Could this really be true? Well a laboratory test carried out by Dr Adam Collins, Senior Tutor in Nutrition at the University of Surrey indicates that the amount of fat we burn changes based on whether we eat before or after exercise – and this appears to be different for men and women.
Our bodies use two main types of fuel: fat and carbohydrates – and early indications from Dr Collins’s study suggest that we could increase the amount of fat we burn just by timing when we eat with when we exercise. I was interested to discover more – like many others I had thought that exercising on an empty stomach was the way to go, but now that school of thought appears to be changing.
Michael Moseley’s team from the BBC programme “Trust Me I’m a Doctor” worked with Dr Collins on an experiment to see if it might be possible to change our eating habits around exercise to increase the amount of fat our bodies are burning throughout the day. Here are the results of that experiment:
Experiment 1: The laboratory test
Adam’s initial experiment had shown that for young men, eating carbohydrates (CHO) before exercise significantly decreased the amount of fat their bodies were burning for the 3 hours afterwards, whilst they were resting (n=10, p = 0.02, Wilcoxon matched pair tests, repeated measures 2-way ANOVA, multiple t-tests).
When he did a similar experiment with both men and women, he and his team found that whilst the men still burned less fat if they had eaten carbohydrates before exercising (n=7, p<0.05), the women burned MORE fat if they had eaten carbohydrates before exercising (n=8, p<0.05).
* Significantly different between genders: p≤0.05; † Significantly different between treatments: p≤0.05
This experiment was repeated on a single brother and sister pair, Jess and Josh, and got similar results.
Jess and Josh results:
Experiment 2: The long term effect
In order to see whether this effect measured in the laboratory could actually be significant in the real world, “Trust Me I’m a Doctor” teamed up with Adam and his research group to recruit 30 volunteers to take part in a longer term experiment.
Thirteen men and seventeen women who did not normally do a lot of exercise we chosen and for four weeks they all took part in three supervised classes a week: high intensity training, Zumba and Spin classes.
All participants had a drink both before and after each exercise class, but one of their drinks was a placebo (with no calories), whilst the other was a carefully calorie-controlled hit of carbohydrate. No one knew who was taking which drink or 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, they were tested on how much fat they were burning whilst at rest (as well as a range of other measures such as weight, waist circumference and blood sugar/fat levels).
Whilst all the women ended up burning slightly more fat at the end of the experiment, those who were taking carbohydrates before their exercise were burning more.
Difference between men and women:
Difference between men and women:
How it works
Men and women burn fat and carbohydrate in different ways.
Men are very much ‘carbohydrate burners’ – if as a man you eat carbohydrate then your body is going to burn it rather than fat. Just giving the men carbohydrate at any time in our experiment made them burn a bit less fat! However, given that we all have to eat (and carbohydrate is an important part of our food), 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 clearly 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 carbohydrate. Moreover, women are much better at conserving carbohydrate during exercise. So when women eat carbohydrate soon after exercise, this is effectively overloading them with fuel, and interferes with the body’s ability to burn fat.
The amount of carbohydrate in our tests is probably the equivalent of a piece of toast, or a small bowl of cereal – and for men ‘not eating before exercise’ means about 90 minutes before exercise, and for women ‘not eating after exercise’ similarly means for about 90 minutes after.
Although our study was quite small, put together with the evidence from the laboratory experiments, it does seem worth us all making that simple adjustment to when we eat in order to maximise the amount of fat that our bodies burn throughout the day.
So what do you think? These studies are always interesting and I would like to see more work in this area as there does seem to be a lot of conflicting advice available on the internet today.