Blaming weight gain on a slow metabolism is a common catch cry from many struggling to lose weight. Most people though would be better blaming the food they’re eating, or the exercise they’re not doing, as the real culprit.
First, let’s consider the term ‘metabolism’. It means the process by which the body converts food into energy. In fact, someone with a ‘slow metabolism’ would not get all of the available energy from food and would actually lose weight!
A much more relevant term – and this is what most people mean when they talk about metabolism – is metabolic rate. This is the energy (measured in kilojoules) a person expends over the course of a day just to keep the body functioning. Maintaining body temperature, breathing, blood circulation and repairing cells are all essential requirements for a functioning body. These processes are always happening and use a lot of energy.
Your basal metabolic rate (BMR) is the absolute minimum amount of energy you need just to exist, without any activity or the metabolic costs of digesting and absorbing food. BMR constitutes the largest component of your total daily energy expenditure and represents around two-thirds of an average adult’s energy requirements.
An accurate BMR can only be measured by monitoring the amount of oxygen inhaled and carbon dioxide exhaled. The person must be in their most restful state so these measurements are taken in the morning, after an overnight fast, with the person lying down in a comfortable environment.
Your basal metabolic rate is influenced by your body’s composition. Muscle requires more energy to function than fat. That’s why men, who typically have a higher muscle mass than women, will generally have a higher BMR than women. Other factors include:
Height (the taller, the higher the BMR due to a larger skin surface area for heat loss)
Growth (e.g. pregnancy, childhood)
Fever and stress
Higher muscle mass (males typically higher than females)
Smoking and caffeine
Environmental temperature (heat and cold both raise BMR)
As we get older, we tend to gain fat and lose muscle. This explains why your basal metabolic rate tends to decrease with age. Fasting, starvation and sleep can also decrease your BMR.
There are a variety of online calculators that use different equations to estimate your BMR, based on your age, sex and body weight. But when it comes to weight loss, knowing your BMR is largely irrelevant.
If you want to lose weight and your current diet and physical activity plans aren’t moving it, then you either need to eat less, move more – or, preferably, both.
So, can a ‘sluggish metabolism’ be blamed for weight gain? With the exception of certain endocrine disorders such as hypothyroidism or Cushing’s syndrome, the answer is a clear ‘no’.
Overweight people actually have higher BMRs than those of a healthy weight and this increases as more weight is added. As someone gains more weight from storing more fat, the body needs to support that excess mass to carry it around. Imagine you had to live with a 20 kg weight tied around your waist. You would struggle to deal with this for the first few weeks, but over time you would build up extra muscle – especially in your legs – to help manage it. More muscle equals a higher metabolic rate at rest.
Another common reason a slow metabolism is blamed for weight gain is the perception that an overweight person eats very little and can still put on weight. Ignoring the well-described correlation between the degree of someone’s excess weight and the amount of food they under-report consuming, research shows people tend to eat more than they think.
Increases in portion sizes is thought to be affecting what people now consider an ‘average’ portion size for meals they serve themselves at home – a phenomenon called ‘portion distortion’. The bigger a person is, the more likely they are to overestimate what a ‘normal’ portion size is.
So is it possible to speed up metabolism? There are many pills, supplements and foods that claim to boost the metabolism and burn fat. Most of these claims are unproven. Some substances such as caffeine and chilli do have a small effect, but not in supplement form. In any case, increasing your metabolism isn’t a shortcut to weight loss and may come with unintended side effects such as increased heart rate.
If you’re struggling to lose weight, it’s probably time to reassess your diet and exercise levels.