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Decreased appetite

It is also possible that interval sprinting may result in suppressed appetite, which could contribute to belly fat loss. Studies on rats have shown that they eat less after they perform hard exercise. The mechanisms underlying this effect are not known, but hard exercise may reduce hunger by releasing hormones that decrease appetite. For example, corticotropin releasing factor, a powerful hormone that depresses appetite, has been shown to increase in rats and humans during hard running and swimming exercise. Although human studies have shown a large decrease in appetite after intensive aerobic exercise, this effect lasts only for a short time.

The effect of high-intensity sprinting on appetite suppression has been investigated by one study, which examined the effect of intensive exercise on the appetite of obese adolescents.
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Appetite was assessed before and after a 6-week high-intensity exercise and diet intervention. The intensive exercise program increased the energy expenditure of the adolescents, however their appetites did not increase in line with their energy output. Why individuals do not eat more after hard, intensive exercise is unclear but animal studies have shown that appetite centres in the brain are affected by blood lactate levels. Lactate levels are increased in the blood when people do hard anaerobic exercise such as interval sprinting. Interestingly, in animals, injections of lactate have been shown to suppress appetite. Thus, it is feasible that the increased blood lactate levels brought about by interval sprinting may contribute to suppressed appetite in humans.

Decreased postprandial lipemia

Consuming saturated fat or fructose in 3 meals per day can result in elevated triglyceride (also called triacylglycerol) levels in the blood for up to 18 hours. This is called postprandial lipemia and is discussed in greater detail in Chapter 4. It has been discovered that people with high levels of fat in their blood after eating also tend to have greater belly fat stores. Impressively, just 1 bout of acute, moderately hard aerobic exercise lasting 40 minutes resulted in significantly lower fat in the blood after consumption of a high-fat meal even as long as 12 hours after exercise. Recently, we have shown that interval sprinting has a similar effect. Twenty minutes of interval sprinting at night reduced by about 20% the fat (triglycerides) in the blood of young women who ate a high-fat meal the next morning.
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The mechanism underlying this effect is believed to be the ability of moderately hard exercise to increase an enzyme called lipoprotein lipase, which is located in the muscle capillaries. One of the major roles of lipoprotein lipase is to remove triglyceride from the blood in the circulation into the muscles that have been exercised. The increase in lipoprotein lipase found after exercise usually peaks after about 4–6 hours but stays elevated for up to 18 hours post exercise.

This effect of exercise on lipoprotein lipase may also play a critical role in the belly fat reduction found after interval sprinting. As discussed, the major hormones that induce fat release from belly fat cells are the catecholamines. Interval sprinting, in contrast to moderate aerobic exercise like walking, results in significantly greater blood levels of catecholamines. We have discovered that belly fat is far more sensitive to the effect of catecholamines than are the fat cells beneath the skin, so more fat is released from the belly fat cells during interval sprinting than from other fat stores. However, all belly fat that is released goes directly to the liver via the portal vein, where most of it is repackaged as triglyceride and secreted back into the circulation. So where does this fat in the form of triglyceride go? Although studies have not yet determined the destination, it is feasible that the increased triglyceride circulating in the blood, similar to the elevated triglyceride found after a high-fat meal, is shuttled into the skeletal muscle by the increased levels of lipoprotein lipase caused by exercise. This sequence of events is illustrated in Figure 4.

How much body and belly fat is it possible to lose after interval sprinting?

The amount of total body fat loss expected to happen after involvement in any exercise program can be assessed by estimating the energy cost of the exercise. For example, the largest amount of fat the body of an untrained individual can burn during a bout of aerobic cycle exercise has been estimated to be around 0.6 gram per minute. Assuming an optimal fat metabolising rate of 0.6 gram per minute for one 60-minute session of cycling exercise would result in an energy usage equivalent of about 36 grams of fat. In optimal circumstances, a 12-week aerobic exercise program consisting of cycling 5 times a week for 60 minutes each session at a moderately hard exercise intensity could theoretically result in a fat mass loss of around 2.9kg, after adding the potential 13% more fat burning that may occur after the exercise has stopped. Over a year of exercise, fat loss could theoretically be around 12.6kg.

However, as mentioned previously, fat loss from aerobic exercise is usually much less than this amount because of a number of factors, such as compensatory eating and reduction in other forms of daily physical activity. Also, a range of individual, physiological and medical factors likes genes, fat-burning ability and thyroid dysfunction may impede fat loss in certain individuals.
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In contrast, some people who exercise may lose much greater amounts of body fat, which most likely reflects the fact they’ve undertaken a starvation diet at the same time as they start their exercise program. Consequently, the most likely explanation for large increases or decreases in total fat mass after exercise is a change in diet. As we’ll see in Chapter 4, starvation diets don’t work for most people in the long term and are associated with a number of health problems. A reasonable amount of total body fat loss to expect after 15 weeks of interval sprinting is around 3.0kg.

The decrease in dangerous belly fat, however, is difficult to estimate, although it is known that belly fat is easier to lose than subcutaneous fat. For example, as mentioned earlier, in our previous study men lost 17% of their belly fat, 9% of their total subcutaneous fat and 5% of their subcutaneous abdominal fat.
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Thus, percentage-wise, they lost 3 times more belly fat than abdominal fat after 12 weeks of interval sprinting. Similar to our study with women, waist circumference of these men was reduced by 3.5cm after 6 weeks of interval sprinting. Importantly, waist circumference was highly correlated with belly fat, suggesting that just 6 weeks of interval sprinting training, which involved eighteen 20-minute interval sprinting sessions – only a total of 6 hours of interval sprinting – produced a significant reduction in the amount of the individuals’ belly fat.

