Dear Dr Adam,
What causes a slow metabolism, and is it actually possible to speed it up?
Blaming an increased difficulty to lose weight on a “slow metabolism” is common, but in truth, it’s a bit of an old wives tale. People sometimes think they’re overweight because they have a slow metabolism and that it’s all down to simply not being able to biologically metabolise things quickly enough. Really, it depends what you mean by slow metabolism.
A lot of new research is being done on how people metabolise meals, to see if we can identify different types of people who respond to a fixed meal in a different way to others. At the moment, we simply don’t know. We can identify that we have polymorphisms in our genome, we might even have differences in our microbiome, but how that translates into differences in how we process meals is all speculative at the moment.
There is probably some element of truth that some people respond differently to others. That difference, however, is most likely a very subtle one. For instance, the findings won’t reveal that someone could eat a jacket potato and receive a massive spike in glucose that stays elevated for four hours, while someone else who eats a jacket potato has no response whatsoever. It’s simply never going to be that dramatic.
Further complications come from the fact that we don’t know the other variable which is the differences in the food matrix itself. If you’re having a jacket potato with beans, as opposed to one with tuna, those two things may have the same amount of carbohydrates, they might even have the same amount of protein, but the two matrixes could cause different responses.
“Someone who is overweight or obese will have a higher metabolism than someone who’s thin. Part of that is because there’s more of them to metabolise, but it’s also because when you’re gaining weight, your body increases your metabolic rate as a way to offset your weight gain”
What I’m trying to say is that determining how people differ in terms of metabolism is a more complicated subject than many think.
In the broadest sense, when we talk about whether one person can have a biologically slower metabolism than someone else we’re talking about whether two people of the same size can have a different metabolic rate, i.e. burn more/less calories doing nothing than someone else.
I’ve done measurements in metabolic rate in hundreds of people and the variability between two people of the same size – once you correct for gender, body size, even body composition – will always be pretty much the same. The difference you see in two individuals of the same size is so subtle that you wouldn’t even be able to measure it if you were then trying to monitor your calorie intake and expenditure.
The only time it becomes noticeably relevant is when you have some form of pathology, and the classic example of that is if you have an underactive thyroid. If your thyroid isn’t working properly then that is a mediator of your metabolic rate and all other things related to your energy cost, hence why you have to go on thyroxin.
But even when you measure the metabolic rate of people with an underactive thyroid, the actual measurement is not hundreds of calories lower than someone with a normally-functioning thyroid. I will say, however, that prevalence of hypothyroidism is fairly high, particularly in women. One in ten women could have an underactive thyroid, especially if you’re post-40 or post-childbirth.
The other key point to make is that someone who is overweight or obese will have a higher metabolism than someone who’s thin. Part of that is because there’s more of them to metabolise, but it’s also because when you’re gaining weight, your body increases your metabolic rate as a way to offset your weight gain.
When that person starts to lose weight as a result of diet or exercise, it creates an energy deficit that in turn causes a decrease in your metabolic rate. Here’s where the idea of a “slow metabolism” becomes true, because it reduces in response to your weight loss efforts. Not to say that it makes it difficult to lose weight per se, but it becomes harder to maintain that rate of weight loss indefinitely.
To have your nutrition questions answered by Dr Adam Collins, please email firstname.lastname@example.org