Understanding metabolism and how to influence it


Understanding metabolism is key to managing your energy, weight, health and performance. This article provides a brief overview of what metabolism is, five facts on metabolism and five tips to optimise yours!

What is metabolism?

Metabolism is essential for life. It is a series of chemical reactions that takes place in every single cell of the body that uses the sugar (glucose) from food consumption alongside the oxygen from the air that you breath in to generate energy (adenosine triphosphate ATP). Metabolism takes place in every single living cell in the body as energy is necessary for every single process that takes place in life.

What is metabolic rate?

Metabolic rate is the speed at which energy is generated. Different parts of the body are more or less metabolically active and therefore have different metabolic rates – for example, the metabolic rate of the heart and kidneys is highest, followed by the brain and then the liver (Wang et al. 2010). Collectively the brain, liver, heart and kidneys account for approximately 60-70% of Resting Energy Expenditure (REE) (the amount of energy required for you to be alive) in adults, even though their combined weight is <6% of total body weight! Skeletal muscle comprises 40-50% of total body weight and accounts for approx. 20-30% of resting energy expenditure (Javed et al. 2010).

Interestingly, small variations in the mass of high metabolic rate organs may significantly affect resting energy expenditure and reduce the role of age, race, and sex in explaining resting energy expenditure (Javed et al. 2010). Additionally, decreases in REE with increasing age may be partly due to age-associated changes in the relative size of fat free mass compartments.

It is important to note that metabolic rate is dynamic!

Components of metabolism: total daily energy expenditure (TDEE)

TDEE can be divided into resting energy expenditure (REE) and non-resting energy expenditure (NREE). REE is also known as basal metabolic rate (BMR) and is the amount of energy required for you to exist. NREE can be split into multiple components: the energy expended for non-exercise movement such as normal daily activities and fidgeting (non-exercise activity thermogenesis (NEAT); the energy expended for the process of ingesting, absorbing and metabolising nutrients from the food that you eat (thermic effect of food (TEF); the energy expended during formal exercise (exercise activity thermogenesis (EAT).

Whilst NEAT usually accounts for approximately 8-15% of TDEE, there can be huge variability in NEAT levels. It is indicated that there could be a potential difference of 1000 calories expended for same weight individuals due to NEAT (Levine et al. 2006). NEAT can be anything from how you move when you’re at work, to tapping your toes, to walking up and down the stairs. A further subdivision of NEAT may be termed Spontaneous Physical Activity (SPA) – so more of the fidgeting and the tapping. Whilst one determinant of NEAT is environmental/behavioural, research indicates that genetics also play a role in levels of NEAT (Levine et al. 2006).

Exercise Activity Thermogenesis (EAT), makes a smaller than expected contribution to TDEE, since the average individual only exercises for approximately 2 hours per week and TEF makes an approximate contribution of 10% to TDEE.

What factors affect your Basal Metabolic Rate (BMR)?

Age: there may be a moderate decline in BMR as an individual ages due to loss of muscular tissue. Recent research indicates that a person’s metabolism slows by about 3% each year until you are in your 20s, when it levels off into a new normal. This data suggests that metabolisms do not really begin to decline until after the age of 60 and the slowdown is gradual with a 0.7 % per year. Therefore, a person in their 90s may need 26% fewer calories than someone in their midlife. This research also indicates that the reduction in age-related changes in body composition is due to cellular changes (Pontzer et al. 2021).

Body size: in general, a larger body has a greater BMR due to larger organs and more a greater proportion of metabolically active tissue.

Body composition: fat free mass is the dominant factor affecting BMR. Research indicates that each kilogram of lean tissue exerts approximately five times more effect on BMR than fat tissue (Johnston et al. 2005). This really flies the flag for strength training and building or at the very least maintaining muscle mass.

Drugs: for example, caffeine and nicotine.

Ethnicity: BMR varies according to racial group.

Environmental conditions: exposure to cold causes an increase in BMR in order to create the extra need for heat. Prolonged exposure to increased temperatures results in an increase in BMR.

Gender: in general men have a faster metabolism than women due to body size, and a greater proportion of muscle mass and lower proportion of fat mass.

Genetics: your genes influence your metabolism, meaning that some individuals have a faster metabolism than others.

Hormones: an underproduction of thyroid hormones, sex hormones, growth hormone, stress hormones etc can negatively influence your BMR.

Nutrient Insufficiencies: Certain nutrient insufficiencies may play a role in reduced hormone production and cellular energy metabolism.

Gut Microbiome and gastrointestinal imbalances: Emerging research demonstrates the influence that the gut microbiome and other gastrointestinal imbalances may indirectly have on metabolic function.

What other factors affect how much energy you burn?

BMR may be temporarily altered by short term factors and NREE dynamically responds to energy balance and lifestyle factors.

Short term factors

Short term factors such as high levels of stress hormones, illnesses, changes in environmental temperature and being in a prolonged fasted or starvation state will temporarily alter BMR.

Adaptions to metabolism during weight loss / gain

During weight loss, TDEE consistently decreases due to a combination of factors: a drop in BMR due to loss of metabolically active tissue and reduced NREE – due to reduced EAT and NEAT (Trexler et al. 2014). Similarly, in response to overfeeding, TDEE has been shown to increase, even under conditions where individuals are unable to formally exercise, levels of NEAT have been shown to account for approximately 60% of TDEE (Levine et al. 1999).

