Introduction – What are Diet Breaks and Why Would we Use Them?
Depending on who you ask inside our industry people might tell you that diet breaks are the best thing since sliced bread for breaking weight loss plateaus OR…
That they are a total waste of time…
If you’ve followed my content for any period of time you’ll know that the answer probably falls somewhere in the middle, and any approach should be dependent on the nature of the client that you are working with.
Let’s talk a little history around the diet break to see why they came about in the first place.
The concept of the diet break was beginning to gain steam probably around 6 or 7 years ago as a tactic to include in a fat loss phase in order to mitigate some of the adaptations that inevitably occur when lowering calorie intake.
For those unfamiliar, there are many adaptations your body goes through as you’re losing weight, and one of those is downregulation in energy expenditure beyond what would be expected from having a lower body mass, so the thought was like “hey, maybe if we take things slower, include time periods where we take dieting stress off the body and include more food, there won’t be as many negative adaptations”.
The idea has been tested in scientific literature numerous times in the past 6-7 years; for example, we have the famous MATADOR study that showed both more fat loss and less of a decrease in resting energy expenditure in the group that took diet breaks.
However, we also have conflicting data, and the study we’ll be talking about today is one of those pieces of conflicting data that didn’t necessarily show any positive effects to taking diet breaks.
This particular study had some names you might recognize, as many of these study authors are also in the fitness space as science communicators – including Bill Campbell, Eric Trexler, and Menno Henselmans.
Let’s go ahead and dive into the study, and we’ll have some commentary on the methods and why this study might have found different results than previous studies.
The Study
If you’re following along, I’m going to break down some specifics about the study which include some of it’s limitations. Reason being, there are many studies that people love to cherry-pick, and many times the results of a certain study cant be extrapolated to the general population as a whole because of certain limitations. These are important to know to understand why we might be seeing conflicting data in the literature.
Population
First, let’s look at the study population. Paying attention to the population of a study is a very important factor for figuring out whether the findings of the study can be extrapolated to other populations, or for identifying a reason why previous studies found different results. For example, a study that finds efficacy for L-carnitine supplementation for weight loss in an obese cohort with type 2 diabetes absolutely cannot be extrapolated to people who are just looking to lose 20-30lb.
The study population used here were already decently lean, young, resistance-trained females. What does resistance trained mean? Well, to be included in the study, they had to have had at least 6 months of continuous, productive resistance training experience. These individuals also have built in resilience due to age, as well, which we have to take into account. The average age of the participants was 21 years old in the control group and 23 years old in the intervention group. The average bodyfat percentage for these women was around 24% as well, which would roughly translate to 14-15% for a male.
All of these aspects are very different from 90% of the diet break or intermittent energy restriction literature we have so far. Most are done in 40-60 year old obese men or mixed gender cohorts that most likely don’t exercise; at least that wasn’t a requirement.
The Intervention
Prior to the intervention, the researchers used a technique that we’re probably all familiar with to get a good idea of their energy expenditure; they asked the participants to track their regular food habits and bodyweight over a period of 14 days. The average maintenance calories were around 1700-1800, which put the average diet calories around 1250-1400 depending on the participant. The participants also underwent RMR testing via indirect calorimetry before the start of the study, but also each week the study went on to assess any drops in RMR.
The participants were randomized to the intervention group or the control group. Both groups were going to be in a 25% energy deficit for 6 weeks total; however the intervention group took 1 week diet breaks back to maintenance after the 2nd and 4th weeks of restriction. In terms of diet composition, protein was set to 1.8 g/kg or 0.8g/lb, and then the rest of the calories were split 60% from carbohydrates and 40% from fat.
Basically, the control group was at a 25% deficit for 6 weeks, and the diet break group’s entire period was 8 weeks, but they were only in a deficit for 6.
The researchers also wrote and supervised their resistance training programs, which was a basic 3-day per week alternating upper-lower split. This is also very different from most diet break research out there, that doesn’t include resistance training at all.
What was Measured?
The participants were required to upload their myfitnesspal data to a spreadsheet for the researchers to view all throughout the study to check on compliance.
The participants filled out a 5-point questionnaire at the beginning of, and each week throughout the study to assess a few psychological aspects. These questions included:
- Hunger over the past seven days; fullness over the past seven days; desire to eat over the past seven days; ease of sticking to the diet for the past seven days; and motivation to diet for the week ahead.
The researchers did a good thing here since many of the studies done so far just assess the physiological aspects and not necessarily any psychological aspects.
As I mentioned, RMR was measured each week, body weights were tracked daily, and body fat percentage and FFM was determined before and after the intervention.
The Results
Both groups lost about 1.5% bodyfat. The diet break group did end up losing more body weight in general; 1.7kg compared to 0.7kg.
Now, if they both lost the same amount of bodyfat but the diet break group lost more weight, then you might think this came from fat free mass, and that’s correct. The diet break group lost, on average, 0.4kg of FFM while the continuous diet group gained 0.4kg of FFM. However, this wasn’t statistically significant, meaning it could have been due to chance. I’ll comment more on that in a bit, however.
In terms of resting metabolic rate, it didn’t change at all for either group throughout the study; it was around 1440 on average at the start, and remained at 1440 after. Which does make sense, since we know most of the reduction in energy expenditure comes from downregulation in NEAT whenever someone is dieting. There certainly can be reductions in RMR as seen in studies like the biggest loser study, but it’s mostly NEAT.
Most of the psychological aspects weren’t too different between groups. Hunger and desire to eat increased about the same between both groups, but scores for ease of sticking to the diet or motivation to diet for the week ahead weren’t too terribly different between groups.
