When it comes to losing weight, people often want know the best way to shed excess pounds – and there’s no shortage of fad diets or fitness crazes claiming to have the “secret” to fat loss.
One theory even suggests that exercising at around 60 percent of your maximum heart rate will bring our bodies into a so-called “fat burning zone”, optimal for losing weight.
But does this “fat burning zone” even exist?
First, it’s important to understand a little about our metabolism. Even if we were to sit at our desk all day, our body still needs “fuel” to meet energy demands. This energy comes from carbohydrates, proteins, fats and phosphates.
However, the rate at which we use them, and how much we have available, varies between people. It depends on a number of factors, such as dietary intake, age, sex and how hard or often we exercise.
Generally, exercising at lower intensities – such as sustained walking or light jogging – doesn’t require as much effort by our muscles as sprinting, for example. This means the amount of energy needed by the body is lower, so energy supply predominantly comes from fats.
But as exercise intensity increases, fat can’t be metabolised fast enough to meet increased energy demand. So the body will use carbohydrates, as these can be metabolised more rapidly. This means there is indeed an exercise intensity where fat is the predominant energy source.
At the lower end of this spectrum is our resting state. Here, the number of calories our body needs to function is considerably low, so the body primarily metabolises fat to use for energy.
This means the potential “zone” for metabolising fat is between the rested state and the level of exercise intensity where carbohydrates become the dominant energy source (in terms of percent contribution to energy demand).
But this is a wide range, which lies between a resting heart rate of around 70 beats per minute to around 160 beats per minute during moderate effort exercise (such as cycling at a constant speed where holding a conversation becomes challenging), where the crossover from using fat to carbohydrates for energy occurs.
The issue with such a wide zone is that the person exercising wouldn’t necessarily be optimising their ability to metabolise fat, because as the exercise intensity increases there’s a gradual change in the balance of fat and carbohydrates your body uses for energy.
Fat burning zone
So how can we know at which point our body will switch from using fat to other fuels for energy? One approach researchers take is assessing how much fat is being used for energy during different exercise intensities.
By measuring how much air a person expels during an exercise test which gets progressively harder, physiologists have been able to calculate the relative contributions of fat and carbohydrates to meet the exercise demand at different intensities.
The highest amount of fat burned is called the “maximal fat oxidation rate” (or MFO), and the intensity this occurs at is termed “FATmax”.
Since this method was first used by researchers, studies have shown that as the intensity rises from around 40-70 percent of a person’s VO₂ max – which is the maximum amount of oxygen a…