Recent headlines warning of concerns such as heart risks or danger to teenagers have put a new spotlight on a diet trend that has long been the popular epitome of a healthy lifestyle: intermittent fasting. “Intermittent fasting’s image has been deeply tarnished—and quite rightly so,” says Stefan Kabisch, a physician at the endocrinology and metabolic medicine department at Charité–University Medicine Berlin. “The hype was never really backed up by good data in humans.”
Advocates have claimed that temporarily abstaining from food for long stretches of the day can improve health and longevity. In the most common fasting schedule, people don’t eat for 16 hours—often skipping breakfast or dinner—and make up for it in the remaining eight hours (the so-called 16:8 method). Some people may severely limit food intake every other day (alternate-day fasting) or take a fasting day twice a week (the 5:2 method).
The effects of such practices can be difficult to study. “The main problem with randomized controlled trials on intermittent fasting is that there are so many variations,” Kabisch explains. “The data is therefore very heterogeneous, which makes it difficult to assess effects.”
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Why Take Long Breaks from Eating?
Modern society has given way to lifestyles that encourage overeating, says Tinh-Hai Collet, a diabetologist and a professor at the Geneva University Hospitals. “This promotes obesity and diabetes.” Experts have called diabetes a global epidemic. Approximately 590 million people worldwide have diabetes, according to the latest 2025 reports from the International Diabetes Federation—and more than 90 percent of them have type 2, often called adult-onset diabetes.
One of the main issues in type 2 diabetes is that those who are affected absorb more energy than they use. A hallmark of the disease is insulin resistance, in which cells no longer effectively respond to the hormone insulin. This means cells take in less sugar from the blood, and blood sugar levels rise. “Intermittent fasting almost always improves the cells’ response to insulin,” says Stephan Herzig, a professor at the Technical University of Munich and head of the Helmholtz Diabetes Center at Helmholtz Munich. “As a result, the concentrations of sugar, fats and cholesterol in the blood also fall.” A 2023 study, led by Mark Ezpeleta of the University of Illinois Chicago, suggests that alternate-day fasting—especially when combined with exercise—also mitigates fatty liver disease, in which fat builds up on in the liver, unrelated to alcohol consumption.
A closer look at the research, however, shows such statements may not be as clear-cut as they are often presented to be. For example, the researchers on the 2023 liver fat study compared volunteer groups that underwent different eating and exercise schedules: alternating days between a reduced calorie intake and an unrestricted diet; only completing an exercise program; combining the fasting schedule with exercising; or not undergoing any intervention. The group that fasted and exercised at the same time saw the best results. Yet there was no comparison group in which people didn’t alternate days but still had the same calorie intake as the fasting participants. “This is an unfair comparison,” Kabisch says. And similar gaps in study design “run through the entire literature on the subject.”
Two meta-analyses of people with diabetes or obesity concluded that intermittent fasting had a positive effect on weight, blood sugar, blood pressure and blood lipids (triglycerides). “In most of the underlying primary studies, however, the fasting subjects were compared with people who did not change their diet,” Kabisch says. As a result, the subjects in the intervention groups lost weight—but, logically, the control groups did not, because the researchers didn’t apply any calorie restrictions. “You only see strong effects of intermittent fasting compared to people who were not given any diet or exercise guidelines,” Kabisch explains. “But as soon as you compare it with equivalent calorie reduction, the benefits of intermittent fasting disappear.”
And the supposed improvement in insulin resistance is also not always demonstrable. In another meta-analysis, no effects were found in this regard. In other studies, the improvements were small.
As Good as Other Diets for Losing Weight
The proven benefits of temporary fasting appear to be that people eat less and lose weight as a result. Scientists at the University of Bath in England tested this explicitly in 2021: they compared intermittent fasting with a diet that had no time restrictions but the same calorie amount. And lo and behold, there was no advantage to fasting in terms of weight or various blood measurements. Abdominal fat actually melted away more in continuous eaters. “This study has a good design for evaluating intermittent fasting in comparison,” Kabisch says. “The finding that [intermittent fasting] is not a superior method for weight loss is consistent with results from other studies.”
“The longer-term studies showed that the positive effects of intermittent fasting are not significantly different from those you would have if you simply ate fewer calories every day,” Herzig adds.
Many people find it easier to skip a meal than to eat less at each one—but eating behaviors and weight tendencies vary greatly among individuals. “It is quite possible that some people reward themselves with particularly high-calorie food after fasting phases and do not significantly reduce their overall energy intake as a result,” Herzig says.
In addition, evidence suggests intermittent fasting makes you hungrier than continuous calorie restriction. A meta-analysis on fasting during Ramadan (a holy month of the Islamic calendar when people don’t eat between sunrise and sunset) supported this finding with a blood analysis: the concentration of the hormone ghrelin, which triggers hunger, was significantly higher in the people who fasted.
Ramadan offers a general, tried-and-tested natural experiment in temporary abstinence from food, and researchers have drawn one important conclusion from related studies: intermittent fasting may pose a significant risk for people with type 2 diabetes—a group that should particularly benefit from it, according to popular literature. For example, in a 2023 study of various Arab countries, a third of participants had increased glucose levels during Ramadan fasting compared with the period before the month started. “We see a significant deterioration in blood sugar levels in diabetics during Ramadan,” Kabisch says. “However, this is also due to the fact that very sweetened foods are often eaten at night.”
