Extended periods of dieting can lead to physiological, metabolic and psychological changes that may adversely impact the likelihood of continued weight loss at the desired rate and / or performance in athletes. This includes increased hunger and reduced adherence resulting in higher calorie intake, metabolic adaptation, increased rates of loss of muscle mass, low energy availability and associated health and performance impacts, and broader impacts on training, recovery and adaptation.
Instead of one long period of dieting, individuals can adopt an ‘intermittent energy restriction’ (IER) approach to dieting. Here periods of energy restriction (i.e. a ‘diet’) are interspersed with periods of days or weeks of ‘refeeds’ or ‘diet breaks’, where energy intake is increased to maintenance or a slight surplus (NOT a free for all excessive surplus!!). Many athletes have the belief that this supports more effective weight loss and maintained performance by mitigating some of the potential adverse effects of extended dieting. The question is, does it?
Most studies into IER have been performed in sedentary and overweight or obese individuals. The results of these studies cannot necessarily be extrapolated to athletes or the physically active, who are typically leaner and metabolically distinct. In this blog, we look at what we do and don’t know about athletes, dieting and continuous versus intermittent energy restriction. This is what I call a ‘mixed’ blog … it is not deep dive science, but there are key references included for the main points raised so that you can go and read more. And a final point: the only ‘athlete’ data we really have is in resistance training athletes … and these results cannot also necessarily be extrapolated to endurance athletes!
HUNGER, APPETITE AND DIET ADHERENCE
A large part of appetite is hormonally regulated, and two key hormones in this response are leptin, peptide YY, and ghrelin. Leptin reduces appetite, peptide YY promotes satiety and ghrelin increases appetite. During a diet leptin and peptide YY levels fall and ghrelin increases (Maestu et al 2008; Campbell et al 2020; Peos et al 2021). And the longer a diet continues, the greater the magnitude of these changes: in part as fat mass falls, a key place where leptin is made in the body. So as dieting continues, appetite can increase. This may result in reduced adherence to a diet … because the greater your appetite the harder it is to keep caloric intake down!
Consistent with this, in the recent ICECAP trial by Peos et al (2021a), dieters were also asked to report on their hunger, fullness, desire to eat and satisfaction around eating at the start and end of a diet. At the end of a continuous period of 12 weeks dieting these all moved unfavourably.
Leptin and ghrelin may not be the only factors responsible for such ‘diet fatigue’. Psychological factors driven by dieters avoiding foods they enjoy, watching others eat freely, and feeling ‘deprived’, together with the depressive impact of low energy intake on general mood state, may also play a role. Particularly if individuals are undertaking a particularly restrictive diet. Indeed, such a state (and post diet rebound excess calorie intake) has been reported in bodybuilders after long periods of restrictive dieting for a competition (Walberg-Rankin, Edmonds, & Gwazdauskas 1993).
Can diet breaks and refeeds restore leptin and ghrelin to their pre-diet levels and relieve some of the psychological stress? Possibly. The only study to look at this in detail in trained individuals is the recently published ICECAP trial by Peos et al (2021a). Here resistance (strength) trained individuals either dieted for 12 continuous weeks (CER group) or undertook 4 x 3 week diets broken by 1 week diet breaks (IER group). No difference was found in the fasted leptin and ghrelin levels between the different diets. However, peptide YY was preserved in the IER group and this group reported significantly higher levels of satisfaction and fullness, and reduced hunger and desire to eat at the end of the diet compared to the CER group. So, it may be that IER can help reduce diet fatigue and thereby increase dietary adherence over the long term. Of course, this must be balanced with the fact that an IER approach will take a longer actual amount of time for the same number of weeks dieting and this may be psychologically unfavourable for some individuals. In the ICECAP trial for example, the CER group reached the end of their total diet period after 12 weeks, whereas this lasted 15 weeks for the IER group (12 weeks + 3 x 1 week diet breaks).
METABOLIC ADAPTATION AND WEIGHT LOSS
In the section above we were talking about the drive and desire to eat calories, i.e. energy intake. Now we are going to look at calorie burn, i.e. energy expenditure. Our total energy expenditure is comprised of our:
Resting metabolic rate (RMR) (the energy we need to keep our organs functioning at rest, i.e. the energy needed to simply keep us alive)
Thermic effect of food (TEF) (the energy we need to digest our food)
Non exercise activity thermogenesis (NEAT) (the energy for movement that is not specifically exercise we are intentionally undertaking, e.g. fidgeting and wandering around the house)
Exercise activity thermogenesis (EAT) (the energy needed for the exercise we do intentionally, e.g. our training as an athlete)
During dieting and weight loss, typically all aspects of energy expenditure fall … and if energy expenditure falls, your calorie deficit reduces, and so your rate of weight loss slows. TEF falls because we inevitably need less energy to digest the lower amount of food we are eating. EAT falls because we are lighter and so need less energy to move our body in space. NEAT falls for the same reason, and it seems as part of the adaptive response to preserve energy … dieters seem to fidget less for example! And RMR also falls.
