Intermittent fasting: is it really healthy, and who is it not suitable for? (Doctor explains)

## Introduction

Intermittent fasting has become extremely popular. People say that it helps with weight loss, improves metabolic health and blood-sugar balance, improves gut health, detoxifies the body, and even extends life. Today we will examine the scientific and research basis behind each of these claims so that you can decide whether it is right for you.

I am Dr. Elisheva, an anesthesiologist. Here you will get clear, direct medical information about a healthy lifestyle—what is proven, what is a myth, and how to make informed decisions about your health, without drama or trends.

## What is intermittent fasting?

Intermittent fasting means limiting your eating hours to defined times of the day. There are several different patterns. The most common is 16:8, in which you eat only during eight hours of the day, but there are more extreme versions such as 18:4, as well as patterns in which you eat normally five days a week and fast completely for two days.

It is important to say that the number 16:8 is not based on research or on a particular medical mechanism. It is simply a pattern that became popular because it is convenient and seems to work reasonably well on average, and more and more studies were then built around it. It is not a magic number or a commandment from above. Our bodies do not have stopwatches: all metabolic processes happen gradually and also vary between people.

## What happens to the body during a fast?

Our cells need glucose—that is, sugar—to produce energy, but that glucose can come from different sources. After a meal, the body uses the energy that is most readily available: the glucose from the food we ate. As more time passes after the last meal, the body begins breaking down glucose stores, called glycogen, in the liver and muscles.

After roughly 12 hours of fasting, it also begins breaking down fatty acids more significantly in order to produce glucose from them. Fat breakdown becomes most significant somewhere between 12 and 18 hours, but this varies greatly. It depends on our metabolic state, physical activity, the size of the last meal, and many other factors. When people talk about 16 hours of fasting, that is a general estimate of the time it may take to reach this state. Some people need less time and others more time to reach substantial fatty-acid breakdown.

## Does it help with weight loss?

One of the main claims is that intermittent fasting helps with weight loss. Because the aim is to reach the point at which fatty-acid stores are used, it seems logical that it should help. But it is important to remember that breaking down fat during one fasting period does not necessarily mean that, when we look at the result over a week or another period, we will have lost weight. Weight loss ultimately depends on energy balance—whether we use more energy than we take into the body.

What we can say with confidence is that intermittent fasting generally helps people eat less. A large meta-analysis of 99 studies involving more than 6,000 participants showed that intermittent fasting and calorie restriction produce similar results. This is probably because intermittent fasting simply causes us to reduce calories.

Another claim is that it reduces appetite compared with a calorie-restriction diet. One study examined this and concluded that it did not affect appetite, but I have a few reservations about that study. First, it did not examine when people ate. You can follow 16:8 while changing the hours every day, or you can keep the same hours every day. I genuinely believe that using the same hours each day reduces hunger because the body learns not to be hungry at certain times; it becomes part of our biological clock. Second, the study did not separate hunger during the eating hours from hunger during fasting hours, instead measuring hunger in general. The important question is whether a person who fasts is hungry during the fasting hours, not necessarily during the eating hours.

Intermittent fasting also creates a more significant behavioral change. For many people, clearly defining their eating hours is easier than counting calories at every meal, so it is easier for them to remain disciplined.

Does it cause weight loss? Yes, but it is not a miracle diet that is a thousand times more effective than ordinary calorie restriction. In my personal opinion, however, it is easier to follow.

## Does it improve metabolic health and extend life?

There are also claims that intermittent fasting improves blood-sugar levels, lowers blood lipids, and even extends life. Here too, studies suggest that the answer is yes—but to roughly the same extent as another healthy calorie-restricted diet without fasting. The positive effects are probably mainly related to the weight loss itself.

Does it extend life? There is no study that has tested this claim. There is evidence that calorie restriction in general affects pathways related to aging. If intermittent fasting ultimately reduces calorie intake, it may perhaps extend life, but there is no clear research proving this.

## Does it improve autophagy?

If you have not heard this term, where have you been? Just kidding. Autophagy is an important process in which a cell, in a sense, eats itself to break down and recycle damaged proteins and components such as mitochondria that are not functioning properly. It is an essential mechanism that helps cells maintain healthy components and function well. It happens in the body all the time, but there are claims that intermittent fasting enhances and makes this process more efficient.

It is important to understand that this does not mean detoxification or removing toxins from the body. It is an intracellular recycling process that improves cellular function.

Does intermittent fasting improve autophagy? We know that restricting food and fasting increases the process in laboratory cells and animals, but it is very difficult to measure directly in humans. One study followed 120 people with obesity who practiced intermittent fasting for six months and found an increase in a blood marker associated with autophagy. However, the researchers did not test whether there was an actual clinical health effect. A blood marker does not always translate into a real clinical benefit.

It is also still unclear exactly how much fasting is required to produce this effect, and it is not necessarily after precisely 16 hours. Intermittent fasting may improve autophagy, but we do not yet know the meaning of this finding.

## Does it improve digestive health?

Many people say that their digestive symptoms feel better with intermittent fasting. The idea is that it may affect gut bacteria—the microbiome—and perhaps improve digestive health that way. Studies disagree: some found changes in the microbiome and others did not, and in the studies that found changes, the effect of those changes is not very clear.

