You may have seen workouts and exercises promoted as fat burning. Even certain machines at gyms suggest workouts in fat-burning zones. But what does this really mean?
Fat utilisation during exercise
During low-moderate intensity exercise, fat, in the form of fatty acids, is the primary fuel
used for energy. As exercise intensity increases, this switches to carbohydrates, in the form of circulating glucose and muscle glycogen, as the primary source of energy (Figure 1). (1) Fat provides a larger energy yield than carbohydrates, however, requires more (around 2.4x) oxygen to metabolise than carbohydrates. Carbohydrates, however, release energy quickly with lower oxygen cost. Therefore, for low-moderate intensity exercise, we can use more fat for energy as there is more oxygen and time available to metabolise it. As exercise intensity increases, our oxygen demand increases and carbohydrates become more efficient to use as they provide energy more quickly, requires less oxygen and time to metabolise.
Figure 1: Exercise expenditure and fuel utilisation. FFA= free fatty acids; VO2 max = maximal volume of oxygen consumption. (2)
So when cardio machines suggest fat-burning workouts, this should mean the workout will keep you working at a low-moderate intensity, usually calculated by measures like your heart rate. Low-moderate intensity means you’ll be working at around less than 65% of your VO2 max (maximal volume of oxygen consumed), but an easy way to know this if you can talk or hold a relatively short conversation without gasping for air.
"Burning" fat vs burning calories
Whilst you will mostly be utilising fat during a low-moderate intensity workout, this does not mean, however, mean you are losing body fat during that particular workout.
To lose body fat, you need to be in a calorie deficit where total calories expended are more than calories consumed over a long period of time (Figure 2). The best way to achieve a sustainable calorie deficit, ie., where you can actually lose body fat and not just water, is mainly through your diet. Incorporating movement you enjoy and strength-based training will also help.
Figure 2: Overview of the principles of weight gain, weight loss and energy balance.
Can specific exercises burn fat in specific areas?
You may have also come across exercises that claim to burn fat in specific areas. For example, claims that sits up can burn belly fat. As above, the only way to lose body fat is through an overall calorie deficit, i.e, sit ups alone will not lead to losing body fat. It is also not possible to target where your body loses body fat. Your body composition and where you lose body fat is largely based on your genetics.
What burns more calories?
In terms of what burns more calories, high vs low intensity workouts, you can burn the same amount calories in a shorter amount of time in a high intensity workout than a low intensity workout. However, it’s harder to sustain high intensity exercise for a long period of time.
Should you eat a high fat diet to improve performance in low intensity, endurance activities?
Since fat is the primarily energy source in low-moderate activities, a thought that may come to mind is that eating a high fat diet or high fat foods before a low-moderate intensity workout will help you perform better and longer. A high fat, low carbohydrate diet can increase the rate of fat oxidation/utilisation during endurance (low-moderate intensity) exercise and spare muscle glycogen (carbohydrate) use. (3,4) However, it has been found that high fat diets or eating fats before exercise has little to no effect on performance in endurance sports. (5,6,7)
So, in summary:
Fat burning exercises/workouts do not mean you will be burning body fat (and therefore losing weight) during that exercise/workout.
Fat burning during a workout applies to "burning" fat for energy and this is derived from fatty acids in your blood and other sources.
Fat is primarily used for energy over carbohydrates in low-to-moderate intensity exercise. In high intensity exercise, carbohydrates, from circulating glucose in your blood and from muscle glycogen, are the main source of energy.
A high fat diet or eating fat before exercise does not improve performance in endurance exercise.
Which ever type of workouts you do is up to you, what you enjoy and your training goals!
References:
Hargreaves, M., Spriet, L.L. Skeletal muscle energy metabolism during exercise. Nat Metab 2, 817–828 (2020). https://doi.org/10.1038/s42255-020-0251-4
Romijn, J. A. et al. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. Am. J. Physiol.265, E380–E391 (1993).
Vukovich, M. D. et al. Effect of fat emulsion infusion and fat feeding on muscle glycogen utilization during cycle exercise. J. Appl. Physiol.75, 1513–1518 (1993).
Odland, L. M., Heigenhauser, G. J., Wong, D., Hollidge-Horvat, M. G. & Spriet, L. L. Effects of increased fat availability on fat-carbohydrate interaction during prolonged exercise in men. Am. J. Physiol. 274, R894–R902 (1998).
Asker E Jeukendrup, Sarah Aldred. Fat supplementation, health, and endurance performance. Nutrition, Volume 20, Issues 7–8, 2004, Pages 678-688.
Burke, L. M. et al. Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling. J. Appl. Physiol. 89, 2413–2421 (2000).
Burke, L. M. et al. Low carbohydrate, high fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers. J. Physiol. (Lond.) 595, 2785–2807 (2017).