It is this fall in RMR that is particularly interesting, as it appears to fall to a greater extent than would be predicted by the amount of weight we have lost (Trexler, Smith-Ryan, & Norton 2014). In a recent study of a combat athlete during weight loss, RMR appeared to fall 6% more than predicted across 8 weeks on dieting where caloric intake matched the estimated RMR, and 13% more than predicted in a final week of dieting where caloric intake was just over half of estimated RMR (Langan-Evans et al 2020).
We call this ‘metabolic adaptation’ or ‘adaptive thermogenesis’ where our body appears to become more efficient at using calories to keep us alive and / or stops doing non-essential things to preserve energy when it is in short supply (because we are restricting calories) (Trexler, Smith-Ryan & Norton, 2014). This reduction in RMR may be driven, at least in part, by the disruption in thyroid hormone and the fall in leptin that is seen in dieting. These hormones help regulate metabolic rate and energy expenditure.
It has been theorised that diet breaks and refeeds may help restore hormone levels sufficiently during the diet breaks or refeeds to avoid this disproportionate fall in RMR. Does it in athletes or trained individuals? It does not currently seem so (much to many people’s surprise, as diet breaks are frequently cited as ‘metabolic boosts’!). Campbell et al (2020) compared CER and IER in strength trained individuals, and whilst they found only a significant decrease in RMR in the CER group, RMR was not significantly different to the IER group. And in the ICECAP trial there was no significant difference in RMR between the IER and CER group (Peos et al 2021a). And in neither study did total weight lost differ between the IER and CER groups.
LOSS OF FAT FREE MASS
During dieting, we don’t just lose fat. Muscle will be lost too. This is not ideal, particularly for athletes who are relying on their muscle mass for performance. Nutritional strategies such as high protein diets can help preserve muscle mass during dieting, as can resistance training (Helms, Aragon & Fitschen 2014).
The more aggressive the rate of weight loss and the leaner the athlete, the greater the proportion of weight loss that is likely to come from muscle. Reports indicate that as much as 1/3 of weight lost in lean bodybuilders preparing for competition may be muscle (Withers et al 1997; Van der Ploeg et al 2001; Garthe et al 2011; Helms, Aragon & Fitschen 2014). In males the fall in testosterone associated with dieting may be in part responsible for this (Maestu et al 2010; Rossow et al 2013).
Can diet breaks and refeeds help preserve muscle mass by providing periods where the body has sufficient energy and building blocks to repair and rebuild muscle, as well as potentially restoring hormone levels such as testosterone? TBC! In the ICECAP trial (Peos et al 2021a) there was no difference in the amount of fat free mass lost in the IER and CER groups. In the Campbell study (2020), dry fat free mass appeared to be preserved in the IER group and not in the CER group. More research is needed!
TRAINING, RECOVERY AND ADAPTATION
Dieting can have an adverse impact on training, recovery and adaptation. Unsurprisingly! If you are eating less energy than you are burning, you are going to have limited energy and building blocks to fuel training, and to refuel, recover and adapt in response to training. The greater the rate of weight loss, the greater these potential impacts (because the bigger the calorie deficit and therefore ‘lack’ of energy and resources) (Garthe et al 2011). And it can take time for performance to recover once weight loss stops (Rossow et al 2013).
Like the question of fat free mass, can diet breaks and refeeds help preserve performance in the short and long term, by providing periods where the body has sufficient energy to fuel training and energy and building blocks to repair and adapt in response to training? A subsequent paper from the ICECAP trial (Peos et al 2021b) suggests maybe. During the diet break period, muscular endurance significantly increased. Whether a short-term boost in training performance has significant long term impacts on strength, power and – ultimately – physiology and long term performance is not clear. However, it is certainly something that warrants further research. Any boost in performance would have to be weighed up against the fact a diet with refeeds or breaks will ultimately take more time and therefore may extend the dieting period further into crucial training cycles.
LOW ENERGY AVAILABILITY (LEA) AND RELATIVE ENERGY DEFICIENCY SYNDROME (RED-S)
This topic is in some ways closely linked to the fall in RMR, as it represents the basic functioning of the body. LEA is where an athlete consumes too few calories for their needs such that, after the calories burned in exercise have been taken away, they don’t have ‘enough’ calories (energy) left to support all the normal functions of the body. It is possible to lose weight without descending into LEA, however the more aggressive the rate of weight loss and / or the leaner physique you are trying to hit and maintain, the higher the risk of LEA. Evidence suggests that nutrient periodisation that balancing periods of dieting and low body mass (e.g. in competition season) with periods of higher energy intake and recovery may help avoid RED-S and maintain athlete health (Iraki et al 2019; Stellingwerff, Morton & Burke 2019). This is a little distinct to diet breaks and refeeds as it is looking at extended seasonal periods of altered energy intake. However, it would be valuable to explore whether diet breaks and refeeds during periods of dieting may further reduce the risk of LEA and RED-S (although the failure to positively restore RMR may indicate potentially not). This has not been directly addressed in research yet, as far as I am aware.
In conclusion … a very interesting topic! A lot of unanswered questions, and things we still need to learn. Based on the limited research we currently have in individuals who train, diet breaks do not have a clear benefit on body composition. However, diet breaks may aid adherence by reducing ‘diet fatigue’, and provide periods where training performance and / or load can be increased because of the increase in energy and macronutrient intake. And this may have a beneficial effect on body composition and performance long term. Time and more research will tell!
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