You hear all about metabolism and how great it is when it’s fast. People wish they had a fast metabolism when they are constantly struggling to lose weight. To fuel this, there are many people who throw around and perpetuate myths about metabolism.
In this article, we will dispel many of the myths about metabolism, but before we do that, we need 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 is also how quickly your body processes certain substances, such as caffeine, acetaminophen, etc.
But since this is a fitness article, we’re referring to the number of calories burned.
Original source: here.
We must also distinguish between resting metabolism (how many calories you burn doing nothing), 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 biggest contributors to resting metabolism are:
- Your body temperature.
- Your body weight.
And when it comes to exercise metabolism, the two biggest determinants are:
- Your body weight.
- Your average heart rate during exercise.
Simple, right? These 2 factors will account for over 95% of all the calories you burn. When you look at it through this lens, many myths about metabolism begin to fall apart. Let’s take them down one by one.
Overweight people have a slower metabolism
In fact, it’s the opposite. Overweight people have a fairly fast metabolism. Think about it – It takes more calories to maintain a larger body than a smaller one.
Usually, overweight people are not overweight because they have a slow metabolism. They are overweight for other reasons (eating more calories than they think, insufficient physical activity, etc.).
And it’s not their fault either. There are many behavioral, psychological, and logistical factors that combine to make weight loss difficult, but metabolically, there’s usually (not always) nothing wrong with it.
I have a slow metabolism
To go back to the previous point, most overweight people do not have a slow metabolism. But a slow metabolism definitely exists, and it’s less than most people think.
Let’s put some numbers to it. If you want to know how many calories you’re actually consuming, simply multiply your body weight (in pounds) by 15. If you exercise, add the calories you burn through exercise.
Let’s use an example. Let’s say the person in our example is 200 pounds and doesn’t exercise. So this person is consuming 3000 calories/day if they are not gaining or losing weight. “But it can’t be. My food tracking app tells me I’m only eating 1800 calories/day.’ I know, I know. But as I’ve written in a previous article, the accuracy of popular nutrition apps is appalling.
So if our 200 kg person has a “normal” metabolism, they consume 3000 calories per day and burn 3000 calories/day. But what about people with slow metabolisms? How slow is slow? What is the difference between a fast metabolism and a slow metabolism? According a study, is only about 15%. In other words, if a 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 this is the comparison of fast and slow metabolism. Abnormal and slow metabolisms.
So yes, slow metabolisms are definitely real, but the magnitude of “how slow is slow” isn’t that slow.
I am X years old. I have a slow metabolism.
A very great study examined the metabolism of 8-day-old and 95-year-old subjects. They found that strictly speaking of metabolism, there is no difference between 20 and 60 years. Zero. Does this interval increase? For some people yes, for others no. But if weight is gained, it is not due to a drop in metabolism. It is largely due to changes in activity levels and/or diet. Maybe when you were 20, you played pickup football a few times a week, and when you’re 60, you don’t anymore.
After 60, metabolism slows down by up to 0.7% per year, which is not much. Thus, any weight gained beyond 0.7% per year is not due to a slowing of the metabolism.
Menopause slows down the metabolism
As I discuss in my upcoming book on menopause, it is debatable whether menopause slows metabolism. Some research finds no slowing at all, up to age 60, and some research shows a slowing of metabolism, but only up to 0.6% per year. Not so much.
Again, if body fat is gained during and after menopause, it is largely due to reasons other than metabolism.
Eating breakfast speeds up metabolism
Let’s go back to the beginning of this article – the 2 main 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 quite a bit. For every 1 degree Celsius your temperature rises, 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 raise the body temperature by even 0.5 degrees. So no, breakfast does not speed up the metabolism.
There can be great reasons to eat breakfast. But what it doesn’t do to a great extent is that it doesn’t speed up the metabolism.
Drinking lemon water in the morning speeds up the metabolism
First of all, I don’t know who thought of it, but lemon water is still lemon water in the afternoon and evening. It’s not like lemon water speeds up the metabolism (it doesn’t, by the way) in the morning, but it doesn’t in the afternoon.
Lemon water does not affect your body weight or temperature. So it does nothing to your metabolism.
If you just like the taste of lemon water, great. Drink it. But not under the illusion that it will speed up your metabolism. It won’t happen.
Certain foods speed up metabolism
No they do not. Eating doesn’t change your body temperature (by much), nor does it change your body weight in an acute sense (chronically, yes, of course but not acutely).
For example, chili peppers are said to speed up the metabolism. They just make you feel warm without actually raising your 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 your metabolic rate are your body weight and your body temperature. Does skipping a single meal change your weight? Not really. Does it change your body temperature? No. So does it slow down your metabolism? Also no.
Muscles speed up metabolism
This is actually true, but we really need to quantify the extent to which it speeds up your metabolism.
Each pound of muscle burns about 6-10 calories per day. But that’s not the whole story. Body fat also burns calories. It burns about 2 calories per kilogram per day.
So for the “gain muscle to burn fat” theory to make sense, you’d need to gain a lot of muscle. Like 15-20 pounds of muscle. And this is 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 lose fat. There are many great reasons to gain muscle, including:
- Improved bone density.
- Better mobility and overall function.
- Better blood sugar control.
- Low blood pressure.
- Reduced inflammation.
But Losing fat is not a good reason to gain muscle.
Exposure to cold water speeds up the metabolism.
Exposure to cold water has become very fashionable. There may be benefits to your immune system. There may be benefits for your cardiovascular system, but many people do it to speed up their metabolism.
The theory goes that “if I submerge my body in cold water, it must burn calories to maintain its temperature.” All true. But we are missing some important points:
- How much is the metabolism increased?
- How long does it stay elevated?
In a study, participants were immersed in cold water up to their necks. The water was 14 degrees. This is very cold and unpleasant. They spent an hour in this water. During that hour, their metabolism roughly tripled. After they got out of the water, their metabolism returned to their original value within about 40-60 minutes.
Not bad, but let’s put some numbers on it. The average person burns about 1 calorie per minute at rest. If their metabolism tripled, they would burn 3 calories per minute. But don’t be impressed just yet. You know what else burns 3 calories per minute? A light walk, housework, gardening, etc. The most moderate or even vigorous exercise can burn significantly more. People can burn up to 12-14 calories per minute during exercise.
