Have you tried running hungry? We spoke to an expert to find out if it’s worth the effort
What you eat before a run is not a decision to be taken lightly. You need to tread a fine line, making sure you provide your body with the energy required without upsetting your stomach or eating so much that you feel sluggish on the run.
What are the potential benefits of fasted running?
Advocates of fasted running claim a number of benefits, from improved competitive performance to positive changes in body composition to enhanced economy of movement. However, many of these claimed benefits are not supported by research, with much ambiguity over whether performance is actually improved and weight loss realised when compared with non-fasted runners. What is clear from studies, however, is that fasted running promotes numerous physiological changes that are health-positive.
Fasted running enhances mitochondrial biogenesis – an increase in the number of mitochondria in the body. These are the batteries energising our muscles, so a greater number of mitochondria is one of the main adaptations we seek through training to improve our physiological potential. All training has a positive impact but to varying degrees. Fasted running has been shown to stimulate mitochondrial biogenesis more effectively than non-fasted running.
Additionally, fasted running improves our fat-burning potential – the ability to mobilise and convert fat for fuel, offering the potential to change body composition. At the same time our regulation of insulin, the hormone which allows the mobilisation of carbohydrates for fuel, is improved, lowering our resting blood sugar levels.
These molecular changes, and some further adaptive changes, have major health consequences, lowering cholesterol, improving cognitive function and reducing the risk of cardiovascular disease and diabetes. And these benefits don’t come with obvious possible disadvantages, such as a noticeable increase in the perceived effort needed when running at a constant speed, or the need to catch up by eating more after fasted exercise.
Are there any downsides to fasted running?
Too much fasted running can hamper your carbohydrate metabolism, hindering performance at higher intensities and, in all likelihood, at race pace. It can prolong your recovery from exercise, hinder immune function, reduce muscle mass and disrupt the menstrual cycle.
So, fasted running is best used in moderation and should be carefully placed within programmes. It’s good for slower, longer efforts and you should not completely eliminate non-fasted days. These will help with recovery, keep the immune system in order and ensure your carbohydrate-powered metabolism stays tuned.
Does fasted running improve running performance?
There is not much scientific evidence showing that fasted running improves performance. The research that has been undertaken is inconclusive.
Numerous studies indicate that there is no significant advantage from fasted as opposed to non-fasted exercise, whereas a study of French triathletes showed that they improved their 10km time trial performance, economy and body composition.
Above 80% of aerobic power, we are reliant on mobilising carbohydrates for fuel and the carbohydrate metabolism can produce 1,500 calories an hour. So when we perform at race pace, we rely heavily on carbohydrate metabolism and the fat metabolism plays a supporting role – it helps us preserve our glycogen and carbohydrates for longer and therefore prospectively last for longer. This is important in endurance and ultra-endurance events but less so in shorter and higher-intensity races.
Can fasted running help with weight loss?
Studies do not show that fasted running promotes weight loss any more than non-fasted running. It is believed that this is primarily because the carbohydrate metabolism generates more thermogenic activity, burning fuel for longer after exercise and negating the fat-burning benefits during exercise.
However, what is not in doubt is that a better-balanced metabolism has numerous health benefits as detailed above, and provides a better foundation for health and exercise.