The concept of the fat-burning zone has become highly appealing to fitness enthusiasts
THE HEALTH and fitness industry is shrouded in many myths and fallacies. Many of the exercise guidelines published in the health media can be contradictory and confusing and often form part of a marketing ploy to sell the latest training device or exercise programme.
In many cases these are gimmicks with no scientific evidence to support their effectiveness.
Perhaps one of the most controversial is the concept of the "fat-burning zone". For many people engaging in regular physical activity and exercise, the primary goal is weight management and specifically reducing overall body fat.
Out of this quest for identifying the optimal training method to reduce body fat, the principle of the fat-burning zone was probably born. The term suggests that there is an optimal exercise intensity at which the body is somehow able to oxidize or burn fat as its preferential energy source. As a result, the concept of the fat-burning zone has become highly appealing to fitness enthusiasts and personal trainers alike.
To understand this concept, it is first useful to be aware of the basic energy systems and fuel sources within the body necessary for sustaining physical activity and daily living.
Energy is stored within the body primarily using one of three sources: carbohydrate, fat and protein. Protein is used as an emergency energy supply when other stores are low and is also involved in the repairing and sustaining of muscle mass.
Carbohydrate is stored in the body as glycogen, and to a smaller degree as blood glucose, and is a higher grade fuel which is used during more intensive activity. The only downside is that there is only a limited supply available which, depending on individual fitness levels, would be sufficient to sustain up to about 60 minutes of intermittent activity, or approximately 90 minutes or steady exercise - for example, running at marathon pace.
In contrast, there is a more plentiful supply of fat within the body stored in various sites as adipose tissue or within the muscle and for the typical average adult they would have sufficient body fat to run about 50 marathons! The only down side is that fat can only be burned as a fuel at relatively low exercise intensities.
Put simply then, the ability to mobilise fat as an energy source appears to be a product of exercise intensity and duration. If you take the analogy of running a marathon to illustrate this point, the typical recreational athlete will have a sufficient supply of glycogen to sustain marathon race pace for about 90 minutes.
Once these high grade energy stores are depleted, the body needs to shift over fat as its primary fuel supply. The only problem with this is that the body's fat stores can only be used to sustain activity at a much lower intensity, hence the concept of a runner "hitting the wall".
Based on this premise, those supporting the concept of the fat-burning zone advocate low intensity long duration activity to target the fat metabolism and increase fat-burning capabilities. In terms of exercise prescription, this fat-burning zone has typically been suggested to be at an estimated work intensity equivalent to about 50-60 per cent of age predicted maximal heart based on the formula of 220 minus your age.
Therefore to hit the zone, a typical 20 year old should work at a heart rate of about 100-120 beats per minute. In reality however, due to a number of factors such as variations in maximal heart rate, individual physiological make-up and training status, it is impossible to target a specific point where suddenly the fat metabolism is somehow magically switched on and fat is burned while carbohydrate stores are conserved.
Another consideration is that while working in this fat-burning zone might be a useful starting point for someone engaging in regular physical activity for the first time, it is questionable that this level of activity would be sufficiently intensive to promote other health parameters such as enhanced cardiovascular fitness.
In this regard, the role of exercise intensity and in particular more intensive activity may be actually more important for weight management than targeting the fat-burning zone. Although you will burn a greater percentage of fat at lower intensities, by working at a higher exercise intensity, you will burn far more total calories and similar absolute calories in the form of fat. This could be achieved by using interval training which involves a series of repeated bouts of intense activity separated by short rest periods.
An additional advantage of working at higher exercise intensities is that once you finish your workout your metabolism does not immediately fall back to resting levels. In fact, your energy expenditure will remain elevated for some time even though you are no longer exercising!
Intensive training has also been shown to lead to elevated resting energy expenditure which means you will burn more calories throughout the day even when non-active.
When it comes to weight loss there are no quick fixes or magic cures.
The best way to reduce body fat is through a balance of good nutritional practices combined with regular exercise training appropriate to the individual's current level of physical fitness.
Although it is not fully clear if the optimum training regimen to maximise improvements works, it does appear that exercise intensity plays an important role.
• Dr Giles Warrington is a sport and exercise physiologist and lecturer in the School of Health and Performance in Dublin City University (DCU)