Fat burning is a subject many trainers get wrong. The common belief is that you should train at a low intensity because your body uses fat for energy at that level. In reality, weight loss depends on a long-term energy deficit, not the mix of fuels your body is using at any given moment.
Fat burning at low vs high intensity
At 40–60% of maximal oxygen uptake, fat supplies around 50% of your energy needs. This is where the recommendation to take it easy if you want to lose body fat comes from. But that approach overlooks one fact: at a low intensity, fat accounts for a larger share of energy expenditure, but you burn calories much more slowly (around 4–5 kcal/min) than under a high training load (around 10–13 kcal/min).
A simple thought experiment: does jogging 5 km burn more body fat than running the same 5 km at race pace? In fact, calorie expenditure and fat loss are essentially similar—you simply reach the same result sooner when you run faster.
What the research shows
10 years ago, Gaesser and Rich compared two groups of college-aged men who trained on a cycle ergometer 3 days a week for 18 consecutive weeks. One group cycled for 50 minutes per session at 45% of VO2 max, while the other cycled for 25 minutes at 85% of VO2 max.
Energy expenditure per session was within 300–350 kcal in both groups. After 18 weeks, both groups had lost the same amount of body fat—an average of 1.5 kg. The reduction in body fat percentage was also similar. Although the low-intensity group burned more fat during each workout, the high-intensity group burned more fat after training.
Fat burning after training
Bahr and Sejersted had 6 men train for 80 minutes at three different intensities: 29%, 50% and 75% of VO2 max. The higher the training load, the further the respiratory quotient fell after training, indicating greater fat oxidation. After high-intensity training, the effect lasted up to 14 hours.
The logic is simple: if carbohydrates have been used up during training, your body has to turn to fat for recovery. The harder the workout, the more depleted your carbohydrate stores are and the longer the afterburn lasts.
Wilmore adds that although fat makes up a larger proportion of the fuel mix at low intensities, you still burn more fat in grams at higher intensities, provided the training duration is similar. Training at 70–75% of VO2 max burns considerably more calories from fat than a workout of the same duration at 40–60%.
Practical takeaways
If your goal is to lose body fat, you need to be in an energy deficit—to lose 450 grams of body fat, you need to burn around 3500 kcal more than you eat. It does not matter whether those calories come from fat or carbohydrates.
If you are significantly overweight or new to training, starting at a lower intensity makes sense—it prevents injuries and gives your body time to adapt. The American College of Sports Medicine recommends at least 30 minutes of moderate physical activity every day of the week. You could aim for a weekly energy deficit of around 1000 kcal through training.
As a general rule, an hour-long low-intensity workout is equivalent to around 30 minutes of high-intensity training. Your choice depends on your fitness, health and what you enjoy enough to keep doing week after week.
Translator’s note: it all depends on what you eat, too. You can be active for an hour every day, but if you consume 1500–2000 kcal more than you need each day, nothing will change, and you may even keep gaining weight. For fat burning, I recommend combining different activities, intensities and methods.
FAQ: fat burning in practice
Does fasted cardio burn more fat?
The short answer: not overall. When you train fasted, a higher percentage of the energy you use may come from fat, but over the course of a day or a week, the energy deficit is still what counts. Choose the option that agrees with your digestion and gives you the energy to train regularly.
Should I always train at a high intensity?
No. Mix it up. One or two higher-intensity sessions a week, plus a few easier workouts, allow for better recovery than pushing yourself to the limit every day.
How long does the afterburn last?
It depends on training intensity. In Bahr and Sejersted’s experiments, fat oxidation remained elevated for up to 14 hours after high-intensity training. After low-intensity training, the afterburn is considerably shorter.
References
- Nieman, DC. Nutrition and physical performance. In: Fitness and Sports Medicine: An Introduction. Palo Alto, CA: Bull Publishing Company; 1990:221-269.
- Gaesser GA, Rich RG. Effects of high- and low-intensity exercise training on aerobic capacity and blood lipids. Med. Sci. Sports Exerc. 1984; 16:269-274.
- Bahr R, Sejersted OM. Effect of intensity of exercise on excess postexercise post VO2 consumption. Metabolism. 1991; 40:836-841.
- Gatorade Sports Science Institute. Commonly asked questions regarding nutrition and exercise. Sports Sci. Exch. Roundtable. 1992; 3:2.
- Centers for Disease Control and Prevention and American College of Sports Medicine. Summary Statement: Workshop on Physical Activity and Public Health. Sports Med. Bull. 1993; 28:7.
- American College of Sports Medicine. The recommended quality and quantity of exercise for developing and maintaining fitness in healthy adults. Med. Sci. Sports Exerc. 1990; 22:265-274.
Translated by: Janar Rückenberg
Come and train! ArtGym
