Q Will eating a few sweets before a run boost my energy and my speed?
A Since they're almost pure sugar (sucrose and glucose), most sweets have a high glycaemic index (GI) of around 75-80, which means they can deliver a rapid blood sugar boost. Conventional advice is to avoid sweets and other high GI foods pre-exercise in case they trigger a surge of insulin and a resultant blood sugar dip (hypoglycaemia), but the truth is that it's all down to portion size.
For example, jelly beans contain a meagre 2g of sugar per sweet. A small handful (30g) consumed before a run gives you just 25g of sugar - half the amount used in studies to determine the GI rating of foods - which is enough to fuel 30 to 60 minutes of running and produces only a moderate blood sugar rise.
This concept of portion size and GI is called "glycaemic load" (GL) and gives you a more accurate idea of how a food behaves in your body.
GL is calculated by multiplying a food's GI by the carbohydrate per portion, and dividing the result by 100). So the GL of 30g of jelly beans is 19 (76 x 25g divided by 100 = 19) - a medium GL food, despite its high GI (76) - which would be perfectly safe blood sugar-wise to consume pre-run. However, if you were to double the portion size to 60g (50g sugar) you'd get a GL of 38 ("high") - enough to send your blood sugar levels soaring. This carries a bigger risk of hypoglycaemia, faintness and dizziness.
Stick to modest portions of sweets before a run. Eat roughly 15 minutes before you set off and ensure the portion supplies no more than 25g sugar. If you plan to run longer, take some sweets to eat on the run. Begin to eat them after about 30 minutes and aim for 12-15 jelly beans per hour.
Anita Bean, RW nutrition editor