Aerobic exercise: Does it really speed up fat loss?

Despite the popularity of moderate intensity aerobic exercise, the majority of research shows that it has a minor effect on the rate of weekly fat loss. This is despite organisations such as the American Council on Exercise claiming that up to three pounds of fat loss per week is a realistic target. However, there are studies indicating that aerobic exercise encourages healthier eating patterns - more physically active people are less likely to eat foods with a high fat content. Individuals including physical activity as part of a weight loss strategy are also far more likely to keep in shape than those who rely on dieting alone.

Aerobic exercise defined

January is a busy time for most exercise centres. Most clubs seem full of enthusiastic newcomers keen to remove the excess fat they managed to accumulate during the festive season. The countless number of treadmills, rowing machines and steppers that adorn most gyms are usually in constant demand, based on the claim that moderate aerobic exercise will speed up fat loss.

A typical program normally consists of steady state aerobic exercise (such as cycling or walking) at 70-80% of maximum heart rate. This type of workout can last anywhere between 20 and 60 minutes, depending on the fitness level of he individual (Utter et al., 1998). Although most people begin these programmes with the intention of sticking with them, the reality is often very different. After six months, half of those who begin an exercise programme will have given up (Leith, 1992), deciding they simply don't have the time to train regularly (Dishman, 1991).

Why most people give up on exercise

You and I know there are usually two reasons behind any decision - the real reason, and the one that sounds good! Individuals claiming that "they don't have the time" are often hiding the fact that their expectations were not met. Simply put, they were not making the kind of progress they were promised. The time and effort being invested in the exercise programme were not matched by any kind of tangible benefit. In other words, exercise did not produce a measurable change that was significant enough to justify the amount of effort being put in.

On an intellectual level, most people are aware that exercise can play an important role in enhancing longevity, reducing the risk of disease, and improving quality of life. However, any good salesperson will tell you that we don't make our decisions based on intellectual reasoning. We base them on emotion. Most of us exercise because, at a very basic level, we simply want to look and feel better. We exercise because it appeals to our sense of vanity and pride. We want to feel good about ourselves.

The problem comes when there is a conflict between what you expect from an exercise programme, and what you actually get. One of the keys to making sure exercise becomes a habit is to establish precisely what results you can expect from moderate aerobic exercise. And that's where the trouble lies. It's common for many individuals starting an exercise programme to expect to lose around two pounds of fat each week. Even the American Council on Exercise suggests that up to three pounds of fat loss per week is an achievable goal. But is this a realistic target? Can an aerobic exercise programme really produce such rapid changes in body composition?

Aerobic exercise has a small effect on the rate of fat loss

Some answers come from a recent review of several hundred weight loss studies, conducted by Dr Wayne Miller and colleagues at The George Washington University Medical Centre (Miller et al.,1997). The team examined 493 studies carried out between 1969 and 1994. Miller and his associates wanted to determine whether the addition of aerobic exercise to a restricted calorie diet accelerated weight loss. Twenty-five years of weight loss research showed that diet and aerobic exercise provides only a very marginal benefit (in terms of weight loss) when compared to diet alone.

TABLE 1. Average weight loss over a 15-week period

Method Weight Loss Aerobic exercise 3.3kg (7.3lb) Restricted calorie diet 7.8kg (17.2lb) Exercise and diet 9kg (19.8lb)

This is not the only research to cast doubt over the effectiveness of moderate aerobic exercise. A study completed at Appalachian State University also showed little effect on body composition over a 12-week period (Utter et al 1998).

The research team assigned a group of 91 obese women to one of four groups. Group one followed a restricted calorie diet (1,200 - 1,300 calories per day), group two performed moderate aerobic exercise for 45 minutes, five days each week, while a third group combined the exercise and diet programme. The fourth group acted as controls.

TABLE 2. Fat loss following a 12-week programme of diet and exercise

Method Weight Loss Aerobic exercise 1.3kg (2.9lb) Restricted calorie diet 6.8kg (15lb) Exercise and diet 7.2kg (15.8lb)

"Moderate exercise training," says Alan Utter, the researcher leading the study, "has a minor, nonsignificant effect on fat mass."

Despite the popular support for aerobic training, it does not appear to significantly accelerate fat loss, even when combined with a low calorie diet. When performed without restricting calories, moderate aerobic exercise has only a very small effect on body fat levels.

Why moderate aerobic exercise is so ineffective

This isn't surprising when you consider how many calories are contained in a pound of fat. Each pound of fat contains the equivalent of approximately 3,555 calories (McArdle et al., 1991).

