It is known that lipoprotein levels are directly related to the process of atherosclerosis and therefore to the occurrence of CVD. Although total serum cholesterol has been found to be related to CVD, its atherogenic effect depends on the structure of the cholesterol or, in other words, on the ratio between low-density lipoprotein cholesterol (LDL) and high-density lipoprotein cholesterol (HDL).
It is assumed that LDL may act directly or indirectly to cause endothelial damage, with subsequent proliferation of arterial smooth muscle cells resulting in an accumulation of lipids and a progression to atherosclerotic plaque formation. HDL, on the other hand, is assumed to be protective against CVD; HDL seems to be responsible for carrying cholesterol from peripheral tissue, including the arterial walls, back to the liver where it is metabolized and excreted.
Besides HDL and LDL, very low-density lipoprotein cholesterol (VLDL) and plasma triglycerides (TG) also need to be considered. Although the atherogenic effects of VLDL and TG are not firmly established, both are assumed to be risk factors for CVD. It is further assumed that during exercise, fatty acids are freed from their storage sites to be burned for energy production.
Several studies suggest that human growth hormone may be responsible for this increased fatty acid mobilization. Growth hormone levels increase sharply with exercise and remain elevated for up to several hours in the recovery period. Other research has suggested that, with exercise, the adipose tissue is more sensitive to either the sympathetic nervous system or to rises in circulating catecholamines. Either situation would increase lipid mobilization.
The fact that children who were very active during adolescence show the largest decrease in physical activity between adolescence and adulthood. This unfavourable change in physical activity was found to be related to unfavourable HDL values at adult age. This finding, although preliminary, suggests that it is important to maintain high levels of physical activity throughout life.
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