In today's blog post, Jack, who holds a Master's degree in Human Nutrition and is currently undertaking a PhD at the University of Auckland from the school of biological sciences, will be discussing the delicate science of energy expenditure.
Image credit: https://tom-draws.com/
A physiological balancing act
Firstly, it is imperative to understand that changes in body weight simply cannot occur without an underlying perturbation to energy balance. This statement is fundamentally true whether an individual’s goal is to lose weight or gain weight. Energy balance is only present when equilibrium is found between energy intake (caloric input) and energy expenditure (caloric output). However, due to the inevitable day-to-day variability in both energy input and output, it is important to note that meaningful weight change only typically manifests when an energy surplus or deficit is sustained for several days or weeks. Specifically, when one’s total energy intake continually exceeds one’s total energy expenditure, an individual is in positive energy balance and will thus gain weight. Inversely, when an individual’s energy expenditure proceeds to exceed their energy intake, they are in negative energy balance and will subsequently lose weight. This delicate process is in line with the unequivocal first law of thermodynamics delineated by the most festive-sounding scientist of all time, Rudolf Clausius, in 1850.
A Christmas calor
The million-calorie question
Even Simone Biles would be beaming in awe to learn of the balancing capabilities of the human body’s homeostatic control. An individual may freely consume and expend 1 million kilocalories in any given year, yet the vast majority of individuals do not experience drastic annual changes in body weight. This exemplifies the body’s remarkable regulation of energy homeostasis. However, despite the impressiveness of this balancing act, it is now recognised that clinically significant weight gain can emerge with continued energy surpluses of just 50-100 kcal/day. Moreover, today’s society is superfluously besieged with highly palatable, energy-dense foods that are making it harder and harder for our bodies to effectively regulate chronic energy balance. This obesogenic environment has evolved over such a relatively short period of time which has left our bodies no time to adapt to keep up! Consequently, the average adult is gaining an additional 0.3-0.8 kg/year, and over many years it is these small expansions of body fat stores that are responsible for the burgeoning global prevalence of individuals with overweight and obesity that we see today.
In order to lessen the pernicious consequences of obesity on both a personal and global level, a widespread focus has aptly surrounded the restriction of energy intake for weight management. However, a far smaller emphasis has been placed on the enhancement of energy expenditure for the maintenance of energy balance (especially outside the bounds of exercise), despite an equally important contribution. The remainder of this article will discuss the breakdown of total energy expenditure and touch upon how we may be able to target the individual components to optimise our energy output.
Turning up the heat
Total daily energy expenditure can ultimately be divided into three overarching components: basal metabolic rate (BMR), thermic effect of food and physical activity. Physical activity can also be broken down into further sub-components; namely exercise activity thermogenesis, non-exercise activity thermogenesis (NEAT) and spontaneous physical activity (SPA) (see figure below).
From Chapter 6 ‘Energy Balance and Body Weight Regulation (Dulloo & Schutz, 2017) in Human Nutrition
Basal Metabolic Rate
The lion’s share of total energy expenditure is attributed to BMR, which typically accounts for around 60-75% of daily caloric output. Hence, for a daily energy expenditure of 2500 kcal, BMR would explain approximately 1500-1875 kcal/day.
But what is BMR? BMR is the amount of energy (kcal) that an individual expends to maintain the functioning of fundamental bodily processes in a rested state. This includes breathing, osmoregulation, blood circulation, continued brain and nervous system function, the regulation of body temperature etc. Although the heart and kidneys are the most metabolically active tissues, the liver is the greatest contributor (Not to scale) to basal metabolic rate (~27%), followed by the brain (19%) and skeletal muscle (18%). Skeletal muscle is actually approximately 35 times less active than cardiac and renal tissue, but its sheer mass and marked inter-individual variability makes this tissue critical for the determination of resting energy expenditure. Accordingly, fat-free mass (FFM) is widely recognised as the strongest predictor of BMR and hence commonly utilised in estimative equations of resting metabolic rate.
Can we enhance our basal metabolic rate and if so, how? The short answer to this first question is yes, however, the key part remains in whether we can increase our resting energy expenditure to an extent that possesses meaningful implications for weight management. Targeting tissue-specific contributions to energy expenditure reveal some clear snags…
“This is not what I meant when I said I needed a detox!”
Hence, research into this area has rightly been dominated by studies investigating whether tangible increases in muscle mass can engender important elevations in BMR. Research indicates that for every kilogram of skeletal muscle an individual gains, BMR will be increased by approximately 13 kcal/day. Clearly, this is not a substantial impact and suggests that considerable muscular gain is needed in order to induce clinically significant changes in BMR.
Nonetheless, the health importance of muscle for the regulation of numerous processes in the body should not be overlooked. Moreover, it should be remembered that any elevation in BMR from skeletal muscle mass gain is in addition to any calories expended from the process of strength training etc. Muscle mass maintenance is also essential during periods of caloric restriction. It is known that weight regain, which occurs in ~80% of individuals after an initial weight loss, is exacerbated by a compensatory reduction in BMR which is driven by underlying reductions in skeletal muscle mass. Hence, resistance training must form a key component of any weight loss programme to offset the body’s evolutionary resistance to fat loss.
