The controversy over intermittent fasting: it is neither miraculous nor can it be said to be dangerous for the heart

For a few years now, intermittent fasting has been in fashion. These types of diets, which restrict the hours of the day in which you can eat, are promoted by all kinds of famous people and have a good image among aging researchers. Reviews of studies that test the virtues of fasting show that it helps to lose weight and improve other associated factors, such as hypertension or glucose levels. However, as is often the case with diet, it is not easy to know what causes the changes seen in experiments, and long-term studies are also lacking.

Recently, in a presentation at an American Heart Association conference, results were provided from following more than 20,000 adults in the United States for an average of eight years. In that period, those who restricted eating time to eight hours a day saw their risk of death from cardiovascular disease increase by 91%. Among those who had an eating window of more than eight hours, but less than ten, the risk increased by 66% compared to those who ate without that time limitation.

If the result were confirmed, the new fad diet would be a risky health practice, but the study is far from being a definitive condemnation of intermittent fasting. To begin with, this is a work that has not been published in a scientific journal nor has it been reviewed by other experts in the field. Details about the work are still scarce and the study has important limitations.

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Truths and illusions about intermittent fasting

To find out what the study subjects ate, researchers from the Chinese universities of Wuhan and Jiao Tong, and the American universities of Northwestern, Harvard and Massachusetts Lowell, used the responses from the Annual National Health and Nutrition Examination Surveys (NHANES). , of its acronym in English) between the years 2003 and 2018. The participants explained in two interviews, with less than two weeks apart, what time they had eaten the previous day and then it was assumed that they maintained those same habits during the later years. Next, the authors of the work cross-referenced that information with the Centers for Disease Prevention and Control (CDC) database of deaths from 2003 to 2019.

Observational studies can see a relationship between two phenomena, but causality is much more complicated. “Someone has been able to start a fasting diet [in which they eat in an eight-hour window and fast for the rest of the day] because the doctor has told them that they need to lose weight; and we know that living with excess weight is a risk factor for heart and circulatory disease,” the British Heart Foundation explained in a note. It is also possible that a person has this type of eating pattern because he works at night or because he has a very stressful job that does not allow him to stop to eat. In these cases, their working conditions would represent a cardiovascular risk factor, regardless of diet.

There is a historical paradox in observational studies, which illustrates the need for caution in the face of work such as the one presented a few days ago on fasting. For decades, studies on the health effects of alcohol show that people who claim to never drink have higher mortality rates than those who drink occasionally or in small amounts. But researchers doubt this result due to the difficulty in finding groups of people who never drink and, furthermore, who do not do so because they have been alcoholics or have illnesses.

Insufficient research on fasting

Although studies in animal models offer very positive results and some molecular analyzes in humans show how fasting promotes the appearance of proteins with protective effects against some metabolic or brain diseases, studies on diet, especially long-term, are very complex. Victor Wenze Zhong, professor at Shanghai Jiao Tong University and first author of the study, emphasizes the need for studies that follow up for years to fully understand the effects of fasting on health and believes that his results encourage the application of “a “more prudent and personalized approach when giving dietary recommendations, ensuring that they are adapted to the person's health status and the latest scientific evidence.” However, Zhong acknowledges that, although the study identified an association between some types of fasting and an increased risk of death from cardiovascular problems, “this does not mean that time-restricted eating causes death [from cardiovascular disease].”

Previous studies have found that using intermittent fasting to lose weight causes greater loss of muscle mass than diets that allow you to eat for a longer period of time. Since muscle is linked to a healthier metabolism, this could explain the link between fasting and cardiovascular problems, but the link is far from proven. For now, the most common recommendation from experts is to doubt miracle diets and, especially if you have health problems, consult a doctor before adopting a way of eating that, although it has promising results, is still in a initial research phase.

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