Our bodies use two main types of fuel: fat and carbohydrates. Research at Surrey's Faculty of Health and Medical Sciences reveals that timing when we exercise and eat can affect how our bodies burn fat.
Dr Adam Collins, Programme Director for BSc and MSc Nutrition at the University of Surrey, has discovered that the amount of fat we burn changes based on whether we eat before or after exercise – and this is also different for men and women.
Dr Collins said: “Most people are using exercise to get healthier, lose weight, or burn fat rather than maximise performance like athletes do. Hence the strategies for eating and exercise are different for competitive athletes.
"Most people exercise for health, not performance.”
Dr Collins’s original research suggests that untrained females may benefit from consuming food prior to exercise and avoid eating during recovery, whereas untrained males may benefit from waiting until recovery to consume food.
In order to see whether this effect measured in the laboratory could be significant in the real world, BBC2s Trust Me I’m a Doctor teamed up with Dr Collins and his research group to recruit 30 volunteers to take part in another experiment.
Thirteen men and 17 women were selected who did not normally do a lot of exercise, and for four weeks they all took part in three supervised classes a week: high intensity training, Zumba and spin classes.
All of them had a drink both before and after each exercise class, but one of their drinks was a placebo (had no calories in it), whilst the other was a carefully calorie-controlled hit of carbohydrates.
No one knew who was taking which drink when. Seven of the men were taking the carbohydrate drink before exercising, whilst six were taking it afterwards. Seven of the women were taking the carbohydrate drink before exercising, whilst ten were taking it afterwards.
At the beginning and end of the experiment, researchers tested how much fat they were burning while at rest (as well as a range of other measures such as weight, waist circumference and blood sugar/fat levels).
While all the women ended up burning slightly more fat at the end of the experiment, those who were consuming carbohydrates before their exercise were burning more.
Meanwhile, all the men were actually burning slightly less fat at the end of the experiment, but those who were consuming carbohydrates after their exercise were better off. There were no significant differences in their weights or waist circumference, but their blood sugar levels changed in the same way as their fat burning.
It’s evident that men and women burn fat and carbohydrates in different ways. Men are ‘carbohydrate burners’ – if as a man you eat carbohydrates then your body is going to burn it rather than fat.
However, given that we all have to eat, it is better for men to eat after exercising if they want to burn fat. This is because, after exercise, men will use that carbohydrate to replace the carbohydrate in their muscles rather than burn it for fuel and will continue to burn fat instead.
For women, the results show that eating before they exercise is better than eating after if they want to burn fat. Women’s bodies tend to burn fat more easily than men’s, and are not fuelled so much by carbohydrates.
Moreover, women are much better at conserving carbohydrates during exercise. So when women eat carbohydrates soon after exercise, this is effectively overloading them with fuel, and interferes with the body’s ability to burn fat.
- For women, eating about 90 minutes before exercise is better than eating after if they want to burn fat
- The key message for women is: don’t eat carbohydrates immediately after your exercise. You don’t have to eat before, just be mindful of what you eat after
- It is better for men to eat around 90 minutes after exercise if they want to burn fat
Although the television study was small, put together with the evidence from the laboratory experiments, it does seem worth making that simple adjustment to when we eat in order to maximise the amount of fat our bodies burn throughout the day.
This feature was originally published on 26 February 2016.