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This is an excerpt from Physical Activity and Health-2nd Edition by Claude Bouchard,Steven N. Blair & William Haskell.
Research has repeatedly shown that exposure to regular, frequent bouts of physical activity stimulates physiological and metabolic changes that benefit health. It is helpful to classify these as either (a) chronic effects, that is, adaptations to training acquired over weeks or months, or (b) short-term, acute responses to each individual session of activity. Health-related adaptations to training are dealt with in other chapters. This chapter describes selected acute responses that are clearly related to health outcomes and explains their relevance. The extent to which these responses benefit health depends on the type, frequency, and regularity of activity and the extent to which particular acute responses persist into the postactivity period. For people who achieve the minimum recommended amounts of physical activity (Haskell et al. 2007, p. 1423), acute responses should be stimulated on five days per week (for moderate-intensity activity) or three days per week (for vigorous intensity). For reasons often related to statistical power and logistical considerations, the experimental models used to study acute health-related responses have relied mainly on planned, structured exercise, as opposed to physical activity performed during daily living. For this reason, the term exercise predominates in this chapter rather than physical activity. Studies of short periods of detraining are included because they illustrate that the changes to health outcomes that are rapidly lost when regular training is interrupted are mainly attributable to acute effects. Intuitively, unstructured periods of physical activity may be expected to stimulate acute responses that are qualitatively similar to, but less conspicuous than, those arising from planned sessions of exercise.
Lipids and Lipoproteins
Lipoproteins are particles that transport triglycerides and cholesterol in the blood plasma. They have a hydrophobic lipid core and an outer surface layer that allows the particle to mix with the watery plasma. Lipoproteins are classified according to their density, which in turn reflects their composition (see “Characteristics of the Main Classes of Lipoproteins”). There are clear links between markers for disordered lipoprotein metabolism and heart disease: Plasma concentrations of total cholesterol, low-density lipoprotein (LDL) cholesterol, and triglycerides are positively associated with the incidence of coronary heart disease, and there is a clear inverse relationship between high-density lipoprotein (HDL) cholesterol concentration and heart disease incidence. Changes to the concentration of lipoprotein lipids arising from a session of exercise are therefore of interest because of their implications for cardiovascular risk.
When considerable amounts of energy have been expended, changes to plasma concentrations of triglycerides, total cholesterol, or HDL cholesterol may be observed immediately after an exercise session. For example, men and women who completed the 1994 Hawaii Ironman Triathlon exhibited a nearly 40% decrease in triglycerides and decreases of around 10% in total and LDL cholesterol compared with prerace values. Events such as a marathon can result in an increase in HDL of about 10% after the race. These changes are independent of changes to plasma volume during these endurance events. On the other hand, a session of moderate-intensity exercise of relatively short duration does not lead to clear changes in lipoprotein variables measured immediately afterward. However, these measurements do not reveal the extent of the influence of such an exercise bout on lipoprotein metabolism.
Even a modest session of activity makes important inroads into the body's energy stores, leading to a prolonged period of metabolic “recovery.” Thus, when blood samples are obtained hours (rather than minutes) after exercise, effects on lipoprotein metabolism are clear. Decreases in triglycerides and increases in HDL cholesterol measured in the fasted state 24 h after a session of exercise have consistently been observed. The design of the HERITAGE Family Study was such that chronic adaptations to training could be distinguished from acute effects. In samples obtained 24 h following the last exercise session, researchers found that total cholesterol and very low-density lipoprotein (VLDL) triglycerides were lower after 20 weeks of training than at baseline, but there were no changes in samples obtained 72 h posttraining. In other words, these reductions were clearly acute responses to the last bout of exercise. The main factor influencing the extent of acute changes to triglycerides and HDL is the amount of energy expended during the exercise session, irrespective of exercise intensity. It may be that around 4.2 MJ (1,000 kcal or the equivalent of walking more than 16 km [10 mi] ) need to be expended to elicit statistically significant changes.
These changes reflect the functional relationship that exists between plasma concentrations of triglycerides and HDL cholesterol. Hydrolysis of triglyceride-rich lipoproteins (chylomicrons and VLDL cholesterol) by the enzyme lipoprotein lipase (LPL) is accompanied by the transfer of cholesterol and other surface materials from triglyceride-rich particles into HDL. Thus, rapid removal of triglyceride-rich lipoproteins is associated with an increase in cholesterol carried in HDL. Prior exercise enhances triglyceride clearance rates, probably by increasing the activity of LPL in muscle—the rate-limiting step in triglyceride clearance. However, there appears to be little cumulative effect of repeated daily sessions of exercise on LPL activity, explaining why the decrease in triglycerides is essentially an acute effect of exercise that dissipates within 24 h.