Voluntary fasting is a practice that dates back to ancient times, mainly for spiritual and religious reasons. The popularity of this new trend has been growing in recent years. The number of online searches for “intermittent fasting” is estimated to have increased by around ten thousand times over the past decade. As a practice, it is conceived not solely as a diet to aid weight loss, but also with the aim of improving health through certain metabolic markers.
Intermittent fasting is defined as any action aimed at not eating during variable and controlled periods of time. The aim is to reduce the total daily calorie intake and lower insulin level in the blood so as to reduce the synthesis of body fat.
There are different kinds of intermittent fasting, the most well-known being the day-long fast, which lasts less than 24 hours. This method divides the days into two separate time frames, one for fasting and one when meals can be eaten. The initial recommendation is generally to start with a 12/12 routine, which would mean fasting for 12 hours and leaving the remaining 12 hours for meals. From there, stricter models can be followed, such as 14/10 (fasting for 14 hours, with 10 hours for meals), 16/8, 18/6 or even 20/4.
There is also the weekly fast, which lasts for 24 hours or more. This consists of fasting completely for one day of the week, allowing an intake of no more than 500 calories, or on two separate days of the week (the 5:2 method).
It’s important to bear in mind that the data on this practice in humans is still limited and of little scientific rigour, and almost no controlled tests have been carried out. Those that do exist involve only a few people, who are monitored for a brief period of time and with significant limitations in the methodology.
Despite the differences in the designs of the studies analysed, in general terms fasting may produce certain health benefits in people who can safely tolerate the long periods of fasting that are involved, within programmed and balanced guidelines, and always monitored by a nutritionist.
The two conclusions that all the studies agree on are, firstly, that it helps with weight loss. Systematic reviews on the effectiveness of intermittent fasting conclude that it can be a valid option to help lose body weight with an effectiveness that is similar to other diets, where the aim is also to restrict the daily calorie intake.
Secondly, it has been observed that with intermittent fasting some biochemical parameters relating to inflammation can improve.
However, uncontrolled intermittent fasting may lead to other health risks, such as low blood sugar levels, headaches and irritation. Furthermore, for people with an eating disorder, restricting meals could lead to a loss of control in their intake.
Current scientific evidence has shown that the main strategies that help with weight loss are achieving a negative energy balance and proper monitoring of one’s diet. Intermittent fasting, therefore, could be an option for people with specific profiles who can follow the guidelines, provided it forms part of a healthy eating plan and lifestyle. However, although initial results seem promising, more solid evidence is needed before making any general recommendations.
Author: Alba Andreu, Nutritionist and Dietitian, Hospital Clínic de Barcelona