Intermittent fasting (IF) is a popular fat loss protocol. A few years ago, the mostly animal-based studies on IF were promising. More human research is now available, as well as more practical experience. How useful is IF actually and in particular for the natural bodybuilder?

INTRODUCTION

We look at IF from three angles:

• health;
• fat loss;
• muscle growth.

WHAT IF MEANS

IF is not a diet as is often thought. IF does not determine what you can and cannot eat, but only when you can and cannot eat. People use IF to lose weight, improve their health and simplify their lifestyle.

Fasting is certainly nothing new; our body is set up for fasting. In prehistoric times we regularly had a period in which little or no food could be eaten. Our fat reserves help the body through periods of scarcity.

IF METHODS

There are different ways to fast. The three main methods are:

• the 16/8 method: you fast for 16 hours a day and eat during the remaining 8 hours a day. This usually boils down to skipping breakfast, starting eating at 12:00 and not eating anything after 8:00 PM. This is the most popular form of IF.
• the alternate day method: fasting every other day;
• the 5/2 method : an eating pattern in which you eat your normal amount of calories 5 days a week and eat extremely little on 2 days (men 600 calories, women 500 calories per day). The big advantage of this method is that you eat little ‘only’ two days a week, so that your body is less inclined to go into low-power mode (cutting back on NEAT).

IF AND HEALTH

IF is primarily intended for weight loss, but the necessary health benefits are also attributed to it, to which IF owes part of its popularity.

Healthline lists the most important health benefits, based on scientific research:

• Insulin resistance: IF can reduce insulin resistance, lower blood sugar and decrease fasting insulin levels by 20-31%, which should protect against type 2 diabetes.
• Anti-inflammatory: IF reduces inflammation in the body through autophagy, the clearing of waste cells.
• Good for the heart: IF lowers the bad LDL cholesterol in your blood and ensures a healthier blood pressure.
• Good for the brain: IF increases the brain hormone BDNF and may promote the growth of new nerve cells. IF may also protect against Alzheimer’s disease.
• Longer lifespan, at least judging by rat studies. Only decades from now will we know if this also applies to humans.

There are two important caveats to these purported health benefits.

In the first place, the health benefits are often partly or entirely due to the realized fat loss, not specifically to fasting.

Second, there is still a lack of human research, as concluded in a 2015 review and noted by scientist and muscle growth specialist Brad Schoenfeld in a 2018 interview. And in a more recent interview with Men’s Health, Schoenfeld says:

Diets will help you control your cholesterol, diabetes and other conditions simply because weight loss will help you control those conditions. The purpose of a diet or eating pattern is to facilitate weight loss.

Schoenfeld’s colleague Menno Henselmans is (still) more skeptical about the health benefits of IF. He bases his argument partly on a recent study in which 139 people were followed for a year during a calorie-restricted diet: one group with a continuous diet, the other group with IF according to the 16/8 method. The outcome: There were no significant differences between the groups in changes in body composition or metabolic risk factors, including insulin sensitivity, at 6 or 12 months.

Henselmans about this:

A new study now adds to the growing evidence that intermittent fasting has no meaningful health benefits independent of calorie restriction.

Schoenfeld and Henselmans gain support from a new meta-analysis that concludes that fasting itself provides no health benefit; it is the energy deficit that makes the difference:

Compared to continuous energy restriction, intermittent energy restriction did not show significant long-term differences in anthropometric, cardiometabolic, inflammatory, or appetitive outcomes in the included studies.

Conclusion: more (human) long-term research is still needed to draw firm conclusions about IF and health.

IF AND FAT LOSS

Any health benefits are a bonus, but most people fast anyway because of the fat loss. After all, by not eating for part of the day you create a calorie deficit throughout the day, provided you don’t overstep the mark within the eating window. To this end, it is important that you use a calorie app, especially in the beginning.

The big question is whether IF is more effective for fat loss than a continuous food intake diet. Based on three scientific reviews, from 2015,  2016 and 2022, this is not the case: IF is no better than regular weight loss, but no worse either. Brad Schoenfeld on his 2022 review:

IF is a viable weight loss method, but not superior to other approaches. The mechanisms are consistent with the energy balance model; by limiting the daily eating window, it helps create a consistent calorie deficit that promotes a decrease in body mass over time.

Update 12/30/20: a new three-month study with more than 300 participants compared five popular dietary approaches to see which is most effective for fat loss. These included a 16-hour intermittent fasting diet, an 18-hour intermittent fasting diet, and a 5:2 alternate-day fasting diet with two very low-calorie days and five maintenance days. The result? After three months, there were no significant differences between the groups in their weight, waist or BMI changes. Coach Menno Henselmans says about this:

In general, the evidence at this point is quite compelling that the most effective diet for fat loss is the one that allows you to create a sustainable energy deficit. It doesn’t matter if you achieve that through ketosis, fasting, food restriction or IIFYM.

IF is therefore above all a way of losing weight, a personal choice based on advantages and disadvantages.

ADVANTAGES

Perhaps the most important advantage of IF is psychological: you don’t have to worry about eating and drinking for a longer period of time. For some people, that is more pleasant than being allowed to eat, but with major restrictions.

Brad Schoenfeld on that:

Fasting can help people control their appetite and food intake.

Another advantage is that you can eat whatever you want, provided you do it within the eating window and provided the total energy balance is negative. And preferably as healthy as possible. As a bodybuilder, make sure you meet your protein quota: eat about 2 grams of protein per kilogram of body weight per day.

Furthermore, IF is easy for some to fit into their lifestyle. For example, if you don’t take breakfast, you already have a large fasting window, including the night’s sleep. IF is therefore not so heavy. This is also an advantage of IF that emerged from a meta-analysis, as noted by Manno Hensemans:

IF can make energy restriction easier for people who aren’t naturally hungry in the mornings. In this case, delaying the first meal of the day usually leads to a reduced appetite during the day and lower energy intake.

