Summary Livestock and laboratory animals show compensatory growth when they are fed ad libitum following a period of restriction feeding. Lysine is a major limiting essential amino acid in the diets both for humans and animals. We hypothesized that changing dietary lysine levels from deficient to sufficient induced compensatory growth in young rats. We elucidated the effect of lysine sufficiency on the dynamics of hormones, relevant to muscle protein synthesis and degradation, insulin-like growth factor-I (IGF-I) and corticosterone, and on the expression of proteolytic-related genes in skeletal muscle during compensatory growth. Lysine sufficiency where the dietary lysine level was increased from 0.46% to 1.30% after 2 wk of subjecting the rats to the lower lysine level induced 80% enhancement of growth rate of rats. During compensatory growth with the lysine sufficiency, fractional muscle protein synthesis rates were higher whereas fractional muscle protein degradation rates were lower than those of the control group ( p Ͻ 0.05). After lysine sufficiency, the expression of atrogin-1/MAFbx mRNA was decreased in gastrocnemius muscle ( p Ͻ 0.05). With the lysine sufficiency, serum IGF-I concentration increased ( p Ͻ 0.05) whereas serum corticosterone decreased ( p Ͻ 0.05). These findings suggest that compensatory growth with lysine sufficiency is due to a change of hormone levels before and after changing diets, resulting in incrementation of protein synthesis and suppression of protein degradation of skeletal muscle. Key Words compensatory growth, lysine, rat, IGF-I, corticosteroneOne of the important subjects in nutritional science is to elucidate how humans and animals respond and adapt to transitions of nutritional status and changes of metabolic reaction that occur during the adaptation.There are numerous reports about adaptation of animals to excess or deficiency of nutrients or energy in diets. Compensatory growth is an example of adaptation to nutritional status. Livestock and laboratory animals show compensatory growth when they are fed ad libitum following a period of restricted feeding or fed adequate energy following a period of energy restriction ( 1-3 ). Previous studies in rats and pigs showed that when animals that had been fed a protein-free or a low protein diet were refed a diet containing a sufficient amount of protein, they developed faster than animals fed a control diet particularly for the first few days after the diet replacement ( 4-6 ). Promoted protein accumulation during compensatory growth is due to a larger difference between the rate of muscle protein synthesis and the rate of muscle protein degradation compared to normal growth. The phenomenon has been considered a transitional state in adapting to the nutritional status. Both anabolic and catabolic hormones may be involved in this response. All of these previous studies indicate that sufficiency of dietary protein and amino acid levels plays a role as a trigger for compensatory growth in animals. However, these studies provide no ...

Two experiments were conducted to elucidate the nitrogen (N) balance of pigs exhibiting compensatory growth when changing the dietary lysine levels from deficiency to sufficiency. Experiment 1 elucidated whether pigs exhibited compensatory growth with dietary lysine sufficiency. Twenty 6-week-old males were assigned to one of two treatments: control and LC (lysine and control). Control pigs were fed a control diet throughout the 24-day experimental period, whereas LC pigs were fed a low lysine diet until day 21 of the experiment, followed by the control diet until the end of experiment. The dietary lysine sufficiency treatment induced an 80% increase in the growth rate of LC pigs (P < 0.05). Experiment 2 focused on the N balance of pigs that exhibited compensatory growth with dietary lysine sufficiency. Eighteen 6-week-old males were assigned to one of three treatments: control, LC, and LL (low lysine). LL pigs were fed a low lysine diet throughout the 24-day experimental period. Pigs that exhibited compensatory growth with dietary lysine sufficiency tended to retain a higher amount of N than control pigs (P = 0.10). These finding suggest that the compensatory growth induced in pigs by dietary lysine sufficiency was partly attributable to a higher level of N retention.

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