In addition to launching a new exercise regimen, the New Year is traditionally a time when many people reconsider their eating habits. In recent years, intermittent fasting has become a popular habit – and has been credited with some health benefits, whether to manage overweight, Chronic diseases or falling energy levels. But what exactly is intermittent fasting? And does all the hype surrounding it stand up to scientific scrutiny?
The term intermittent fasting covers several approaches, each based on different principles. It is important to note that regardless of the method used, the restrictions only affect food intake, never water.
The verdict of science?
The results vary depending on the strategy adopted.
With the “Eat Stop Eat” and 5:2 approaches, relatively few scientific studies have been conducted. The little data we have has shown that they can effectively help us lose weight and improve certain metabolic parameters such as fasting blood sugar. For example, nutritionist Surabhi Bhutani has shown that using the 5:2 method for three months results in weight loss of 3 to 6 kg in participants.
However, both methods are very restrictive and can cause Side effects days of total fasting or severe calorie restriction – hunger, negative effects on mood and risk of hypoglycemia.
In the longer term, restriction also increases the risk of developing or worsening eating disorders, as well as yo-yo dieting. These patterns often appear after the individual has tried to lose weight by restricting himself: despite the initial progress, the deprivation is likely to generate frustrations which will promote the return of old eating habits.
The most studied method is the one with a daily food intake but limited in time. Two “time slots” are often observed:
When food intake begins with breakfast and ends in the late afternoon – known as “early time-restricted feeding”.
When food intake begins with lunch – known as “time-restricted late feeding”.
This approach appears to improve metabolic regulation and reduce the risk of metabolic diseases. However, these benefits vary depending on the time slot chosen. When food intake begins in the morning, studies have observed weightloss and improved insulin sensitivity.
Conversely, there are less or no benefits to start meals at noon and finish them in the evening. Ram Babu Singh’s team (Halberg Hospital and Research Institute, India) also showed positive results in participants who only ate in the morningand not among those who ate in the evening after 8 p.m.
Why such a difference ?
Research suggests our internal clock and circadian rhythms may have something to do with it. Indeed, the advantage of eating only in the morning is that the periods of food intake and fasting coincide with our biological clock.
In our previous article, we explained to you that in response to light cycles, our body produces hormones cyclically to adapt our food intake to the body’s energy needs: the optimal time to eat is therefore around 8 or 9 a.m. from morning (when the sun rises) to 7 p.m. (when the sun begins to set, depending on the season).
Skipping breakfast and eating after 7 p.m. disrupts circadian rhythms, and increases the risk of developing metabolic diseases.
However, although time-restricted eating appears to be a good approach to metabolic health, there is still a lot to understand about how it works and how to optimize its effects. Work in 2022 showed no difference in weight loss between opting to eat early or late in the morning. It did, however, have an effect on appetite during the day – this time to the former’s advantage.
And beyond the time of day when it seems best to eat, other factors may come into play which are not always measured in the studies carried out: quality and quantity of food absorbed, duration of the fasting period (which may be ‘extend from 12 to 20 hours a day), etc. It should also be remembered that each individual has their own metabolism and may react differently to fasting. New, better controlled and more comprehensive studies are therefore needed to confirm the potential benefits of these methods and to understand the mechanisms involved in their effects.
In practice, what to do?
The most suitable method for not disturbing one’s circadian clock (and thus limiting the risks of frustration or eating disorders) seems to be time-limited food intake by synchronizing meals with circadian rhythms.
Thus, a typical day could be organized with a hearty breakfast in the morning taking place between 6 a.m. and 8 a.m., lunch around noon and finally bringing forward dinner so that it takes place between 4 p.m. and 6 p.m., depending on the season.
It is not necessarily easy to reconcile with his social life. It can be complicated to practice intermittent fasting for a family, when you practice a sports activity in the early evening or when you work in the evening until 7 or 8 p.m.
One solution would be to opt for a hearty breakfast and a low-calorie meal in the evening – preferably without carbohydrates or sugars, so as not to risk shifting your biological clock.
Chrononutrition is becoming increasingly popular, and intermittent fasting appears to be effective in improving metabolic health. That said, we have seen that it is not a panacea. And we have to make sure that the periods of fasting and food intake are consistent with our biological clock.
Faced with the many existing methods, and the potential risks, patients and healthcare professionals still face a lack of information. Further research is essential to better understand their effects. Currently, there is not yet a general consensus on the ideal time to eat/fast, nor on the optimal length of each period. In addition, these parameters may differ from person to person, depending on their genetic makeup, history and lifestyle. It is therefore important to consider the use of this dietary strategy with qualified health professionals, with a view to implementing a healthy and balanced diet that will limit the risk of complications.