Over the past decade there has been some exciting new research into the benefits of fasting. Studies have shown periodic fasting can help lower blood pressure and cholesterol, reverse Type II diabetes, and help with weight loss. This research has led to a whole host of intermittent fasting and time restricted eating diets. In intermittent fasting diets (such as the 5:2 diet), you eat a very low number of calories 2 or 3 days a week, then eat normally 4 or 5 days a week. In time restricted eating diets, you increase the amount of time each day where you eat no food at all. These range from ones where you restrict your eating to only 8 hours a day, to an extreme form where you are only allowed one meal a day.
Fasting diets all promise the same thing: So long as you avoid eating during the fasting times, you can eat whatever you want during the feeding times and lose weight. This makes them incredibly attractive, as you are not asked to give up your favourite foods or stop drinking alcohol.
The science behind these diets usually revolves around insulin. Insulin is made by the pancreas and released when there are high concentrations of glucose in the blood (such as after a meal with carbohydrates). Insulin tells cells, particularly muscle cells, to absorb the glucose from the blood. Glucose is then stored in the muscles and liver as glycogen. When glucose levels fall, as happens between meals, glycogen can be released back into the bloodstream.
However, glucose is not our only source of energy; we use fats as well. In fact, free fatty acids are our primary source of energy at rest. High insulin levels after a meal ensure free fatty acids are stored in muscles and fat cells. Falling levels of insulin between meals then causes the release of stored glucose and the breakdown of fat into fatty acids for energy. The problem is for people with chronically high levels of insulin. If insulin levels are always high, either through constant snacking on sweet treats or type II diabetes, then there is never a strong signal to breakdown fat. This makes it very difficult to lose unwanted body fat.
This is rationale behind fasting diets. The idea is to drastically reduce insulin levels to force the body to use fat as fuel. And they usually work – blood sugar and insulin levels go down, and body fat is reduced.
However, such diets can be hard to maintain long term and can come with several unpleasant side effects. For example, the brain runs solely on glucose so drastically reducing glucose stores can lead to headaches, dizzy spells, mood swings and feeling fuzzy headed. As well, most people find they are very tired during the fasting times and find it harder to exercise. Such symptoms may pass as the body adapts to using fat for fuel, but not always.
Of course, all of these diets rely on the person eating a normal amount of healthy food during the feeding times. Yes, eating 500kcal a day will create a significant calorie deficit for that day. But this only leads to weight loss if the person doesn’t turn around and eat 4000kcal of MacDonald’s every other day of the week. Likewise, limiting food intake to 8 hours only works if the person actually skips a meal, rather than trying to cram the same number of calories into a smaller time frame. Let’s say that the meal I’m skipping is my 250kcal bowl of breakfast cereal. If missing it means I am starving by lunchtime, I may eat much more than an extra 250kcal to compensate. After all, I missed breakfast so I deserve a large slice of chocolate cake with rice cream, right?
Fundamentally, any weight lost on these diets is due to the same reason as on every other diet: eating fewer calories. Yes, there are health benefits to fasting. Periodic fasting may even assist with fat loss. But it is a myth that simply by not eating one day, you can eat whatever you want the next and still lose weight.