Should you eat, or not, before you exercise?

The question of whether or not you should eat before exercise depends on a lot of variables.  Can your stomach handle food while you’re active?  Are you competing in an event, or just casually jogging?  Is your work-out very strenuous, or just casual?   Some new research suggests that there’s another variable to add: are you trying to lose weight?   The results suggest that exercising on an empty stomach may encourage your metabolism to burn fat more effectively.

In brief the study went like this: 14 subjects had to cycle 3 days/week.  After the cycling, they did an intense session of cycling an hour later.  Half of them fasted during the cycling, the other half were not told anything in particular about eating.  The fasting group performed worse during the intensive cycling, but they burned more fat while doing it.

This isn’t all that surprising.  When muscles don’t have any fuel available from digestion, they tap into stored energy: fat.  Muscles run more efficiently on carbohydrate energy, which is why sports drinks contain carbohydrates instead of fat.  Competitive athletes want to perform at their best and they want readily available fuel.  But what if you aren’t training to be a competitive athlete?  If your goal is to burn more fat, it might make sense to exercise on an empty stomach and rev up fat burning.

Believe it or not, this topic is controversial among nutrition professionals.

  1. Athletes:  Training on an empty stomach forces fat burning.  But athletic performance could be adversely affected, as the study showed.  Some sports nutrition experts believe training once per week while fasting teaches muscles to burn fat more effectively.  If you’re training for an endurance sport, when fat becomes an important fuel source, this might be helpful.
  2. People with eating disorders: anorexics in particular are obsessed with fat avoidance, so the idea that fat can be burned by exercising on an empty stomach could drive someone to even more destructive behaviors.  In extreme cases, an anorexic person has so little body fat that all muscle fuel burning is severely impaired anyway.  But certainly, this is not a reasonable choice for someone dealing with an eating disorder.
  3. Only muscle fat is burned: some fat is stored in muscles for just this kind of “emergency” energy need.  So if you exercise on an empty stomach, you burn the fat already stored in your muscles, not fat from your fat tissue.  While this is technically true, if you combine this practice with a lower-calorie diet, your muscles will have to replenish that stored fat from somewhere, most likely from fat tissue.  And the longer or more intense your activity, the more fat fuel will be necessary.  So if a half-hour jog burned up your muscle fat stores, and you jogged another half hour, the additional energy has to be drawn from fat stores.  As long as you don’t make up for that later by overeating, you would create a slow drain on fat stores.
  4. This was a small study, and the purpose was not to investigate weight loss.  Both true arguments.  But I don’t think it means you can’t draw conclusions.  Certainly, this study and other similar findings shows that more research is needed to clarify some of the concerns.  The fat burning/weight loss question is significant, given the growing obesity epidemic.

Take Away Message:  If you’re a highly trained athlete, fueling for performance is your primary goal, and carbohydrates and eating sometime before a workout remain important.  For the average person, with no major health problems, just trying to lose weight, this approach may turn out to be helpful.  But of course, it’s an individual situation.  If you know you need to eat something before a workout, stick to easily digested items like a banana or toast.  This study most certainly did not say that eating a whole lot of fat before exercise was at all helpful.

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