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Medical Myth: ‘My Slow Metabolism Makes Me Fat’

by | May 17, 2020 | +SCIENCE

People who struggle to lose weight often blame their difficulty achieving a healthy weight on their “slow metabolism”. So is this a real barrier to weight loss, or is the real culprit an excess of food and a deficit of exercise?

First, let’s consider the term metabolism. It means the process by which the body converts food into energy. So, far from being responsible for weight gain, someone with a truly slow metabolism wouldn’t get all of the available energy from the food they eat 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.

running in city Unsplash

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 you are, the higher your BMR will be, due to a larger skin surface area for heat loss)
  • growth during pregnancy or childhood
  • fever and stress
  • smoking and caffeine, and
  • 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.

As we age, we tend to lose muscle and gain fat. 

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.

With an increase in body size, there is also a change in internal organ size and fluid volume, which further increases the metabolic rate.

Larger portions are the norm. Unsplash

Another common reason a slow metabolism is blamed for weight gain is the perception that an overweight person eats very little and still gains weight. But research shows people tend to eat more than they think and will typically report eating less food than they actually do as their weight goes up.

Increasing portion sizes may also affect what people now consider an average portion size for meals they serve 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 metabolism and burn fat. Most of these claims are unproven. Some substances such as caffeine and chili 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.


This article first appeared on The Conversation and is republished here under a Creative Commons license.

About The Author

Tim Crowe

Associate Professor, Deakin University. As an Advanced Accredited Practising Dietitian, I have spent the majority of my career in the university teaching and research environment. The academic role in nutrition and dietetics includes teaching across undergraduate and postgraduate nutrition courses in subject areas of nutritional physiology, nutritional biochemistry, diet and disease, sports nutrition and research methodology. A major theme of my research has centred on clinical nutrition research and the role that nutrition plays in the management of chronic disease states.

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