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Is It Bad to Exercise on an Empty Stomach?

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With some fitness gurus advocating exercising on an empty stomach to maximize fat loss, Greatist investigates: does working out on an empty tank really speed up fat loss, or will it simply lead to burnout midway through a workout [1]?

To Eat Or Not to Eat — Why It Matters

Don’t give in to those tummy grumbles just yet— studies suggest working out on an empty stomach increases the rate muscles use fat as fuel during exercise [2]. In other words, exercising after a fast might stimulate greater fat breakdown than working out soon after a meal [3]. Exercising after a fast may even help prevent drops in blood sugar that can cause dizziness, weakness, and even fainting during a tough workout [4].

But don’t pass on that pre-workout snack just yet. While the body relies more on fat to get through a workout while on an empty stomach, that won’t necessarily make a difference when it comes to losing weight. Other research suggests the actual amount of fat and calories burned with an empty tummy is ultimately no different from the amount of fat burned working out after eating a snack (though the snack itself adds on extra calories to that daily intake) [5].

Running on Empty — The Answer/Debate

While that pre-workout snack brings extra calories along with it, research suggests munching on something before hitting the gym may significantly improve exercise endurance [6]. And the benefits of a pre-workout snack may even extend beyond exercise. In one study, women who ate carbohydrates prior to exercise consumed less over the remainder of the day than those who exercised without fueling up [7] [8].

But what makes the perfect pre-workout fuel? The snack choice itself may simply be a matter of personal preference, but remember: adequate nutrition to help the body recover will always be a must. Starving the body in the hopes of shedding pounds faster could cause the body to compensate by slowing down its metabolism (and potentially undermining weight loss goals).

Works Cited

  1. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Van Proeyen, K., Szlufcik, K., Nielens, H., et al. Research Centre for Exercise and Health, Department of Biomedical Kinesiology, K. U. Leuven, Leuven, Belgium. Journal of Applied Physiology, 2011 Jan;110(1):236-45.
  2. Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans. De Bock, K., Richter, E.A., Russell, A.P., et al. Exercise Physiology and Biomechanics Laboratory, Faculty of Kinesiology and Rehabilitation Sciences, K.U.Leuven, Tervuursevest 101, B-3001 Leuven (Heverlee), Belgium. The Journal of Physiology, 2005 Apr 15;564(Pt 2):649-60.
  3. Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans. De Bock, K., Richter, E.A., Russell, A.P., et al. Exercise Physiology and Biomechanics Laboratory, Faculty of Kinesiology and Rehabilitation Sciences, K.U.Leuven, Tervuursevest 101, B-3001 Leuven (Heverlee), Belgium. The Journal of Physiology. 2005 Apr 15;564(Pt 2):649-60.
  4. Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Van Proeyen, K., Szlufcik, K., Nielens, H., et al. Research Centre for Exercise and Health, Department of Biomedical Kinesiology, K. U. Leuven, Leuven, Belgium. Journal of Applied Physiology, 2011 Jan;110(1):236-45.
  5. Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake. De Bock, K., Derave, W., Eijnde, B.O., et al. Research Center for Exercise and Health, F.A.B.E.R. - K.U.Leuven, Tervuursevest 101, B-3001 Leuven. Journal of Applied Physiology, 2008 Apr;104(4):1045-55.
  6. Increased fat availability enhances the capacity of trained individuals to perform prolonged exercise. Pitsiladis, Y.P., Smith, I., Maughan, R.J. Department of Biomedical Sciences, University Medical School, Aberdeen, United Kingdom. Medicine and Science in Sports and Exercise, 1999 Nov;31(11):1570-9.
  7. Effect of carbohydrate ingestion during exercise on post-exercise substrate oxidation and energy intake. Melby, C.L., Osterberg, K.L., Resch, A., et al. Nutrition and Metabolic Fitness Laboratory in the Department of Food Science and Human Nutrition at Colorado State University, Fort Collins, CO 80523, USA. International Journal of Sport Nutrition and Exercise Metabolism, 2002 Sep;12(3):294-309.
  8. Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance. Hargreaves, M., Hawley, J.A., Jeukendrup, A. Centre for Physical Activity and Nutrition Research, School of Health Sciences, Deakin University, Burwood, Vic 3125, Australia. Journal of Sports Sciences, 2004 Jan;22(1):31-8.