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Monday’s medical myth: ‘my slow metabolism makes me fat’

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…

If you’re not meeting your weight-loss targets, you need to eat less or move more. Flickr/lism

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.

Andy in NYC

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 you 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. ArmyStrongPA

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. Chieee

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 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.

Join the conversation

15 Comments sorted by

  1. Jonathan Nolan


    I'd hazard a guess that the phenomenon people are actually referring to when they talk about their metabolism is in fact the amount and speed leptin is produced in their brains in response to food intake. That is a real chemical that does have real effects on how much people eat. To say that people are wrong simply because they dont' know the correct term or chemical for the real difficulty they have losing weight sounds a bit unfair to me.

  2. John Harland

    bicycle technician

    Production of a satiety hormone is quite different to the rate at which food energy is used. It is not merely the name that many people have wrong. but the entire concept.

    If people are using biochemical processes as an excuse, they do have some obligation to get the process and the terminology correct.

    There is nothing unfair in blowing people's purportedly-scientific excuses if those have no basis in science.

  3. John Harland

    bicycle technician

    Oops. Sorry, meant to check the precise role of leptin before hitting "send".

    I had it wrong in my memory and apologise for the comment.

  4. Rosemary Stanton

    Nutritionist & Visiting Fellow at UNSW Australia

    Well said, Tim.

    In my experience, the 'slow metabolism' claim becomes so ingrained for some people that they firmly believe it is why they cannot lose weight. As a result, they pay less attention to what they consume.

    I also hear the opposite claim about 'fast metabolism' from many very thin young women who actually eat very little butdo not want to be thought of as having an eating disorder.

    In over 45 years of clinical practice, I have yet to see the person whose weight is a mystery…

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  5. Ebony Jackson

    logged in via Twitter

    It is not that hard to work out. If you eat crap and never get off the get fatter.

    I remember a long time ago being anorexic - fibbing about how much I ate and embelishing my actual intake of food and unexplained weight loss.
    Privately I exercised like a maniac - and starved myself almost to death.

    Weighing 33 kg is just ugly!

    These days I eat healthily and walk to the shops at least 3 times a week. I notice I am a little overweight at 60 kg...but I know the reason...snacking on icecream too often!

  6. Regan Forrest

    logged in via Twitter

    "Slow metabolism" may be among the Top 10 List of Excuses but there would still seem to be considerable individual differences in BMR that may be at least partly genetically determined.

    My partner is slim as is all his family. Many of my family members are overweight. He can stay slim while eating what he wants; for me it is a constant struggle. If I ate as much as he did, I'd be the size of a house within a week!

    (N.B. we both exercise regularly)

  7. John Wright


    Dear Tim interesting article.
    Having spent the last 4 years developing and testing an Indirect Calorimeter that can be used by primary healthcare practitioners, I can certainly attest to the considerable number of subjects whose expectation of their metabolic rate is considerably different from the measured results.

    Frankly most people have no idea, many consider themselves to be the physiological norm, and all are therefore very surprised when there results are substantially different…

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  8. Harrison McIntosh

    Head of Mathematics Department

    As someone who has recently lost almost 20kg and looking to lose more, I reject the let's blame the fat person mantra.

    I did not over eat, and I know I had a slow metablism because I had it tested. Why? Because I ate too little. Research reported overseas (forgot the link) shows that the size and type of fat cells produced by couch potatoes will cause them to gain weight regardless of how much they eat. I've mostly been quite active, even as an overweight person.

    I went to doctor after…

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    1. Harrison McIntosh

      Head of Mathematics Department

      In reply to Harrison McIntosh

      NB. My metablosim was "tested" in the sense of comparing what I ate and weight loss/gain over a week compared to what was "normal" with a sports dietician. Just to be clearer.

    2. Anna Cox

      youth worker

      In reply to Harrison McIntosh

      Just posted but did not do as reply to you - Thanks for posting something that is true and relevant
      How is the weight loss going?

  9. Anna Cox

    youth worker

    thankyou Harrison - you hit the nail on he head! I, like you, have ended up eating next to nothing as a result of similar ignorant feedback and now find my metabolism has not been functioning well as a result.
    Shame on these ignorant ppl (esp those that are allegedly from within the medical profession)
    What diet ended up working for you if you see this????

  10. Anna Cox

    youth worker

    I would like to know why the author, Tim Crowe, did not even bother to respond to comments made by Harrison in any way, shape or form.
    Not very impressive if he cannot respond to views that differ from his own

  11. Rich Nation

    logged in via Facebook

    I am a 51 year old woman who has dieted myself fat! If u dont think dieting and starvation can mess up ur metabolism u are wrong! I am nearly suicidal over this problem, I am a diet addict and fear food! The only periods in my life since age of 15 that I wasnt on and extremly low calorie diet was when I was only eatting one meal a a day when my kids were young and by most peoples standards that is still a diet. It scarry when even professionals dont beieve u! I managed tp lose 75 lbs on hcg…

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  12. David Graham Jones

    logged in via Facebook

    My BMR is 2800, I consume around 1000 calories a day (I use some PC software to record every mouthful), I swim 1/2 km 5 times a week, use the gym for an hour 3 times a week, play a round of golf once a week. That helps keep my BG and cholesterol down but I don't lose weight, haven't done so for ages. What would you suggest is the problem? Please don't suggest that I'm in starvation mode, I think you'll find that weight loss might slow down but shouldn't stop, that's why people can starve to death. I've reduced my calorie intake over the years from 2300, down to 1800 then 1200 and in fact my Endocrinologist suggested 800 although nutritionists have said don't go below 1200.
    I'm well aware that people underestimate what they eat, which is why I weigh everything and use PC software to record everything. Your article doesn't really reflect the problem that some people have in losing weight, it's not as simple as "cutting down".