Quite apart from the aesthetics of a slim toned body, for most sportsmen and women, lower levels of body fat equate to better performance. Andrew Hamilton explains the nuts and bolts of fat burning and how you can manipulate your training to burn more fat…

INTRODUCTION TO FAT BURNING
Although increased fat burning has beneficial implications for sport performance, many people who exercise regularly do so for general fitness, health and aesthetic reasons rather than to increase fat burning per se. So if you wander into almost any gym and ask people why they train or what spurred them to begin training, it’s hardly surprising that weight loss comes right at the top of the list!

Any exercise programme that promotes increased fat burning therefore can help you to reach and maintain your target body weight more easily, bringing you all the associated health and kudos benefits sooner rather than later. Together, these facts explain why there’s such an interest in fat burning and how to maximise it during exercise.

EXERCISE INTENSITY
One of the most powerful ways to manipulate the proportion of fat used to produce energy is to adjust the intensity of your exercise. The question that follows therefore is how hard should your aerobic training be to maximise fat burning? As we’ve hinted above, lower intensities favour fat burning but as the intensity increases, carbohydrate becomes more and more important until at very high intensities, almost all of the energy to fuel exercise comes from carbohydrate burning and none from fat-burning.

Figure 1 shows some actual data gathered from recreational cyclists. You can see that that as the exercise intensity (in watts) increases, the rate of fat burning increases, reaching a maximum of around 35 grams per hour at 180 watts. Above 180 watts, the amount of fat burned drops off rapidly so that by 300 watts, it’s contributing virtually nothing. Carbohydrate burning increases steadily too but at around 180 watts (just as fat burning drops off) it jumps dramatically so that by 300 watts, it’s contributing 100% of the energy for exercise.


THE FUNDAMENTAL LAWS OF FAT BURNING
Body fat can be thought of as form of stored chemical energy. Each pound of body fat contains approximately 3500kcals of energy, so to lose a pound of body fat over any given time period, you have to burn 3500kcals more energy than is contained in the food and drink you consume. This can be achieved either by increasing your energy expenditure (ie by training/exercising more), or reducing your calorie intake (eg by following a calorie restricted diet), or, even better, by a combination of the two.
Although there seems to be a small degree of genetic variability that makes the process of fat burning somewhat easier in some individuals than others, these fundamental laws of chemistry and physics (energy in versus energy expended) are immutable.
However, while you can’t overcome the laws of thermodynamics, the good news is that by manipulating your exercise intensity/duration and food intake/timing, you can increase the proportion of your expended energy that is derived from fat burning, which (as we will see) can yield significant benefits.


FUNDAMENTAL FACTS
Before we get into the nitty-gritty of maximising fat burning, it’s important to understand that the fundamental laws of chemistry and physics still apply (see box 1) and there’s absolutely no way of circumventing them. If you want to lose weight, you have to create a ‘negative energy balance’ in your body – ie expend more calories than you consume. However, by enhancing fat burning, you increase the proportion of your expended calories that will come from stored body fat. A second important fact that needs to be born in mind is that all of us, all of the time are burning all three types of food calories (fat, carbohydrate and protein) to produce energy. The contribution from protein is minimal except when vigorous exercise is performed in the absence of carbohydrate (a subject we’ll discuss in part II of this article) so in actual fact, most of your energy is derived from carbohydrate and fat.It follows therefore that increasing the proportion of your expended energy from fat is associated with a reduced proportion from carbohydrate and vice-versa. You can think of it as a kind of seesaw effect. As the proportion of carbohydrate burning increases, fat burning decreases; as the proportion of fat burning increases, carbohydrate contribution decreases.A third fact that needs to be hammered home is that if maximising fat burning is desirable, aerobic type exercise (also known as endurance, cardiovascular or stamina training) such as jogging/running, cycling, swimming, rowing, skipping, X-country skiing etc should form the mainstay of any exercise programme. That’s because during aerobic type exercise, energy is provided by combining carbohydrate, fat and (to a small extent) protein calories with oxygen. The role of oxygen is especially important here because:

There’s an unlimited supply in the air around you and even a lean body contains a very large amount of stored chemical energy in the form of body fat. Providing you don’t exercise so vigorously as to ‘run out of puff’ (ie oxygen), you can continue to exercise for long periods without fatiguing, which helps you burn a lot of calories in total;
Unlike carbohydrate (the other main fuel for the body), fat needs an abundant supply of oxygen in order to be converted to energy. Provided your exercise intensity isn’t too severe (ie you have enough oxygen fl owing around your body), you can derive a large proportion of energy throughout this aerobic exercise from fat.
Very high-intensity exercise like sprinting or lifting weights (resistance training) uses different metabolic pathways to produce energy, most of which comes from carbohydrate burning. This is why it’s not an effective way to enhance fat burning, although some resistance training can enhance a weight loss programme by helping to sustain or increase muscle mass (see later).