Fat burning exercise and nutrition strategies – Matt Lovell

The best exercise and nutrition methods for fat burning are hotly debated. Matt Lovell cuts through the science with some practical suggestions.

Intuitively, having a low proportion of body fat would seem to be advantageous for athletic endeavour, increasing the efficiency of power transfer from respiring muscle. Assuming that the same power output is achieved, reducing surplus weight will increase the resulting speed of movement for actions such as throwing, running, hitting or lifting. In addition, adipose tissue is a metabolically active organ that uses the body’s resources and excesses can be associated with a number of adverse health outcomes. Considering sports individually, some sports have an inherent requirement for a low fat mass, such as weight-categorised sports, load-bearing activities like running, and sports judged on the aesthetics of performance.The act of fat burning and losing body fat, however, often represents a delicate balance between providing adequate energy for performance, while maintaining a necessary calorie deficit in the diet. In order to maximise the use of fat as an energetic substrate to as high a degree possible, the energy systems and nutritional requirements specific to an individual athlete must be taken into account.

Unless the athlete is affected by a particular deficiency, nutrition alone cannot increase fat loss or improve performance. Rather, certain energy deficits can stimulate fat burning, while training stimuli induce adaptations assuming that adequate rest is taken and nutrients are available for repair. Therefore nutrition must be seen as part of an integrated strategy to support training, while undertaking the necessary exercise to achieve fitness and fat-loss goals.

Exercise and fat burning

During exercise, the use of carbohydrate or fats as fuels represents a compromise between the greater energy available from complete fatty acid oxidation (9 kcal/gram from fat as opposed to 4 kcal/gram from carbohydrate) and the greater power production made possible from carbohydrate utilisation (1). Carbohydrate oxidation is faster and therefore starts to predominate over fat burning as exercise intensity increases. Therefore, longer durations of lower intensity exercise are likely to rely more on fat, while shorter, higher intensity activity depends more on carbohydrate.

Although it’s accepted that low intensity, prolonged exercise depends most heavily on fat for fuel, sprinters are commonly amongst the leanest athletes in sport, despite doing less overall work in a typical training session. Elite sprinters typically show body fat percentages between 6–7 per cent, and exhibit significantly lower body fat percentages than sub-elite competitors (2). Whether a greater capacity for high-intensity exercise augments leanness or whether elite performance is made possible by lower body fat, is debatable.

However, even in sub-elite athletes, average exercise intensity rather than work output is more strongly associated with leanness, even if supported by higher levels of 
fuel consumption (3). Clearly the situation is more complex than it first appears.

High intensity exercise

Although sprinting uses mainly glycolysis and the phospho-creatine systems for energy provision, high-intensity exercise has been seen to better induce the body’s synthesis of mitochondria; the power-plants of the cell (4,5). This observation has been accredited with the increased excess Post Exercise Oxygen Consumption (EPOC) seen in sprinters (6,7). Essentially, higher levels of EPOC following a training session increases the energy consumed in recovery, which seems to be greatly contributed to by fat burning. This rise in metabolic rate is seen most following exercise at very high intensities (supra-maximal – i.e. above what is usually possible with prolonged aerobic training) [6]. High-intensity compared to lower-intensity training has been seen to cause a nine-fold increase in body fat skinfold reduction, despite a lower energetic cost of exercise (8). So, although high-intensity exercise must be predominantly carbohydrate fuelled, basal metabolic rate can be increased, maximising fat burning and increasing overall daily energy expenditure.

Sub-maximal exercise

There is no escaping the fact, however, that long bouts of endurance exercise represent
an effective way to increase the duration and therefore the work output of training. Increasing energy expenditure is still the most important factor in fat burning and fat loss. If these longer bouts of exercise are performed at sub-maximal exercise intensities, then both the total energy expended, and the relative proportion of energy derived from fat will increase (1). Endurance training is also a major stimulus for adapting to fatty acid oxidation, increasing the subsequent capacity for fat burning during sub-maximal exercise (9). Steady state training, between 60 and 80% VO2max, will also decrease reliance on carbohydrate utilisation for a given exercise intensity, increasing importance of fat oxidation (10,11,12). So, sub-maximal exercise also plays an integral role in fat loss, despite imposing a lesser post-exercise stimulus.

Resistance exercise

If fat loss, rather than weight loss is the goal, this can be supported and maintained by gaining lean mass. Basal metabolic rate is proportional to lean mass (13), meaning that resistance exercise is particularly relevant for fat loss, while the energetic costs of exercise and recovery will be increased following hypertrophy of muscle. In addition, resistance exercise is also a powerful stimulus for fat loss and can increase heart rate for sustained periods. The magnitude of heart rate elevation induced by resistance exercise is frequently within a range that stimulates fat burning and it incurs a high energy cost proportional to the size of the load and number of repetitions completed. ‘Metabolic perturbations’ caused by resistance exercise have been shown to cause large EPOC demands, increasing energy consumption for a number of hours post-exercise, which have been postulated to be caused by increased lactate metabolism, glycogen resynthesis, increased catecholamine action and energetic demands of heat dissipation (14). Such energy demands are greater than those of low-intensity aerobic exercise (6).

Nutrition to support exercise for fat burning


One of the most important indicators of nutritional status is insulin, produced from the pancreas in response to elevated blood sugar. Its actions are opposed by the adrenal hormones, released during exercise, stress and starvation. Insulin indicates the ‘fed state’ and promotes carbohydrate utilisation or storage. Carbohydrate and fat vie for dominance as fuel within the body. The use of these two fuels is reciprocal to a degree, with each metabolic pathway inhibiting the functions of the other. In the fed state, the presence of carbohydrate can reduce the propensity of circulating fat to become the body’s predominant fuel. In starvation (or in response to exercise with little available carbohydrate) the hormonal and nutritional state of the body encourages fat burning, while decreasing carbohydrate utilisation: stress hormones such as cortisol and adrenaline are released, encouraging the use of fat as a fuel.

