Article by Glenn Cranham B.Sc

Intermittent fasting is increasing in its popularity, and is linked with a variety of health benefits (http://drummondeducation.com/hunger-games-intermittant-fasting-does-it-work/). But what are the guidelines when it comes to training? Can fasted training boost fat loss, or increase athletic performance? Well this is an area of some debate and seems to depend upon your goals.

Maximising fat loss

A recent study compared male subjects undertaking a moderate intensity running protocol in a fasted or post breakfast state. They found that those exercising in a fasted state burned up to 20% more fat than those that ate breakfast. Surprisingly they also found that fasted exercise didn’t increase appetite, hunger, or food consumption later in the day. However, questions still remain regarding fat burning for women exercising in a fasted state.

Maximising Endurance

A number have studies have investigated whether cardiovascular endurance training in a fasted state, may lead to adaptations that increase an athlete’s ability to burn fat over carbohydrate. This ‘fat adaptation’ could be glycogen sparing which may be beneficial to an endurance athlete in a competition.

Studies comparing fasted and non-fasted athlete’s in training over several weeks found that this adaptation does in fact occur, and that the fasted athletes not only burned more fat, but increased their ability to burn fat in preference to carbohydrate.

However, many further studies have shown that this increased ability for fat oxidisation, does not lead to increases in endurance performance during competition. Furthermore, there may well be a detrimental effect on cardiovascular adaptations through training whilst fasted.

Bodybuilding

Bodybuilding diets are a subject of much controversy and those that wish to investigate further can look here for some good information https://jissn.biomedcentral.com/articles/10.1186/1550-2783-11-20. But what are the recommendations regarding fasting?

Sports scientists agree that the primary fuel for a high load (75-85% 1RM) resistance training session is glycogen (carbohydrate). With depletions of up to 40% in the working muscles subjected to multiple sets. So the effect of starting a session in a fasted state may be detrimental for two reasons.

Firstly, when commencing a session in an already glycogen depleted state, your ability to perform multiple sets and the load which you can manage are adversely affected. Although some proponents argue that the loads are unimportant, as long as the muscle fibres are being damaged and all sets are performed to failure.

Secondly when your body doesn’t have the necessary glycogen available, it need may produce it by the breakdown of your lean muscle mass (gluconeogenesis). This can have the net effect of actually reducing lean mass rather than building it. That said, some studies found that using branched-chain amino acids (BCAAs) as a supplement can reduce the rate of muscle breakdown during fasted exercise. With a further study showing that fasted exercise followed immediately by a carb/protein & leucine supplement drink actually increased the anabolic response in muscles. It seems the jury is still out.

High Intensity Performance

The evidence here seems pretty conclusive that exercise beyond the lactate threshold, whether resistance, cardiovascular or combined; is negatively affected by either fasting or ketogenic diets.

When one considers that the fuel for this type of exercise would be primarily glycogen and creatine phosphate. It’s logical that without some carbohydrate it’s very difficult to generate maximal power and perform at your peak.

So it would seem that if weight loss is your goal, moderate intensity fasted training may very well be beneficial. Whereas fasted training for exercise performance is likely to be detrimental, and very unlikely to be beneficial (unless cutting weight for competition). When it comes to bodybuilding it may be best to conclude that fasted training without additional supplementation and careful nutrient timing. Is unlikely to lead to any beneficial gains and may well lead to a reduction in lean body mass.

If you want to learn more about nutrition and training, why not book onto our Sports Nutrition course.