You already know what you eat and how much you eat can has a profound influence on your body fat. But did you know when you eat might be important as well?
Until recently, most dietitians and exercise professionals would have told you that it does not matter when you eat with regard to body fat gain or loss. For fat loss, the standard response has long been that the total caloric balance over time is all that matters. Yet recently, results from well-controlled studies have lent support to the idea that when you eat could have a substantial influence on stimulating fat loss, enhancing insulin sensitivity, and preventing weight gain.
Several studies have shown that fasting before an exercise session may enhance fat loss and increase glucose sensitivity when compared to exercise after eating. Exercise following a twenty-three hour fast showed increased fat mobilization and utilization in young healthy males compared to the same population in a non-fasted state.2 Additionally, intramyocellular triglyceride content (stored fat in the muscles) decreases more readily, glycogen re-synthesis following a workout is enhanced, and the exercise intensity corresponding to maximal fat oxidation is increased, when sustained exhaustive exercise takes place in a fasting state.1,5,6
Put simply, the body seems to be more adept at using stored muscle fat for energy (as opposed to carbohydrate) and replenishing its glycogen stores once exercise has ceased when we exercise after fasting. These studies have lent support to the idea that, at least for a young healthy population, the body is better at burning fat for fuel in the fasted condition.
Many people dismiss the idea of fasting before exercise because of the known performance improvements of eating prior to exercise. We have long known that eating before exercise improves both the peak intensity and the length of time exercise can be maintained, compared to exercising in a fasted state.3,4 Thus, if our goal is to compete at our top sustained performance level, eating before exercise and even during exercise are vitally important to the outcome.
Performance improvements aside, many exercisers probably do not care if they shave off two minutes from their half marathon time or are able to bike ride at a higher intensity for two or more hours. In fact, I would argue, most people who exercise are doing so to enhance their physical appearance and maintain their health, not necessarily to maximize performance on any one particular task. Central to the idea of these outcomes is optimizing body composition and regulating a healthy body weight. In this case, a strategy that enhances fat loss and increases insulin sensitivity would be the ideal choice for many people.
In 2010 a well-done study by Van Proeyen et al.6 may have convinced many people once and for all of the benefits of exercising after fasting. The researchers compared three groups of young males who were each being fed a hyper-caloric and high fat diet designed to cause weight gain. The control group ate the excess calories and fat and did not exercise. The carbohydrate group consumed calories before and during lengthy sustained morning exercise sessions four days a week for six weeks. The fasted group ate nothing before or during lengthy sustained morning exercise sessions four days a week for six weeks BUT made up for the missed calories by consuming them later in the day. Thus, there was no daily caloric intake difference between the groups. What happened?
Fasted exercise enhanced insulin sensitivity, prevented weight gain, and enhanced adaptations to cellular glucose transport by increasing muscle GLUT4 protein when compared to the other two groups.
Simply put, all the groups were fed a similar hyper-caloric high fat diet designed to cause weight gain and insulin resistance. In spite of equivalent caloric intake between the groups, the group that exercised in the fasting state fared far better than the other two groups in terms of weight gain, body fat, and insulin sensitivity. Exercise in the fasted state seemed to ward off some of the negative outcomes of bad nutrition.
So what does all this mean for you?
It depends on your personal situation and goals. If your goal is to maximize fat loss, you have a family history of type 2 diabetes, you want to get rid of that annoying roll around your middle, or you are entering into the holiday season where extra caloric intake is likely, exercising in the morning after fasting for 12 to 14 hours may be the right choice for you. It seems clear that lengthy exercise sessions after a period of fasting may help with IMTG fat oxidation, weight gain from excess calories, and increasing glucose sensitivity (At least for a young and healthy male population). Thus, morning exercise on an empty stomach may be very beneficial to you. On the other hand, if your goal is to enhance performance, maximize muscle gains, or feel your best during a workout, it seems that consuming the right kinds and amounts of nutrients before exercise is the right choice for you. (Check with your doctor before making such changes in your program)
The truth is, as with most things in life, what strategy you take will always depend on your individual goals. Just remember—no matter what workout methodology you choose, as long as you are consuming a low-quality, highly processed, high-caloric diet accompanied by an inconsistent exercise program, you will not meet any of your long term goals. You must identify your goals so you can create an individually tailored diet and exercise plan to meet your specific needs. This will not only lead to your best results, it will lead to the perpetual motivation only continual growth and personal achievement can accomplish.
*Opinion section—I personally highly recommend morning exercise in the fasted state for those who are cleared by their physician. The enhanced fat oxidation during the workout, combined with improved glycogen storage from the post workout meal, provide fantastic fat-burning results and terrific recovery for the next workout. Just be sure to talk to a registered dietitian (RD) about the right balance of nutrients for your post workout meal.
1. De Bock, K. et al. (2005). Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans. Journal of Physiology, 564: 649-660.
2. Dohm, G.L., et al. (1986). Metabolic responses to exercise after fasting. Journal of Applied Physiology, 64(4): 1363-1368.
3. Schabort, E.A., et al. (1999). The effect of a preexercise meal on time to fatigue during prolonged cycling exercise. Medicine and Science in Sports and Exercise, 31(3): 464-471.
4. Sherman, W.M., Pedan, M., & Wright, D. (1991). Carbohydrate feedings 1 hour before exercise improves cycling performance. American Journal of Clinical Nutrition, 54: 866-870.
5. Van Proeyen, K. et al. (2011). Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Journal of Applied Physiology, 110(1): 236-245.
6. Van Proeyen, K. et al. (2010). Training in a fasted state improves glucose tolerance during fat rich diet. The Journal of Physiology, 588: 4289-4302