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Intermittent fasting (IF) is a dietary regimen that involves eating or fasting on a schedule. During the feeding period, people can eat whatever they want. During the fasting period, there is a total absence of calories and eating. People who do IF might:
- eat only every other day.
- eat for a some hours in the day, and fast for most of the other hours.
Why do people do it? IF is mainly used as a weight management tool. Supporters of IF believe that the diet helps them lose weight and regulate their metabolism. Supporters also like prolonged fasting so their bodies can use more fat stores for energy. This logic is taken up a notch for some who say that IF improves blood pressure and cholesterol levels (though these two things tend to happen with weight loss, in general).
So, is the wait worth it? Let’s look at the pros and cons. Since we are a physically active bunch, our comparison will mainly consider IF schedules that let us eat daily, but with restricted hours.
| Pros | Cons |
| IF does not restrict food groups. Unlike Paleo, keto, Whole30, Zone, and other fad diets, you can still eat whatever foods you want! | There is potential muscle wasting. If you are not eating enough protein throughout the day, your body will break down your lean tissue for amino acids. Eating a ton of protein at once won’t help you, because protein is not stored as energy. Instead, the excess amounts get stored as fat. |
| IF does not limit calories. This makes the diet even more simple to follow! …but calorie restriction IS an option. Calorie restriction is usually necessary if you are trying to lose weight. In fact, studies that show when people lose weight with IF, it is simply because their average daily calorie intake dropped. | Your workouts may suffer. Depending on your eating schedule and the time of your workouts, you may not have the energy you need to carry you through a workout. The lack of glucose and glycogen stores may cause fatigue, which makes strength training very difficult. |
| The structure is simple. Holding back from eating for many hours may not be the EASIEST thing to do, but the structure itself is simple (and some people really like structure). For example, if you trying to avoid nighttime snacking, the structured schedule might be the thing for you. | You may not optimally adapt from workouts. If your IF schedule restricts you from eating after exercise, then your muscles will lack the fuel they need to recover. Whether you are doing a WOD, surfing, or jogging, your muscles need protein and carbohydrates to optimally adapt. |
| IF can improve sleep rhythm and cortisol levels. Some people who do IF report having less hunger at bedtime and better sleep satisfaction. | Fasting can cause you to lose focus. Your brain runs on glucose, but your glycogen stores can only last you about 12 hours (even less if you exercise). |
| IF might not fit your lifestyle. Eating on a set schedule might not be compatible with your family, work, or social life. | |
| IF is not safe for those with eating disorders, diabetes, pregnant and breastfeeding women, and medications that require food. Even though we are an active bunch, we have different states of health and should ALWAYS consult a dietitian before making dramatic changes to our diet! |
A few things to keep in mind:
- Since there are many IF schedules. Every study looks at the effects of eating on different schedules. In other words, a study that shows results for eating every other day can’t be looked at the same way as one where you eat 12-hours on and 12-hours off.
- Many IF studies are in animals, not just humans. Also, humans don’t live in a lab. We are free-living and thinking people, with social influences, values, and other things that inform our food choices.
- The research has no conclusion on IF. The thing with fad diets is that they’re new, and research takes a lot of time. While scientists are busy being thorough, some people jump the gun and share nutrition advice that simply isn’t proven.
The bottom line is: Due to the lack of convincing research, there are currently no strong recommendations for IF for weight loss or any other health condition. If your goal is to lose weight, IF can help. However, ample research shows that the reason IF-weight loss is successful is because of the good old fashion practice of cutting back on calories every day. For active folks like you, it is most beneficial to focus your energy on when you SHOULD fuel, and not when NOT to fuel. Next week, we will talk about nutrient timing. Please add your questions in the comments!
Sierra
References
Cioffi, I. et al. (2018). Intermittent versus continuous energy restriction on weight loss and cardiometabolic outcomes: a systematic review and meta-analysis of randomized controlled trials. Journal of translational medicine, 16(1), 371. https://doi.org/10.1186/s12967-018-1748-4
Harvard School of Public Health. (N.D.). Diet Review: Intermittent Fasting for Weight Loss. Retrieved from https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/intermittent-fasting/
Jamshed, H. et al. (2019). Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients, 11(6), 1234. https://doi.org/10.3390/nu11061234
Orenstein, B. (2014). Intermittent Fasting: The Key to Long-Term Weight Loss? Today’s Dietitian. 26(12), p.40. https://www.todaysdietitian.com/newarchives/120914p40.shtml
