What do we know about so-called "intermittent fasting"?

Every person needs a specific number of calories per day (energy expenditure) to sustain the body's biological functions. These calories come from the food and beverages we consume. However, when we consume more calories than we need, an imbalance can occur, leading to overweight or obesity.

Excess weight is a pandemic and in Mexico it is a public health problem that is directly related to the development of chronic degenerative diseases such as diabetes, hypertension and cardiovascular problems. To reduce the risks of these diseases, it is suggested to be active, exercise and modify our diet, among other tools used to promote weight loss.

Regularly, dietary modifications or interventions used in the treatment of obesity include calorie restriction, reducing the food we consume. However, other strategies have emerged with the same objective, among them intermittent fasting.

What is intermittent fasting?

Intermittent fasting is a broad term that encompasses a variety of programs that manipulate meal timing, using fasting in order to improve overall health through weight loss. These programs regularly include periods of abstinence from food longer than the fasting we do when we sleep at night (approximately 7-8 hours) and are accompanied by a decrease in food intake (caloric restriction), although this restriction is not maintained every day, so it can be combined with other dietary interventions.

Fasting programs are classified in three categories: 1) intermittent fasting alternated in days, 2) complete fasting for 24 hours and 3) restriction in meal times.

Intermittent fasting alternating days: consists of alternating fasting days with days comprising a single meal, which represents approximately 25% of the calories needed in a day.

Complete fasting for 24 hours: this is one of the simplest forms of fasting, and consists of not eating for 24 hours, one or two days a week, although some programs include minimum food consumption (25% of calories needed) on fasting days. This same program can be accompanied by optional restrictions on food consumption on non-fasting days.

Restriction in meal times: this program involves following the same eating routine every day, dividing the day with a certain number of fasting hours and the remaining hours as a feeding period; for example, the person can eat in a six-hour time window and fast for the remaining eighteen hours of the day, or eat in an eight-hour window and fast for sixteen hours. Similarly, this program can be accompanied by optional restrictions on food consumption at designated meal times.

It should be clarified that, although the word "fasting" implies abstinence from food, many programs include a minimum consumption of food, as long as it does not exceed the 25% of calories that each person needs.

The benefits of intermittent fasting

Some of the benefits that have been reported in the short term are the reduction in elevated blood glucose, cholesterol and triglyceride levels, as well as weight loss. Also, although the evidence so far has proven the effectiveness of the different fasting programs, they have not been tested in the long term and their use continues to be a controversial issue, especially because of the different characteristics of each fasting program.

There are health risks when following very low calorie diets, including nutrient deficiencies and electrolyte imbalances, so you should always seek help from a health professional for follow-up and not follow generalized recommendations from the media or social networks.

It has been shown that diets very low in calories or with fasting periods do not produce greater weight loss in the long term, compared to diets with less severe restrictions. In addition, a reduction in food intake can trigger a variety of biological adaptations that promote regaining lost weight after severe food restriction (rebound effect). Subjecting the body to prolonged periods of fasting can promote modifications in the control of energy homeostasis (the body's natural and normal way of regulating energy use).

The scientific literature shows negative results in the signals that regulate food intake in the hypothalamic centers, so this weight loss strategy can lead to health detriments due to the overexpression of orexigenic neuropeptides accompanied by increased food intake in response to prolonged periods of fasting. The alterations caused in the hypothalamus and adipose tissue that are evidenced in animal models are warning "signals" to consider when using this type of strategies, since there is a lack of studies on the effect on the Central Nervous System (CNS) in the long term.

Therefore, it is good to keep in mind that the available scientific evidence suggests that the best strategy to achieve and maintain long-term weight loss is to change one's eating and lifestyle habits.

It is also important to remember that, like any nutritional intervention, intermittent fasting requires the supervision of health professionals (physician and nutritionist) trained to evaluate and recommend the fasting program that best suits the characteristics of each person (gender, age, diseases, etc.). The above in order to avoid adverse health effects or weight gain after abandoning this program.

Source: Humberto Astiazarán García, researcher of the Nutrition Coordination of the Food and Development Research Center (CIAD), and clinical nutritionist Herminia Mendívil Alvarado.