A few months ago I was chatting with a gentleman who was having difficulty losing weight despite doing ‘all the right things’. Almost every morning he would start the day with a vigorous and quite lengthy exercise session. He would ‘fuel’ this session with a bowl of cornflakes. I suggested he pull back a bit on the cornflakes, on the basis that it would likely cause his blood sugar and insulin levels to skyrocket, which in turn would impair his body’s ability to mobilise fat for burning during the exercise session.
Not one for doing things by halves, he declared he would just stop eating the cornflakes and do the exercise on an empty stomach. Hey presto, the last I heard he was thrilled with the results.
This conversation came back to me today when I was reading a study which explored variety of nutritional approaches to enhancing fat-burning during exercise . Most attention in this paper is paid to the nature of carbohydrate consumed prior to exercise. Specifically, the authors put forward the idea that carbohydrates of relatively high glycaemic index (generally disruptive to blood sugar) will lead to high levels of insulin that inhibit what is known as lipolysis. Lipolysis is a word used to describe the process through which fat is mobilised from cells (usually fat cells) to appear in the bloodstream as ‘fatty acids’. These fatty acids can make their way to the muscle cells to be burned during exercise, though this is unlikely to happen to the same degree if they end up ‘trapped’ in the fat cells as a result of the influence of insulin.
The authors concede that the evidence regarding the influence of the glycaemic index of food eaten prior to exercise on fat-burning is not utterly consistent. However, taken as a whole, there is more than enough evidence to think that there are benefits to be had from avoiding spikes in sugar and insulin prior to exercise. To quote from the paper:
“… studies have demonstrated up to 2-fold increases in the amount of whole-body fat oxidation during treadmill running. This occurred during exercise intensities ranging from 50 to 70% VO2max in both males and females, differing in activity levels and differences even occur during the first 15 min of exercise. Furthermore, similar findings are seen with carbohydrate intakes ranging from 1 to 2·5g/kg BM [body mass] and when the pre-exercise meal is ingested from 30min, up to [12 hours] before exercise.”
Another interesting note from this paper concerned the impact of the sugar fructose on fat-burning. Although fructose is a low glycaemic index food, the authors point out that it inhibits fat burning during and after exercise to a greater extent than glucose. There is some thought that one of fructose’s effects is to increase levels of lactate – which is known to inhibit lipolysis.
I’m not a big believer in aerobic exercise for weight loss, but I do think that it’s perhaps worth bearing in mind that what one eats prior to exercise can influence fat burning and any weight loss results that might be achieved. Also, for those engaged in endurance exercise, enhanced fat-burning can reduce reliance on carbohydrate as a fuel, and reduce the risk of ‘glycogen depletion’ and ‘hitting the wall’ (also known as “bonking”). Cornflakes, with their high glycaemic index and relatively fructose-rich nature make a pretty poor choice of pre-exercise food.
So, what should people eat prior to exercise? Maybe nothing.
I know we’re often told we should have something inside our stomach during exercise to ‘fuel’ our efforts, but I’m not sure this is always a good idea. If someone exercises in the morning and can do this on an empty stomach, so much the better I think. Insulin levels are likely to be low and this might help fat-burning. It might, essentially, force the body to dig into its fat reserves for the energy it needs during the exercise. As long as there is not undue weakness or lightheadedness, I see no issue with exercising on an empty stomach.
This may not be the best approach for those seeking to crack their personal best for the marathon. But for common-or-garden exercise, I think it’s the way forward for most.
1. Gonzalez JT, et al. New perspectives on nutritional interventions to augment lipid utilisation during exercise. British Journal Nutrition epub 5 December 2011.