Some research indicates the presence of adaptive thermogenesis during underfeeding, that is a decline in TDEE exceeding what is expected by the loss of body mass (Trexler et al. 2014).

Five facts about metabolism

The largest component of the energy you expend is at rest

The amount of energy you expend at rest, your BMR is approximately 60-80% of your TDEE. Therefore, the amount of energy you expend during physical activity is considerably less – approx. 10-30% which is why exercise sometimes may be regarded as delivering disappointing results during weight loss journeys.

The amount your metabolism slows down during your adult years is negligible

Many people use age as a reason for their “slowing” metabolism, especially when they become middle aged. In reality the biggest drop in metabolism is after you’re an infant! When you get to your 20s it actually appears to stabilise until you then reach the age of 60, when it begins slowing down a little more (approx. 0.7% per year).

Your metabolism is dynamic

Which means that it responds to energy balance. In general, when individuals are overfed, they tend to expend more energy, and when they are underfed, they conserve more energy. This takes place through a variety of factors: the energy you expend through digesting and absorbing food, the energy you expend through exercise, the energy you expend through NEAT and a change in overall body size. NEAT is highly variable depending on the individual and these differences may occur due to both environmental and genetic reasons.

The best way to increase your metabolic rate is through building lean muscle

Muscle mass may contribute approximately 20% to TDEE and is one of the few factors that you can influence. Although your organs are for more metabolically active, you cannot adjust these, and physical activity accounts for a much lower proportion of TDEE than imagined, approx. 5-10%.

Your metabolism will not stop you from losing weight

Many people use their metabolism as a reason for not being able to lose weight and / or sometimes gain weight. However, the reason is always the energy balance (unless there is some rare case of a genetic defect). There is a degree of variation for individual metabolic rate and therefore you do have to base your energy deficit or energy increase on you as an individual.

It is also worth noticing that your metabolism will adapt as you lose or gain weight and therefore your energy needs will need to change. The number of calories you require at a higher body weight will be proportionately higher than the number of calories you require at a lower body weight.

Top tips to optimise your metabolism

Eat more protein

Protein is the macronutrient with the highest thermic effect. It will also contribute to building lean muscle to increase your BMR.

Include resistance training

The act of lifting heavy weights, whether that be a barbell, dumbbells or your body is energy requiring. When done regularly, muscle protein breakdown and muscle protein synthesis take place, an energy requiring process which subsequently results in the development of lean muscle mass. An increase in lean muscle mass increases your metabolic rate.

Eat adequate calories

Inadequate calories to meet your energy balance will likely result in a decrease in exercise energy expenditure and NEAT, resulting in an overall decrease in energy expenditure. Whilst a reduction in calories may result in reduced weight, managing this calorie deficit appropriately and methodically can assist with mitigating adaptive thermogenesis – a process that takes place when metabolic rate adjusts at a greater amount than predicted from body mass loss.

Take movement snacks

Planned movement snacks is an intentional way of increasing NEAT which may be the greatest predictor of weight gain or loss. A simple set of press-ups, squats, lunges or star jumps every hour keeps you on your toes and your metabolism up.


Including sprints, whether that be sprinting on foot or on the bike is a great way to increase resting energy expenditure. This is because sprints increase your oxygen debt, meaning that your body must work harder even at rest post sprints to recover.

Key take aways:

Metabolism is defined as: a series of chemical reactions that takes place in every single cell of the body in order to generate energy.

Resting energy expenditure, also known as basal metabolic rate can be impacted by a variety of factors: including age, gender, race, body size, body composition and genetics.

Metabolism is a dynamic process: that responds and adapts to energy balance.

You can influence your metabolism by: changing your body weight, body composition, the food that you eat and the amount and type of activity that you do.

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  1. Trexler et al. (2014). Metabolic adaptation to weight loss: implications for the athlete.
  2. https://jissn.biomedcentral.com/articles/10.1186/1550-2783-11-7
  3. Wang et al. (2010). Specific metabolic rates of major organs and tissues across adulthood: evaluation by mechanistic model of resting energy expenditure.
  4. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2980962/
  5. Javed et al. (2010). Brain and high metabolic organ rate mass: contributions to resting energy expenditure beyond fat-free mass.
  6. https://academic.oup.com/ajcn/article/91/4/907/4597328
  7. Levine et al. (1999). Role of non-exercise activity thermogenesis in resistance to fat gain in humans.
  8. https://pubmed.ncbi.nlm.nih.gov/9880251/
  9. Levine et al. (2006). Non-exercise activity thermogenesis: the crouching tiger hidden dragon of societal weight gain.
  10. https://www.ahajournals.org/doi/full/10.1161/01.ATV.0000205848.83210.73
  11. Johnston et al. (2005). Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age and circulating thyroxine but not sex, circulating leptin or triiodythronine.
  12. https://academic.oup.com/ajcn/article/82/5/941/4607670?login=true
  13. Pontzer et al. (2021). Daily energy expenditure through the human life course.