and triolein label showed rapid appearance in the circulation and rapid utilization. The rate of appearance of oleic acid and triolein in the circulation from the gastrointestinal tract was high at hatch and did not change with age. However, the rate constant for disappearance from the plasma increased with age and after 6 d posthatch disappearance curves fluctuated with little decline over 12 h. Following the introduction of labeled lipids more label was found in acylglycerides with time after injection but age had no effect. Lipids introduced via the yolk sac were present in higher proportions in fractions corresponding to low-density lipoprotein (LDL) than lipids introduced via the gastrointestinal tract. Glucose appearance in the circulation from the gastrointestinal tract was low at hatch and increased more than two-fold by 3 d posthatch. The rate constant for disappearance of glucose from the circulation did not change with age. It appears that lipoproteins synthesized maternally or in yolk transport lipids at hatch, but their concentration decreases posthatch, and this could reduce utilization of circulating lipids. Uptake of glucose increases after hatch, thus, allowing glucose to become a major source of energy as the yolk lipids are depleted.