Intermittent fasting: Is the hype justified?

Lately, it seems that people don't count calories anymore, but instead count hours.

For example, one person says "I eat nothing for 16 hours by skipping breakfast and then I eat what I want for 8 hours!", the other says "I eat nothing for 20 hours by skipping breakfast AND lunch and then I eat what I want for 4 hours!," and then another arrives and says "That's nothing! I eat nothing 2 days every week and the other 5 days I eat what I want!"

These are all forms of so-called "intermittent fasting."

Intermittent fasting is reputed to be able to help you lose weight and maintain your muscles while dieting, and even to ensure that you build muscle better and become healthier overall, according to proponents of this strategy.

But first: Why is intermittent fasting so popular right now in the first place?

This is probably at least partly due to the fact that a few - at first glance interesting-looking - studies that have been published on the subject, and also to the fact that it is a SIMPLE way of eating.

What could be easier than eating exactly the same as usual, just skipping one or a few meals? You don't have to weigh everything, and you don't have to eat low carb or low fat or give up certain foods - you just have to eat fewer meals.

Well, claims are nice, but you are probably reading this blog article to get the most objective assessment of the effectiveness of intermittent fasting.

That's why we are doing it this way now. Intermittent fasting is supposed to help you lose weight, maintain muscle, build muscle, and get healthier. These are four points that we will now take a closer look at one by one.

Losing weight

Let's start with the first point, losing weight. Proponents of intermittent fasting claim that during a normal weight loss diet the body gets used to the constant calorie deficit quite quickly, so that it soon stops losing weight.

In this case, a "normal" diet refers to one where you simply eat a little fewer calories at each meal than you normally would. This is not supposed to be the case with intermittent fasting.

To explain this by way of example: It is basically claimed that if you eat 3 cheeseburgers a day within a short period of, say, 6 hours (and nothing else), you will lose weight better than if you eat the same 3 cheeseburgers within a longer period, say 14 hours.

Is there any truth to this claim? What do the studies say about it? Before we start with the studies, there's one thing that should be mentioned first: Most studies on intermittent fasting have been conducted on animals.

Relatively few studies have been done on people, even fewer on people who are physically in good health, and even fewer on people who are healthy AND strength train. This means we need to be very careful about what studies we look at and how transferable those results are to YOU.

For example, if you are healthy and of an average weight and do strength training regularly, then it is dangerous to apply to your own life the results of studies using, for example, overweight subjects with diabetes and no training experience.

Yes, and we probably don't even need to go into the fact that this is even more tricky with animal studies. However, this does not mean that these studies do not provide us with any added value.

In fact, the results of these studies can give us a very rough indication of the direction we should take, and are particularly useful for scientists who are considering which ideas to test on humans in the future.

Studies on humans are much more complex and expensive than studies on animals. Therefore, studies in humans are often only conducted if there are already promising results from animal studies on the relevant topic.

Okay, but what do the studies say about intermittent fasting and weight loss? There is a large 2015 literature review by Seimon and others. This review includes ONLY human studies and ONLY studies where fasting was done at least one COMPLETE day a week.

The result was that at the same level of weekly calorie deficit, no difference was seen between intermittent fasting and a normal diet in terms of weight loss.

Now, of course, you might say, "Yes, but what if I don't fast for a whole day, but just skip one meal a day". That was not considered in this review. But there is a 2015 meta-analysis by Schoenfeld and others that looked at exactly that scenario.

The authors examined and then summarized all relevant studies on the topic of number of meals per day and the effects this had on weight loss. Again, the results were: No matter how many times a day you eat and no matter how long the eating period, weight loss is the same if the calorie deficit is also the same.

Of course, this does not exactly recommend intermittent fasting, but it does not fully discredit it either. Now, of course, most people don't count calories in a robust way.

Let's look at an example: Angelica usually eats something for breakfast, lunch, and dinner, and then she eats a bag of chips in front of the TV in the evening from 8:15 to 10:15.

If Angelica then starts intermittent fasting and decides to eat only from 12 to 8 p.m., the following could happen: She skips breakfast. That's not difficult for her anyway, since she actually doesn't like eating anything in the morning. Instead, the first meal she eats is lunch.

Then she eats a little more than usual - e.g. takes 5 potatoes instead of 4. She is hungrier because she skipped breakfast. The next meal following this is dinner. She also eats a little more than usual here. For example, she eats a little more rice and sauce.

Then it's already 8 p.m. and for her it's clear that "now it's fasting time and I won't touch the bag of chips that I would normally have eaten in the evening!"

Now, if we add up all the calories she ate, it is very likely that she took in fewer calories overall than with her previous diet without intermittent fasting, even though she ate a little more than usual at lunch and dinner.

That is, she ate in a calorie deficit without counting calories. Thus, intermittent fasting is a relatively simple method for Angelica to get into a calorie deficit and thus lose weight without having to count calories.

