If you are going on a one-hour run and want to shift body fat, should you wait for up to an hour after running before you eat a meal, to aid the increase in general metabolic rate, or will you burn more calories if you eat before you train?

Your question relates to two concepts: the thermic effect of feeding and excess post-exercise oxygen consumption (EPOC), commonly called "exercise afterburn". Both can raise your metabolic rate - but hold on before you get too excited.

Digesting and absorbing nutrients requires energy - around 10% of the energy consumed at any one meal. So if you eat a 400-calorie meal, your metabolic rate will burn only a potential extra 40 calories.

EPOC refers to the increased uptake of oxygen post-workout, and the consequent increase in your metabolic rate. This varies according to how long and hard you exercise (the harder the session, the greater the potential afterburn); how fit you are (the fitter you are, the quicker your body will return to its normal metabolic rate); and what exercise you do (resistance exercise seems to have less impact on raising short-term metabolic rate than vigorous, weightbearing cardiovascular exercise).

Since with either category the potential additional increase in calories burned is relatively small and your primary concern is body fat, I'd urge you to pay more attention to your overall energy balance. Focusing on the number of calories you are consuming, compared with those you are expending through day-to-day activity, as well as structured exercise will reap greater rewards.

· Joanna Hall is a fitness expert (joannahall.com). Send your exercise questions to Weekend, 119 Farringdon Road, London EC1R 3ER (weekend@theguardian.com).