I'm training to do my first triathlon. I work shifts, so training and eating can be erratic. I want to refuel correctly, but not increase my body fat.

Effective refuelling involves paying attention to what you eat before, during and after sessions. You'll notice a big improvement in alertness and performance.

Pre-fuelling involves eating early in the day, to replenish vital muscle glycogen stores. Failure to pre-fuel is a common mistake that can not only affect performance but also increase the risk of injury; you'll be more likely to compromise on nutritional quality after training, too, and to refuel on excess empty calories, thereby tipping the balance towards potential body fat gains.

When training later in the day, make sure you have had two meals and one snack beforehand. If training early in the morning, after a night shift, say, your midnight meal is critical to a high-performance workout.

Fuelling during training should focus on hydration. To this end, make your own sports drink: add a pinch of salt to fruit juice (this aids fluid uptake) and dilute with water (too concentrated a juice decreases rate of absorption). For longer sessions of one to three hours, a more solid food snack is required - try dried fruit or an energy bar.

In the two-hour window after exercise, eat more high glycaemic index foods to replenish muscle glycogen stores faster and to help repair any muscle tissue damage. While body fat is important, don't get too hungry, aim to make moderate changes to your diet, and experiment until you find what works for you.

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