User:Littlewikigal/Compensatory growth

Article Draft
also seen in pigs. mention overcompensation.

Effects
Compensatory growth in an organism can have impacts on the organism's behavior and muscle development. These changes can occur during or after the period of compensation, and can lead to changes in behavior, movement, and body composition. During compensation, animals may experience changes in food intake, body composition (increased fat and lessened muscle), metabolism, and the endocrine system. Animals undergoing compensatory growth have been seen to have impaired muscle development after regrowth, which leads to impairments in locomotion and escape movements. Behavioral impacts such as increased risk-taking and aggression have also been seen, both during and after regrowth. Animals often have impaired movement due to lessened muscle development, yet they have been observed taking time during feeding, despite the risks of extended time spent eating. Brown trout (Salmon trutta) who have been treated with growth hormone after a period of deprivation have been observed to increase their growth rate but decrease their anti-predator responses, such as swimming in waters more accessible by predators. Increased aggression and decreased sexual displays have also been seen in animals undergoing compensatory growth.

Mechanism
In animals, homeostatic and homeorhetic processes are involved in the abnormally high growth rates. Homeostatic processes usually affect compensatory growth in the short term, whereas homeorhetic processes usually have a long-term effect.

First, during nutrient starvation, a reduction of basal metabolism takes place. The gut tissues are the first tissues to be reduced in weight and activity. Then, during the realimentation (re-feeding) phase, an increase in feeding enables more dietary protein and energy to be contributed for tissue growth instead of basal metabolism. The gut tissues are the first to increase in weight, followed by muscle tissue and finally adipose tissue.

The exact biological mechanisms for compensatory growth remain poorly understood. However, it is known that during the restriction period, levels of growth hormone (GH) are increased by the pituitary gland, while the number of GH receptors decrease. There is a decrease in insulin-like growth factor (IGF-1) as well. When this restriction is severe, amino acids are released from muscle cells and catabolic hormones circulate to initiate gluconeogenesis processes. Then, when compensation begins, the GH levels continue to remain high while insulin levels spike, allowing the GH to be used for growth processes. At first, lean muscle is built, but after a few weeks of refeeding, fat begins to accumulate on the organism. IGF-1 and thyroid hormones, on the other hand, approach normal levels much sooner.