Faced with so much noise, the scientific evidence And clinical experience offers a more nuanced picture: intermittent fasting can be a useful tool for some peopleBut it is far from being a magic solution and is not without risks, especially if practiced without supervision or in vulnerable profiles.
What is intermittent fasting really and why has it become so popular
Simply put, fasting means going for a period of time without eating, and intermittent fasting transforms something so commonplace into a Organized schedule of eating and fasting windowsThe most popular model is the one called 16/8: about 16 hours without consuming calories and 8 hours in which all the day's meals are concentrated, which in practice usually translates into Skipping breakfast and starting to eat at midday.
From a scientific standpoint, several specialists agree that fasting is considered to occur when the period without food lasts at least [number of] hours. From there, other variations emerge: schemes of 14 or 16 hours of fasting per day, the so-called 5:2 diet (five days of normal eating and two of very reduced intake), or alternate-day fasting, where days with severe calorie restriction are alternated with days of normal eating.
Much of its appeal lies in the fact that, unlike other diets, it doesn't require weighing food or counting calories. Many users perceive intermittent fasting as an easier method to integrate into their routine, something that some experts summarize as an approach that “It doesn’t require bringing a calculator to the table”provided that a certain order is maintained in the schedules.
However, behind this apparent simplicity lie complex physiological mechanisms and intense scientific debate. Medical students and nutrition experts who have reviewed the scientific literature describe a divided landscape between studies in favor, others critical, and numerous simplistic headlines which do not always reflect all the nuances.
How intermittent fasting works in the body: from calorie balance to autophagy
The basis of weight loss remains the same, with or without fasting: the body responds to a simple balance of energy coming in versus energy being spentWhen you consume fewer calories than you burn, you lose weight; if the opposite happens, you accumulate fat. In this sense, intermittent fasting often works, in part because it helps certain people to spontaneously reduce what they eatby concentrating the intake in fewer hours.
Several studies have observed that the longer we remain awake and have access to food, the more likely we are to consume additional calories. In studies with hospitalized people, who They got up earlier and they had more time to eat, consuming, on average, about 300 extra calories a day compared to those who slept a little longer. Translated to daily life, shortening the eating window can mean a reduction in energy that, in the long term, translates into weight loss.
But the interest in intermittent fasting isn't limited to weight loss. After about 10-12 hours without food, glucose reserves begin to decline, and the body enters what some authors describe as a state of... “metabolic silence”Insulin levels decrease, glucagon levels increase, insulin sensitivity improves, and the body uses stored fat more intensely as an energy source.
In that context, the following takes center stage: autophagyAutophagy is a cellular "recycling" mechanism by which cells eliminate damaged components and waste. This process is not a social media fad: the 2016 Nobel Prize in Medicine specifically recognized research on autophagy. Prolonged fasting contributes to its activation, although it is not the only way to do so, and it is unclear to what extent these mechanisms translate into measurable clinical benefits in humans in the long term.
What the studies say: benefits, limitations, and the role of calories
Research on intermittent fasting in humans has proliferated in the last decade, yielding results that, taken together, are less spectacular than some headlines suggest. One of the most frequently repeated conclusions is that this approach is often more effective than making no changesHowever, it does not demonstrate a clear superiority over other diets when calories and food quality are equal.
A relevant example is the study ChronoFastThe study, conducted in Germany by the German Institute of Human Nutrition and the Berlin University of Medicine, followed 31 overweight or obese women who alternated between two eating schedules for two weeks: an early meal (8:00 a.m. to 16:00 p.m.) and a late meal (13:00 p.m. to 21:00 p.m.). In both cases, the following conditions were maintained: exactly the same calories and nutrients newspapers.
The results were clear: when total energy intake did not change, no significant improvements were observed Neither weight, nor glucose tolerance, nor insulin sensitivity, lipid profile, nor inflammatory markers were affected. In other words, in this specific case, modifying meal timing without reducing calories did not provide measurable metabolic benefits.
The researchers point out that many previous studies describing positive effects of intermittent fasting likely benefited from a unintentional reduction of calorie intakeIn other words, people ended up eating less almost without realizing it by limiting the hours in which they could do so, and that would explain a good part of the improvements observed.
Even so, other studies have found specific advantages to certain schemes. One study published in Annals of Internal Medicine analyzed the call fasting 4:3This method involves drastically reducing calorie intake for three non-consecutive days a week, while on the remaining days people eat more freely, although prioritizing healthy foods. In this trial, people assigned to the 4:3 fasting lost about a 7,6% of their body weight in one year, compared to 5% of those who followed a moderate daily calorie restriction equivalent in weekly calories.
In addition to weight loss, the group that practiced intermittent fasting showed improvements in blood pressure, cholesterol levels, and fasting glucoseSome experts point out, however, that it is possible they also ate somewhat less than planned on their "free" days, which again introduces the caloric factor as a likely explanation.