“Fasting is definitely harmful at an older age because it signals the body to break down muscles.” —Eline Slagboom, epidemiologist
In 2020 a poster by researchers at Shanghai Jiao Tong University School of Medicine in China and their colleagues made headlines when it was presented at an American Heart Association conference in Chicago. (The research has not yet appeared in a peer-reviewed journal.) In it, the scientists postulated an increased risk of death in people who fasted intermittently. They referred to 20,000 people in the U.S. who took part in the National Health and Nutrition Examination Surveys (NHANES) between 2003 and 2018. On average, the participants were observed for eight years. It was found that people who took at least 16-hour breaks from eating were almost twice as likely to die from a cardiovascular event (mainly heart attack or stroke) than those who ate more often. The study has received a lot of criticism, however, partly because its design cannot prove causation. And it is also possible, for example, that people who were already ill abstained from meals in order to lose weight; this group’s mortality would then be higher because of underlying diseases, such as diabetes and morbid obesity—not because of breaks in eating.
Nevertheless these results have made proponents of the diet trend more cautious. “Until recently, short-term studies all showed positive effects of intermittent fasting,” Herzig says. “Now there are more and more long-term observations with initial indications that it could also have negative effects, especially on cardiovascular disease.”
Better Not to Skip Breakfast
Another meta-analysis, covering more than 200,000 participants from Japan and the U.S., examined the extent to which skipping breakfast affected health. Over a 17-year observation period, the risk of cardiovascular disease was 21 percent higher in people who generally did not eat breakfast than in people who ate regularly in the morning. In other studies, those who mostly skipped breakfast showed an increased risk of obesity (48 percent) and type 2 diabetes (between 12 and 21 percent) and a higher mortality rate from cardiovascular diseases or cancer.
Kabisch cautions against extrapolating too much from the findings. “These are epidemiological studies that do not prove causality,” he points out. “Distortions are likely because skipping breakfast is often associated with poverty, smoking, alcohol consumption, poor-quality nutrition and lack of exercise.”
In addition, dietary patterns were often only surveyed selectively—for example, for the last day, the last week or the last few months before the study—and were therefore not necessarily representative. Nevertheless, these studies do not provide good arguments that intermittent fasting could benefit health.
Muscle Mass Decreases
It’s often only in studies’ fine print that one particular dark side of the intermittent fasting trend is acknowledged: concerns about muscle mass loss. This was the case in a 2025 study. According to the authors, co-led by Manuel Dote-Montero and Antonio Clavero-Jimeno, both at the Sport and Health University Research Institute in Spain, intermittent fasting offers no advantage over a Mediterranean diet in terms of abdominal fat loss, regardless of the time window of the fasting period. One graph showed that those who fasted lost muscle mass—but this was not the case for those who ate the same number of calories throughout the day. The same pattern can be found in a meta-analysis of the effects of different forms of interval fasting: the study participants’ fat-free mass (their muscle mass) decreased everywhere.
“Our body utilizes 20 to 40 grams of protein per meal,” Kabisch explains. “But an adult needs 70 to 90 grams per day. So if you pack the entire amount into two or even a single meal, you can easily slip into a protein deficiency.” In some cases, this even results in no weight loss during intermittent fasting, and instead to muscle loss and fat buildup.
“Fasting is definitely harmful at an older age because it gives the body the signal to break down muscle,” says Eline Slagboom, a professor of epidemiology who is focused on aging research at Leiden University in the Netherlands. “That’s the last thing you need if you want to stay fit and independent in old age.”
Kabisch generally advises against intermittent fasting for people older than age 50.
“If you’re healthy, you won’t benefit from intermittent fasting—you can’t improve an ideal metabolism.” —Tinh-Hai Collet, diabetologist
People with preexisting conditions should also be careful. “Diabetes patients, for example, have to adjust the dosage and timing of their medication,” Collet says. “They should only practice intermittent fasting in consultation with a doctor.” The same applies to those who take medication for hypertension because breaks between meals can also lower blood pressure.
“People with eating disorders and cancer should also definitely avoid intermittent fasting,” Collet advises. People with tumors often lose a lot of weight, which worsens their prognosis; any food restriction can exacerbate this effect. The German Heart Foundation advises against fasting altogether in the case of pronounced cardiac insufficiency. People with other heart conditions who want to lose weight should aim to reduce their calorie intake by a maximum of 500 to 800 kilocalories.
Experts say that it’s only advisable for healthy people up to middle age to try intermittent fasting to lose weight. “However, the positive effects only last as long as you keep up the eating breaks,” Herzig says. Intermittent fasting is by no means a guarantee of a healthy life.
“A balanced diet and plenty of exercise are important,” Collet says. “But if you’re healthy, you won’t benefit from intermittent fasting—you can’t improve your metabolism.”
This article originally appeared in Spektrum der Wissenschaft and was reproduced with permission.
