You hear all about the metabolism, and how great it is when it’s fast. People wish they had a fast metabolism when they have ongoing difficulties losing weight. To feed into it, there are plenty of people throwing around and perpetuating myths about the metabolism.
In this article, we’ll bust many of the myths about metabolism, but before we do that, we have to give a definition of what metabolism is. Metabolism is the speed of chemical reactions in the body. It’s not strictly how many calories you burn. It’s also how quickly your body processes certain substances, like caffeine, acetaminophen, etc.
But since this is a fitness article, we’re referring to the number of calories burned.
Original source: here.
We also need to differentiate between resting metabolism (how many calories you burn when you’re not doing anything), and exercise metabolism (how many calories you burn during exercise). At the end of the day, the number of calories you burn is extremely simple.
By far, the two greatest contributors to resting metabolism are:
- Your body temperature.
- Your body weight.
And when it comes to exercise metabolism, the two greatest determinants are:
- Your body weight.
- Your average pulse during exercise.
Simple, right? Those 2 factors will account for over 95% of all the calories you burn. When you view it through that lens, a lot of myths about metabolism start to crumble. Let’s bust them one by one.
Overweight People Have a Slower Metabolism
Actually, it’s the opposite. Overweight people have pretty fast metabolisms. Think about it – it takes more calories to sustain a larger body than a smaller one.
Usually, overweight people are not overweight because they have a slow metabolism. They’re overweight for other reasons (eating more calories than they think, not enough physical activity, etc.).
And it’s not their fault, either. There are many behavioral, psychological and logistical factors that combine to make it difficult to lose weight, but from a metabolic perspective, there’s usually (not always) nothing wrong.
I Have a Slow Metabolism
To piggy back on the previous point, most overweight people don’t have slow metabolisms. But a slow metabolism definitely does exist, and it’s less than most people think.
Let’s put some numbers to this. If you want to know how many calories you’re really consuming, just multiply your body weight (in pounds) by 15. If you exercise, add in the calories burned through exercise.
Let’s use an example. Let’s assume the person in our example is 200 pounds, and not exercising. So this person is consuming 3000 calories/day if they’re not gaining or losing weight. “But it can’t be. My nutrition tracking app says I’m only eating 1800 calories/day.” I know, I know. But as I’ve written about in a previous article, the accuracy of popular nutrition tacking apps is atrocious.
So if our 200-pound person has a “normal” metabolism they’re consuming 3000 calories per day, and burning 3000 calories/day. But what about people with a slow metabolism? How slow is slow? What’s the variation between a fast metabolism and a slow metabolism? According to one study, it’s only about 15%. In other words, if one 200-pound person has a fast metabolism and burns 3000 calories per day, a 200-pound person with a slow metabolism will still burn 2550 calories per day. And that’s comparing fast and slow metabolisms. Not normal and slow metabolisms.
So yes, slow metabolisms are definitely real, but the magnitude of “how slow is slow” is not that slow.
I’m X Years Old. I Have a Slow Metabolism.
One very large study looked at the metabolism of people as young as 8 days old, and as old as 95 years old. They found that strictly speaking about metabolism, there is no difference between the age of 20 and 60. Zero. Is weight gained during this time? For some people, yes, for others, no. But if weight is gained, it’s not due to a drop in metabolism. It’s largely due to changes in activity levels and/or nutrition. Maybe when you were 20, you would play pickup soccer a few times a week, and you’re not doing that anymore when you’re 60.
After 60, metabolism does slow down by up to 0.7% per year, which isn’t a lot. So any weight gained beyond 0.7% per year is not due to metabolism slowing down.
Menopause Slows Down the Metabolism
As I discuss in my upcoming menopause book, it’s debatable whether menopause slows down the metabolism. Some research finds no slowdown at all, until age 60, and some research does show a slowdown in metabolism, but only up to 0.6% per year. Not that much.
Again, if body fat is gained during and after menopause it’s largely for reasons other than metabolism.
Eating Breakfast Speeds Up the Metabolism
Let’s go back to the beginning of this article – the 2 major determinants of your resting metabolic rate are:
- Your body temperature.
- Your body weight.
Does breakfast change your body temperature? Maybe very slightly – but it won’t give you a fever. A fever actually speeds up your metabolism, because it changes your body temperature significantly enough. For every 1 degree Celsius that your temperature increases, you burn 10-13% more calories. If a normal body temperature is 36.5-36.8 degrees Celsius, and 37.0 degrees is considered a fever, breakfast does not elevate body temperature by even 0.5 degrees. So no, breakfast does not speed up the metabolism.
There may be great reasons to eat breakfast. But what it doesn’t do to any great extent is it doesn’t speed up the metabolism.
Having Water with Lemon First Thing in the Morning Speeds up the Metabolism
First of all, I don’t know who came up with this, but water with lemon is still water with lemon in the afternoon and evening. It’s not like water with lemon speeds up the metabolism (it doesn’t, by the way) in the morning, but doesn’t do it in the afternoon.
Water with lemon doesn’t affect your body weight, or your temperature. So it doesn’t do anything to your metabolism.
If you just like the flavour of lemon water, great. Drink it. But not under the illusion that it’ll speed up your metabolism. It won’t.
Certain Foods Speed Up the Metabolism
No they don’t. Food doesn’t change your body temperature (to any great extent), nor does it change your body weight in an acute sense (chronically, yes, of course it does, but not acutely).
For example, it’s said that chili peppers speed up the metabolism. They just make you feel warm without an actual elevation in body temperature.
The same can be said for other foods.
Skipping a Meal Slows Down the Metabolism
Again, let’s go back to our main principle: the two things that determine the speed of your metabolism are your body weight and your body temperature. Does skipping a single meal change your body weight? Not really. Does it change your body temperature? Nope. Therefore, does it slow down your metabolism? Also no.
Muscle Speeds Up the Metabolism
This one is actually true, but we really have to quantify the extent by which it speeds up your metabolism.
Each pound of muscle burns around 6-10 calories per day. But that’s not the whole story. Body fat also burns calories. It burns around 2 calories per pound per day.
So for the “gain muscle to burn fat” theory to be meaningful, you’d have to gain a lot of muscle. Like 15-20 pounds of muscle. And that’s extremely difficult (and takes about a year for men, and 2-3 years for women).
So gaining muscle to burn fat is not the most direct way to fat loss. There are many great reasons to gain muscle, like:
- Improved bone density.
- Better mobility, and overall function.
- Better blood sugar control.
- Lower blood pressure.
- Reduced inflammation.
But fat loss is not a good reason to gain muscle.
Cold Water Exposure Speeds up the Metabolism.
Cold water exposure has gotten very trendy. There may be benefits to your immune system. There may be benefits to your cardiovascular system, but lots of people are doing it to speed up their metabolism.
The theory goes “if I submerge my body in cold water, it has to burn calories to maintain its temperature.” All true. But we’re missing a couple of important points:
- By how much is metabolism elevated?
- For how long does it stay elevated?
In one study, participants were submerged in cold water up to the neck. The water was 14 degrees. That’s seriously cold and unpleasant. They spent an hour in this water. During that hour, their metabolism approximately tripled. After they got out of the water, their metabolism returned to baseline within about 40-60 minutes.
Not bad, but let’s put some numbers to this. The average person burns about 1 calorie per minute at rest. If their metabolism tripled, they would have burned 3 calories per minute. But don’t be impressed yet. You know what else burns 3 calories per minute? A light stroll, housework, gardening, etc. More moderate or even intense exercise can burn significantly more. People can burn as much as 12-14 calories per minute during exercise.