Growth hormone (GH) is essential for body growth but as normal growth occurs over a relatively short time period and GH secretion continues throughout life it is not surprising that GH has many other functions including both acute and chronic effects on protein metabolism and body composition. Many of the actions of GH are mediated both directly and indirectly through insulin-like growth-factor-I (IGF-I) acting in an endocrine or paracrine manner.

The balance between the rates of protein synthesis and breakdown determines whether the body is in protein balance, protein loss or protein gain (Figure 8.1). The change in these rates in response to physiological and pathological processes is determined by non-hormonal factors, for example, nutritional status, exercise, growth and infection, and hormonal factors. During growth in young animals, protein synthetic rates and protein breakdown rates are higher than in adult animals and the increased protein breakdown may allow the remodelling of muscle enabling growth to take place (1). During muscle hypertrophy induced by exercise both protein synthesis and breakdown increase but synthesis exceeds breakdown and there is a net gain of lean body mass (LBM) (2). Conversely during starvation both rates of protein synthesis and breakdown decrease but breakdown exceeds synthesis and there is a net loss of LBM (3). In acute serious illness the resultant catabolic state is due predominantly to an increased rate of protein degradation (4).

It is customary to refer to body composition as consisting of the fat mass (fat) and the LBM (consisting of predominantly protein). LBM does not change when protein synthesis and breakdown are in balance.