How does the body adapt to changes in energy intake?

When considering either a muscle building or fat loss diet one of the main considerations is to estimate the right amount of calories we need to consume to reach our goals. In order to calculate the amount of calories we need to form some form of starting point. There are a few different techniques and formulas that can be used to estimate our calorie intake. These include the use of food diaries and food logging aps alongside monitoring of bodyweight and visual changes in body composition. There are also numerous different formulas that can estimate calorie requirements that use body mass, lean body mass and levels of activity amongst other measures.


All of these factors can influence the main outcome we are interested in which is our total daily energy expenditure (TDEE). TDEE consists of a few different components some of which we can influence through training, diet and general lifestyle considerations. However, other things to a certain extent are out of our control. Awareness of these things and adjustment to the areas we can control can help us make the most out of our muscle building or fat loss programs.


TDEE is made up of our basal metabolic rate (BMR), non-exercise activity thermogenesis (NEAT), thermic effect of food (TEF) and exercise activity thermogenesis (EAT). NEAT, TEF and EAT can be grouped into what is known as our non-resting energy expenditure whereas BMR makes up our resting energy expenditure.


BMR is basically the amount of calories we would require to maintain our current mass if we were completely at rest and accounts for around 70% of our TDEE. BMR can be impacted by dieting because the process of being in a consistent calorie deficit, influences hormones like leptin and thyroid hormones which are direct regulators of metabolic rate. The impact of dieting on BMR is largely out of our control, however there are some strategies we can use to minimise dieting’s effects on BMR. These include preserving lean muscle tissue, a key component of BMR, by having a higher protein diet and the use of resistance training.


The use of periodic higher calorie days or weeks during a diet might also have some limited benefits in restoring BMR by helping to recover leptin levels and improve thyroid output. That said long term reductions in calorie intake will reduce BMR which will only be recovered after calories have been reintroduced back into the diet for longer periods. This is one of the reasons why after fat loss diets (especially rapid losses in body fat found with ‘crash diets’ that are often accompanied with muscle loss) a return straight back to pre-diet calorie intakes can lead to rapid increases in body fat if not accounted for. It is for this reason it is suggested that a rapid drop in calorie intake may have a more damaging effect on BMR, so longer more gradual periods of dieting create a better environment to maintain BMR.


TEF is basically the heat loss that is accompanied with digestive processes of certain foods; it was thought that for this reason increased meal frequency might influence fat loss; however TEF is related to the size of the meal so smaller more frequent meals have a comparable TEF to larger less frequent ones. Protein does however have the highest TEF of the macronutrients, again showing the benefits of a high protein diet in helping to illicit fat loss and maintain TDEE. TEF typically accounts for around 10% of TDEE and although not large compared to BMR, this is still a sizeable chunk of non-resting energy expenditure (about 33%) with non-resting energy expenditure accounting for around 30% of TDEE.


EAT is basically our exercise activity and accounts for around 5% of TDEE in most people. This is a key component which we can influence in regards to increasing our output to maintain or increase TDEE even when BMR might be reduced. Training regularly and with high energy demand sessions is a great way to boost TDEE. It is for this reason that those who are hard trainers can typically get away with eating more food than those who do not and maintain lower levels of body fat. This is important when dieting as the natural thing to do when energy intake becomes lower is to reduce training output. It is during dieting periods where a strong mentality during training can go a long way to accelerating fat loss as reductions in calorie intake then have less of an impact on expenditure.





Strength may drop in some people, but focussing on maintaining consistent training and progressing output instead of reducing it becomes increasingly important. As BMR becomes lower and fat loss becomes more difficult it fundamentally leaves two options, either increase output or reduce food intake. Increasing EAT by adding in extra sessions or more volume in a session is a way to maintain calorie intake from food whilst breaking through fat loss plateaus. For those looking to gain weight that have a naturally fast metabolism it might mean that assessing training volume and increasing the amount of rest can be a way to reduce general energy output and allow adequate recovery to promote muscle growth.


NEAT is really what’s left over in terms of energy output and includes things like walking, working, sitting, general moving about and fidgeting. Studies have shown that in those who are on a calorie restricted diet that they suffer a reduction in NEAT. Some of this can be obvious, such as generally being lazier and avoiding tasks as far as possible (my housework definitely gets neglected when dieting) and things like using transport when under normal circumstances you would walk. There are also more subtle changes such as a reduction in fidgeting, which we have limited conscious control over.


NEAT contributes to around 15% of TDEE and 50% of our non-resting energy expenditure. Being aware of these things can help maintain fat loss in a really simple and effective way by avoiding the laziness that we can suffer when energy intake is relatively low to demand. Again for those looking to get to low levels of body fat, fighting the urge to avoid normal daily activities can make a big difference. Even increasing NEAT for those who have stagnated in their fat loss can be a useful tool, taking the stairs instead of lifts, walking/cycling instead of driving and making a habit of going for walks or choosing weekend activities that are not sedentary in nature all add up. This is also important for those looking to gain weight as awareness of how much NEAT we perform on a daily basis will influence the amount of calories we require (this is not an excuse to not do the hoovering or walk the dog!).


In summary TDEE can be influenced by some things that are within our control as long as we make a conscious effort to control them. BMR is to a certain extent out of our control but the impacts of dieting on BMR can be reduced by employing sensible dieting strategies, maintaining food intake as high as possible and the inclusion of resistance training and a high protein diet.