When we talk about metabolism, the first thing that comes to mind, for many, is that you must have a fast metabolism to lose weight or the more overweight you are, the slower your metabolism must be.
But there’s a lot more to it - metabolism is a complex biochemical process which, in recent years has become somewhat skewed by various myths around its part in weight loss (spoiler alert: being overweight isn’t the result of a ‘slow’ metabolism and we will get to that).
In this article, Polly Smith and Tom Rothwell, members of the Nutrition driver at AXA Health, separate the fact from fiction.
Your metabolism is a set of life-sustaining chemical reactions that occur in the body such as breathing, digestion and growth and repair of tissues.
When we talk about metabolism in this article, we are referring to your basal metabolic rate (BMR). It is important to understand that your BMR does not have a speed. Your BMR is the number of calories needed to preserve the life-sustaining functions, when we are resting. BMR accounts for 70% of your body’s total daily energy requirements and energy expenditure. The other components of metabolism include thermic effect of food (TEF), exercise activity thermogenesis (EAT) and non-exercise thermogenesis (NEAT).
Energy balance is the balance between the energy we take in from food vs the energy we expend. If the energy we take in is larger than the energy that we expend, we will put on weight. If the energy we take in is less than the energy we expend, we will lose weight.
To understand metabolism fully we must also understand what our body expends energy on day to day. There are 4 different elements that make up our total daily energy expenditure.
The first is your BMR. Your BMR accounts for around 70% of your daily energy expenditure.
The second is the thermic effect of food, or TEF. TEF describes the calorific cost of digesting, absorbing and metabolising food. TEF accounts for around 10% of your daily energy expenditure.
The third component of our total daily energy expenditure is activity, which can be broken down into two categories: exercise activity thermogenesis (EAT) and non-exercise activity thermogenesis (NEAT). EAT is all the energy that we expend while performing intentional exercise sessions, such as going to the gym, running, cycling, swimming. EAT accounts for around 5% of our daily energy expenditure. NEAT on the other hand is the energy that we expend performing tasks involved in our daily life, such as tidying up, moving around the office or changing platforms on the commute to work, even fidgeting. NEAT accounts for around 15% of our daily energy expenditure, which is quite a lot more than the amount of energy we typically expend on exercise. For this reason, NEAT is useful tool to use in weight management.
Your BMR is dependent on your body weight (and what that weight is composed of, e.g. muscle and fat mass), age, gender and our genes.
The larger your body and your body weight, the higher your BMR will be. This is because a larger body requires a larger amount of energy than a smaller body does to carry out daily bodily processes. What your body weight is made up of will also have an impact on your metabolic rate. Muscle cells require more energy to maintain themselves than fat cells do, so having a higher percentage of your body weight as muscle will mean you have a higher metabolic rate.
Men tend to have a higher BMR than women and this comes down to women having a higher percentage of essential body fat than men. This is because at some point in their life a woman may need to nourish a developing foetus, and so they have higher fat stores in anticipation of pregnancy.
Our BMR is also impacted by our age. Naturally, as we age, we’ll experience a certain degree of age-related muscle loss, which results in losing tissue that has a high metabolic demand. In other words, less muscle results in lower metabolic rate. You can of course, minimise any reductions in your metabolic rate by taking part in resistance exercise, at least twice a week for adults.
Our genetics may play a role in determining your metabolic rate, however this hasn’t been established by researchers yet and we don’t have a clear enough picture to be able to determine how particular genes may or may not influence your metabolism. What we do know is that your genetics can influence fat storage and capacity to build muscle. So, whilst there is no clear direct relationship between our genetics and our BMR, there is a possible secondary link between the two.
There are two ways of answering this question. Put simply – no your metabolism does not affect your weight, in fact it’s the other way around. Your weight plays a large role in determining your basal metabolic rate. The larger your body weight, the larger your BMR will be, i.e. the larger the number of calories required to keep those life-sustaining processes going.
Contrary to popular belief, people with a higher body weight do not have a ‘slow’ metabolism, they will actually have a higher metabolic rate than someone carrying less weight.
There is however a more nuanced approach to answering this question. As we’ve previously discussed, your body weight and its composition have a large role to play in determining your basal metabolic rate. As you lose weight your BMR will reduce, this is called metabolic adaptation, or adaptive thermogenesis. Some diets draw people in by promising rapid loss of large amounts of weight, which is very enticing but not advisable from a metabolic standpoint. These diets can often have long lasting implications and can cause an individual to struggle to manage their weight in the long run.
