Intermittent fasting can be defined in a few different ways.
For the purpose of this discussion, I will be relying on the following definition: Regularly eating during a specific time period. For example, only eating from noon-8 PM, essentially skipping breakfast. Some people only eat in a 6-hour window, or even a 4-hour window. According to NerdFitness, “During the “fasted state,” your body doesn’t have a recently consumed meal to use as energy, so it is more likely to pull from the fat stored in your body, rather than the glucose in your blood stream or glycogen in your muscles/liver” (1). Furthermore, the blog goes on to say, “The same goes for working out in a “fasted” state. Without a ready supply of glucose and glycogen to pull from (which has been depleted over the course of your fasted state, and hasn’t yet been replenished with a pre-workout meal), your body is forced to adapt and pull from the only source of energy available to it: the fat stored in your cells!” The theory is that both the fasted state and exercise will increase insulin sensitivity, so when someone does eat, their body will utilize the fuel more efficiently to build muscle—insulin increases protein synthesis in the body (2). Additionally, the exercise-induced growth hormone response (EIGR) has a role in increasing fat metabolism, although recent research does suggest that endurance training can reduce this impact over time (3). Other blogs such as Lean Gains, Scrawny to Brawny, and even Scientific American also subscribe to this belief that intermittent fasting builds muscle and burns fat better with exercise than working out and eating regularly.
Who participates in intermittent fasting? This diet technique is contraindicated in those who might be pregnant, nursing, have diabetes, suffer from hypoglycemia, or have other health conditions. It has been popularized in athletes in recent years because of the previously mentioned ‘increased muscle and fat metabolism.’ A large concern arises in regard to decreased cognitive and physical activities—conventional wisdom preaches to eat a healthy breakfast for a healthy brain. A strong double-blind research study from the American Journal of Clinical Nutrition came to the conclusion that, “Cognitive performance, activity, sleep, and mood are not adversely affected in healthy humans by 2 d of calorie-deprivation when the subjects and investigators are unaware of the calorie content of the treatments” (4). According to this, fears of ‘starvation mode’ are unwarranted. So intermittent fasting will not harm a healthy person, exercise increases fat metabolism through EIGR, but the question remains: does it really make a difference in weight loss or improve exercise?
Overall, the literature examining intermittent fasting is limited. One study found that intermittent energy restriction (IER) was as effective as continuous/daily energy restriction (CER) (5). Having said that, the study defined IER as a 25% restriction delivered as a VLCD for 2 days per week, with no restriction on the other 5 days per week. This is very different from my previously stated definition. Consequently, this is another weight loss option for those that find it hard to limit their calories by 25% every day. Another study suggests while both are effective for weight loss, less fat-free mass was lost with IER than CER, so IER would be better for retaining lean mass (6). In terms of athletic performance, one study examined the fasting practice of Ramadan and its effects on Islamic athletes. Overall, the researchers found decreased performance in some athletes and not others, and suggest further research to determine the mechanisms (7). Somewhat more troublesome than decreased athletic performance is a study that concluded long-term intermittent feeding, but not caloric restriction, leads to redox imbalance, insulin receptor nitration, and glucose intolerance (8). A relatively sparse amount of literature has emerged recently to suggest that women may react more negatively to intermittent fasting than men, but much more research is needed in this area (9).
In conclusion, athletes should consult their coaches and trainers before considering intermittent fasting to determine if it will possibly affect their performance. For you, intermittent fasting might be a good weight loss technique given you don’t have any contraindications, don’t experience severe side effects, and discuss it with your physician. More research must be conducted before intermittent fasting can be recommended as a proven technique for sustainable weight loss, especially since the definition of intermittent fasting can vary, including everything from periodic multi-day fasts to skipping a meal or two on certain days of the week, and few long-term studies have been conducted.
1. Kamb, Steve. (2013). The Beginner’s Guide to Intermittent Fasting. NerdFitness. https://www.nerdfitness.com/blog/2013/08/06/a-beginners-guide-to-intermittent-fasting/ Accessed 3/29/16.
2. Proud, C. G. (2006). Regulation of protein synthesis by insulin. Biochemical Society Transactions, 34(2), 213-216.
3. Godfrey, R. J., Madgwick, Z., & Whyte, G. P. (2003). The exercise-induced growth hormone response in athletes. Sports Medicine, 33(8), 599-613
4. Lieberman H. R., Caruso C. M., Niro P. J., et al. (2008) A double-blind, placebo-controlled test of 2 d of calorie deprivation: effects on cognition, activity, sleep, and interstitial glucose concentrations. American Journal of Clinical Nutrition. 88(3):667–676.
5. Harvie, M. N., Pegington, M., Mattson, M. P., Frystyk, J., Dillon, B., Evans, G., ... & Son, T. G. (2011). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. International journal of obesity, 35(5), 714-727.
6. Varady, K. A. (2011), Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?. Obesity Reviews, 12: e593–e601.
7. Chaouachi, A., Leiper, J. B., Souissi, N., Coutts, A. J., & Chamari, K. (2009). Effects of Ramadan intermittent fasting on sports performance and training: a review. International journal of sports physiology and performance.
8. Cerqueira, F. M., da Cunha, F. M., da Silva, C. C. C., Chausse, B., Romano, R. L., Garcia, C. C., ... & Kowaltowski, A. J. (2011). Long-term intermittent feeding, but not caloric restriction, leads to redox imbalance, insulin receptor nitration, and glucose intolerance. Free Radical Biology and Medicine, 51(7), 1454-1460.
9. Ruper, Stefani. (2012). Shattering The Myth Of Fasting For Women: A Review Of Female-Specific Responses To Fasting In The Literature. Paleo for Women. http://paleoforwomen.com/shattering-the-myth-of-fasting-for-women-a-review-of-female-specific-responses-to-fasting-in-the-literature/ Accessed 3/29/16.