Fat calories make you fat, protein calories make you lean. A calorie is a calorie, is a calorie. However, portions of fat are denser in calories than portions of protein. One gram of protein equals four calories. But one gram of fat equals nine calories, says nutritionist Joy Bauer. So it's easier to get fat eating fatty foods than it is eating lean protein.

Don't eat dairy if you're trying to lose weight. Low-fat dairy gives value for your money, nutritionally, says Bauer. 'As long as you're not lactose intolerant, low-fat dairy is fine.' Furthermore, a recent study published in the Journal of the American College of Nutrition shows that women who got most of their daily calcium from dairy foods actually lost weight and body fat over two years – regardless of exercise.

You can munch as many low-fat snacks as you want and still lose weight. Say it with us again: calories count. Many low-fat foods compensate for lost flavour with additional sugar. That's why the difference in calories between low-fat and normal-fat snacks is often minimal. Consider the biscuit. A medium chocolate-chip biscuit contains about 50 calories, with 20,5 calories from fat. A low-fat biscuit has 45 calories, with 13,9 calories from fat. Less fat, true. But the calorie difference is almost nil.

Obesity is the result of eating too much sugar. In fact, sugar and fat are both responsible – as are the phrases 'double-dip it' and 'I'll take the large'. The most common reason for obesity? Too much food, not enough exercise. It's that simple, Bauer says.

Don't eat bananas, grapes, carrots or beets. They have a high sugar content. Ridiculous, says Bauer. A banana has 18 grams of sugar, a half-cup of grapes about seven grams and both are between 70 and 110 calories per serving. Half a cup of carrots has only five grams of sugar, the same amount of beets has four-and-a-half grams, and both are between 25 and 35 calories. They have decent quantities of fibre, carotenoids, potassium and folate, too.

Give up sugar, and you'll stop craving it. Repeated studies have never confirmed that people really stop wanting sugar, says nutritionist Sharon Akabas, research assistant at Columbia University's Institute of Human Nutrition.

There is some evidence that eating a high-sugar snack in the evening makes a person hungrier in the morning, but trying to document a simple relationship between hunger and food intake is very difficult. 'People eat when they're hungry and they eat when they're not.'

If you're eating a salad, you're safe. A salad may seems like your smartest friend – until it's not. To the average dieter, it's as if putting anything on a bed of lettuce negates its calories. But take a closer look. Is it smothered in oily dressing? Are the greens and tomatoes merely a vehicle for chicken, blue cheese and bacon?

Eating lettuce burns calories. Ditto for grapefruit and celery. 'Don't we wish,' says Marion Nestle, chairman of the department of nutrition and food studies at New York University. If you ate nothing but lettuce, grapefruit and celery, you would lose weight. You'd also be very sick, because there would be no protein or fat in your diet.

Eat a high-protein diet and the kilograms will melt off. Typically, high-protein diets are low in carbohydrates, which means a substantial source of calories is eliminated. Cutting carbohydrates also means the body naturally loses water quickly: a high-protein diet taxes the kidneys, thus dehydrating the body. But ultimately, say nutritionists, a high-protein diet is neither safe, nor effective.

Many of these diets advocate a lot of meat, which means a lot of animal fat – not great for the heart. And most people can't stick to a high-protein diet in the long haul.

Sports bars are a great meal substitute.
The misconception is that these are somehow protein bullets, going straight to the biceps. 'Choose the right kind, and it's better than skipping a meal altogether,' Akabas says. 'But some of them are only marginally healthier and lower in fat than a candy bar.' Look for a bar that has five grams of fat or less and at least three grams of fibre. Fibre slows the body's absorption of sugar, giving a more sustained release of energy throughout a workout – or during an after-lunch slump.

Drinking water during a meal helps you lose weight. False. 'Water helps the body separate and digest food and aids the process of elimination,' notes nutritionist Barbara Crosby. 'But a normal amount of water has no effect on caloric intake.'

Diet sodas help you lose weight. Mental weight maybe – there's less angst attached to a diet soda. But at least one study, says Bauer, showed that people who drink diet sodas make up the 'saved' calories with after-dinner snacking.

Don't eat carbohydrates in the morning or you'll get hungry sooner. Or – eat carbohydrates in the morning to keep the hunger pangs at bay. Here's a wacky concept: why not eat a variety of nutrients? Crosby notes, 'Protein will sustain you throughout the day, and a little simple carbohydrate – toast, fruit – will give you instant energy. Eggs, a slice of wholegrain toast and half a grapefruit would be a perfect start to a day.'

Certain foods – pizza and pasta, for example, must be avoided if you want to stay thin. No. The best diets involve budgeting, notes Bauer. Try to incorporate roughly 90-per cent healthy foods and about 10-per cent 'fun' foods (meaning, for many women, starch and sweets) into your diet.

Don't eat after 6pm. It's the total number of calories eaten during the day that counts. But research has shown that people who eat late at night tend to be overeaters. 'I think it's important to have the bulk of your caloric content before the evening meal to give the body fuel when it's necessary to perform,' Crosby says. Even so, an after-dinner chocolate mint is fine. A whole chocolate cake is not.