You may have heard the claim that you should run more slowly to burn more fat. My local gym has signs suggesting that if you want to burn fat, you should exercise in a “fat burning” heart rate zone.
This idea is based on a misunderstanding of the science. It is true that when you exercise at a lower intensity, your body is able to draw more of the energy directly from fat. But it is not correct to conclude that exercising at this lower intensity will result in your having lower overall body fat.
There are two reasons why the claim about a “fat burning zone” is wrong.
- First, it does not matter where the fuel comes from while you are exercising; in the end (ie later in the day) your body will replenish and rebalance your energy stores. So a calorie surplus will always end up increasing your body fat; and a calorie deficit will always end up reducing your body fat. The source of fuel while you are exercising is irrelevant.
- Second, what matters is the total amount of calories you burn; not the proportion that come fat. If you exercise at lower intensity, a higher proportion may come from body fat, but this may be a smaller absolute number of calories.
In other words, the fat burning zone is a myth caused by misunderstanding the science.
To lose fat, you need to eat fewer calories than you burn. The extra calories you burn will be taken from your body fat.
Suppose your daily calorie requirement is 2800 kcal per day; and suppose that you consume 2710 kcal per day. Then you are consuming 90 kcal a day fewer than you are burning. Because fat is about 9 calories per gramme, you will lose about 10 grammes of body fat a day.
You can reduce weight either by reducing your calorie intake (eg by eating smaller portions) or by increasing your daily calorie requirement (eg by exercising more) or both.
If you do decide you want to reduce your body fat, you should not attempt to lose weight too rapidly. In particular, you should not aim to lose more than ½ kg a week; nor should you try to consume less than 85% of your daily equilibrium calorie needs. This is to avoid illness, and because if you eat less than this, your body goes into starvation mode, your metabolism slows down, and your rate of weight loss decreases.
The best approach to estimating how fast you should be losing weight is:
- decide how much running you realistically intend to do per week;
- estimate your equilibrium daily calorie requirement at that level of exercise, given your current bodyweight;
- work out how many calories a day you need to eat to lose weight without consuming less than 85% of your daily needs or losing more than 500g a week.