I very often get asked questions relating to metabolism and how it is affected by diet and exercise. There seems to be a great deal of confusion about metabolism, so I hope to clarify some of the main questions clients come to me with.
So firstly, let’s address the question at the forefront of your mind: ‘What is your ‘metabolism’?
Your metabolism is the sum of all of the chemical reactions taking place within your body. Your metabolism can be viewed as your ‘internal furnace’. Your metabolism is comprised of 3 main components: -
Basal Metabolic Rate (BMR) - The amount of energy required to maintain basic life processes. In sedentary populations, BMR accounts for around 60% of total daily energy expenditure (see diagram below).
Thermic Effect of Food (TEF) - The amount of calories you burn digesting and absorbing your food.
Activity Thermogenesis - Can be split into Non-Exercise Activity Thermogenesis (NEAT) and exercise-related activity. NEAT refers to any energy expenditure of daily living that is not formal activity, such as walking to work, taking stairs, fidgeting, shopping etc.
The figure above demonstrates the relative contribution of the different components of the metabolism. As you can see, BMR represents the largest component of total daily energy expenditure (TDEE).
Your metabolism is what you might call a ‘regulated system’. A regulated system attempts to maintain itself around some pre-determined level. A good example to illustrate what a regulated system is would be a thermostat: You set a pre-determined temperature to maintain. The thermostat will continually monitor temperature and when temperate increases the air conditioning will kick in to cool the temperature. The end result is that the temperature of your house will stay, within some range, around the temperature you have set the thermostat to.
Myth #1: “I have a slow metabolism”
The number 1 determinant of your basal metabolic rate (BMR) is your lean body mass. Simply put, a larger individual with a greater amount of muscle mass will require more calories to maintain weight than a smaller individual. Smaller people require less energy.
Your metabolism can both increase and decrease, which is known as ‘adaptive thermogenesis’. You aren’t genetically born with a slow metabolism, but it can slow due to to a number of factors, such as: -
Low calorie intake (relative to your needs)
Low activity levels
No or little exercise and training
As mentioned earlier, your metabolism is a regulated system. If calorie input is decreased such that you are in a calorie deficit, your body will compensate by moving less.
Myth #2: Prolonged dieting will have a permanent adverse affect on metabolism.
We know from the above that your metabolism can be influenced by external factors, such as energy intake, exercise etc. However, can your metabolism be permanently damaged?
Zinchenko & Henselmans (2016) undertook a systematic review of studies that reported weight loss and weight regain of individuals who were initially within a healthy weight range, in addition to research on under-nourished individuals, such as patients with anorexia nervosa, before and after recovery.
They discovered that whilst your metabolism can decrease after long-term dieting, Positive energy balance undoes the negative metabolic effects of AT and causes RMR to return to a level that is appropriate for the body composition at that time. The findings here show that human metabolism is highly plastic and rapidly adapts to changes in energy availability and body composition. This stands in contrast to the hypothesis of an inflexible metabolism that is susceptible to metabolic damage during prolonged caloric restriction. As such, the presence of metabolic damage in non-obese individuals is not supported by the current literature.
Essentially Zinchenko & Henselmans (2016) found that an increase in energy intake will result in the restoration of metabolic rate and that permanent damage didn’t occur.
Myth 3: “I Lost Weight Easily to Begin with, Now My Progress Has Plateaued”
So you embark on a diet, reduce your calorie intake and take up some exercise. You see some immediate changes in body composition. Clothes that once felt tight now start to feel looser. Work colleagues comment that you look like you’ve lost weight. You feel great! This is easy!
Then… disaster strikes! You hit the dreaded weight loss plateau that everyone talks about. No matter what you do, you cannot seem to lose any more weight.
So is there an in-built mechanism that stops you from losing weight?
Let’s review what happens when calorie intake is reduced: -
Body weight goes down, reducing (albeit slightly) your restoring metabolic rate - you need slightly fewer calories to maintain weight
The amount of food you consume decreases, reducing (albeit slightly) the amount of calories expended by the thermic effect of feeding (TEF).
Your Non-Exercise Activity Thermogenesis (NEAT) decreases - you subconsciously move less. You become more lethargic. You start to take the escalator rather than the stairs. You jump in the car rather than using your bicycle.
Appetite and hunger signals increase
Thyroid hormone output decreases
Metabolic rate slows, we burn less calories during exercise, NEAT goes down. Hunger goes up, appetite goes up, food is tastier and we’re more attentive to it. (Lyle McDonald)
So essentially you can see that your body becomes protective. It doesn’t know the difference between you wanting to lose weight so you can look good on the beach, and you starving to death. All it can react to is the internal stimulus of ‘I don’t have enough energy, I’m having to use fat tissue, stored glycogen and amino acids for energy’. From an evolutionary perspective this protective mechanism makes a lot of sense. When we were hunter-gatherers such protective mechanisms would have been essential for survival as humans would have gone extended periods without any food.
fundamentally, dieting is just controlled starving to death (Lyle McDonald)
So what can be done to off-set the slowing of metabolism as you diet?
It must be emphasised here that as weight loss occurs, a persons’s Resting Energy Expenditure (REE) will naturally decline, so this will naturally alter what someone’s maintenance calorie intake will be. Therefore it is important to periodically re-calculate someone’s calorie needs to ensure the deficit there are in is sufficient to elicit the appropriate rate of weight loss.
It is not the scope of this article to go into how large calorie deficits should be. However, as a general rule, the more body fat you have to lose, the greater the calorie deficit you are able to utilise without running the risk of losing lean muscle mass. The leaner an individual becomes, the more conservative you have to be with caloric deficits, as there will be less body fat available as a fuel source and hence the risk of losing muscle mass increases.
At this point the role of resistance training in the process of weight loss and improved body composition should be emphasised. Engaging in resistance training throughout the process of dieting is strongly advised. Resistance training elicits a range of morphological and neurological adaptations that contribute to changes in muscle function with respect to size, strength and power. Not only can these adaptations support improved athletic performance, but can also improve health-related musculoskeletal function (Egan & Zierath, 2013). Resistance training, through enhancement of physical activity-related energy expenditure, elevation of post-exercise basal metabolism, and preservation of lean mass can compensate, at least partly, for the decrease in energy expenditure in response to low energy intake (Major et al, 2007). Performing resistance training will assist in the increase in muscle tissue, which will, in turn, help to increase a person’s BMR.
Take Home Points: -
Your metabolism is not ‘fixed’ but rather is highly plastic and reacts rapidly to changes in energy availability and body composition
Permanent damage to your metabolism is unlikely - studies have found that your metabolism will recover, even under the most extreme circumstances
A plateauing of weight loss should be expected and planned for. This is where the utilisation of diet breaks and non-linear dieting approaches can be useful