A question of fat burning

The point of view of Asker Jeukendrup

Burning fat has become an obsession. Some of us want to burn it to lose weight or otherwise have a good physique, others to be faster. That said, what’s the evidence that says we should be burning fat? Most importantly, is it good for us to burn it?

People desire to burn fat primarily for 3 reasons:
1. weight loss
2. the effects on training
3. health

1. Slimming down

It is the #1 reason. For example, weight that’s typically accumulated after the Christmas holiday is the result of increased body fat. Well, if we want to eliminate it, we have to burn it. Even athletes who have little fat mass and aim to become even leaner are not infrequently interested in the metabolic process that leads to burning more fat.

2. The effects on training

Fat burning has a good reputation because we associate it with being well trained.

"Trained athletes have a greater ability to burn fat. Weeks or months of training have modified their muscles, allowing them to burn fat with greater speed. Be careful though, it's good to remember that the athlete is ALSO able to burn more carbohydrates...."

Let's make an analogy with the automobile. If the engine is more powerful, it can use more fuel and, therefore, it can go faster. If the intensity (i.e., speed) is moderate, the body reduces its dependence on carbohydrates, allowing for savings in glycogen (carbohydrate, that is) stores. And that should translate to better performance.

3. Health

The ability to burn fat is also associated with better health. How did this come about? Through observation. If the body loses the ability to burn fat properly (see due to lack of physical activity or poor diet) the risk of running into metabolic or health disorders increases. Those who train their bodies to burn more fat will have some benefits to their wellbeing in return.

A look at the scientific evidence

Having outlined the top three reasons why we care about burning fat, let's take a look at the scientific evidence.

• If we aim to lose weight, we need to have a negative energy balance. In other words, we need to burn more calories than we ingest. If we burn more fat but continue to overeat, we will not lose weight. Of course, if the energy balance is negative, you will need to make sure that the weight you lose is mostly fat. This is where fat burning becomes critical. For weight loss purposes, the take-home message is that a negative energy balance is far more important than fat oxidation.
  
  
• We have seen that trained athletes are good at burning fat. In general, this is true, but it isn’t always the case. Each of us can certainly improve our ability to burn fat through training, hence achieving greater fat burning efficiency. However, there are significant individual differences, and the starting point for different people can turn out to be completely different.

We are not (yet!) able to understand all the factors behind these differences. In all likelihood they have a genetic origin. In one study, for example, we found that some sedentary individuals were able to burn fat in similar amounts to a highly trained athlete.

• Finally, the health effects. These too are related to the ability to burn fat, not just the ability to have burned some fat. The amount of fat you are able to burn during physical activity is really small. Typically, we're talking about 0.3 to 0.5 g per minute during exercise (in extreme cases, up to about 1 g per minute). This means that in order to burn 1 kg of fat, we need to exercise from 33 to 56 hours!

The real benefits come from boosting the motor and increasing the number of mitochondria. This means that even in resting conditions we will be able to burn more fat. Not only that, for those who are not very active, reducing carbohydrates will help them. However, the most substantial benefits will come from exercise.