The truths and myths of intermittent fasting

In the absence of real scientific evidence to support the benefits of this booming practice, experts warn that it should be taken with a grain of salt

intermittent fasting
Rita Puig-Serra Costa
Jessica Mouzo

The tradition of fasting goes back a long way in different cultures and religions. Lent, Ramadan and Yom Kippur come to mind; even Hippocrates, the father of modern medicine, wrote that eating when you are sick amounts to “feeding your illness.” But although caloric restriction practices have always been present in human history, in recent years they have gone from the temples to the streets, driven by social media trends and the growing scientific interest in their potential health effects. And it is there, between fads and science, fluttering in endless controversy, that a phenomenon known as intermittent fasting grows — one with the theoretical power to heal, but with little evidence in real life.

Actress Gwyneth Paltrow, a devoted follower of this practice, has coffee for breakfast, bone broth at noon and some vegetables as a very early dinner, as she explained on the podcast The Art of Being Well. That is her method, but it is not the only one. There are different fasting patterns and each individual, within these programs, decides what to eat. The most common models of intake and abstinence, however, are the ones known as 5/2 — not eating for two (non-consecutive) days a week — and 8/16, which consists in eating anything for eight hours a day and fasting the rest. Although these methods entail a caloric restriction, this may not happen if, for instance, a person uses those eight hours to consume all the calories that someone else would have eaten throughout the entire day.

These practices have been credited with some beneficial properties, such as the potential to improve the metabolic state and reduce weight, as well as the possibility to treat diseases such as diabetes or cancer. However, beyond the social media noise there is controversy, as well as conflicting positions among the scientific community. “There is suggestive evidence in experimental models, but we have no evidence in the case of humans. The existing information is interesting, but insufficient in humans,” explains Antonio Zorzano, head of the Complex Metabolic Diseases and Mitochondria laboratory at the Institute for Research in Biomedicine in Barcelona, Spain.

The hypothesis about the potential advantages of intermittent fasting comes from animal experimentation. “There is a lot of accumulated information that shows that if you apply caloric restriction to an animal, 70% or 50% of what it used to eat, it becomes metabolically healthier and lives longer,” continues Zorzano. If an animal eats less, if calories are chronically restricted, it gains less weight and has less adipose tissue, and this lower adiposity improves other parameters: it has less resistance to insulin and uses more fatty acids instead of glucose to produce energy (ketosis).

However, this avalanche of metabolic phenomena is not so easily reproduced in humans, adds Francisco Botella, coordinator of the Nutrition Area of the Spanish Society of Endocrinology and Nutrition. “For a human to develop ketosis, many more hours have to pass. With intermittent fasting patterns, ketosis is hardly achieved,” he says. A scientific review published in the New England Journal of Medicine acknowledges that, although intermittent fasting does improve health in animal models, it still remains to be determined whether people can sustain intermittent fasting for years and potentially gain the benefits seen in animals.

Hypertension and glucose

The experts agree that where the evidence is clearest — albeit limited — is in the fact that a person can lose some weight through intermittent fasting and, as a result, slightly lower their hypertension or glucose levels. “But this metabolic benefit has not been shown to entail a change in quality of life or mortality. There are no studies covering more than a year,” adds Botella.

The lack of long-term research in humans affects the potential to make recommendations. “The principle of prudence tells me to wait. For example, we know that there is a connection between muscle strength and longevity: with age, muscle mass decreases and sarcopenia [loss of muscle mass, strength and function] appears. I would like to be sure that this type of dietary manipulation doesn’t alter the capacity to have muscular strength,” says Zorzano. Fasting has also been reported to cause weakness, dehydration, headaches, difficulty concentrating, low blood pressure and fainting.

Jordi Salas-Salvadó, a researcher at the Rovira i Virgili University in Tarragona (Spain) and an expert in the study of the Mediterranean diet, adds that intermittent fasting is a practice that is “difficult to maintain over time.” Botella agrees: “It is not quite compatible with our social life [...] the abandonment rate is very high.”

Biochemist Valter Longo, one of the great advocates of intermittent fasting, examined the available scientific evidence in an article that was published in Nature Aging, where he emphasized that, according to research conducted on humans, this dietary technique “improves sleep, attenuates age- and diet-induced deterioration in cardiac performance and improves blood pressure and lipid accumulation.” But he is also aware of the potential adverse effects, and suggests that this type of intervention “should be limited to short periods and applied only to people with conditions in which regular intermittent fasting has been shown to be effective.”

It is not for everyone, the experts agree. It is contraindicated in pregnant women, people who have undergone bariatric or stomach surgery, people with an underlying pathology such as kidney failure, or individuals who are vulnerable to an eating disorder. “The great danger is that someone with a contraindication does it and we find ourselves before an undernourished cancer patient, a serious accident in a diabetic person or a hypoglycemia with kidney failure,” warns Botella.

Starving cancer to death

Cancer is one of the fields where intermittent fasting seeks to redeem itself, and Longo is one of the great advocates of the use of this practice for patients with oncological conditions. On paper, his hypothesis is remarkable: he proposes to starve cancer to death. “If a cancer patient is deprived of food before undergoing chemotherapy, normal cells will respond by putting up a protective shield; meanwhile, the tumor cells will disregard the order to protect themselves, becoming vulnerable and prone to being eliminated, while the damage to the healthy ones is reduced,” he states in his book Fasting Cancer.

But oncologists call for prudence. Juan de la Haba, a member of the Spanish Society of Medical Oncology, says that people must be cautious in real life. “It is justified because tumor cells have a greater caloric need and a lesser capacity to adapt to the restriction, while a healthy cell has a lower caloric requirement and adapts better to the caloric restriction,” he points out. However, the evidence is very limited: “We have studies in rodents that justify it. There is a very good preclinical line. But in the laboratory, we have been curing cancer for a long time. In epidemiological studies, fasting has some relevance, but there are conflicting results. There is no solid scientific basis to make recommendations to patients.”

The process is more complex than it might seem, and there are many variables involved. “Not all tumor cells have the same metabolic rate. Those with a rapid cell growth have a high metabolic rate, but usually respond well to chemotherapy. Tumor heterogeneity, and that of the population, is so great that a mechanism as generic as caloric restriction is a strategy that’s doomed to failure,” states the oncologist.

These practices can even be counterproductive for patients. If someone has sarcopenia and cachexia (loss of muscle mass and weakness) and fasts, losing more weight, it can aggravate the condition. “Food also plays an important role in the physical, mental and emotional well-being of the patient. And sometimes, eating a piece of cake is accompanied by a feeling of guilt because they think that their illness is going to get worse,” laments De la Haba, who adds: “It involves a personal sacrifice, and it is typically accompanied by the purchase of supplements that are usually not cheap.”

Salas-Salvadó agrees that, when it comes to cancer, the evidence is little and the studies are few. He warns: “There are many charlatans that can be harmful, because people with cancer are looking for a miracle.”

A success with lab animals does not always apply to humans. From a biochemical point of view, the experts admit, the possibilities of intermittent fasting are “tremendously interesting,” but these do not necessarily translate into a benefit for the patient. The scientific world still has many doubts: first of all, if intermittent fasting actually entails any benefit for the patient — and, if so, which type is the best, for how long, with what food and who could benefit from it. Salas-Salvadó admits his reservations: “I am completely skeptical. I have serious doubts that this will have a long-term beneficial effect. But I find it super interesting.”

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