This week’s expert is Ollie Chrzanowski-Smith from the Centre for Nutrition, Exercise and Metabolism at the University of Bath. Ollie completed a PhD in exercise and fat use and has published articles in world-leading scientific journals on the topic.
Key question: What fuels do we burn during exercise and does it matter?
As Rob introduced in Article #2, the body needs energy for functions that keep us alive, and for any physical activity. The source of energy used by all cells in the body is a molecule known as adenosine triphosphate (ATP). For most types of exercise, we use a combination of two fuels to provide this ATP, either carbohydrates or fats. The human body stores energy in different tissues; for carbohydrates this is mainly the liver and skeletal muscle (as something called glycogen), whereas fat is mostly stored in adipose (fat) tissue with a small amount in skeletal muscle. The human body has a massive capacity to store fat (e.g. >100,000 calories worth in the fat tissue of a 75 kg man with a body fat of 15 %, compared to ~3000 calories as carbohydrates).
To keep things simple at the start, let’s say you are in the fasted state, so you haven’t eaten your breakfast yet. In this state, the ratio of fat to carbohydrate burning is shifted towards fat, although your liver is supplying a steady amount of carbohydrate (sugar) to the bloodstream, to keep tissues such as the brain working.
When you start being physically active, your energy expenditure goes up, so you start burning more energy and typically more carbohydrate and more fat. Various factors alter the ratio of how much carbohydrate compared to fat is used as a fuel during exercise. Broadly speaking, as the intensity of the exercise gets harder you burn more carbohydrates compared to fat. Whereas if you exercise at lower intensities or for longer periods of time (~2 hours) and without eating carbohydrates, the ratio is shifted towards fat burning (for some good studies on this topic click here and here). There is some nice research showing that if you exercise with very low amounts of carbohydrate in your muscle (glycogen) you might get some boosted adaptations. However, this is very difficult to achieve for most people as it can involve several intense bouts of exercise in close succession.
During low-to-moderate intensity exercise (an intensity where you can easily-to-just about hold a conversation when exercising), you can alter the ratio of the fuels you burn by changing what and when you eat around exercise. If you exercise in a fasted state (e.g. before having your breakfast) you burn more fat (and less carbohydrate) during exercise compared to if you completed the same exercise in a fed state, after eating a breakfast with carbohydrates like porridge or toast (where exercise would be increasingly fueled from the carbohydrate in the meal). For a great review click here.
‘So if I increase fat burning during exercise, will I lose more body fat?’
This is an attractive prospect but not necessarily true. As covered by Rob in Article #2, changes in body weight and to a large extent body fat are determined by energy balance, which is energy intake compared to energy expenditure. This means that even if you burn a higher amount of fat during exercise, if you replace the calories you expend during that exercise by eating and drinking more, you may return to energy balance. Therefore, in the long-term your body weight and body fat would be unlikely to change. Put simply, increased fat burning does not always increase fat loss, and this has been shown in studies that have altered the amount of fat burnt during exercise, but showed no additional benefits for weight loss.
There is still lots to learn in this area. For example, there is an idea that increasing fat burning during exercise may preserve carbohydrate stores in the body (i.e. you don’t use up your glycogen as much), which may alter what we feel like eating after exercise. There is also some new research that increasing fat burning during moderate intensity exercise, may have some health benefits. Want to learn more about that? Stay tuned for an upcoming blog post.
Take home message: We burn a mixture of carbohydrates and fats to provide energy. At rest and during low intensity exercise, this ratio is more towards fat, but at higher exercise intensities the ratio swings towards carbohydrates. If you exercise after eating carbohydrates you also burn less fat. However, for weight loss and changes in body fat, your overall energy balance is still a very important consideration.
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