As many people frantically try to lose weight and detox themselves at this time of year, they might be torturing themselves needlessly. Hadley Freeman exposes 10 sacred food myths that might make you think twice if you've given up Guinness and sweets in pursuit of a healthier lifestyle

Bad news, Popeye . . . spinach doesn't give you muscles of iron after all, plus nine other foodie myths dispelled:

1. Spinach is the richest source of iron: How many poor children out there have suffered at the hands of spinach propagandists, aka parents? Spinach, they would have us believe, is the best source of iron available. This belief is partly due to a typing error by a food analyst who, many years ago, put a decimal point in the wrong place when listing iron content, resulting in a 10-fold overestimation.

In fact, many other green, leafy vegetables are better choices for those seeking to iron themselves up - spinach has only 1.7 milligrams per 100 grams, whereas kale has 2 milligrams. The other problem is that spinach contains oxalic acid, which actually leaches iron out of the system. Says nutrition scientist Brigid McKevith, "Meat is actually a better source of iron than any vegetable, as the body absorbs iron from meat, such as steak, more easily." Surely that's more fun than a dish of wet leaves?

2. Adding salt to water will make it boil more quickly: You can always tell the cuisine king in a student household. They are the ones who chuck salt into their pot of water with a knowing flourish before dumping in the pasta. The theory is that salt makes water boil quicker, but the inverse is actually true.

Salt does lower the temperature at which water freezes (which is why it is a good thing to put down on icy pavements), but it raises the temperature at which it boils, so boiling will take longer.

While we're on the subject of cooking the pasta, there's also not much point in adding oil to stop things sticking together, because it just floats to the surface of the water.

3. Oysters are an aphrodisiac: The only real connection oysters have with human relations is that they are high in zinc, which is said to be good for male fertility. But it's really the lascivious eating method that is the reason for oysters' association with hot lovin'.

Moreover, as the BBC TV drama Tipping the Velvet showed, oyster eating is really more appropriate as a symbol for Sapphic love. Whether this is the reason that men still extol the aphrodisiac nature of oysters is perhaps not best pondered on these pages.

4. You need to drink two litres of water a day: Another year gets under way, another article about the importance of "cleansing" our toxin-ridden systems with water. But while nutritionists and health writers maintain a pretty self-righteous front on this issue, the medical establishment has long been less certain. A recent article in the American Journal of Physiology pointed out that the water content in food will contribute to this two-litre amount, which opens up the wonderful possibility of eating a tub of ice-cream and claiming that you are doing it for rehydration purposes.

5. "Sealing in" the flavour of meat: This is what people think they are doing when they are simply browning meat for a casserole or whatever - "locking in" all those juices and flavour. The surface caramelisation you get from frying the meat comes from the Maillard reaction, which is caused by a reaction between carbohydrates and proteins on the surface. But there's no "sealing" going on - all that sizzling is in fact the water being driven out and boiling off on the pan.

6. Beer is good for your hair: In the name of dedicated research, I actually tried this the other night (another slow night in at home) and can authoritatively report that it made my hair look like an oil mishap and my shower smell like a Frog and Firkin. Moreover, the beer foamed up on my head, giving me a head on my head, which may be a nice pun, but is hardly the height of glamour.

The theory is that the yeast in beer will, in industry parlance, "nourish" one's hair. But if we're dealing with the real world as opposed to that of the land of fashion freaks, stick to letting the beer nourish you orally, not follically.

7. Eating too much sugar can lead to sugar addiction: The number of warning tales about sugar at the moment, especially in women's magazines, is enough to lead you to think that certain scaremongers have confused sugar with what newspapers call "crack cocaine".

" 'Addiction' suggests physiological dependency, and I don't think that is really what is happening when it comes to sugar," says McKevith. Instead this is likely to be a case of what she refers to as "habit eating" - nothing more than the fact that people eat what they always have in their cupboards, or associate with particular friends or activities. Biscuits, yes; a handful of sesame seeds, no.

8. You should eat fruit at the end of a meal: Folk who actually go for the fruit plate for dessert are the bane of modern restaurant life. This is comparable, in terms of sheer smugness, to people saying, "Actually, just water for me, thanks". But according to Ian Marber, founder of The Food Doctor, the sugars from a post-prandial piece of fruit can ferment in the colon while your main meal is being digested, which "can lead to bloating and wind", which is hardly something to feel self-righteous about. Fruit, he says, is better eaten alone, separate from your main meal.

9. Celery has "negative" calories: Along with the theory that the best way to get spotted by a modelling talent scout is to hang around the shopping precinct with your mum, the belief that it takes more calories to ingest celery than you gain by eating it is held dear by 15-year-old girls. "Celery has very few calories - only seven per 100g - but as anyone who has put in the effort on a treadmill knows, you have to put in a lot of effort to burn off even such a small amount, and chewing won't do it," says McKevith.

10. Guinness is very fattening: Five Big Macs' worth of calories, isn't it, in one pint of Guinness? Or was it five Snickers bars? Whatever, you are guaranteed to get a similar smug fact from some smug git at the bar when you order your Guinness. Well, just tell them that Guinness Draught is actually lower in calories than most premium lagers - 196 calories per pint as oppposed to lager's 210 calories. This is largely due to the fact that Guinness is relatively low in alcohol. Bottoms up.