Fat Burning: using body fat instead of carbohydrates as fuel

Fat burning is a very popular and often-used term among endurance athletes. But is it really important to burn fat – and, if so, how can it best be achieved? Professor Asker Jeukendrup looks at what the research says

The term ‘fat burning’ refers to the ability to oxidise (or burn) fat, and thus to use fat – instead of carbohydrate – as a fuel. Fat burning is often associated with weight loss, decreases in body fat and increases in lean body mass, all of which can be advantageous for an athlete.

It is known that well-trained endurance athletes have an increased capacity to oxidise fatty acids. This enables them to use fat as a fuel when their carbohydrate stores become limited. In contrast, patients with obesity, insulin resistance and type II diabetes may have an impaired capacity to oxidise fat. As a result, fatty acids may be stored in their muscles and in other tissues. This accumulation of lipid and its metabolites in the muscle may interfere with the insulin-signalling cascade and cause insulin resistance. It is therefore important to understand the factors that regulate fat metabolism, and the ways to increase fat oxidation in patients and athletes.

Fat oxidation during exercise

Fats are stored mostly in (subcutaneous) adipose tissue, but we also have small stores in the muscle itself (intramuscular triglycerides). At the onset of exercise, neuronal (beta-adrenergic) stimulation will increase lipolysis (the breakdown of fats into fatty acids and glycerol) in adipose tissue and muscle. Catecholamines such as adrenaline and noradrenaline may also rise and contribute to the stimulation of lipolysis.

As soon as exercise begins, fatty acids are mobilised. Adipose tissue fatty acids have to be transported from the fat cell to the muscle, be transported across the muscle membrane and then be transported across the mitochondrial membrane for oxidation. The triglycerides stored in muscle undergo similar lipolysis and these fatty acids can be transported into the mitochondria as well. During exercise, a mixture of fatty acids derived from adipocytes and intramuscular stores is used. There is evidence that shows that trained individuals store more intramuscular fat and use this more as a source of energy during exercise (1).

Fat oxidation is regulated at various steps of this process. Lipolysis is affected by many factors but is mostly regulated by hormones (stimulated by catecholamines and inhibited by insulin). The transport of fatty acids is also dependent on blood supply to the adipose and muscle tissues, as well as the uptake of fatty acids into the muscle and into the mitochondria. By inhibiting mobilisation of fatty acids or the transport of these fatty acids, we can reduce fat metabolism. However, are there also ways in which we can stimulate these steps
and promote fat metabolism?

Factors affecting fat oxidation

Exercise intensity – One of the most important factors that determines the rate of fat oxidation during exercise is the intensity. Although several studies have described the relationship between exercise intensity and fat oxidation, only recently was this relationship studied over a wide range of intensities(2). In absolute terms, carbohydrate oxidation increases proportionally with exercise intensity, whereas the rate of fat oxidation initially increases, but decreases again at higher exercise intensities (see figure 1). So, although it is often claimed that you have to exercise at low intensities to oxidise fat, this is not necessarily true.

In a series of recent studies, we have defined the exercise intensity at which maximal fat oxidation
is observed, called ‘Fatmax’. In a group of trained individuals it was found that exercise at moderate intensity (62-63% of VO2max or 70-75% of HRmax) was the optimal intensity for fat oxidation, whereas it was around 50% of
for less trained individuals (2,3).

However, the inter-individual variation is very large. A trained person may have his or her maximal fat oxidation at 70%VO2max or 45%VO2max, and the only way to really find out is to perform one of these Fatmax tests in the laboratory. However, in reality, the exact intensity at which fat oxidation peaks may not be that important, because within 5-10% of this intensity (or 10-15 beats per minute), fat oxidation will be similarly high, and only when the intensity is 20% or so higher will fat oxidation drop rapidly (see figure 1).

exercise intensity and fat oxidation
This exercise intensity (Fatmax) or ‘zone’ may have importance for weight-loss programmes, health-related exercise programmes, and endurance training. However, very little research has been done. Recently we used this intensity in a training study with obese individuals. Compared with interval training, their fat oxidation (and insulin sensitivity) improved more after four weeks steady-state exercise (three times per week) at an intensity that equalled their individual Fatmax (4).

