What if I told you that all you need to do to lose weight is a calendar and tells the time? These are the basics for successfully following an intermittent fasting diet.
But can it be that simple? Does it work? And what is the scientific basis for fasting? As a registered dietitian and expert in human nutrition and metabolismI often get such questions.
Simply put, intermittent fasting is defined by alternating fixed periods of fasting with periods when eating is allowed. One method is: fast every other day† On “fasting days” adherents of this form of fasting should not consume more than 500 calories per day; on “holidays”, which occur every other day, they can eat freely, with no restrictions on the types or amounts of food eaten.
Other methods are the increasingly popular 5:2 method† This form of fasting includes five days of feasting and two days of fasting per week.
Another variation is based on time-restricted eating. That means followers must fast for a certain number of hours — usually 16 to 20 a day — while freely consuming food within a designated four to eight hour period.
To answer these questions, it helps to understand the basics of human metabolism.
The human body needs a continuous supply of energy to stay alive, and the food we eat provides us with this energy. But because eating is often followed by periods without eating, there is a complicated set of biological pathways to meet the body’s energy needs between meals.
Most pathways function at some level all the time, but they fluctuate after a meal in a predictable pattern that the fed-fast cycle† Cycle time frames can vary depending on the types of foods eaten, the size of the meal, and the person’s activity level.
So what happens, metabolically speaking, after we eat? Consuming carbohydrates and fats leads to a rise in blood glucose and also lipid levelsincluding cholesterol and triglycerides.
This triggers the release of insulin from the pancreas. The insulin helps tissues throughout the body to absorb the glucose and lipids, which provide the tissues with energy.
Once energy needs are met, the leftover glucose is stored in the liver and skeletal muscle in a condensed form called glycogen. When glycogen stores are full, excess glucose is converted to fatty acids and stored in adipose tissue.
About three to 18 hours after a meal – again, depending on a person’s activity level and the size of the meal – the amount of circulating blood glucose and lipids returns to baseline levels. So tissues then have to rely on fuel sources already present in the body, namely glycogen and fat. A hormone called glucagon, secreted by the pancreas, helps to facilitate the breakdown of glycogen and fat to provide the body with energy between meals.
Glucagon also initiates a process known as: gluconeogenesis, which is the synthesis of glucose from non-dietary sources. This helps in maintaining the right level of blood glucose levels.
When the body enters a true fasting state — about 18 hours to two days with no additional food intake — the body’s glycogen stores are depleted, and tissues such as the heart and skeletal muscles begin to rely heavily on fats for energy. That means an increase in the breakdown of stored fats.
“Ah!” you could say. “So intermittent fasting is the key to ultimate fat burning?” Well, it’s not that simple. Let’s run through what happens next.
While many tissues adapt to using fats for energy, the brain and red blood cells require a continuous supply of glucose. But when glucose is unavailable due to fasting, the body begins to break down its own proteins and convert them to glucose instead† However, since protein is also crucial for supporting essential bodily functions, this is not a sustainable process.
When the body enters the starvation state, the body goes into self-preservation mode and a metabolic shift occurs in an attempt to conserve body protein. The body continues to synthesize glucose for those cells and tissues that absolutely need it, but the breakdown of stored fats also increases to provide energy for tissues such as skeletal muscle, heart, liver and kidneys.
This too promotes ketogenesis, or the formation of ketone bodies – molecules produced in the liver as an energy source when glucose is not available. In the starved state, ketone bodies are important sources of energy, as the body is unable to use fat alone for energy. This is why it is incorrect when some proponents of intermittent fasting claim that fasting is a way to “only burn fat” – it is not biologically possible.
What happens if you break the fast? The cycle starts again. Blood glucose and lipids return to basal levels and energy levels in the body are seamlessly maintained by switching between the previously described metabolic pathways. The nice thing is, we don’t even have to think about it. The body is well equipped to adapt between periods of feasting and fasting.
If an “all-or-nothing” diet approach to weight loss sounds appealing to you, chances are it just works. Indeed, intermittent fasting has led to clinically significant amounts of weight loss. Intermittent fasting is also possible reduce disease risk by lowering blood pressure and lipid levels in the blood.
On the other hand, numerous studies have shown that the weight reduction by intermittent fasting diets is: no bigger than weight loss on a standard calorie-restricted diet.
In fact, the weight loss caused by intermittent fasting is not due to spending time in some magical metabolic window, but rather to reduced overall calorie consumption. Dieters usually don’t do that on holidays fully compensate for lack of food on fasting days. This results in mild to moderate weight loss. About 75% of the weight is fat mass; the rest is lean body mass. That’s about the same ratio as a standard low-calorie diet†
If you do continue with intermittent fasting, keep a few things in mind. First, there are no studies on the long-term safety and efficacy of following this type of diet. Second, studies show that intermittent fasting isn’t getting enough certain nutrients†
Exercise is something else to consider. It helps maintain lean muscle mass and may also contribute to increased weight loss and weight maintenance in the long run. This is important because almost a quarter of weight loss with any diet is muscle tissue, and the efficacy of intermittent fasting for weight loss has been demonstrated for only a short time†
Once you stop following an intermittent fasting diet, you will most likely regain the weight. This is a critical consideration as many people find the diet difficult to follow long term. Imagine the challenge of planning parties and fasts for six months around family dinners, holidays, and celebrations. Then imagine doing it all your life.
Ultimately, the best approach is to follow an eating plan that meets current dietary recommendations and fits your lifestyle.
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