In general, fat is only mobilised significantly during aerobic activity. However, because of the energy deficit produced from periods of anaerobic activity (i.e. oxygen debt) and a lack of glucose in the system, fat may be called in to make up this deficit after the activity. The limitations, therefore, come in the body’s ability to provide oxygen to the working muscles.

There is one further requirement we need to introduce here to understand this fully. Lactic acid is a by-product of anaerobic glycolysis. The build-up of tactic acid develops exponentially with increases in exercise intensity, until a ‘threshold’ is crossed where extended exercise can no longer be continued. This is called the ‘lactate threshold’ or ‘anaerobic threshold’ which is the point at which lactic acid production is greater than its rate of removal (although in reality, this is probably not a well defined cut-off point but a phase). As a result, a build up of lactic acid occurs and this is generally considered to be the limiting factor in performance. In practical terms it is where someone gets so exhausted and ‘out of breath1 that they have to stop what they are doing and allow the oxygen debt to be repaid. The anaerobic (lactate) threshold indicates a point above which fat utilisation becomes negligible.

The amount of fat used during exercise therefore becomes dependent on two things:

(1) the total amount of energy used during physical activity

(2) the proportion of this which is below the anaerobic threshold.

Both, in turn, are dependent on the amount and type of exercise carried out, and the aerobic capacity and anaerobic threshold of the individual concerned.

A confusion in the fitness industry often arises in relating these events. It is often argued, for example, that although the proportion of fat utilisation is lower as a result of high intensity exercise, the total amount of energy used is higher, and therefore the absolute amount of fat burned for a given individual will be greater at high intensity.

Comparing total energy and fat use at 70 per cent VO2 max versus 50 per cent VO2 max (equivalent to moderate intensity exercise) over a set time period. Fat utilisation at 70 per cent VO2, shown on the right hand side of Figure 12.3, is 40 per cent of energy use compared to 50 per cent at 50 per cent vo2. The higher total energy use at 70 per cent over a 30 minute period (i.e. 206kcal v 146kcal at 50 per cent VO2) means that the absolute amount of fat oxidised is greater at 70 per cent VO2 (i.e. 82kcal of fat energy) than at 50 per cent VO2 (73kcal fat energy). This general view has support from some clinical research, although other work suggests greater fat utilisation at lower intensity.

It has thus been suggested that higher intensity exercise (i.e. around 70-80 per cent of VO2 max) will always result in greater absolute fat use (even though this type of activity is both de-motivational and potentially dangerous for fat people!) However, this ignores the fact that fat metabolism is related to aerobic fitness and is therefore a graded function of aerobic capacity, which in turn is inversely correlated with body fat levels. The theoretical differences in oxidative capacity at the same levels of relative exercise intensity for an unfit versus a fit individual. It can be seen from this that as the relative intensity of exercise increases, the ability to oxidise fat decreases at a much greater rate in the unfit than the fit individual. Hence, exercise at low-moderate relative intensities is more likely to provide greater absolute fat utilisation for the fat, unfit person than exercise at a higher intensity. The cut-off point is not definitively known and needs to be more closely researched, but there are indications that exercise at around 40-65 per cent VO2 max is optimal in these people.

Again of interest is the finding that fat oxidation plateaus early for the unfit, but continues to increase over time for the fit. The difference in fat utilisation appears to be not so much in release of fat from the fat cells, as demonstrated by the increase in fatty acids in the blood, but in the uptake and combustion of fat by the muscle tissue itself. It seems that fit muscle has a greater supply and is able to ‘soak up’ intra-muscular fat stores more readily than unfit muscle.

The association between fitness and fatness is not a direct one. It is possible, of course, for two individuals of the same aerobic capacity to have different levels of body fat and this seems to be determined by a number of factors, in particular, genetics and gender. In general though, it’s reasonable to suggest that a fat person is likely to be at the lower end of the aerobic fitness scale.

*141\186\4*

There are still large sections of Western populations who are classified as totally sedentary or inactive, i.e. they don’t do any regular physical activity in their leisure time. It’s quite likely that a large proportion of these people also make up that section of the population regarded as overweight or obese. They are unlikely to spring from their lounge chairs into an aerobics class, even though they may wish to decrease their own creeping corpulence. To them, fitness is anathema. They’d like to be less fat, and possibly more healthy in the process, but they have no real desire to break world records, or be highly ranked among the triathlon set. And they don’t want to miss out on too many of life’s little luxuries to get rid of their excess body fat.

There’s another reason why fitness and fatness are less correlated than thought in the past. Much of the traditional nutrition and exercise advice for increasing fitness is now no longer regarded as appropriate for fat loss. This knowledge has come about through research in the area of exercise physiology, down to the microcellular level, particularly since the early 1980s. The same has not yet happened in the body fat area and scientific knowledge on fat physiology is only just starting to accumulate.

*2\186\4*

Related Posts:

• FAT MOBILISATION THROUGH EXERCISE