Dietary effects – The other important factor is diet. A diet high in carbohydrate will suppress fat oxidation, and a diet low in carbohydrate will result in high fat oxidation rates. Ingesting carbohydrate in the hours before exercise will raise insulin and subsequently suppress fat oxidation by up to 35%(5) or thereabouts. This effect of insulin on fat oxidation may last as long as six to eight hours after a meal, and this means that the highest fat oxidation rates can be achieved after an overnight fast.

Endurance athletes have often used exercise without breakfast as a way to increase the fat-
capacity of the muscle. Recently, a study was performed at the University of Leuven in Belgium, in which scientists investigated the effect of a six-week endurance training programme carried out for three days per week, each session lasting one to two hours(6). The participants trained in either the fasted or carbohydrate-fed state.

When training was conducted in the fasted state, the researchers observed a decrease in muscle
use, while the activity of various proteins involved in fat metabolism was increased. However, fat oxidation during exercise was the same in the two groups. It is possible, though, that there are small but significant changes in fat metabolism after fasted training; but, in this study, changes in fat oxidation might have been masked by the fact that these subjects received carbohydrate during their experimental trials. It must also be noted that training after an overnight fast may reduce your exercise capacity and may therefore only be suitable for low- to moderate- intensity exercise sessions. The efficacy of such training for weight reduction is also not known.

Duration of exercise – It has long been established that oxidation becomes increasingly important as exercise progresses. During ultra-endurance exercise, fat oxidation can reach peaks of 1 gram per minute, although (as noted in Dietary effects)fat oxidation may be reduced if carbohydrate is ingested before or during exercise. In terms of weight loss, the duration of exercise may be one of the key factors as it is also the most effective way to increase energy expenditure.

Mode of exercise – The exercise modality also has an effect on fat oxidation. Fat oxidation has been shown to be higher for a given oxygen uptake during walking and running, compared with cycling(7). The reason for this is not known, but it has been suggested that it is related to the greater power output per muscle fibre in cycling compared to that in running.

Gender differences – Although some studies in the literature have found no gender differences in metabolism, the majority of studies now indicate higher rates of fat oxidation in women. In a study that compared 150 men and 150 women over a wide range of exercise intensities, it was shown that the women had higher rates of fat oxidation over the entire range of intensities, and that their fat oxidation peaked at a slightly higher intensity(8). The differences, however, are small and may not be of any physiological significance.

Nutrition supplements

There are many nutrition supplements on the market that claim to increase fat oxidation. These supplements include caffeine, carnitine, hydroxycitric acid (HCA), chromium, conjugated linoleic acid (CLA), guarana, citrus aurantium, Asian ginseng, cayenne pepper, coleus forskholii, glucomannan, green tea, psyllium and pyruvate. With few exceptions, there is little evidence that these supplements, which are marketed as fat burners, actually increase fat oxidation during exercise (see table 1).

nutrition supplements and evidence on fat metabolism
One of the few exceptions however may be green tea extracts. We recently found that green tea extracts increased fat oxidation during exercise by about 20%(4). The mechanisms of this are not well understood but it is likely that the active ingredient in green tea, called epigallocatechin gallate (EGCG – a powerful polyphenol with antioxidant properties) inhibits the
catechol O-methyltransferase (COMT), which is responsible for the breakdown of noradrenaline. This in turn may result in higher concentrations of noradrenaline and stimulation of lipolysis, making more fatty acids available for oxidation.

Environment – Environmental conditions can also influence the type of fuel used. It is known that exercise in a hot environment will increase glycogen use and reduce fat oxidation, and something similar can be observed at high altitude. Similarly, when it is extremely cold, and especially when shivering, carbohydrate metabolism appears to be stimulated at the expense of fat metabolism.