In the cardiovascular and metabolic field, recent reviews, including publications in journals such as JAMA Network OpenThey have pointed out that many intermittent fasting interventions achieve short and medium term weight reductionsHowever, maintaining it beyond the first year is not always easy, and in many cases a "rebound effect" occurs.
Intermittent fasting, heart, and potential risks
Beyond the potential benefits, some specialists warn of the risks of idealizing intermittent fasting. Cardiologists who have addressed this topic on social media and in the press remind us that the heart... “He doesn’t understand fashion” and that any sudden change in eating habits can have consequences, especially if there is an underlying illness.
In people with a history of hypertension, arrhythmias, or other heart problemsProlonged periods without food can be accompanied by drops in blood pressure, dizziness, intense fatigue, or an increase in physiological stress, especially during the initial adaptation phases. In these situations, the body may perceive prolonged fasting as a form of stress and activate compensatory hormonal responses.
There are also doubts about the impact of systematically skipping breakfast, a common practice in 16/8 diets. Some observational studies have associated the Absence of breakfast is associated with a higher risk of cardiovascular mortality.While these studies have limitations and may be influenced by other lifestyle factors, they still serve as a reminder that not all fasting methods are equally suitable for all individuals.
The most common side effects when starting intermittent fasting include intense hunger, headache, irritability, tiredness or difficulty concentrating, especially during the first few days or with overly aggressive protocols. In many people these symptoms lessen over time, but in others they become a reason to quit or for the program to become an additional source of stress.
Specialists in digestive health, endocrinology, and clinical nutrition emphasize that, in populations with diabetes, kidney or liver disease, eating disorders, pregnancy, breastfeeding, or advanced ageThe general recommendation is to avoid intermittent fasting or, if necessary, only consider it under close medical supervision.
A critical perspective: when intermittent fasting isn't the best option
Within the field of nutrition itself, there are clearly skeptical voices regarding the enthusiasm surrounding intermittent fasting. Some registered dietitians and nutritionists with extensive clinical experience warn that this practice, when presented as a near-miraculous solution, can distancing the population from basic food education.
From this critical perspective, the main risk is that the person ends up focusing all their efforts on when he eats, not what he eatsIt is emphasized that no one learns to eat well simply by not eating for hours, just as one doesn't learn to play the violin by not practicing. For those who already have a complicated relationship with food, introducing periods of strict restriction can encourage the development or worsening of eating disorders. eating disorders which tend to have a worse prognosis.
These professionals point out that the rise of intermittent fasting is part of a context of nutritional illiteracy and an abundance of contradictory messagesSocial media and video platforms have given a voice to a large number of content creators who, without specialized training, recommend extreme protocols or generalize personal experiences without qualifying risks or contraindications.
Meanwhile, some experts are talking about a certain “nutritional nihilism”Faced with a deluge of studies with conflicting conclusions, many people end up thinking that no recommendation is reliable and that "anything goes." This climate of confusion allows eye-catching proposals like intermittent fasting to gain visibility, while basic concepts such as increasing the consumption of fruits, vegetables, and legumes are relegated to the background.
Within this framework, the harshest criticism of intermittent fasting is not so much directed at the tool itself—which may be valid in certain cases—as at its indiscriminate use, without individual analysis and without being accompanied by a clear improvement in the overall quality of the diet and lifestyle.
What the Blue Zones and the experience of long-lived populations show
Beyond clinical trials, another focus of interest is the way people eat in so-called blue zones, regions of the world where the proportion of centenarians in good health is especially high. Researchers like Dan Buettner have studied the habits of populations of these regions for years. Okinawa (Japan), Ikaria (Greece), Sardinia (Italy), Loma Linda (California) or the Nicoya Peninsula (Costa Rica).
In these places, intermittent fasting isn't practiced as such, nor is there a formalized "miracle diet." What is observed is that the inhabitants tend to eat especially during daylight hours and they leave a period of 12-14 hours between the last meal of the day and the first of the next, which in practice resembles a long overnight fast, but not strictly planned.
Their diet is based primarily on local and minimally processed productsThe diet includes fiber-rich and antioxidant-rich vegetables, legumes, seasonal fruits, whole grains such as oats or brown rice, olive oil, nuts, seeds, and, to a lesser extent, oily fish and some fermented dairy products. The consumption of red meat and ultra-processed foods is generally low.
Buettner and other researchers point out that moving from a typical Western diet, rich in processed foods, to a pattern that prioritizes plant-based and natural foods It can add several years to life if adopted early, and even provide notable benefits in old age. But they insist that the key for these populations is not counting hours of fasting, but rather that their Environment and routine promote healthy habits without conscious effort..
