EFFECT OF DIETARY-PROTEIN QUALITY, FEED RESTRICTION AND SHORT-TERM FASTING ON PROTEIN-SYNTHESIS AND TURNOVER IN TISSUES OF THE GROWING CHICKEN

The effect of dietary protein quality and quantity on fractional rates of protein synthesis (k(s)) and degradation (k(d)) in the skeletal mu scle, liver, jejunum and skin of young growing chickens was studied. C hickens were either fasted overnight or were fed at frequent intervals , using continuous feeders, with equal amounts of a diet containing so ya-bean meal as the sole protein source, unsupplemented, or supplement ed with either lysine or methionine. Each of the three diets was provi ded at 2 or 0.9 x maintenance. On the higher intake, birds on the unsu pplemented diet gained weight, lysine supplementation decreased and me thionine supplementation increased body-weight gain (by -23% and +22% respectively). Birds fed at 0.9 x maintenance lost weight; supplementa tion with methionine or lysine did not influence this weight loss. Non e of the dietary regimens had significant effects on protein synthesis rates in any of the tissues, thus the mechanism whereby muscle mass i ncreased in response to methionine supplementation appeared to be a de crease in the calculated rate of protein degradation. Similarly, on th e 0.9 x maintenance diet the failure of the animals to grow appeared t o be due to an increase in the rate of protein degradation rather than an effect on synthesis. Conversely, muscle k, was decreased in fasted chickens previously fed on the unsupplemented diet at 2 x maintenance , and in birds which had received the 0.9 x maintenance diet fasting r esulted in a similar reduction in protein synthesis in muscle; k, in t he liver and jejunum was also significantly decreased. The effect of f asting, unlike the effect of supplementation or restriction of the die t, appeared to be due to changes in the rate of protein synthesis.