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Fasting has been practiced for centuries, whether for religious or spiritual reasons. Yet, it is only recently that it has become a popular topic for those wanting to lose weight or improve their overall health. In 2012, journalists Michael Mosley and Mimi Spencer published their book The Fast Diet in which they detail their new dieting plan. Their plan involves drastically restricting calories for 2 days (below 600 calories per day) and eating normally the other 5 days of the week. Of course, one of the authors used this plan and reports improving his health and losing more than 20 lbs. But does it actually work?
It is important to note that since then, there have been many fasting plans. They can limit eating to a certain number of hours, suggest skipping a few meals, or even tell you to fast for 24 hours 1-3 times a week.
In theory, if you are limiting the time you have to eat, you are eating less over the course of the week. Eating fewer calories equals to pounds lost. Fasting also forces your liver to use up all the glycogen (sugar stored in your liver) for energy. Since the body’s priority is to provide enough energy to your brain, your body has to use fat to make sugar.
Unfortunately, there isn’t a lot of research on this way of eating, and the research we do have is mostly conducted in animals, and we know we are different! So far, the studies have shown that with alternate-day fasting, we can reduce our incidence of diabetes, reduce LDL (the “bad cholesterol”), and lose about one pound a week with most of it coming from body fat (exactly what we want).
We would think that reducing caloric intake would lead to poorer performance due to fatigue, even leading to fewer training days due to not being able to train on fasting days. As expected, studies conducted on athletes who have been fasting (usually for a religious reason) do show a negative impact on their performance due to fatigue. If we do not consume enough calories, especially carbs before an event, our sugar stores in our body will be low and not sufficient to fuel our moving bodies! Just like a car, if it does not have enough gas, it won’t be able to go as far as it normally does. Although we do not have enough evidence with different types of fasting methods to give recommendations, our understanding of nutrition and performance says that fasting will reduce performance in all types of physical activity.
Overall, research in this area is spotty. What we do know is that any plan that reduces calorie intake will likely lead to these same results. If the diet is too hard to follow, it will not work long-term, and the studies were only carried out on a short-term basis. We need to think about how sustainable this is in the long-term. With diets, the one that works best is the one that you are able to follow long-term!
In terms of performance, it does not seem to provide any advantage to be fasting. If anything, it will negatively impact your performance by increasing fatigue.
It is important to note that anyone with any history of eating disorders or disordered eating should not attempt this. It is also not recommended for pregnant or lactating women, children, those with diabetes, or anyone taking medication.