The most fundamental aspect of any fat loss programme is to create a caloric deficit - to expend more calories than are consumed. Unfortunately, moderate aerobic exercise has only a moderate caloric requirement - around 187 calories per session (Utter et al., 1998). Based on this estimate, it could take up to 19 moderate aerobic workouts to lose just 0.45kg (1lb) of fat.

This might come as a surprise to those of you using the calorie counters on exercise machines to monitor energy expenditure during a workout. Unfortunately, these digital readouts are not always accurate. The most reliable way to assess energy expenditure during exercise is to measure oxygen consumption. Each litre of oxygen that you consume generates approximately five calories of energy. For example, if you were to exercise for 30 minutes and consume 30 litres of oxygen, you would have expended approximately 150 calories (five calories x 30 litres). Without directly measuring oxygen consumption, it's difficult to establish an accurate estimate of energy expenditure during a workout.

A second factor affecting the reliability of calorie counters is the difference between net and gross energy expenditure. Gross energy expenditure refers to the energy cost of exercise plus the metabolic rate. Net energy expenditure refers to just the energy cost of exercise. Calorie counters often display gross energy expenditure - so they don't accurately represent the additional energy that is used during exercise. In fact, during a 45 minute workout, net and gross energy expenditure can differ by almost 30% (Utter et al., 1998). The moral? Don't always believe what the machine tells you.

Another popular misconception is the idea that aerobic exercise increases caloric expenditure AFTER a bout of exercise, thus making a further contribution to fat loss. Unfortunately this is not always the case. Excess post-exercise oxygen consumption (the name given to the increase in caloric expenditure following a workout) is more likely to occur after high intensity exercise. Moderate aerobic exercise has very little effect on post-exercise metabolic rate (Sjodin et al., 1996). Furthermore, when an increase in physical activity results in a caloric deficit (as would occur with diet and exercise), there is evidence to show that the metabolic rate does not rise at all (Sjodin et al., 1996).

Does this mean that aerobic exercise is a waste of time? Definitely not.

Aerobic exercise reduces the risk of gaining weight

There is a good deal of evidence to suggest that aerobic exercise can help to prevent the regain in weight that is often seen following a period of dieting (Buemann and Tremblay, 1996). It can be extremely difficult to adhere to the kind of low calorie diet needed to maintain weight loss. As a result, many individuals soon return to their original weight.

Moderate aerobic exercise has been shown to play a vital role in sustaining a healthy body weight. The National Weight Control registry contains a listing of subjects who have lost at least 13.6kg (30lb) of body weight, and kept it off for at least 12 months. Although the individuals on the register used a number of different strategies to reduce body fat, there was one thing they had in common; a commitment to regular exercise. They regularly expended at least 1,306 calories through physical exercise each week (McGuire et al., 1998). Scientists from George Washington University also found that regular exercise helps to maintain higher levels of weekly fat loss 12 months after starting a diet (Miller et al., 1997).

Aerobic exercise promotes healthier patterns of eating

Physical activity as part of a fat loss strategy also appears to encourage healthier eating patterns. Individuals who exercise frequently may perceive food as 'activity fuel', and place greater importance on the quality of their diet. Californian researchers have identified a direct relationship between physical activity and various markers of dietary quality (Johnson et al., 1998). They found that physical activity was associated with healthy eating habits, such as eating fruit and vegetables. More physically active individuals were also less likely to eat foods with a high fat content. "There is a consistent pattern of findings," the researchers concluded, "indicating that more physically active young adults have more healthful diets."

Moderate intensity aerobic exercise may also reduce fat under the skin to a greater extent than diet alone. Fat in the body is stored in three main areas - under the skin, surrounding the internal organs, and between muscle cells. Fat stored under the skin (known as subcutaneous tissue) is one of the main reasons many people decide to lose weight. It's the fat you can pinch; around the waist, arms or hips. Japanese scientists have discovered that a moderate aerobic exercise programme can reduce subcutaneous tissue to a greater extent than diet alone (Abe et al., 1997). In contrast, diets appear to have a greater effect on the fat that surrounds the internal organs (known as visceral fat).

The bottom line

Moderate aerobic exercise does have a role to play in reducing body fat. However, it's important to be clear precisely what to expect from a programme of this type. On a weekly basis, you can expect an additional fat loss amounting to between 0.11kg (0.2lb) (Utter et al., 1998) and 0.22kg (0.5lb) (Miller et al., 1997), depending on the frequency and duration of the exercise.

Although this might not seem like a great deal, it's important to remember that moderate aerobic exercise is a key factor in preventing weight regain following a restricted calorie diet. The majority of research shows that individuals including physical activity as part of a weight loss strategy are far more likely to keep in shape than those who rely on dieting alone.

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