Rapid and significant weight loss leads to significant reductions in BMR and have also been shown to be unsustainable, with around 80% of people regaining their weight in 1-5years1. This kind of weight loss can result in permanent down regulation of your BMR, i.e. a permanent lowering of your BMR. An example of this is shown in individuals who competed in the television show ‘The Biggest Loser’ which featured people losing on average 39% of their body weight over a 30-week period. Participants regained around 70% of the weight they lost over the 6 years following the competition. Their BMR was reduced on average by 76%, yet upon regaining the weight their BMR did not return to baseline and appeared to be reduced even further, despite regaining the weight3.
An example of how weight loss can impact your metabolic rate is shown in a study conducted by Ashtary-Larky et al., (2017)2. Participants were randomised into one of the study groups; either rapid or slow weight loss. Those in the rapid weight loss group lost at least 5% of their body weight in 5 weeks and those in the slow weight loss group lost at least 5% of their body weight in 15 weeks. Whilst BMR was reduced in both groups, the rapid weight loss group’s BMR was reduced significantly more than the slow weight loss group, despite losing the same amount of weight. This shows us that quick weight loss can result in a significant and long-lasting lowering of our BMR, which can impact your weight in the future. Issues can arise in maintaining the weight you have lost, and it may also be difficult to lose weight in the future.
You may have seen or heard of certain foods or drinks that claim to boost your metabolism, such as; green tea, black coffee, spices and certain energy drinks. Unfortunately, the evidence on these is weak. Caffeine itself, has been shown to increase our metabolism, but from a weight management perspective, long-term studies have not seen effects on body weight.
One thing that has been shown to increase our energy expenditure, specifically the TEF, is protein. Around 5-10% of the energy provided from carbohydrates is used to digest, absorb and metabolise this nutrient, whereas for fats, only 0-3% of the energy is used. But to digest, absorb and metabolise protein, 20-30% of the energy provided by protein is needed. For example, if a protein source provides us with 100 calories, around 20-30 calories will be used to digest, absorb and metabolise this nutrient. So, the more protein we consume, the greater TEF and ultimately an ‘increase’ in our energy expenditure. This is one of the reasons why protein plays a big part in many weight loss programmes. Lean protein rich foods come from animal products such as; chicken, turkey, red meats, milk and other dairy products, and fish is also classed as a protein rich food. Vegetarian sources of protein include; soy, tofu, tempeh, chickpeas, beans, and lentils.
There are many supplements and pills that claim to boost metabolism, burn fat or block absorption of nutrients. In truth, there is very little evidence that supports the claims these supplements make. Some of the supplements you might see that make these claims are; products labelled as ‘thermogenic’, green tea, ketones, caffeine, conjugated linoleic acid and there are many more! If it sounds too good to be true, it generally is!
If you’re looking lose weight unfortunately you cannot ‘boost’ or ‘speed up’ your metabolism. What you can do is increase the amount of energy you are expending day to day. The basics are a good place to start – as unexciting as that might sound! Increasing your physical activity or exercise levels and your protein intake as part of a balanced diet, will stand you in good stead to maintain a healthy weight.
1. Anderson, J. et al., (2001) Long-term Weight-Loss Maintenance: A Meta-Analysis of US Studies. Am J Clin Nutr, 74(5): 579-84.
2. Ashtary-Larky, D. et al., (2017) Rapid Weight Loss vs. Slow Weight Loss: Which is More Effective on Body Composition and Metabolic Risk Factors. Int J Endocrinol Metabol, 15(3): e12349.
3. Fothergill, E. et al., (2016) Persistent metabolic adaptation 6 years after The Biggest Loser competition. Obesity (Silver Spring), 24(8): 1612-1619.
4. Galgani, J. & Ravussin E. (2008) Energy metabolism, fuel selection and body weight regulation. Int J Obes (Lond), 32 (Suppl 7): S109-S119.
5. Larsson, L. et al., (2019) Sarcopenia: Aging-Related Loss of Muscle Mass and Function. Physio Rev, 99(1): 427-511.
6. Trexler, E. et al., (2014) Metabolic adaptation to weight loss: Implications for the athlete. J Int Soc Sports Nutr, 11(1): 7