For general symptoms such as abdominal pain and bloating, only small preliminary studies support an improvement. Feeling better is probably related to less snacking throughout the day and not eating at night. The opposite can also happen: eating enormous meals during the eating window can actually worsen digestive symptoms.

We do know that the small intestine has a cyclical pattern of contractions whose purpose is to move along food that has not yet been digested, improving the digestive process. Time without eating allows it to work properly. That does not mean 16 hours are required; simply not snacking between meals may be enough to maintain normal function.

## Does it improve concentration and focus?

A sense of unusual clarity during fasting hours is common. Unfortunately, a large meta-analysis of more than 3,000 participants did not find an objective improvement in cognitive performance.

There is a plausible explanation for the common feeling: it may relate to avoiding the heaviness and fatigue that follow a large breakfast, and to not having to think about food. I am not dismissing these experiences—I feel them myself—but, as usual, we sometimes need more research to confirm them.

## Myths worth debunking

Does fasting detoxify the body? No. Your liver and kidneys remove toxins from your body; fasting does not make you eliminate more toxins.

Can you eat whatever you want during the eating window? Absolutely not. If you put unhealthy processed food into your body, it will harm you whether or not you fasted for 16 hours beforehand. When people say that you can eat whatever you want, they mean that you generally do not need to impose deliberate calorie restriction during the eating window because it often happens naturally—that is the attraction. As for what you eat, choose healthy food if you want to be healthy.

Does the timing of the eating window matter? For example, if you follow 16:8, is eating from noon until 8 p.m. equivalent to eating from 8 a.m. until 4 p.m.? A large study of 2,200 patients found that earlier eating was preferable. People whose eating window was earlier and who finished eating by 5 p.m. lost more weight than those who finished after around 7 p.m., and they also had a small reduction in blood insulin levels.

This is probably related to something already well established: our metabolic response is better earlier in the day, meaning that we use energy more efficiently. It is therefore preferable to choose a relatively early eating window and avoid eating late at night.

## Who may not be a good fit?

I strongly recommend consulting a doctor before trying intermittent fasting if you have diabetes—especially diabetes treated with insulin, because of the risk of hypoglycemia—if you are pregnant or breastfeeding, significantly underweight, or have a history of eating disorders; if you are a child or adolescent; or if you have a serious chronic illness in which changing eating habits can affect the condition or treatment, such as heart, kidney, or liver failure.

This does not necessarily mean that intermittent fasting is forbidden for every person in these groups. These patient groups were not studied adequately, so a doctor should help decide whether it is suitable on a case-by-case basis.

## Are there risks to intermittent fasting?

One concern sometimes raised about intermittent fasting is loss of muscle mass: the idea that after a prolonged fast, the body may break down muscle to produce energy. In general, our bodies always break down a little muscle for energy, but muscle breakdown as a significant energy source usually occurs after more than 18 hours of fasting.

As with calories and weight loss, the more important question is whether our overall protein balance is positive or negative. The risk of a negative protein balance rises if the fast does not leave an eating window large enough to consume sufficient protein, and if you are not building muscle through resistance training.

You may also have heard about the 2024 American Heart Association report involving 20,000 Americans, which found a 91% increase in cardiovascular mortality among people who ate within an eight-hour window. The headline sounds frightening, but this was an observational study that simply gathered information about eating patterns. The data were based only on self-reporting. There was no information about what people ate or the hours in which they ate, and there was no analysis of other factors that could affect the result, such as underlying illnesses.

The finding is worth examining in a follow-up study, but for now there is no reason to panic. The association itself described the result as something requiring further investigation, not as evidence of a causal relationship between intermittent fasting and cardiovascular mortality.

## A beginner's guide

There is no proof that one fasting method is better than another—a daily eating window versus ordinary eating days combined with fasting days. In my opinion, fasting for entire days is much harder, but it is a matter of personal preference.

If you choose a daily eating window such as 16:8, it is preferable to choose an earlier window. The most common pattern I hear is starting to eat at noon and finishing at 8 p.m., but it is actually better to move the window earlier when possible.

Do not begin immediately with 16:8. Start with 12:12 and gradually shorten the eating period. Remember that 16:8 is not a magic number that suits everyone. A 14:10 pattern may be enough for you to get results and feel good, while another person may only see results after shortening the eating window further.

During the fast, you may and should drink anything without calories. That includes water, of course, but also black coffee and tea without sugar.

See whether it suits you. Some people, including me, feel excellent with intermittent fasting—less tired, more focused, and less hungry—while for other people it simply is not a good fit.

## Summary

Intermittent fasting is a very effective tool that helps you eat in a more structured way, snack less, lose weight, and improve metabolic health. The other proposed effects are not all proven, and some still require deeper research.

If it works for you and you feel good while doing it, as I do, continue. If it does not fit your life, do not worry: there are many other ways to achieve the same health outcomes.

Remember that the best eating habits are those that allow you to eat healthy, whole, unprocessed food with enough vitamins and protein.

I hope this video helped. If you have more questions about intermittent fasting, write them in the comments. If you want more videos and content about a research- and science-based healthy lifestyle, subscribe to the channel, and I will see you in the next video.