New information about the benefits of intermittent fasting has generated constant debate. The popular practice, frequently highlighted by digital influencers, is the target of scientific analyzes to determine its real effectiveness and safety for human health, according to surveys carried out on August 23, 2026.
This eating method, which can vary between restricted food days and specific eating windows, has gained great notoriety as a weight loss tool, supposedly surpassing traditional calorie reduction. In addition to the weight issue, intermittent fasting has potential positive effects on reducing the risk of cancer and preventing cognitive decline. The widespread dissemination of these claims contributed to their great popularity.
However, the most recent evidence has raised doubts about the veracity of these promises. Depending on the research evaluated, intermittent fasting can exhibit varying levels of effectiveness when compared to other eating plans.
A specific survey even indicated that this practice can, in fact, be harmful, increasing the chances of developing cardiovascular diseases.
Scientists often disagree about the results, as do the influencers themselves, said Krista Varady, a professor of nutrition at the University of Illinois at Chicago, in the United States. She stated that “It’s not magic”.
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Questions arise about whether intermittent fasting is being overestimated or whether it really represents a diet capable of prolonging life. Another important question is why scientific evidence is still so inconsistent.
For many researchers, the main barrier to obtaining more conclusive data is also the most difficult to overcome. There is no precise control over “what people are actually eating,” as Varady of the University of Illinois pointed out. This problem is common to several dietary interventions.
The diet of our ancestors and metabolism
The concept behind intermittent fasting seeks to replicate the eating patterns of our ancestors throughout evolution. Mark Mattson, a neuroscientist at Johns Hopkins University in Baltimore, USA, whose work is central to this area, explained that “We didn’t evolve in an environment where we ate three meals a day and still had two snacks.”
When fasting, the body changes its metabolism, ceasing to use sugar as the main source of energy (glycolysis) and starting to burn the fat present in the bloodstream. This metabolic change influences cell behavior, leading healthy cells to recycle old and damaged proteins, a process known as autophagy, which rejuvenates them.
In theory, this change directly translates to the health benefits associated with fasting. In a relevant analysis published in the scientific journal New England Journal of Medicine, Mattson, from Johns Hopkins University, correlated this metabolic transformation with increased longevity and a lower incidence of diseases such as diabetes and cancer.
There is, however, a crucial point. The most convincing evidence for this thesis comes from research on rodents, flies, and worms. For these animals, intermittent fasting proved to be a surprising solution. Mice could eat the same amount of calories as a group that ate freely, but by restricting the daily feeding period, they remained thin and healthy until old age, while the others developed obesity and disease.
Unfortunately, information obtained from research with human beings has proven to be less definitive.
Comparisons between animal and human studies
When analyzing the results related to obesity, it appears that intermittent fasting, depending on its modality, “causes a weight loss of around 5%” in people, as pointed out by Varady, from the University of Illinois.
She said this effectiveness is “as good as any other diet strategy,” but results in humans have not yet matched the remarkable effects seen in mice or the reports circulating on social media.
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Although modest, a small weight reduction achieved by intermittent fasting can, in fact, generate metabolic advantages.
Varady mentioned that “There are some reductions in markers of metabolic risk,” including cholesterol levels and blood pressure. The practice can even reduce the symptoms of Polycystic Ovary Syndrome, now officially designated in 2026 as Polyendocrine Metabolic Ovarian Syndrome (PCOS).
Still according to Varady, the less healthy the person is at the beginning of the intervention, the greater the benefits for their health tend to be. However, these gains tend to be less pronounced in humans than those recorded in mice.
Additionally, more rigorous intermittent fasting methods, such as alternate-day eating, can compromise metabolic health by causing the loss of lean muscle mass.
A similar pattern emerges in claims of improved cognition: There is a notable disparity between data obtained in mice and that in humans. Cancer investigations also present ambiguous findings. Animal studies have correlated intermittent fasting with disease prevention and greater tolerance to the adverse effects of chemotherapy. However, crucial research into the mechanism that worked in mice did not yield the same results in people.
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Researchers designed a specific diet to simulate the biological response of prolonged intermittent fasting.
In principle, this diet should strengthen healthy cells and, at the same time, weaken cancerous cells. However, participants did not manifest the desired metabolic change.
Despite this, the 5:2 diet (five days of normal eating and two days of very restricted consumption) can help women with metastatic breast cancer achieve better results during chemotherapy, both in terms of quality of life and in slowing the progression of the disease.
Faced with a scenario of confusing and sometimes contradictory results, one aspect becomes clear: the advantages of intermittent fasting are less evident in humans than those observed in animals. The question is: why this difference?
Part of the explanation lies in essential biological distinctions. For example, a mouse’s metabolic rate is significantly higher than that of a human, which can lead to erroneous conclusions. Courtney Peterson, associate professor of nutrition at Harvard University’s School of Public Health in the United States, explained that “A 16-hour fast in a rodent is actually equivalent to a multi-day fast in a human, because their metabolic rate is much higher.”
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There is also the fact that the rodents used in these studies are often genetic clones, as Varady highlighted, which does not reflect the great genetic diversity of the human population. She considered that “They are simply a very bad model”.
According to Peterson of the Harvard School of Public Health, “Perhaps the biggest obstacle is not biology, but information. It’s very difficult to know what people are actually eating.”
Amid mixed and contrasting findings, a pattern emerges: The gains from intermittent fasting are less marked in people than in animal studies. The question remains: what is the reason for this?
The challenges of accuracy in food records
One of the reasons why the correlation between diet and its effects was so precise in animals is the absence of doubts about their food intake. Mice and other laboratory specimens are required to follow the imposed diet. They feed only when the researcher offers food and in the exact amount, making it easier to identify direct relationships between the diet and biological or medical results.
In the vast majority of studies involving human participants, controlling nutrient intake is unfeasible. Researchers can advise people on the diet to follow and provide food diaries, either on paper or digitally. However, participants return to their daily routine and make their own choices.
Varady of the University of Illinois at Chicago pointed out that “In nutrition research, we have no way of measuring what people are actually eating.” She added that, “So we are completely dependent on them being honest.”
However, even people with the best of intentions can have memory failures. They may forget to record a meal in their diary, which could lead to the false impression that they have restricted their eating schedule. Alternatively, out of embarrassment at not having adhered to the diet, they may omit items consumed during a period of fasting. Varady estimates that only half of the participants in his surveys hand over food records. Others sometimes modify the information.