Why is it that, when I manage to get to the gym or pool most days and feel that I have pushed myself quite hard, I weigh more than when I am not able to exercise properly for a couple of weeks?

According to the National Register of Weight Loss, monitoring your weight regularly is one of the key ingredients of successful long-term weight loss. However, there are a number of factors that can subtly affect weight fluctuations, and knowledge of these can help keep things in perspective.

Bouts of weight training can increase blood flow to the muscle area concerned, and fluid between the muscle fibres. While this appears not to increase weight drastically in the short term, it can alter the size and potential shape of the muscle. Over four to six weeks, this is important for increased muscle development through protein synthesis, which can increase weight. However, combine it with cardiovascular activity and sensible diet, and the size, shape and health of your body will improve.

Hydration can increase weight by as much as 1-2%, while the composition of your diet - specifically levels of sodium and carbohydrate - affects the amount of water your body holds on to. Some individuals have been shown to be more sodium-sensitive, retaining more water with a sodium-rich diet. One client of mine lost an incredible 5kg in one week when I switched their diet from a high- to low-sodium one. Obviously, calorie intake was affected, but water retention had a significant impact, too.

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