The night before someone runs a marathon, there’s a fair chance they’ll chow down on pasta. The carbohydrate in pasta is supposed to help stock the liver and muscles with glycogen (a form of starch) which can then be used to fuel one’s efforts the following day. Such an approach is said to reduce the risk of glycogen depletion which can lead to a serious downturn in output colloquially referred to as ‘hitting the wall’.
The concept of endurance athletes stocking up on carbs has, I think, fuelled the notion that we should ideally have some sort of fuel inside of us prior to exercise. However, as I explain here, there is an argument for avoiding spikes in blood sugar is seeking to maximise one’s capacity to utilise fat as a fuel during exercise. I think there’s an argument for consuming little or nothing before exercise unless, perhaps, exercise is to be very prolonged.
I was interested to read about a recent study in which the effect of feeding prior to exercise on fat burning was tested . In this study, a group of overweight men were assessed in the morning in each of three conditions:
- A set breakfast with no exercise
- A set breakfast followed by 60 minutes of exercise at 50 per cent maximal intensity
- A set breakfast following 60 minutes of exercise at 50 per cent maximal intensity
Three and a half hours later the men were given lunch which they could eat in unlimited quantity. Intakes at this lunch were essentially the same on all three occasions.
The study subjects were also assessed in terms ‘fat balance’ (the relative amounts of fat consumed and metabolised) over an 8.5-hour period. Overall, fat balance was favourable after exercise compared to no exercise. However, the effects were significantly more favourable if breakfast was taken after exercise, rather than before.
The ‘fat deficit’ equated to 250 calories if breakfast was taken after exercise, compared to 167 calories if breakfast was taken before.
This is just one study, done in a limited number of people, with each condition tested only once and in quite a controlled fashion, so let’s not over-interpret the results. However, there is evidence here which supports the notion that, for the most efficient fat-burning, exercise may perhaps be best performed on an empty stomach.
1. Farah NMF, et al. Effects of exercise before or after meal ingestion on fat balance and postprandial metabolism in overweight men. British Journal of Nutrition. Published online: 26 October 2012