CONS

However, IF is by no means suitable for everyone: it also has disadvantages.

In the first place, it takes a lot of perseverance to eat nothing at all for a long time. We already said that ideally you use it in a lifestyle where you are already used to fasting for an extended period of time. But you’ll still get hungry. Anecdotal evidence does teach: the longer you persevere, the less the feeling of hunger will become.

Sometimes IF leads to compensatory behaviors, with unhealthy food choices and excessive calorie intake during the eating window. The trick is to control yourself during the eating window, which does not alter the fact that you can finally enjoy eating again.

In addition, IF is difficult in social situations where you are expected to eat along. Even if it only has to be a matter of moving your dining window for once.

Certainly in the beginning of IF there can also be fatigue. This also means that you plan your training(s) during eating periods as much as possible.

IF is not suitable for everyone, especially: people with type 1 and 2 diabetes, pregnancy, underweight, people under the age of 18 and people who have had an eating disorder in the past.

CONCLUSION

All in all, we can say that IF can be a useful strategy for fat loss, but not for everyone. It just has to fit your personality and lifestyle. Menno Henselmans:

If fasting fits into your lifestyle, it’s a useful way to reduce your energy intake. If not, that’s fine too: you won’t miss out on any health gains or fat loss.

And also:

IF is a useful diet aid for some, but certainly not a panacea for everyone.

And finally Brad Schoenfeld:

If IF fits your lifestyle, consider using it; if not, choose another option. Weight loss can be achieved in several ways, provided that the diet produces a negative energy balance over time.

IF AND MUSCLE GROWTH

So far we have looked at benefits of IF specifically for health and for fat loss. But what about the effect on muscle building, after all the main goal of the average gym-goer? Unfortunately, not much is known about this and the existing literature shows contradictions.

You can say very generally: food is anabolic (building muscle), not eating is catabolic (muscle breaking down). That is why you can build muscle during a bulk (calorie surplus) and you have to watch out that you do not lose muscle mass during a cut (calorie deficit).

Research shows that you need enough protein for muscle growth or maintenance, about 2 grams per kilogram of body weight per day. While the daily total is critical, it’s best to spread your protein intake evenly throughout the day in portions of 20-40 grams, every three to four hours. Unlike carbohydrates and fats, proteins cannot be stored and kept until the times when they are most needed. More about that in this article.

Seen in this way, IF does not seem ideal for building or maintaining your muscle mass. But it also doesn’t mean that fasting leads to muscle breakdown. After all, we all ‘fast’ seven to eight hours a day during the night’s sleep and are nevertheless capable of building muscle mass. A meta-analysis of eight studies therefore indicates that IF strategies lead to fat loss, without negatively influencing lean body mass. Coach and author Eric Texler concludes:

Timed feeding with an eight-hour feeding window is fine for supporting strength gains and hypertrophy, as long as protein intake is high enough.

Within eight hours you manage to consume at least three large portions of protein with sufficient time in between. So do not fast more intensively than 16/8 and eat about 2 grams of protein per kilogram of body weight daily.

However, another meta-analysis, based on nine studies, concludes that IF may be more easily at the expense of lean mass than continuous dieting: contrary to previous systematic reviews, this study suggested that lean body mass is better preserved with continuous dieting compared to intermittent fasting.

But doesn’t fasting increase growth hormone? Yes, but more human growth hormone does not automatically mean more muscle growth .

Brad Schoenfeld is clear in his opinion of IF in relation to muscle growth and maintenance. In the  interview with Men’s Health he says:

Intermittent fasting can be effective with very few meals from a fat loss standpoint, but it’s not a great strategy from a muscle building standpoint. (…) In any case, fasting does not benefit strength or muscle growth.

Our vision? If you’re bulking, IF probably won’t make much sense. But if you like fasting, don’t go more intense than 16/8 (otherwise you’ll have to eat way too much in a short time frame) and plan your training exactly between two protein-rich meals.

If you’re cutting, IF can be a helpful tool in our experience, especially at fat loss plateaus. Use 16/8 and plan your training exactly between two protein-rich meals. Preferably do not go more intensive than 16/8, so that you can continue to spread your proteins somewhat. Training on an empty stomach is not allowed.

Example :

4:00 pm meal 1
6:00 pm-7:00 pm workout
8:00 pm meal 2
11:00 pm meal 3 (protein powder)

Would you rather apply the 5/2 method? Then make it a Protein Sparing Modified Fast (PSMF): during the fasting days you eat almost no carbohydrates and fats, but you do eat a portion of proteins of 20-40 grams every three to four hours. Protein powders are ideal for that.

CONCLUSION

Intermittent fasting is a popular way to lose weight. This is partly because IF offers health benefits. However, they are by no means all convincingly scientifically substantiated.

Furthermore, it is unlikely that you will lose fat faster with IF than with a ‘normal’ diet at the same number of calories. It is also the other way around.

For muscle growth and strength gains, IF offers no specific benefits. However, the 16/8 protol may be an aid during a heavy cut.

The most important thing when losing weight while maintaining muscle mass is:

• create a medium calorie deficit (20-25% of your maintenance level);
• eat enough protein (~2 g/kg/d);
• spread those proteins as much as possible over the day (in ‘shots’ of 20-40 g);
• do not train on an empty stomach.

FINALLY

The best diet or eating pattern is the one that you can maintain. Everyone has different preferences. If you can do without food for a long time, give IF a chance and see if it suits you.

Originally published May 27, 2022, revised and updated December 30, 2022.