Figure 4. Interval sprinting may enable fatty acids to be transported from belly fat cells into the liver and then back into the circulation in the form of triglyceride, where they are shuttled into the skeletal muscles.

The fitness, muscle mass and insulin-resistance response to aerobic, resistance and interval sprinting exercise

Reducing belly fat is a very important feature of interval sprinting, but there are other important adaptations that have big implications for health, including an increase in aerobic fitness and skeletal muscle mass and a decrease in insulin resistance.

Aerobic fitness

Aerobic fitness, typically called aerobic power by exercise physiologists, is very important for health. Maximum aerobic fitness is typically measured by getting subjects to exercise to exhaustion on a cycle or treadmill. Gases are collected to assess the ability of an individual to deliver and use oxygen to the exercising muscles. Blair and colleagues found that aerobic fitness was strongly associated with early mortality.
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Thus, fitter adults suffered fewer deaths from lifestyle diseases such as cancer, heart disease and stroke. These researchers showed that low aerobic fitness accounted for 16% of all deaths in a large group of American adults. This 16% contribution to total mortality is significantly larger than other risk factors, such as obesity, smoking, high cholesterol and diabetes. It has been well documented that being aerobically fit results in reduced risk for heart disease, metabolic disease, certain cancers and Alzheimer’s disease. The good news is that interval sprinting significantly increases aerobic fitness even though the sprinting exercise is mostly anaerobic. We found that 15 weeks of interval sprinting resulted in a 26% increase in the aerobic fitness of young women, compared to a 19% increase in another group who completed 15 weeks of moderate aerobic exercise.
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The interval sprinting consisted of 3 20-minute sessions per week, compared to 3 40-minute sessions of aerobic cycling exercise. In a second study with young overweight women, we found that 12 weeks of interval sprinting resulted in an 18% increase in aerobic fitness, while a 16% increase occurred in young overweight men.
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Thus, a very important characteristic of interval sprinting is that it results in large increases in aerobic fitness. As interval sprinting is mostly anaerobic in nature, it also results in large increases in anaerobic fitness.

Skeletal muscle mass

Retention of skeletal muscle mass is very important when it comes to your health. For example, women who possess less muscle mass have greater incidence of osteoporosis. We typically lose a significant amount of our muscle mass as we age, and people who go on starvation diets generally lose significant muscle mass. As can be seen in Table 4 (below), women who went on a starvation diet, reducing their daily caloric intake by 50%, lost about 3kg of muscle over a 16-week period. Since it has been estimated that an increase of 1kg of skeletal muscle could use up just under 3kg of fat per year, any loss in muscle mass can result in a reduced incidence of fat loss, so retaining or increasing muscle mass is very important for health.

Unfortunately, even exercising does not stop the loss of muscle when dieting. When women performed either aerobic or resistance exercise while on a starvation diet, they still lost muscle mass but the loss was reduced by between 40% and 50%. It is well documented that participating in aerobic exercise does not change muscle mass, whereas moderately hard resistance exercise may result in increased muscle mass for some. As can be seen in Table 4 (below), 3 interval sprinting studies showed significant increases of leg and trunk muscle mass of 0.3kg and 0.4kg for women and 0.5kg and 0.7kg for men. Our study that examined interval sprinting and dieting is particularly interesting, as we asked overweight women to switch to a Mediterranean eating plan, which resulted in a daily caloric intake decrease of 13%. Despite eating slightly less every day, these women still significantly increased the amount of muscle in their legs (0.3kg) and trunk (0.3kg) and reduced their body fat by 2.6kg after 12 weeks of interval sprinting. These results suggest that combining healthy eating, such as the Mediterranean eating plan, with 12 weeks of interval sprinting may be the optimal way to lose belly fat and enhance muscle mass.

Table 4. A comparison of muscle mass loss with no dieting (ND), moderate dieting (MD) or low-calorie dieting (LCD) with or without aerobic, resistance and interval sprinting exercise.

Adapted from information in studies by Kuk et al., Trapp et al., Dunn et al. and Boutcher et al.
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Insulin resistance

Insulin resistance is measured by assessing the amount of insulin and glucose in the blood. If these levels are high, it indicates that a person’s tissues, especially the skeletal muscles and liver, are becoming resistant to the effects of insulin. Insulin, as you may recall, is released from the pancreas when we eat sugar. It enables the sugar in the blood to enter the tissues, to be used as energy by the cells or to be stored as glycogen for later use. Only insulin and exercise can remove sugar from the blood. Failure to reduce blood sugar levels over time is bad for health and typically leads to the development of type 2 diabetes. Participation in all forms of interval sprinting, aerobic exercise and resistance exercise typically decreases insulin resistance, and we found a large decrease in insulin resistance in 2 of our studies. The women who participated in the 12-week study reduced their incidence of insulin resistance by 31%, while those in the 15-week study experienced a reduction of 36%.
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In our study with overweight young women, their insulin resistance decreased dramatically after 6 weeks of interval sprinting. In comparison, aerobic exercise lasting greater than 12 weeks typically results in decreased insulin resistance of around 10%. Surprisingly, regular resistance exercise also decreases insulin resistance but the exercise has to be moderately hard.