GUIDE: Intermittent Fasting — What the Science Really Says in 2026

The study that shook the scientific community

In March 2024, at the American Heart Association’s Scientific Sessions in Chicago, Victor Zhong’s team from Shanghai Jiao Tong University presented a preliminary analysis with alarming implications. By analyzing data from 20,078 American adults followed for an average of 8 years, the researchers found that those who restricted their eating to less than 8 hours a day had a 91% higher risk of death from cardiovascular disease compared to people who ate within a 12- to 16-hour window. For people with preexisting cardiovascular disease, an 8- to 10-hour window was associated with a 66% increased risk of death from heart disease or stroke.

These figures made headlines around the world. Should we panic? No—but should we ignore them? Not at all. The study’s co-author, Professor JoAnn Manson of Harvard, herself qualified the findings: “Correlation does not prove causation, and we’ll need more research to understand whether the observed associations are causal or merely correlational.” ” The study is observational—it cannot control for all confounding factors. Participants may have adopted a restricted eating window because of a preexisting condition that reduced their appetite, rather than the other way around.

What Experts Are Really Taking Away From This

The Cleveland Clinic analyzed this study and highlighted its major methodological limitations: participants reported on their eating habits for only 2 days out of 8 years, with no control for diet quality, physical activity, or other lifestyle factors. The group eating within an 8-hour window had significantly more smokers, more men, a higher BMI, and more comorbidities—all variables that increase cardiovascular risk regardless of the eating window.

In early 2025, Frontiers in Nutrition published a review summarizing the current state of knowledge: “Although short-term benefits of intermittent fasting have been demonstrated—such as weight loss and improved lipid profiles—these effects appear to be primarily mediated by calorie restriction rather than by the unique attributes of intermittent fasting. The long-term impacts remain largely unexplored. ” This cardiovascular signal, even if imperfect, calls for caution regarding very long-term use.

What this controversy reveals is that the science of nutrition advances slowly and that observational studies on human diet are fundamentally difficult to conduct. No one lives in a laboratory. But to ignore a finding involving 20,000 people on the pretext that it is “merely” observational would be an error in the opposite direction. The right approach: cautious curiosity.

Who Can Benefit from Intermittent Fasting

Despite some variations, a body of evidence suggests that intermittent fasting can be an effective tool for certain individuals. People who struggle to count calories or maintain conscious calorie restriction may find time-restricted eating to be an effective mechanism for self-discipline: by limiting the eating window, one automatically limits opportunities to eat, thereby reducing calorie intake without active calculation. Studies show that it can improve insulin sensitivity in prediabetic patients, with modest but consistent short-term results.

For people with good cardiovascular health, no eating disorders, and who are looking to manage their weight, the 16:8 approach remains a reasonable option according to Harvard Health Publishing—provided that the nutritional quality of meals is maintained during the eating window and that it is combined with regular physical activity to preserve muscle mass.

Contraindications and At-Risk Populations

The science is clear on certain contraindications. Intermittent fasting is not recommended for people with diabetes (risk of hypoglycemia), pregnant or breastfeeding women, people with eating disorders (risk of exacerbation), and people taking medications that require regular food intake. Studies also point to a loss of muscle mass associated with intermittent fasting without resistance training—a particular concern for older adults, for whom sarcopenia is a major risk factor for frailty.

The disruption of circadian eating rhythms is also a growing concern. Consuming the majority of one’s calories in the evening—which some 16:8 practitioners do unintentionally when they shift their eating window to the evening—is associated with poorer metabolic markers than a diet concentrated earlier in the day. The time of day when the eating window occurs is likely just as important as its duration.

Muscle loss is the blind spot in the popularity of intermittent fasting. “Before and after” photos show slimmer figures—but slimmer in what? In fat, certainly, but also in muscle. In the medium term, losing muscle as we age fundamentally undermines our health. This nuance deserves to be stated plainly.

There is a severe lack of long-term studies

The major gap in research on intermittent fasting is the lack of long-term randomized controlled trials—spanning 5, 10, or 20 years. Most of the available studies last between 2 weeks and 12 months. The central question—whether practicing intermittent fasting for decades is beneficial, neutral, or harmful to cardiovascular, neurological, and oncological health—has yet to receive a satisfactory scientific answer.

Many researchers’ hopes rest on autophagy and cellular longevity: studies in yeast, nematode worms, fruit flies, and mice show that fasting prolongs life and reduces the incidence of age-related diseases. These effects occur through the activation of autophagy, the reduction of IGF-1, and the activation of sirtuins. But extrapolating these findings to humans is complex—we live in environments, with diets, and have genomes that are infinitely more variable than those of laboratory organisms.

Personalization as the Future

A promising direction in current research is personalized nutritional medicine: the same diet does not have the same effects on two different people, due to differences in genetics, microbiota, circadian chronotype, and physical activity levels. Continuous glucose monitoring technologies now allow us to see in real time how a specific individual responds to intermittent fasting—and the results vary greatly from person to person. The future is likely not “intermittent fasting for everyone,” but rather “a dietary protocol tailored to your individual biology.”

The idea that personalized nutrition based on genetics and the microbiome could replace one-size-fits-all diets is both exciting and mind-boggling. It means that the dietary advice of tomorrow may be as individualized as a medical prescription. And that “intermittent fasting works” will be replaced by “intermittent fasting works for you—or it doesn’t.”

What Science Tells Us in 2026

In 2026, the honest scientific assessment of intermittent fasting is as follows: it is a potentially useful dietary tool for certain individuals, with real benefits that are primarily mediated by calorie restriction; its specific effects related to the time window remain a subject of debate; and its long-term risks—particularly cardiovascular risks—warrant serious scientific monitoring. It is neither the health revolution of the century nor an immediate danger to the vast majority of healthy adults. It is an approach that deserves to be considered rigorously, in consultation with a healthcare professional, and without the exaggerated claims that surround it.

What You Can Do Right Now

If you’d like to try intermittent fasting, it’s best to proceed with caution: start with the 12:12 (12 hours of nighttime fasting, which simply means not eating after dinner and before breakfast), assess your tolerance, maintain sufficient protein intake to preserve muscle mass, engage in regular physical activity that includes strength training, and never practice it without medical advice if you have a history of eating disorders, diabetes, or cardiovascular disease. Science provides the details—it’s up to you to make an informed choice.

By Maxime Marquette, columnist

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