Intermittent Fasting and Calorie Restriction: what’s wrong with the studies?

Intermittent Fasting and Calorie Restriction: what’s wrong with the studies?

Intermittent Fasting is trendy now, especially for weight loss. Its lesser known sister is Calorie Restriction, on which it’s partly based, although they aren’t the same. But both are getting a lot of attention in the scientific sphere, with studies promoted mainly by organizations with a focus on longevity, such as the National Institute of Aging.

I’ve been reading many of these publications, which you can easily find at PubMed by searching “intermittent fasting” or “calorie restriction”. Also articles more oriented towards popular science. And, although the results seem revolutionary at first sight… when you look at them with a cool head you discover many holes and questions. Those are what I’m going to talk about for the next two posts.

I don’t intend to do here a revision of the studies, but to bring up a series of considerations when it comes to interpreting them. To throw out some questions to ask yourself to analyze them critically.

Intermittent Fasting (IF) or Calorie Restriction (CR)?

I see that, in many publications about IF, when they go beyond the average people level of weight loss method, they associate it to something called calorie restriction. This concept shouldn’t be mistaken for calorie deficit, which just means eating less calories than you burn in order to lose weight.

CR implies living constantly with less calories than our body needs, although its followers would say that what happens is that the recommended amounts are too high and you can live with much less. They believe that, following a diet as restrictive as possible, they’ll be able to extend their lifespan. Even at the cost of their mental health or even, paradoxically, physical, as we shall see.

On the basis of this, there are people who defend IF as a way of “tricking” the body, so they can achieve the same benefits for longevity that CR promises, but without having to actually reduce the total amount of calories they eat; they just have to distribute them in a way that makes them spend more time without eating. Their theory is that, since autophagy —regeneration of cells— occurs when you don’t eat, the more time you don’t eat, the more this process is boosted and that’s the secret to eternal youth.

But think about this

Truth is, we all practice IF. We aren’t eating all the time, we fast between meals. The longest fasting period is the one that goes from your last meal of one day to your first meal of the next, which hopefully lasts at least 8 hours, preferably 10 (it seems to be better not to eat just before going to bed). During those times, our body carries out autophagy. But they think that’s not enough; however, they can’t agree on how it would be enough. There are several ways to practice IF, from fasting completely for 1 or several days a week to eating every day, but only at certain hours.

Alternative ways

You can get benefits out of everything. Low fat, high fat, low carb, high fat… you can find studies on all of those diets that show benefits. Most of those are simply due to following the basic guidelines, the truisms of healthy eating: eating varied, not eating too much ultra-processed food, not eating too much.

Apart from that, each method can have specific benefits. But, if you get the ones from one of them, you’re missing out on the ones from the other ones. Mind you, those benefits are never even remotely comparable in magnitude to the general ones, the ones that all of them share.

Other times, you can achieve the same things through several ways. And doing all of them at once isn’t the best option. Following two things that do the same doesn’t always maximize the benefits. Because you’ll just be insisting in a handful of health markers and be blind to the others. And you’ll still be missing out on the benefits of other things that aren’t compatible. In addition, more of something good doesn’t have to mean better, because the negative effects are also boosted.

And, moreover, biology has limits. There comes a point where, even if you are doing everything possible to improve a certain health marker or reduce the risk for a certain illness, you just can’t improve/lower it anymore. Everything extra you’re doing once you’ve reached that point is futile.

Simpler ways

In the issue we have at hand, we can find benefits for both eating more times a day in smaller amounts and eating less times a day in larger amounts. And, with regards to the specific benefits that are attributed to a CR diet, the biggest ones can be achieved through an alternative way which is much more consolidated scientifically and gratifying for our life: exercise. Among them:

So, basically, the best way to reap all the benefits is to have a balanced diet and exercise. Shocking, I know. And if you want to go one step further, you just need to figure out which specific benefits of specific diets and eating patterns will have a better impact on you. That’s easier than it sounds: you have to forget about diet culture and learn to tune in with your body, listen to its cues and pay attention to how it reacts to different foods, timing, etc. It’s a process, but so worth it.

The argument of the past

A fallacious argument that’s sometimes used in favor of IF is that it was the way we used to eat in the past, when we didn’t have as much food available as we do now. However, I think it should be obvious that the fact that we did something in the past because we didn’t have another option doesn’t mean it’s the best. Just as we shouldn’t accept the modern because it’s modern, we shouldn’t idolize the ancient because it’s ancient. We know that human beings can adapt to many different kinds of diets and thrive.

And, in order to know whether we “are made” for a certain kind of diet, practical experience is the most useful study; when it’s not the right diet, your body tells you, it sends signals (which aren’t always visible, but they can be seen by doctors), it’s not as if you had to guess. As if we had to invent a lifestyle in a lab. That’s pretentious.

