By Nick Vollenbroek
Fasting is getting increasingly popular, especially intermittent fasting is getting more and more attention. This is mainly caused by famous people like Chris Hemsworth and Hugh Jackman, who swear by intermittent fasting, which means that Thor and Wolverine keep up their superhero form by practicing intermittent fasting. Of course, intermittent fasting alone will not turn improve the situation all by itself. People like Chris Hemsworth and Hugh Jackman who say they benefit from intermittent fasting, also have personal coaching with strict workout and dieting regimes, since getting in shape is part of their careers. Most statements on why you should fast are anecdotal at best, which mostly lack scientific support. This article will try to give some scientific support on the positive and negative effects of intermittent fasting, starting off with a few approaches for intermittent fasting.
Time restricted feeding is the most popular way to go, which states you eat during a certain timeframe per day. A 16 hour fast is the most common, with 8 hours to eat during the day. Another intermittent fasting regime is the 5:2, meaning that you’ll eat a significant lower amount of kilocalories on 2 days, e.g. 500. On the other 5 days you eat as per usual. A diet which involves only eating at night is called the warrior diet. This type of fasting is normally done a few times a week. The main advantage the warrior diet has over the normal intermittent fasting, is that you are not tempted to eat unhealthy snacks during the day.
When it comes to weight loss there is not a significant difference when comparing intermittent fasting to a control with constant energy intake, however findings did point towards less loss of fat-free mass during intermittent fasting [1-3]. Indicating that intermittent fasting seems to be a suitable method to lose large amounts of adipose tissue in short amounts of time, with regards to keeping muscle mass loss minimal.
An interesting fact regarding intermittent fasting is that scientists in 1986 showed that human growth hormone (HGH) levels in healthy men were tripled after fasting for 24 hours . HGH is a hormone, as the name already indicates, that is essential in human growth and the development of bodily structures, and therefore this fact is attention-grabbing for anybody attracted to body building and/or strength training.
Furthermore, intermittent fasting could be the answer to a longer and healthier life. A short article recently published in Cell Stem Cell showed that a 24-hour fast in mice causes fat breakdown in progenitor - and intestinal stem cells . This fat breakdown or, more accurately, fatty acid-oxidation improved the function of stem cells in the researched mice, and even in aged mice benefits were found. Based on the findings in the article it was stated that fasting leads to pro-longevity and regenerative effects in diverse species, due to an enhanced (aged) stem cell activity in tissues.
A downside to intermittent fasting is to perform in sports during an intermittent fasting routine. Research showed a decrease of VO2 max by up to 12% in both professional and recreational athletes . Also, in a few enquiries all athletes reported feeling higher levels of fatigue [6, 7]. Performance wise it has a significant downside, but an article comparing normal dieting with intermittent fasting regarding jockeys showed that strength over time was increased when an intermittent fasting schedule was applied .
Other negative effects that can arise during intermittent fasting is binge eating during your non-fasting cycles. Especially people following the 5:2 regime are at risk. During fasting cycles chances are present that irritability and inability to focus will arise, although it is not rare that fasting works the other way around, increasing your senses as some sort of survival instinct. Only experience will tell how you will react on fasting.
Most research is done with fasting periods of 24 hours or longer, since intermittent fasting consists of mostly 16-20 hours of fasting, it is debatable if the positive effects mentioned before are applicable to intermittent fasting. Even when they are applicable there still isn’t enough research to promote intermittent fasting as a virtuous method for gaining lean muscle mass and rejuvenation. Nevertheless, it is safe to say that intermittent fasting is almost completely harmless, so why not give it a shot?
 Patterson, R. E., & Sears, D. D. (2017). Metabolic effects of intermittent fasting. Annual review of nutrition, 37.
 Anton, S. D., Moehl, K., Donahoo, W. T., Marosi, K., Lee, S. A., Mainous III, A. G., ... & Mattson, M. P. (2018). Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity, 26(2), 254-268.
 Harvie, M., & Howell, A. (2017). Potential benefits and harms of intermittent energy restriction and intermittent fasting amongst obese, overweight and normal weight subjects—a narrative review of human and animal evidence. Behavioral Sciences, 7(1), 4.
 Ho, K. Y., Veldhuis, J. D., Johnson, M. L., Furlanetto, R., Evans, W. S., Alberti, K. G., & Thorner, M. O. (1988). Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. The Journal of clinical investigation, 81(4), 968-975.
 Mihaylova, M. M., Cheng, C. W., Cao, A. Q., Tripathi, S., Mana, M. D., Bauer-Rowe, K. E., ... & Freinkman, E. (2018). Fasting activates fatty acid oxidation to enhance intestinal stem cell function during homeostasis and aging. Cell stem cell, 22(5), 769-778.
 Chaouachi, A., Leiper, J. B., Chtourou, H., Aziz, A. R., & Chamari, K. (2012). The effects of Ramadan intermittent fasting on athletic performance: Recommendations for the maintenance of physical fitness. Journal of sports sciences, 30(sup1), S53-S73.
 Chaouachi, A., Coutts, A. J., Chamari, K., Wong, D. P., Chaouachi, M., Chtara, M., ... & Amri, M. (2009). Effect of Ramadan intermittent fasting on aerobic and anaerobic performance and perception of fatigue in male elite judo athletes. The Journal of Strength & Conditioning Research, 23(9), 2702-2709.
 Wilson, G., Pritchard, P. P., Papageorgiou, C., Phillips, S., Kumar, P., Langan-Evans, C., ... & Close, G. L. (2015). Fasted exercise and increased dietary protein reduces body fat and improves strength in jockeys. International journal of sports medicine, 36(12), 1008-1014.