If you're giving up coffee to boost hydration or avoiding eggs to protect your cholesterol, then stop. Here the experts fight back against popular pseudo-science.

By Helen Foster

Fifty per cent of us limit our coffee intake because we believe it makes us lose fluid, but it is safe to hit Starbucks again. "Coffee has no diuretic effect on regular drinkers," says Dr Ann Grandjean of The Center for Human Nutrition in Nebraska. In fact, every cup you drink not only hydrates you, it also raises antioxidant levels in your blood by four per cent for two hours, so it's beneficial too. But stick to just 3-4 cups a day because of its high caffeine content.

Blueberries have held a ‘superfood' title for quite a while now, but research by Texas A&M University found that one plum contained the same amount of antioxidants (and in some cases more) as a handful of blueberries. "People tend to eat just a few blueberries at a time, on top of cereal perhaps - but, of course, they'll always eat a whole plum," says Dr Luis Cisneros-Zevallos, who carried out the research, so plums offer a bigger dose per serving.

When you're worried about your weight, it's easy to be seduced by products saying: "Just 99 cals". But you could be doing more harm than good. "Diet foods such as reduced-calorie dishes can be full of sugar, carbohydrate substitutes and synthetic vitamins," says nutritionist Penny Rushton. "They have little nutritional value and your body is put under a strain to digest them. Eating them also means you miss out on essential fatty acids from good fats."

Worried that you can't eat dinner after 8pm, as your body won't burn off the calories? When scientists studied monkeys who consumed 65 per cent of their calories at night, they found they weighed the same as those eating the same diet, but who had only six per cent of their calories at night. Night-time nibbling is only an issue if it adds extra calories to your diet, such as munching crisps mindlessly in front of the TV. "We eat for reasons other than hunger at night - boredom, stress, tiredness etc," says diet coach Chrissie Webber (www.life-shapers.com). "To break the habit, look at the emotion you're really feeding and address that."

Not according to Dr Bruce Griffin of the University of Surrey. He put people on a two-egg-a-day low-calorie diet for 12 weeks and found no rise in their cholesterol levels - in fact, as the people lost weight, their cholesterol often went down. As a bonus, eggs have also been shown to fill you up better than any other food in the morning, so if you're trying to slim down, an egg breakfast could be the key to success. For extra flavour, try poached eggs splashed with a little Tabasco sauce. Or, we love the Food Doctor Ian Marber's tip of adding a teaspoon of toasted sesame seeds to your scrambled eggs - yum!

It has zero fat and fewer than three calories per 5ml serving, so what's not to love? Perhaps the news that studies show that eating salad with a little fat, such as an oil-based dressing or cheese, helps your body absorb valuable nutrients found in veg, such as beta-carotene and lycopene. Even better, those who ate avocado with their salad absorbed five times more lutein, 15 times more beta-carotene and seven times more alpha-carotene than those who only had salad. So next time you're lunching on greens, don't be afraid to add something ‘naughty'.

Hurrah! You can stop stressing over your two-litre bottle of mineral water. When Doctors Dan Negoianu and Stanley Goldfarb of the University of Pennsylvania looked into the alleged health benefits of drinking this much water they found no evidence that it led to increased detoxification, plumper skin or boosted weight loss. The fact is, the amount of water you need varies from person to person and day to day, so the best way to check whether you're drinking enough is to pay attention in the bathroom. If your urine is darker than pale lemon in colour, then you need a little more fluid, if it's lighter, you want less.

Surprisingly, a cup of decaf coffee contains around 5mg of caffeine, but it won't leave you buzzing - quite the opposite. Dr Crystal Haskell of Northumbria University found that low levels of caffeine seem to impair us. In fact, she discovered that anything under 10mg actually makes us feel more tired and less able to remember things than normal. So ditch the decaf and enjoy a refreshing herbal tea, such as ginger, instead.

When Dr Michelle Palmer of Griffith University in Australia compared the weight loss of people eating six mini meals to three hearty meals, she found both groups lost the same amount of weight. "But the little-and often eaters were more likely to put weight back on - probably because their portion sizes started to increase," she says. If you like to graze, the trick to keeping the weight off is to ensure that each meal will fit onto a side plate - or is no bigger than what would fit into your cupped hands.

It can be, but don't assume that every loaf has perfect health credentials. While the official definition of brown bread is bread containing 10 per cent bran, some brown breads are just white bread with caramel colouring. "Plus, plain brown bread has the same GI rating as white bread, so even the better varieties will still raise levels of fat-storing insulin in your system," says Nigel Denby, author of The GDA Diet - Shop Yourself Thin (Capstone, £7.99). For the healthiest bread, stick to granary or wholemeal, which have about 2.2g fibre per slice.

Recent studies have shown that a watermelon kept at room temperature will double its levels of the antioxidant beta-carotene and gain 20 per cent more of another antioxidant - lycopene - over two weeks than one stored in the fridge. Tomatoes, peaches and bananas also become more nutritious if you store them at room temperature.

Yes, as the latest research says they may lower levels of good bacteria in your body, thereby affecting how you process certain drugs and interfere with your metabolism - but there is one sweetener with health benefits. Called xylitol, it's made from vegetable fibre. In studies at the University of Oulu in Finland it was found to decrease rates of bone breakdown. "Plus there's evidence that it increases the amount of calcium your bones absorb," says nutritionist Antony Haynes. Try Perfect Sweet, £2.69, from Sainsbury's.

The raw food lobby says this is the case because raw foods contain higher levels of natural enzymes than cooked ones. But no one knows exactly what those enzymes do, whereas we do know that antioxidants are good for us - and it's increasingly being shown that some vegetables can't actually release these unless they are cooked. Cooked carrots, for example, release three times more beta-carotene than raw, while cooked tomatoes supply more lycopene. "So, to get maximum nutrients, that means steaming or simmering on a low heat for five minutes," says nutritionist Naomi Beinart.