As for breakfast, the examples described are quite different from the sugary cereals or pastries common in many European countries. In some areas, the overnight fast is broken with olives, sourdough bread and cheeseor with savory dishes based on legumes, rice, avocado, and garden fruits. In other words, nutritious meals are prioritized to start the day, something that many specialists also recommend when practicing any fasting regimen.
Practical experiences: from “metabolic silence” to daily routines
Alongside scientific studies and the observation of long-lived populations, many people in Spain have been incorporating intermittent fasting into their daily lives. Some healthcare professionals recount how, while reviewing the scientific literature during their training, they discovered that fasting was not so much an isolated fad as one more strategy within the range of dietary tools.
Those who tolerate periods without eating well often point out that they find it easier to do two hearty and filling meals within a limited window, instead of three or four small meals that leave them feeling unsatisfied. In these cases, intermittent fasting helps maintain adherence because people feel satisfied during eating times and don't perceive the diet as a continuous sacrifice.
There are also public figures who have shared their experience with this approach. Well-known television presenters in Spain have commented that they have been skipping breakfast for years and concentrating their food intake in the hours before their workday, effectively following a 16/8 pattern. In their account, this routine helps them to maintain a stable weight and feel more clear-headed mentally during the morning.
However, even in these cases, it is emphasized that intermittent fasting is only one part of a broader lifestyle that includes Daily exerciseA combination of cardiovascular and strength training, adequate rest, and a diet based on fresh foods. In other words, it's not presented as an isolated solution, but as one more component within a comprehensive health approach.
Specialists who work with patients emphasize that when fasting is well integrated, it is usually because it respects each person's rhythms, fits their work and family schedule, and, above all, does not conflict with their relationship with food or their previous state of health.
Who can benefit from intermittent fasting and under what conditions?
In light of the evidence and clinical practice, intermittent fasting may be especially useful for adults without serious medical conditions who are looking to a simple way to reduce calories and regulate their meal times, as long as they feel comfortable with periods without eating.
Among its practical advantages are that it doesn't require strict calorie counting, it can be adapted to different lifestyles, and in many cases, it promotes greater awareness about what to eat and when to eatSome studies suggest improvements in markers such as blood pressure, cholesterol, or glucose when fasting is combined with a quality diet and slight energy restriction.
However, there are groups for whom the recommendation is much more cautious. Cardiology, endocrinology, and nutrition professionals agree that intermittent fasting is not the best option, or at least requires prior medical evaluation, in people with diabetes treated with certain drugs, cardiovascular diseases, kidney or liver diseases, as well as in pregnancy, breastfeeding, childhood, adolescence and over 65 years of age.
It is also strongly discouraged for those who have suffered Eating disorder or they have a fragile relationship with food. Introducing periods of total prohibition can reinforce patterns of restriction and binge eating, or intensify the guilt associated with eating.
In any case, both in Spain and in other European countries, scientific societies insist that the step prior to considering intermittent fasting should be to assess, together with a healthcare professional, goals, health status, medication and current habits, instead of simply imitating the scheme seen in a viral video.
Keys to practicing safer and more sustainable intermittent fasting
When intermittent fasting is considered a suitable option, specialists often recommend starting with variations moderate and flexibleinstead of jumping straight into extreme protocols that are difficult to maintain in the long term.
A reasonable guideline for many people is to start with fasts of 12 to 14 hours at nightThis can practically mean eating dinner slightly earlier and breakfast a little later, without skipping it altogether. This eating window is usually better tolerated and reduces the risk of dizziness or pronounced energy crashes.
The quality of the diet during the eating window is crucial. The potential benefits of intermittent fasting are diminished if the eating window is filled with junk food. Ultra-processed products, added sugars, and poor-quality fatsConversely, a diet similar to the Mediterranean diet—rich in vegetables, fruits, legumes, whole grains, fish, olive oil, and nuts—is strongly associated with better cardiovascular and metabolic health, with or without fasting.
Other practical tips include maintaining good hydration During the fasting period, plan meals in advance to avoid excessive hunger that leads to binge eating, and adjust the schedule to circadian rhythms, prioritizing more substantial meals during the day and lighter dinners.
Finally, experts emphasize that any dietary strategy should be evaluated not only for its short-term effects on the scale, but also for its impact on the mental well-being, daily energy, and the ability to sustain oneself over timeAs one of the researchers who has worked with intermittent fasting summarized, "the best diet is the one that a person can sustain for years without it becoming a burden."
Overall, the available research and the experience of professionals suggest that intermittent fasting can be a useful piece in the healthy eating puzzle, but not the only one, nor necessarily the most important. Its positive effects seem to depend less on miracle cures and more on classic factors such as Eat slightly less, choose better foods, respect the body's rhythms, and adapt the strategy to each person.This remains true both in Spain and in the rest of Europe.