Carbohydrate fuelled training


Performance in sports with intermittent, intense bursts of activity is particularly responsive to carbohydrate feeding, enhancing glycogen utilisation. High GI carbohydrate feeding will optimise glycogen use and resynthesis, but a problem arises when we consider fat loss. The reduced fat oxidation from high GI feeding reduces the potential for fat loss.

Depleted state training

Training whilst fasted has been observed to maximise the proportion of fat oxidised, but may pose practical and ethical problems. As well as optimising fat burning during exercise, the cellular signalling induced by depleted state training can up-regulate systems to cope with a low carbohydrate and low energy environment. Cellular levels of the signalling molecule cAMPK are increased (15). This indicator of low energy then up-regulates transcription of enzymes involved in fatty acid metabolism, while also stimulating mitochondrial biogenesis to support compromised energy-provision (16). As sub-maximal, steady state exercise (50-70% VO2max) is reliant on fat more than carbohydrate, a role for fasted, sub-maximal training emerges for fat burning. Carbohydrate fuelling is less of a concern for lower intensity exertion (17), meaning that regular, fasted endurance exercise presents an efficient means of both burning fat, and inducing training adaptations to optimise fat loss. Protein intake during this period may need to be increased due to catabolic effects and increased gluconeogenisis, but such a strategy has been observed to prevent losses of lean mass in the face of energy deficits in athletes (18).

Fat adaptation

The reciprocal relationship between fat and carbohydrate metabolism also occurs chronically via transcriptional regulation of enzymes involved in these areas of metabolism. ‘Fat adaptation’ is the process of increasing the enzymatic capacity for fat burning following the adoption of a low carbohydrate high-fat diet. The effects of this strategy on increasing fat oxidation are unequivocal, with as little as five days fat adaptation being seen to decrease RER values, indicating increased reliance on fatty acids at a given exercise intensity (19). This would theoretically optimise fat loss. However, when possible performance benefits are considered, the evidence is not so clear. Protocols have been evaluated to assess the effectiveness of fat adaptation on varying intensities of exercise. Despite restoring glycogen levels after a period of fat adaptation with 1-2 days of high carbohydrate feeding, the results indicate that fat adaptation still decreases the capacity for carbohydrate utilisation and impairs high-intensity performance in athletes (20).

However, this strategy doesn’t seem to impair exercise other than sprinting, with sub-maximal time-to-exhaustion not being significantly affected and it is possible that medium-intensity exercise may actually be enhanced (17). It is unlikely that this would be of benefit even for elite endurance athletes, however, who typically sustain power outputs of 80-90% of maximum for prolonged periods of time. Considering training for optimising fat loss, fat adaptation may have a role as part of an integrated training strategy, augmenting fat burning from endurance exercise, while not compromising anything other than sprint performance. In addition, the effects are reversible and subsequent glycogen storage may even be enhanced following supercompensation (21). Obviously the demands of a specific athlete must be considered if using this strategy and dietary regimens suitably periodised to prevent a possible negative impact on high-intensity performance.

Considerations of glycaemic index (GI)


If pre-exercise fuelling is deemed necessary to successfully complete an endurance session, low GI feeding prior to exercise may offer a compromise between a depleted state and carbohydrate-fuelled training. Many studies have found that low GI meals promote FA oxidation and carbohydrate sparring in subsequent endurance exercise compared to high GI meals, possibly by reducing circulating levels of insulin. This can compliment endurance training adaptations to increased FA utilisation, while still permitting high intensity, carb-dependent exercise. In addition, low GI foods also aid weight regulation by inducing feelings of satiety, helping an athlete to reduce their energy intake.

Timing and cycling carbohydrate intake

Training adaptations are enhanced by ingestion of carbohydrate after exercise. Not only is the muscle more receptive to carbohydrate uptake and glycogen resynthesis through increased activity of GLUT4 receptors, but the induced insulin release exerts a powerful anabolic/anti-catabolic effect that supports adaptation. It would therefore seem reasonable to recommend the majority of daily carbohydrate be consumed following exercise to minimise the inhibitory effect on fatty acid oxidation that would occur if consumed beforehand. Combined with the increased potential for glycogen synthesis, it is less likely that this carbohydrate will act as a substrate for de novo lipogenesis. This reflects current consensus in sports nutrition, with guidelines of the International Olympic Committee recommending as much as 57 per cent of an athlete’s daily carbohydrate requirement be consumed within the first four hours after exercise (22). It would therefore be advantageous, if pursuing fat burning goals, to refuel for subsequent intense training sessions by limiting carbohydrate intake to post-training recovery meals. This will still allow a carbohydrate intake adequate for an athlete’s bodyweight to be ingested, while losing fat. However, sub-maximal exercise will most effectively stimulate fat loss if undertaken when fasted; meaning recovery from the preceding training sessions can also be compromised with a lower carbohydrate intake. Recovery should be supported with smaller quantities of lower GI carbohydrate than are typically recommended for performance.

Fat burning conclusions

In order to optimise the energetic cost of exercise, as well as to support a high resting energy expenditure in athletes, a fat loss strategy should incorporate both long-duration sub-maximal exercise performed in a depleted state, along with high-intensity exercise fuelled with an adequate carbohydrate intake. Variety is the spice of life!