The effects of training

Exercise training has a number of effects, which cannot be discussed at length in this book. They occur over various time-spans. A single bout of exercise will bring about some metabolic changes that are relevant to the theme of this book.

Expression of muscle lipoprotein lipase increases after exercise, and this increase lasts around 24-48 hours. We can imagine that this is an adaptation allowing the muscle to use more circulating triacylglycerol-fatty acids. It has a clear consequence. The ability of the body to handle incoming, dietary fat is improved within 24 h of a single bout of exercise (Fig. 8.16). Since impaired ability to handle a fat load is a marker of risk of cardiovascular disease, this may be one mechanism by which exercise protects against such disease (discussed in more detail in Chapter 9, see Section 9.4.3). Note, however, that since the increase in lipoprotein lipase activity is fairly short-lived, exercise must be regular to sustain this benefit.

Fig. 8.16 Plasma triacylglycerol (TG) concentrations after a high-fat test meal on two occasions. In one trial, participants had exercised for 2 h at 70% maximal aerobic power 24 h previously ('Exercise trial'); on the other occasion, they had rested the day before ('Control trial'). Both the fasting TG concentration and the rise in concentration after the test meal are significantly reduced in the exercise trial. Data are from eight normal subjects and are shown as mean ± standard error. Data from Malkova et al. (2000).

Fig. 8.16 Plasma triacylglycerol (TG) concentrations after a high-fat test meal on two occasions. In one trial, participants had exercised for 2 h at 70% maximal aerobic power 24 h previously ('Exercise trial'); on the other occasion, they had rested the day before ('Control trial'). Both the fasting TG concentration and the rise in concentration after the test meal are significantly reduced in the exercise trial. Data are from eight normal subjects and are shown as mean ± standard error. Data from Malkova et al. (2000).

Similarly, if tested 24 h after exercise, there is improvement in glucose utilisation in response to insulin (tested, for instance, by infusing both glucose and insulin intravenously). Resistance to the effects of insulin, common in sedentary people (discussed in more detail in Chapter 10, Box 10.2), is a marker of increased risk both of developing diabetes and of developing cardiovascular disease. Again, therefore, this may be a mechanism by which regular exercise helps to protect against these conditions.

Beyond these rather early changes, prolonged training will bring about longer-lasting structural changes in muscle. In the case of anaerobic exercise (such as weight-lifting or sprinting) the changes brought about by training are largely increased muscle bulk and strength. The increase in muscle bulk is mainly the result of muscle hypertrophy rather than hyperplasia: that is, muscle cells become bigger rather than increasing in number. A weight lifter, sprinter or high-jumper will have a higher proportion of Type II fibres than a long distance runner (see Fig. 3.7), but this is not primarily a result of training; it appears to be genetically determined. Rather, he or she is a weight lifter or sprinter because he or she has a high proportion of Type II fibres.

The changes occurring with endurance training are rather more varied. They are listed in Table 8.5. They concern increased ability to deliver O2 and other substrates to the working muscle, and increased ability within the muscle to utilise substrates. Note that the activity of glycogen synthase is usually found to be increased whilst that of glycogen phosphorylase is not; presumably the activity of glycogen phosphorylase is not usually limiting for generation of power in endurance exercise.

Table 8.5 Changes occurring with endurance training. Cardiovascular and whole-body

Increased cardiac output, and ability to increase this during exercise

Improved respiratory function

Increased lean body mass (mainly muscle bulk)

Decreased body fat

Increased bone strength

Structural changes in muscle

Increased density of capillaries

Increased number of mitochondria

Increased size of mitochondria

Increased myoglobin concentration

Metabolic changes in muscle

Increased expression of GLUT4

Increased sensitivity to insulin (discussed in text)

Increased activity of lipoprotein lipase (discussed in text)

Increased activity of oxidative enzymes in mitochondria

(tricarboxylic acid cycle and P-oxidation) Increased glycogen synthase activity

Based in part on Astrand & Rodahl (1977) and on Holloszy & Booth (1976).

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

Get My Free Ebook


Post a comment