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Exercise on an empty stomach can speed up weight loss?

Written by: Ms. Sylvia Lam, Accredited Practicing Dietitian APD, Accredited Dietitian HKDA

There is a wide-spreading sports nutrition myth claiming that exercise on an empty stomach will speed up weight loss. The theory behind assumes that if our body is low in glycogen storage – a storage form of sugar as energy in our body – by not eating in the morning before exercise, our body will mobilize more fat as energy during exercise, resulting in greater weight loss.

An interesting but yet small study in 2017 conducted by the University of Bath in the United Kingdom found that obese men who walked for 60 minutes on an empty stomach activated certain genes, which helped them burned more fat than those who ate carbohydrates before exercise. Another systematic review and meta-analysis in 2016 published in the British Journal of Nutrition stated that aerobic exercise performed in the fasted state induces higher fat oxidation than exercise performed in the fed state. However, in 2014, a study published in the Journal of the International Society of Sports Nutrition compared two groups of people who followed the same low-calorie menu to lose weight, and concluded that whether subjects who exercise on an empty stomach or those who ate before exercise did not have significant weight loss changes between groups.

 

Besides, if one wants to metabolize fat as an energy source during exercise, the duration of exercise has to be long enough.  Some studies indicated that exercise on an empty stomach only benefited people who participated in long-distance cycling or running. For those who needs short burst of force like weight lifting or other resistant exercise, body’s glycogen is the instant of energy and indispensable; therefore, exercise on an empty stomach for these people might not be suitable.

As not only the body will turn to fat but also muscles as an energy source; thus, exercise on an empty stomach might induce muscle loss, Exercise without eating might also create extra stress to the body leading to higher level of cortisol secretion which increases muscle breakdown. Therefore, exercise on an empty stomach might not ideal for people who wants to increase their muscle mass. Some people might also experience fatigue, dizziness and even low blood sugar if exercise on an empty stomach resulting in reduced exercise performance.

 

So, if you really want to try exercise on an empty stomach, the recommendation is to ensure that the duration of exercise does not exceed 90 minutes and to eat enough carbohydrates at dinner the day before. If exercise duration is more than 90 minutes or exercise intensity is high, it is recommended to eat moderate amounts of carbohydrates and protein before exercise to have better exercise performance.

Although exercise on an empty stomach might create an opportunity for the body to use more body fat as energy, but in fact, one has to reach an energy deficit of 3,500 kcal to lose 0.5 kg no matter where the calories come from. To achieve successfully lose weight, reducing the energy input (i.e. reduce food intake) and increasing the energy output (i.e. increase exercise) are the only key strategies, no matter you eat or not eat before exercise.

 

Reference:

  1. Yung-Chih Chen et al. Feeding influences adipose tissue responses to exercise in overweight men. Am J Physiol Endocrinol Metab 313: E84 –E93, 2017.
  2. Alexandra Ferreira et al. Effects of aerobic exercise performed in fasted v. fed state on fat and carbohydrate metabolism in adults: a systematic review and meta-analysis. British Journal of Nutrition (2016), 116, 1153–1164
  3. Schoenfeld BJ et al. Body composition changes associated with fasted versus non-fasted aerobic exercise. J Int Soc Sports Nutr. 2014 Nov 18;11(1):54.