Short- and long-term effects of nitrogen metabolism of feeding protein during mild or severe energy restriction.

The complex relationship between protein intake and energy restriction was studied in acutely and chronically restricted adult rats. Rats were fed either 12 or 24 kcal/day from protein alone or protein plus carbohydrate from 3 to 16 days in one experiment. In another, groups were fed either 0.75 or 1.5 g lactalbumin/day in diets supplying 90%, 80% or 70% of the ad libitum energy intake for 8 or 104 days. Body composition and nitrogen of various tissues were determined. In experiment 1, feeding protein conserved tissue and body nitrogen as long as sufficient adipose stores were maintained, but thereafter had no advantage over lower protein intakes supplemented with carbohydrate. In experiment 2, all rats gained weight in proportion to energy intake. Protein level was protective of carcass nitrogen, water and liver nitrogen in both the short- and long-term at the 80% and 90% level of energy intake. Energy intake was more important than protein in conserving muscle nitrogen. Effects of energy and protein restriction are complex and dependent upon the total amount of protein and energy provided, body reserves and duration of restriction.