Only, of course, this is not something that will work for everyone. Angelica, for example, finds it very easy to not eat anything in the morning. But what about Angelica's husband, who loves his breakfast? Sure, he could skip dinner, but he loves that too!

Probably the better method for him would be to simply eat a little less at each meal. This would also lead to him being in a calorie deficit and thus losing weight.

Preserving muscle

That's about it on the subject of losing weight. What about muscle maintenance during your diet? Do you lose more or less muscle mass during intermittent fasting compared to a normal diet? Let's take another look at the meta-analysis by Schoenfeld and his colleagues mentioned above.

They found that you can maintain muscle just as well if you're only eating 1 or 2 times a day. BUT - and here's the rub: In almost all the studies they looked at, the participants were untrained. In other words, they had no training experience whatsoever.

Since most of you are probably not untrained, you're likely to be more interested in what happens when trained people eat infrequently. Fortunately, there was one study in this meta-analysis that actually looked at trained individuals - specifically boxers.

This study by Iwao and others looked at whether there was a difference in muscle retention when one group of boxers ate only 2 meals per day and the other group ate 6 meals per day. Of course, both groups consumed the same total amount of calories.

The result was that the boxers with only 2 meals per day lost more muscle mass than the boxers with 6 meals per day.

Unfortunately, this was only one study with trained people and boxers are actually more endurance athletes than muscle builders. It is always quite hypothetical to make recommendations based only on one study. That's why it would be great if more studies like this were conducted on trained people in the future.

Nevertheless, this finding is also consistent with the hypothesis found in almost every fitness textbook that by eating more meals per day, muscle protein synthesis is stimulated more often and thus more strongly overall. This protects against muscle loss during a diet (and of course also helps to build muscle).

Building Muscle

But what about when you want to build muscle? Until now, we have only talked about losing weight and maintaining muscle while losing weight. Unfortunately, for this third metric there are no relevant studies. In all studies, the participants were in a calorie deficit and/or did no strength training.

So we don't have any suitable studies, but we can get the opinions of the leading scientists in the field.

Alan Aragon and Dr. Layne Norton suggest that it is optimal for muscle building to eat at least 4 protein-rich meals, approximately equally distributed, throughout the day. Dr. Brad Schoenfeld and Menno Henselmans say that you should eat at least 3 meals.

So the consensus seems to be that at least 3-4 protein-rich meals per day are necessary for optimal muscle building.

The reason is probably again so that muscle protein synthesis is stimulated more often during the day. Indeed, there is probably a cap on how much protein can be used per meal for muscle building.

This does not mean that the body can metabolize, for example, only 30g of protein per meal. That is a myth. The body can metabolize 50g of protein or more in one meal. Nevertheless, in that case it is likely that more of the protein is converted into carbohydrates by the body and is not used for muscle building.

If this is true, it is likely that if you eat (for instance) only once a day and in this meal you consume 180g of protein, then your body will not be able to use nearly as much of this protein for muscle building as it would if you spread this 180g evenly over the course of the day across several meals.

Getting healthier

Losing weight, maintaining muscle, building muscle... but what about health? Many claim that intermittent fasting is healthy. In fact, there are several studies that suggest that intermittent fasting is healthy. But there are three big problems:

First, these are often animal studies. If intermittent fasting makes a rat healthier, it doesn't mean it will work the same for you.

Second, sometimes only the effect of intermittent fasting is studied and then this is not compared with the effect of a normal diet which produces the same level of weight loss. It is important to remember that many people who are overweight and lose weight will see an improvement in some significant health markers through dieting.

So if you improve your blood values through intermittent fasting, this does not have to be due to intermittent fasting, but can simply be due to weight loss. However, you can also lose weight - and thus see these same health impacts - with a standard diet.

The third problem is that the participants in many studies have specific health complications. For example, people with diabetes are often the main study participants. This then limits the extent to which these results can be transferred to people without diabetes.

So if someone who wants to convince you of the health benefits of intermittent fasting cites a study to prove their point, it's always best to look closely at the study to make sure it is truly relevant so you can know whether to trust their claims or not.

For example, the 2015 Literature Review by Seimon and others does not include animal studies, and the effects of intermittent fasting were also always compared to the effects of a normal diet on health. The result was that intermittent fasting had no health benefit over a normal diet.

However, this literature review is already three years old. This year, a completely new study by Sutton et al. was published.

The researchers wanted to find out if intermittent fasting WITHOUT weight loss could improve the health of people with diabetes. The result was: Yes, intermittent fasting made the study participants healthier WITHOUT weight loss. One of the ways this manifested itself was that insulin sensitivity increased and blood pressure decreased.

That's a pretty interesting finding - FOR PEOPLE WITH DIABETES! We don't know if this also works for other people, but it could be a good sign.