The benefits of intermittent fasting are debated: Experts say evidence is not as clear as thought

Intermittent fasting, which involves fasting on certain days of the week or limiting the hours of the day during which one eats, has become one of the most popular nutritional approaches in recent years. It is noted that the method can be easier to implement than standard calorie restriction and can help with weight loss. However, the scientific picture regarding more ambitious benefits, ranging from cancer risk to cognitive decline, is still not clear.

Krista Varady, a professor of nutrition at the University of Illinois Chicago, states that the debate over intermittent fasting is not just about claims on social media. Emphasizing that scientists do not always agree on the existing evidence, Varady assesses it by saying, "It is not a magic bullet."

Research shows that intermittent fasting can lead to weight loss in some people. According to Varady, depending on the type of method, about 5 percent weight loss can be seen in humans. While this rate is remarkable compared to other diet strategies, it does not prove that the dramatic results observed in animal experiments are equally valid for humans.

THERE IS A DIFFERENCE BETWEEN ANIMAL EXPERIMENTS AND HUMAN DATA

One of the theoretical foundations of intermittent fasting is that the body changes its energy usage during periods of fasting. The shift from burning sugar to burning fat and the activation of the autophagy process, where cells clear out damaged components, are among the mechanisms proposed to explain the potential benefits of the method.

Mark Mattson, a neuroscientist at Johns Hopkins University, argues that modern humans did not evolve on a "three meals and two snacks a day" schedule. However, a significant portion of the strongest findings in favor of intermittent fasting comes from studies on mice, flies, and worms. Although positive results are obtained in these experiments, research conducted on humans does not yield results with the same clarity.

Courtney Peterson from the Harvard T.H. Chan School of Public Health states that drawing direct conclusions from animal experiments to humans can be misleading. According to Peterson, a 16-hour fast in a rodent can correspond to several days of fasting in a human due to metabolic rate. Furthermore, the fact that experimental animals are genetically very similar to each other does not reflect the biological diversity in humans.

In human studies, improvements in some metabolic indicators can be reported. It is noted that declines in risk markers such as cholesterol and blood pressure are seen, and symptoms of polycystic ovary syndrome may alleviate in some individuals. Conversely, it is stated that more intensive practices, such as alternate-day fasting, may reduce lean body mass and therefore may not be suitable for everyone.

In areas such as cancer and cognitive functions, the findings are more mixed. Some mechanisms that appeared promising in animal experiments did not yield the same result in humans. On the other hand, there are studies linking some practices, such as the 5:2 diet, to positive results in terms of quality of life and disease progression rate in women undergoing treatment for metastatic breast cancer.

THE HARDEST QUESTION IN NUTRITION RESEARCH: WHAT DO PEOPLE EAT?

According to experts, one of the significant reasons for the uncertainty in intermittent fasting studies is the difficulty of accurately measuring what participants actually eat. While the amount and timing of food can be completely controlled in laboratory animals, researchers in human studies often have to rely on self-reporting and food diaries.

Varady says that only a portion of participants submit complete nutritional records, and some records can be inaccurate or incomplete. An equation developed in 2025 revealed that the tendency for people to underreport food consumption is common. Thomas Wilson, a nutrition researcher at Aberystwyth University, states that approximately 30 percent of people underreport what they eat.

This difference can sometimes be quite large. Techniques that measure energy expenditure more objectively, such as the doubly labeled water method, show that there can be differences of hundreds of calories between the amount of calories people report and their actual consumption. Varady reports that some people state they consume 1,600-2,000 calories per day, but the number can rise to around 3,500 calories in objective measurements.

For this reason, researchers are turning to new technologies to collect more reliable data. Studies are being conducted on food photos, wearable cameras, necklaces that detect chewing, smart forks, and nutrient sensors placed on teeth. However, some of these technologies are still in the prototype stage; camera-based solutions bring privacy debates with them.

For now, one of the most practical approaches is to monitor biomarkers related to macronutrients processed by the body through blood and urine tests. According to Wilson, no single method solves the entire problem; however, a more reliable picture can be obtained when personalized food diaries, biomarkers, and wearable technologies are used together.

Mattson, on the other hand, argues that intermittent fasting studies should better monitor not only dietary patterns but also variables such as physical activity and sleep. This is because these factors, which are strictly controlled in animal experiments, can significantly affect the results in humans.

In conclusion, intermittent fasting is considered a method that can provide weight loss and improvement in metabolic indicators in some people. However, the current evidence is not strong enough to say that the method is a superior solution for everyone or provides broad health benefits. Experts point out that people with health problems, in particular, should evaluate such dietary patterns with professional support.