We like to think that taking regular exercise means we can eat what we want, when we want. If only it were that simple, says Rebecca Hardy.

Many of us believe that exercising gives us a licence to eat fattening foods with no adverse effects. Surely that's the whole point of physical exertion? Not so, say the experts. Any effective exercise regime must be supported by the right food. "It's like fuelling a car," says sports scientist Dr Garry Palmer, fellow of the American College of Sports Medicine. Bad nutrition, he says, can hinder performance, leaving you sluggish and nauseous.

"Many people think going to the gym means they can eat what they want. It can free you up, but not much," says British sports nutritionist Drew Price. "It depends how overweight you are."

Heidi Skolnik, a sports nutritionist who works with the New York Giants football team, says it's all about balancing the calories coming in with the calories going out.

"I often see people who can't understand why they're not losing weight, but, when we look at their food intake, they're eating as if they're running a marathon, not half an hour a day. That might burn 300 calories, but that's one chocolate bar - not a lot of food. Generally, you need more calories only if you are training intensely for more than two hours every day."

The standard advice is that, if you're exercising, your diet should consist of

60 per cent carbohydrates and 10-12 per cent protein. The rest should be fat.

If you want to lose weight, eat more protein and reduce both your carb and fat intake. "Proteins increase your metabolism and chew up calories faster," says Price.

Other weight-shedding advice is to eat your carbs immediately post-exercise, when they are less likely to turn to fat.

Skolnik says sipping a carb-based sports drink during an intense spinning class (lasting an hour or more) helps the body to burn more fat.

Another piece of often ignored advice is to drink enough water.

"Check you are getting enough by weighing yourself before and after," says Skolnik.

The timing of meals is also a big factor. The general rule is to wait three to four hours after big meals and 1 1/2 to two after smaller meals before exercising. This way you'll avoid feeling lethargic.

Or keep a food diary of everything you eat. It is hard to kid yourself when it is all there in black and white. Above all, says Price, see the bigger picture: "There are no bad foods, just bad times and quantities to eat them in."

EXERCISE AND FOOD PLAN

The morning work-out

Early birds need to pep up their blood sugar, refuel their muscles and drink about 475 mls of water to replenish the water lost during sleep. Remember, your body has been on an eight-hour fast. "Dehydration is one of the biggest factors in morning work-outs," says Price.

If you love breakfast, aim for a small but healthy one - such as oats, yoghurt and fruit or brown toast with a little jam - no less than an hour (preferably longer) before you work out. Think little portions: exercising on a full stomach will only make you nauseous.

Breakfast refuseniks can up their blood sugar with half a banana or half a slice of bread before they work out, then afterwards eat a healthy breakfast such as fruit, oats, wholemeal toast and a poached egg, washed down with 475 mls of water.

Is it ever OK to exercise on an empty stomach? It depends on the individual. Dr Palmer recommends that for intensive work-outs, the athletes he works with delay solids and opt for a sports drink instead.

The lunchtime work-out

It is nearly one o'clock, your stomach is growling, but you're determined to hit the gym. You can't eat lunch before a work-out, so, ideally, you should snack on some fruit, nuts and yoghurt at "around 10.30-11am to stabilise your blood sugar", says Price. "Then have lunch after you've exercised."

If you're too busy to remember to do this, nibble a banana or raisins right before you exercise. Sports drinks are advisable only if you are doing high-intensity exercise or working out for more than an hour, otherwise you will be saddled with spare calories. A quick cuppa can help (30-45 mins before), but have a regular coffee, not a double-caramel latte.

"Caffeine is a double-edged sword," says Price. "It allows your nervous system to fire more easily so you can do more work without feeling it. But too much of it can dehydrate you."

Sensible post-work-out lunches include a chicken or salmon salad or sandwich, or tuna and a roast potato, plus fruit.

The after-work work-out

Dodge afternoon munchies by eating a yoghurt and piece of fruit between 3pm and 4pm. This keeps your blood sugars up so that you won't be ravenous by dinner and therefore more likely to overeat.

Skolnik advises eating a small carb snack immediately after your work-out, particularly if you have a long journey home (fitness lore states that you should eat or drink carbs 15-30 minutes after exercise to repair the muscles).

For dinner, have lean protein and plenty of colourful veg, such as salmon with stir-fried vegetables and wholegrain rice; pasta with chicken breast and steamed or roasted vegetables; chicken and vegetable couscous, or lentil-and-bean tabbouleh with vegetables. Avoid eating a mountain of pasta, for example, and loading your body (and waistline) with carbohydrates. Again, make sure you drink enough water - especially if you're drinking alcohol.

Guardian