One of the tenets of exercise nutrition is the importance of the pre-workout meal: a small snack, eaten about 15-30 minutes before exercising, to maintain normal blood sugar levels during the workout and to promote muscle recovery afterward. But findings from a new study published in American Journal of Physiology-Endocrinology and Metabolism (paywall) flips that idea on its head, at least for one group of people – overweight men.
Ten healthy overweight men between the ages of 18 and 35, with a waist circumference roughly between 37 and 43 inches, participated in the study. Large waist size is associated with increased risk for diseases such as diabetes, high blood pressure, and heart disease.
The study took place over a period of about four weeks. First, the researchers measured the maximum amount of oxygen the men needed during intense exercise, called VO2max. Then they asked the men to walk for one hour on a treadmill at 60 percent of their VO2max – equivalent to moderately intense exercise, such as a brisk walk or a slow bike ride – two hours after eating a large meal (about 650 calories). The scientists took blood samples before, during, and after the exercise.
About four weeks later, the researchers repeated the experiment, but this time they asked the men to fast for at least 12 hours prior to the exercise. Again, blood samples were taken before, during, and after the exercise.
The results of the study showed that when overweight men exercised two hours after eating, their utilization of fat stores dropped by as much as 45 percent compared to when the men exercised after fasting. This means that instead of using stored fat to fuel their exercise, the men’s bodies relied on the carbohydrates from their recently eaten meal (in the form of blood glucose).
These findings suggest that eating before exercise may alter the beneficial effects of exercise such as weight loss and metabolic improvements in overweight men.
Chen YC, Travers RL, Walhin JP, Gonzalez JT, Koumanov F, Betts JA, Thompson D. Feeding Influences Adipose Tissue Responses to Exercise in Overweight Men. American Journal of Physiology-Endocrinology and Metabolism. 2017 Mar 14:ajpendo-00006.