However, disinhibition decreased over time for the diet break group and increased for the control group. This is basically a measure of impulse control, and the ability to inhibit yourself from eating, which was better in the diet break group, indicating a potential psychological benefit there. This is an important point.
Analysis – What’s the Deal Here? Should I Use Diet Breaks or Not?
Why might we see no real effects from this diet break study when others have shown results?
First off, there’s a large heterogeneity in these studies, meaning that not every study uses the same type of diet break nor study length nor % energy restriction. Some use alternate day fasting, the MATADOR study used 2 week diet breaks, etc.
As another example, this study was 6 weeks of energy restriction total with 2 weeks of diet breaks in the intervention group, while the MATADOR study was 16 weeks of energy restriction with a total of 14 weeks of diet breaks. Basically, in this study the whole weight loss phase for the trained females taking diet breaks was 8 weeks, and in the MATADOR study it was 30 weeks; significantly more drawn out. There was also a greater deficit – 33% – during the periods of restriction in the MATADOR. A larger degree of restriction has been associated with more BMR downregulation.
Not to mention the study population. The MATADOR was done in obese, sedentary men, whereas this study was in lean, resistance-trained women. What we can conclude then, is that in obese and sedentary men in a 33% deficit, taking longer diet breaks may stem some of the adaptations, whereas it doesn’t seem to have a terribly large effect on what I would deem a mini cut of 6 weeks in leaner, resistance-trained females.
There is really only one other study that was done in a resistance trained population, which was done by Jackson Peos et al in 2021. In that particular study, the deficit during the restriction periods was significantly larger (34%) and the study was longer (12 weeks). At the end of the study, both groups saw RMR decreases that were around the same, so again this showed no benefits to body composition or RMR between either group.
It’s important to note though that there were actual decreases in RMR in that study whereas this one didnt show any. It’s possible if the current study we’re addressing was carried out significantly longer, we might have drops in RMR, but it still probably wouldn’t be different between groups. Then again, we might not, since this study was a 25% deficit vs. a 34% deficit in the Peos study.
We have several more points to consider though.
This group was just in a 25% deficit at around 1250 calories, which yes, seems low, but that’s what it came out to be with more sophisticated measurement tools than we have access to as coaches. There’s about 4 or 5 previous studies, including the Peos study, that clearly show when energy restriction goes to 35-40%+ or protein isn’t kept at least at 0.8g/lb, there’s significant reductions in FFM and RMR.
In this study, the intervention group lost about 3.8lb in 6 weeks, so less than 1lb a week. A very modest rate of loss and much slower than many of our clients expect to go, but this is clearly a rate that doesn’t affect FFM loss or RMR in leaner women, at least. The FFM loss in the diet break group may have had something to do with how the resistance training was programmed, which I’ll touch on in a bit.
Next up, we have to consider starting bodyfat levels. While these women were lean, yes, they weren’t competitive physique athletes dieting down to absurdly low bodyfat levels. To this date, there isn’t a diet break study on competitive physique athletes who have to get down to unhealthy bodyfat levels, where we’d expect severe metabolic adaptation.
Women competing in physique sports typically have to go above a 40% relative deficit to reach this, and do way more activity. There are a lot of anecdotes in the space from folks saying they very much believe diet breaks preserved lean mass in these situations, so I’d be very curious to see an actual study done on physique athletes.
Why would the diet break group have lost more FFM? Well first, it could have been due to chance, but it could have also been due to how the resistance training program was designed. The researchers equated volume between groups throughout the entire study period, meaning the groups did the same volume, however the diet break group did it over 8 weeks whereas the control group did it over 6 weeks, resulting in a higher weekly resistance training volume for the control group.
As an example, the average weekly volume for the intervention group on hamstrings, quads, and biceps was 9 sets per week, whereas in the control group it was 12 sets per week. The researchers also did not control extra activity, meaning any one of the participants in the groups could have been doing extra cardio or have higher step counts.
Psychological Benefits May be Worth it
I think one of the main benefits we see here, that we see consistently across literature on the topic, are the psychological benefits. Sure, 7 out of 8 of the metrics around psychology weren’t different between groups, but disinhibition was, and the more inhibition you have, the better you can stick to a diet, and lower disinhibition has been shown to be much better for weight maintenance after a dieting phase in previous studies.
If you combine this and past research with multiple hundreds of anecdotes in the health and fitness space, we can see that diet breaks can be a huge boon for the psychological portion of dieting and allow people a lot more freedom, which has huge implications for successful weight maintenance and lifestyle change in non-fitness populations.
Bottom Line – Diet Breaks Should be Used on a Case-by-Case Basis
What can we really conclude about the study? We can conclude exactly what the study showed. In fairly lean, resistance-trained females seeking to lose fat, the use of diet breaks within the context of six weeks of a prescribed 25% reduction in energy intake does not improve the efficiency of fat loss and has no beneficial effect on FFM and RMR.
What this means is that we have to consider our client’s population, deficit length, and deficit severity. In relatively lean men and women who are resistance training with a modest deficit of 25%, its probably not going to impart physiological benefits, but if you know your client is going to need a psychological break for better adherence, this can be a huge boon.
If the deficit is larger, the individual is sedentary, and the weight loss phase is significantly longer, we might start to see physiological benefits in addition to psychological benefits.
In any case, I’m looking forward to when the body of literature grows significantly more, and we have studies on physique competitors and other populations who have been left out so far in order to really see where, and what type of diet breaks can be most effective.
References
Madelin Siedler et al. The Effects of Intermittent Diet Breaks during 25% Energy Restriction on Body Composition and Resting Metabolic Rate in Resistance-Trained Females: A Randomized Controlled Trial. Exercise Physiology and Sports Medicine. Journal of Human Kinetics volume 86, 2023, 117–132 DOI: 10.5114/jhk/159960
