The times are a-changin’. After decades of being told to eat six meals a day and energy-sustaining carbs an hour before a workout, it turns out that all our efforts may have been misguided.
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Recent research suggests that our bodies benefit from intermittent fasting (IF), and that exercising during a fast might not be as bad as we think. In fact, an empty stomach workout benefits our muscles, insulin regulation and fat-burning ability.
Is It Better To Exercise On An Empty Stomach?
The short answer: Yes.
Does that mean all that talk about eating before a workout has been meaningless? Not quite. Some studies have shown that eating easy-to-digest carbohydrates an hour before exercising allows athletes to work out longer than if they were in a fasted state.
Another study found that men in a fasted state ran significantly shorter distances in 60 minutes than men who hadn’t been fasting, despite no significant differences in heart rate and perceived exertion.
But while fasting right before an athletic competition probably won’t help you perform better, training in a fasted state just might. And for those of us who care more about our waistlines than marathon times, there are even more benefits.
One breakthrough study showed men aged 18-25 who did intense cardio in a fasted state while maintaining a high-fat, high-calorie diet had improved glucose tolerance and insulin sensitivity compared with those who exercised without fasting and those who didn’t exercise at all.
Of the three groups on the same diet, only those who exercised in a fasted state did not gain weight.
What Happens When You Work Out On An Empty Stomach
The science behind exactly how exercising in a fasted state — or intermittent fasting in general —benefits the body is inconclusive at best. We do know though that it has something to do with insulin and that it also may involve the growth hormone. Furthermore, we know that intermittent fasting can disrupt traditional notions about protein digestion.
Insulin is a hormone that regulates how nutrients get used by the liver, a body’s muscle, and fat cells. Insulin is produced as a response to carbohydrate intake — every time you eat a banana, chew a carrot, swallow a sandwich, or take a sip of juice, your body releases insulin to direct where these bits of food should go.
Insulin therefore helps manage fat storage and nutrient absorption, and insufficient sleep, lack of exercise, and overeating make our bodies less sensitive to insulin. Insulin resistance, in turn, makes us more prone to diabetes, several types of cancer, and obesity, and is a major health concern in the U.S. today.
If you choose to exercise at a time when most of the glucose and glycogen from recent meals have been used up, your body is more likely to look to fat as an energy source. After fasting, your body is especially sensitive to insulin, which it produces every time you eat. Insulin sensitivity helps your body use carbs, proteins, and fats more efficiently, promoting muscle creation and weight loss.
One explanation for why insulin resistance has become such a huge issue for us today is called the theory of thrifty genes. It states that our metabolisms thrive when we fluctuate between periods of feast and famine, and spurts of exercise and rest. This contrasts greatly with the average modern lifestyle, which involves an abundance of food, constant eating and being sedentary. The theory of thrifty genes is one explanation for why a 2005 study showed that intermittent fasting improved insulin sensitivity and glucose uptake rates in healthy men.
This controversial hormone promotes growth in children and adolescents, and has been sought out as an anti-aging elixir by adults. While it can’t reverse your age, growth hormone can increase muscle mass and strength. A 24-hour fast can increase your body’s production of GH 13- to 20-fold.
Now, you might be thinking, “Even if exercising after a fast is good for you in the short-term, doesn’t fasting affect protein digestion and muscle mass?” In fact, studies on young, middle-aged, and elderly adults have shown that you can absorb a whole day’s worth of protein in one sitting. Some studies even suggest that eating only one or two meals per day might help build lean muscle.
At the end of the day, unless you’re an athlete, your best strategy for healthy food consumption is simple: Don’t eat if you’re not hungry.
Despite longstanding health myths, skipping a meal or two — particularly before you exercise — will not damage your body. Intermittent fasting might reverse some of the consequences of a high-fat, high-calorie, sedentary lifestyle, and exercising in a fasted state might help your body use the food you eat later more efficiently. Skipping breakfast for the gym every few days might make your body better at digesting that post-workout snack.
- Steve Kamb. “The Beginner’s Guide to Intermittent Fasting.” http://www.nerdfitness.com/blog/2013/08/06/a-beginners-guide-to-intermittent-fasting/
- Nick English. “Exercising On an Empty Stomach: The Surprising Benefits.” http://greatist.com/fitness/why-you-should-exercise-on-an-empty-stomach
- Effects of Ramadan fasting on 60 min of endurance running performance in moderately trained men. Aziz AR, Wahid MF, et al. British Journal of Sports Medicine, 2010 Jun;44(7):516-21.
- Training in the fasted state improves glucose tolerance during fat-rich diet. Van Proeyen K, Szlufcik K, et al. 2010 Nov 1;588(Pt 21):4289-302.
- Metabolic responses to exercise after fasting. Dohm GL, Beeker RT, et al. Journal of Applied Physiology, 1986 Oct;61(4):1363-8
- Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Heilbronn LK, Smith SR, et al. American Journal of Clinical Nutrition, 2005 Jan;81(1):69-73
- Regulation of protein synthesis by insulin. Proud, CG. Biochemical Society transactions. 2006 Apr;34(Pt 2):213-6.
- Exercise in the fasted state facilitates fibre type-specific intramyocellular lipid breakdown and stimulates glycogen resynthesis in humans. De Bock K, Richter EA, et al. Journal of Physiology2005 Apr 15;564(Pt 2):649-60.
- Effect of intermittent fasting and refeeding on insulin action in healthy men. Halberg N, Henriksen M, et al. Dept. of Muscle Research Centre, The Panum Institute, University of Copenhagen, Denmark. Journal of Applied Physiology, 2005 Dec;99(6):2128-36.
- Growth hormone increases muscle mass and strength but does not rejuvenate myofibrillar protein synthesis in healthy subjects over 60 years old. Welle S, Thornton C, et al. The Journal of Clinical Endocrinology and Metabolism, 1996 Sep;81(9):3239-43.
- Intermittent fasting does not affect whole-body glucose, lipid, or protein metabolism. Soeters MR, Lammers NM, et al. American Journal of Clinical Nutrition, 2009 Nov;90(5):1244-51.
- A controlled trial of reduced meal frequency without caloric restriction in healthy, normal-weight, middle-aged adults. Stote KS, Baer DJ, et al. American Journal of Clinical Nutrition, 2007 Apr;85(4):981-8.
- Protein feeding pattern does not affect protein retention in young women. Arnal MA, Mosoni L, et al. Journal of Nutrition, 2000 Jul;130(7):1700-4.
- Protein pulse feeding improves protein retention in elderly women. Arnal MA, Mosoni L, et al. American Journal of Clinical Nutrition, 1999 Jun;69(6):1202-8.