Those who exercise to lose weight often wonder whether it’s better to eat or fast before a workout. The short answer is that — to lose weight, don’t eat before exercising.
A study published in the American Journal of Physiology — Endocrinology and Metabolism is the first of its kind to show the effects of eating versus fasting on gene expression in fat tissue in response to exercise. This study highlights the different roles fat plays in powering and responding to exercise and seems to suggest that, if you want to lose weight exercising, better not to eat before exercising.
Researchers from the University of Bath in the U.K. studied a group of overweight males. The volunteers walked for 60 minutes at 60 percent maximum oxygen consumption on an empty stomach in one experiment and two hours after consuming a high-calorie carbohydrate-rich breakfast in a comparison experiment. The research team took multiple blood samples after eating or fasting and after exercising. The researchers also collected fat tissue samples immediately before and one hour after walking.
Researchers found that gene expression in the fat tissue differed significantly in the two trials. Specifically, the expression of two genes, PDK4 and HSL, increased when the men fasted and exercised and decreased when they ate before exercising.
How Fasting Stomach Helps in Weight Loss
The rise in PDK4 gene likely indicates that stored fat was burned and used to fuel the exercise instead of carbohydrates from the recent meal. The rise of HSL gene usually happens when fat tissue uses stored energy to support increased activity during exercise.
These results reinforce the view that fat tissue is primarily an energy storage mechanism in human body and carbohydrate is the preferred energy source. When carbohydrate is available as a fuel, stored fat is less likely to be used during exercise. This means that exercise in a fasted state might promote burning of fat tissue, which could lead to weight loss and other health benefits in the long term.
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Journal Reference: Yung-Chih Chen, et al. FEEDING INFLUENCES ADIPOSE TISSUE RESPONSES TO EXERCISE IN OVERWEIGHT MEN. American Journal of Physiology – Endocrinology And Metabolism, 2017; ajpendo.00006.2017 DOI: 10.1152/ajpendo.00006.2017
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