Exercise training

At present, the only proven way to increase fat oxidation during exercise is to perform regular physical activity. Exercise training will up-regulate the enzymes of the fat oxidation pathways, increase mitochondrial mass, increase blood flow, etc., all of which will enable higher rates of fat oxidation.

Research has shown that as little as four weeks of regular exercise (three times per week for
30-60 minutes) can increase fat oxidation rates and cause favourable enzymatic changes(10). However, too little information is available to draw any conclusions about the optimal training programme to achieve these effects.
In one study we investigated maximal rates of fat oxidation in 300 subjects with varying fitness levels. In this study, we had obese and sedentary individuals, as well as professional cyclists (9). VO2max ranged from 20.9 to 82.4ml/kg/min. Interestingly, although there was a correlation between maximal fat oxidation and maximal oxygen uptake, at an individual level, fitness cannot be used to predict fat oxidation. What this means is that there are some obese individuals that have similar fat oxidation rates to professional cyclists (see figure 2)! The large inter-individual variation is related to factors such as diet and gender, but remains in large part unexplained.

maximal fat oxidation as a function of aerobic power

Weight loss exercise programmes

Fat burning is often associated with weight loss, decreases in body fat and increases in lean body mass. However, it must be noted that such changes in body weight and body composition can only be achieved with a negative energy balance: you have to eat fewer calories than you expend, independent of the fuels you use! The optimal exercise type, intensity, and duration for weight loss are still unclear. Current recommendations are mostly focused on increasing energy expenditure and increasing exercise volumes. Finding the optimal intensity for fat oxidation might aid in losing weight (fat loss) and in weight maintenance, but evidence for this is currently lacking.
It is also important to realise that the amount

of fat oxidised during exercise is only small. Fat oxidation rates are on average 0.5 grams per min at the optimal exercise intensity. So in order to oxidise 1kg of fat mass, more than 33 hours of exercise is required! Walking or running exercise around 50-65% of VO2max seems to be an optimal intensity to oxidise fat. The duration of exercise, however, plays a crucial role, with an increasing importance of fat oxidation with longer exercise. Of course, this also has the potential to increase daily energy expenditure. If exercise is the only intervention used, the main goal is usually to increase energy expenditure and reduce body fat. When combined with a diet programme, however, it is mainly used to counteract the decrease in fat oxidation often seen after weight loss (11).


Higher fat oxidation rates during exercise are generally reflective of good training status, whereas low fat oxidation rates might be related to obesity and insulin resistance. On average, fat oxidation peaks at moderate intensities of 50-65%VO2max, depending on the training status of the individuals(2,8), increases with increasing exercise duration, but is suppressed by carbohydrate intake. The vast majority of nutrition supplements do not have the desired effects. Currently, the only highly effective way to increase fat oxidation is through exercise training, although it is still unclear what the best training regimen is to get the largest improvements. Finally, it is important to note that there is a very large inter-individual variation in fat oxidation that is only partly explained by the factors mentioned above. This means that although the factors mentioned above can influence fat oxidation, they cannot predict fat oxidation rates in an individual.

  1. J Appl Physiol 60: 562-567, 1986
  2. Int J Sports Med 24: 603-608, 2003
  3. Int J Sports Med 26 Suppl 1: S28-37, 2005
  4. Am J Clin Nutr 87: 778-784, 2008
  5. J Sports Sci 21: 1017-1024, 2003
  6. J Appl Physiol 104: 1045-1055, 2008
  7. Metabolism 52: 747-752, 2003
  8. J Appl Physiol 98: 160-167, 2005
  9. Nutrition 20: 678-688, 2004
  10. J Appl Physiol 56: 831-838, 1984
  11. Int J Obes Relat Metab Disord 17 Suppl 3: S32-36; discussion S41-32, 1993
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