Glucose, protein and energy metabolism in suckling and ruminating lambs

Abstract

Ruminant lambs were compared with suckling lambs, pair-fed to equal digestible energy intakes. Studies were made of glucose entry rate, by continuous infusion of D-[U-¹⁴C] glucose, protein synthesis, by continuous infusion of L-[side chain-2, 3-³H] tyrosine, and the energy cost of growth, by comparative slaughter.

Ruminant lambs had less body fat (10.1%EBW at 15kg) and higher maintenance energy requirements (755kJME/d/kg⁰·⁷⁵) than suckling lambs of a similar age (15.1%EBW and 544kJME/d/kg ⁰·⁷⁵, respectively). Whole body synthesis calculated from the tyrosine flux was greater at the same growth rate in ruminant lambs (21.6g/d/kg ⁰·⁷⁵) than in suckling lambs (18.1g/d/kg ⁰·⁷⁵), as was the whole body rate calculated from the tissue synthetic rates. Fractional synthetic rates in the liver and muscle tissues were greater in suckling lambs (148%/d and 3.4%/d, respectively) than in ruminant lambs (83%/d and 3.0%/d). Glucose turnover was similar in both ruminant and suckling lambs (8.75 and 10.53mg/min/kg ⁰·⁷⁵ respectively), but the proportion of CO₂ derived from glucose was significantly lower in the former (18.8% compared with 41.9%). Plasma insulin levels decreased at weaning, and the amount of insulin released in response to a glucose tolerance test (GTT) also declined. In suckling lambs fasting plasma insulin and the insulin release during a GTT increased with age, while glucose tolerance decreased, indicating a development of insulin resistance.

It is suggested that the difference in fatness is a result both of a decrease in the energy available to the ruminant lamb, due in part to increased costs of protein turnover, and of the promotion of fat deposition in the suckling lamb, associated with increased circulating insulin levels and a high fat diet.