Natural vs real

I’m not convinced either, along the same lines, by the arguments that say that’s what happens in nature, or that circadian rhythms would make us follow that eating pattern… Our lifestyle doesn’t follow nature. What we would hypothetically do in a world of noble savages doesn’t matter if that’s not viable in our current situation, those are just ramblings and laments over a lost “golden age”.

Perhaps in our “natural state” we would do better with IF eating patterns. But, is it like that in our current lifestyle? In which we get up and have to go to work, study, take care of a family, in which we go to the gym at the moment of the day we can, in which we need to count on other people… Our way of eating must be the one that allows us to carry out all those tasks in an optimal way, not the one that would be ideal if we lived without responsibilities or duties (or artificial lighting) in nature. Or in a lab, as the animals used in studies.

One example. According to some people, our biological clock doesn’t support the idea of having breakfast as soon as we get up —which can be discussed anyway *–. But, if we’re going to be all morning at work or at college unable to concentrate and about to pass out, what matters more? (Note: this doesn’t mean breakfast is indispensable for everyone. I’m saying that those for whom it’s not shouldn’t judge those for whom it is).

Confounding variables

There’s a problem with fact that studies about IF in humans usually include too many variables that can give room to confusion when it comes to interpreting the results. Most are focused on weight loss **, so it’s very complicated to separate which benefits come from that and which ones from the specific method.

Or many times it’s compared to a control group which leads a crappy diet, or whose diet isn’t controlled, or it’s directly compared to the normal diet of the rest of the population. So in the end the only thing they show is that a well done IF is better than a bad diet. We didn’t need a study for that.

The ignored ones

When you read a study, either about IF or CR, pay attention to the description of how the clinical trial has been carried out. When those are done with humans, it’s worth noting that in most of them the dropout rate is quite high. That is, many of the subjects who start an experiment end up leaving it. And that taking into account that they’re a very special population, which lots of willingness, since they’ve signed up for a dietetic experiment. How many more would them be among normal people?

But anyway, what I want to say with this is that, of course, in the end, the results of the study are good, they say that diet works. But in fact it’s only worked for those who have completed it.

Therefore, it’s not trivial to ask ourselves this: perhaps it only works for people… for whom it works? I mean, what would have happened with the subjects who dropped out had they continued? It would have been interesting if, at the moment of leaving the trial, before letting them go, they could have been submitted to an analysis of the same markers that are going to be examined on the people who complete the trial, so as to see whether they were getting the same improvements or not.


Perhaps this kind of diet isn’t appropriate for them? Perhaps they could get the same positive results by following a dietetic pattern that is? Couldn’t it be that each one achieves the optimal results when they learn to know and listen to their body, and that’s why there exist numerous alternatives that work, but not for everyone (as it’s evidenced by the contradictory results of the studies on other diets that have received more attention)? It seems like scientists take for granted that those who drop out do so because they’re weak and lack adherence, but, what if it just wasn’t for them?

The long term

All the studies lack an important element: the long term. They talk about promises of longevity but, how much time are we talking about? How much more time do people who practice IF live compared to those who don’t? I believe it would be almost impossible to know, because the trial would have to run for many years, and making sure there aren’t other differences from person to person.

That is, you can’t grab people who follow a perfect diet but without IF, and people who eat so badly but do IF (although, if even so they lived 10 years more, everything would have been proved… but this is highly unlikely to happen). Nor the opposite —as it’s often done when studies are made to “prove” something’s good—.

And, with regards to CR, just the same, their followers say it delays aging and they’ll live more but, what does this mean, taking into account that life expectancy is already very high? Will they live 1 day, 1 week, 1 month, 1 year… or 10 years more? And, how are they going to live them? Eternal youth because they haven’t aged? Or horrible elderliness with broken bones and lack of energy to carry out any daily activity? Or perhaps they won’t live more at all because other illnesses can kill them faster due to the precariousness of their system?

To be continued…

And, with that question hanging, I put an end to today’s post. In the next one, I’ll answer it talking about the disastrous consequences of CR at a physical and psychological level, making reference to IF too in this last point in order to make a call to precaution.

Remember. Everything is not what it seems. Read everything with a critical eye and be careful when someone wants to present a very specific kind of diet as a panacea. Feed your body without guilt and external pressures <3

*Our cortisol levels are at a peak when we wake up, and eating breakfast lowers them. Some say that’s a bad thing because it’s “disruptive” (aka not natural, because it wouldn’t happen if we didn’t eat). But others say it’s a good thing, since too much cortisol can be problematic and it’s one of the main hormones related to stress. That’s the view supported by most studies (for example, 1, 2 and 3). For me, it seems like a good thing. Perhaps the cortisol peak is actually a sign to make us eat in order to lower it? (At least to some of us).

**As evidenced by this big revisional study. Just search “weight loss” within the document to check it.


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