In summary, muscle is the best target we have for altering our basal metabolism and even if changes to energy expenditure are minimal, the accretion of muscle protein can only do good for our bodies. Moreover, with the delicacy of energy homeostasis described, for an individual on the border of energy balance, a few kilograms of muscle could be the difference between chronic weight gain and chronic weight maintenance.
Thermic effect of food
Do you often find yourself having to take your jumper off or wipe the sweat away from your forehead at the dinner table? In addition to any direct heat transfer coming from steaming hot dumplings or sizzling lamb bhunas, this additional heat production you may experience is derived from the energy costs of digesting and absorbing the main macronutrients that make up a meal (carbohydrate, fat, protein and alcohol!). This increase in energy expenditure consecutive to nutrient consumption (aka thermic effect of food or diet-induced/postprandial thermogenesis) explains 5-15% of an individual’s total daily energy expenditure. Hence, for our reference 2500 kcal/day, would translate to a clinically meaningful range of 125 – 375 kcal/day.
It is now widely known in the literature that, as well as being highly satiating, protein is also the most thermogenic of all the macronutrients. The costs associated with breaking down and storing protein equate to approximately 25% of the ingested energy load. Whilst the thermogenic response to fat and carbohydrate is only around 2-3% and 5-8%, respectively. Hence, for every 100 kcal of protein consumed, approximately 25 kcal would be expended as a consequence of the thermic effect of food.
Moreover, with an abundance of highly palatable foods now available at our fingertips, there has been a huge shift towards food habits resemblant of grazing whereby most people practically spending the entire day in a postprandial state.
Recently, targeting foods of greater thermogenic potential has gained renewed interest in the literature. In addition to high-protein products or meals, various ‘functional foods’ contain a high content of bioactive ingredients which may possess the ability to maximise postprandial energy expenditure. By targeting postprandial thermogenesis with these ‘functional foods’, we may be able to maximise the energy expended subsequent to eating and thus observe meaningful increases in overall daily energy expenditure.
Functional foods, also known as nutraceuticals, possess bioactive ingredients that may provide health benefits and help to minimise the risk of noncommunicable diseases such as diabetes. Some of these bioactive compounds and the mechanisms by which they may enhance thermogenic and fat-oxidising capacity have been discussed in a really exciting paper by researchers in Switzerland (Dulloo, 2011). This appraisal pinpointed certain food products and components which have generated particular allure in the literature, these include: coffee or other products containing caffeine, teas or other products rich in catechins and polyphenols, spices composing capsaicinoids/capsinoids (e.g. chilli, red pepper, ginger), medium-chain triglycerides (MCTs) and polyunsaturated fatty acids (PUFAs). Some interesting research has provided some promise in this area, however, robust clinical studies remain to be conducted to assess whether this can be achieved to a clinically significant degree.
Joe really didn’t understand when I said he should increase the heat output of his food.
When people think of burning energy, they usually associate it with physical activity, or more loosely, exercise. As we have seen in this article, physical activity is only one of three overarching components of total energy expenditure. Physical activity typically explains 10-30% of daily energy expenditure, yet this estimate is slowly retreating to the lower end of this range in industrialised societies. Moreover, volitional exercise (what we might typically think of when we think of physical activity e.g. running, cycling, playing sports) explains just one of two primary subcomponents of physical activity, alongside non-exercise activity thermogenesis (NEAT). NEAT encompasses any activity that results in energy being burnt at a higher rate than being flat out on the sofa watching Netflix. This includes any additional energy expended during occupational activities, leisure activities (non-exercise) or even spontaneous physical activity (SPA), such as fidgeting.
Rebecca insisted that she was working on her resting metabolic rate
Most commonly, we attempt to modulate the amount of energy we expend on a daily basis by undertaking volitional exercise (i.e. going for a run or playing basketball). Yet, quantitatively, the amount of energy an individual expends as part of their working day (NEAT) may actually be more significant still and is often overlooked. Through conscious efforts to keep moving during the day, one may be able to produce meaningful additions to their daily energy expenditure. Hence, if you’re trying to be mindful of your body weight, there is definitely no harm in opting to take the stairs up to your office instead of hopping in the lift. Likewise, getting out for a walk whilst you’re on a meeting call is a NEAT way to boost your energy expenditure!
Thanks, Jack! Don't forget to check out other interesting reads here
Dulloo, A. G. (2011). The search for compounds that stimulate thermogenesis in obesity management: from pharmaceuticals to functional food ingredients. Obesity reviews, 12(10), 866-883.
Dulloo, A. G., & Schutz, Y. (2005). Energy balance and body weight regulation. Human Nutrition, 83.
Mozaffarian, D., Hao, T., Rimm, E.B., Willett, W.C. and Hu, F.B. (2011). Changes in diet and lifestyle and long-term weight gain in women and men. New England Journal of Medicine, 364(25), 2392-2404.
Zhai, F., Wang, H., Wang, Z., Popkin, B.M. and Chen, C. (2008). Closing the energy gap to prevent weight gain in China. obesity reviews, 9, 107-112.