Metabolism

Myth: "Your metabolism increases after exercise, so you burn more kilojoules even though you've stopped exercising."

Bonus! Not only have you just burned off 1600 kilojoules doing aerobics, but you'll keep burning kilojoules for hours afterwards. Better still, the less fit you are, the slower you recover and therefore the more you burn.

Experts call it excess post-exercise oxygen consumption (EPOC), whereby increased metabolism means consuming more oxygen, which in turn burns kilojoules. And while it's not entirely bunkum, one extra biscuit could wipe out that kilojoule debt - and then some.

Most people who are aware of this post-exercise metabolic increase probably overestimate the degree of extra energy burned. And the small amounts involved could well be negated by moving less throughout the day and/or eating slightly more as a 'reward' for exercising.

For any sort of meaningful EPOC effect, you have to exercise long and hard. Low or moderate-intensity exercise - walking or slow jogging, say - would have little to no effect. Even at higher intensities you need to exercise for almost an hour for any appreciable increase in metabolism. Experts argue the level required would be intolerable for the average person, and only serious athletes would be capable of sustaining the required intensity for long enough.

Bottom line:

Any increase in metabolic rate after exercising is likely to be fairly insignificant for the average athlete.

Myth: "You burn more fat if you exercise on an empty stomach."

The theory goes that if you don't eat before exercising your body has to use fat as fuel because there's hardly any sugar to burn. And if you want to lose weight it stands to reason that burning off fat is a good thing.

There's nothing wrong with the basic theory - evidence shows that more fat is burnt after longer fasting conditions than in the few hours after eating, when carbohydrate is the preferred fuel. But in fact it's the total amount of energy burned that's the issue, not whether it's fat or carbohydrate.

You may find that without eating you get tired and don't exercise as hard as you would if you'd eaten beforehand - and therefore you'll burn less total energy.

Depending on how hard and how long you intend to exercise and whether exhaustion might affect your speed and endurance, it may be better to have some fruit, yoghurt or toast at least half an hour before exercising, as well as a drink to help prevent dehydration, and have the rest of your meal afterwards.

Bottom line:

Eating before exercise may help by giving you more energy for a better workout, and it's total energy burn that counts.