Two weeks ago, I was happy in the knowledge that two staples of my diet, namely caffeine and pizzas, were inherently bad for me. Now that's all been overturned. Italian scientists, it seems, have discovered that pizzas act to protect us from all manner of cancers. And they should know. Then it was the turn of the Australians. After a bunch of tests on athletes, scientists at the Australian Institute of Sport in Canberra have concluded that caffeine is not only going to make me more powerful and exercise longer, but it's likely to make me lose weight faster too. If I'd been feeling guilty about what I was consuming, it would all have been for nought.

So what's going on? And more importantly, who am I supposed to believe? Evidently, I'm not alone in my confusion. "You can't expect the public to know the real risks of doing something and make a rational judgment. We don't do it, and why should we?" says risk expert Sir Colin Berry. "Mr Spock might be able to do it, but what a bizarre person he turns out to be." Could it be that scientific studies will tell you whatever you want to hear if you look hard enough?

Having unprotected sex



It's never easy getting scientists to harp on about the benefits of having unprotected sex, but a few have stuck their necks out. Gordon Gallup at the State University of New York asked nearly a thousand women about their sex lives and used standard psychological tests to assess how happy they were.

He found that women whose partners didn't wear condoms consistently came out as happier than those whose partners did. Having ruled out other factors, Gallup says he is convinced semen is the reason. "It suggests there's something in semen that can alter mood," he says. "If you could isolate what it is in semen that appears to be doing this, you might be able to use it as an alternative way of treating depression."

Gallup's latest results pander less to those smelling a male conspiracy: he found that women who don't use condoms tend to be more gutted when their relationships break down, yet get into new sexual relationships more quickly than those that do. "They experience rebound more. It's as if they find semen addictive," says Gallup.

He isn't the only one to endorse the virtues of semen. Scientists at the University of Adelaide have found evidence that exposure to a man's semen makes for a less problematic pregnancy. The team suspect that the semen conditions the woman's immune system, so it is less likely to attack the growing foetus. Male conspiracy theorists take note: the scientists found the positive effects of semen to be strongest if swallowed. Gulp.

Getting stressed



Our reaction to stress reveals what a lumbering beast evolution really is. Stress makes our nervous system pump out a chemical called noradrenaline, which kicks our heart rate up and breaks down body tissue to give us more energy. "That's great if you're trying to run away from a mammoth," says Ashley Grossman, an endocrinologist at St Bartholomew's Hospital, London. "But it's not much use if someone's screaming at you in an office and you can't do Anything but sit there and seethe."

Fortunately our archaic response to stress does have some modern-day benefit. After an hour or so of being stressed, blood levels of the hormone cortisol rise. "Small amounts of cortisol make you process information faster. If you're very stressed in an exam and you're completely lost, your brain will work faster and better," says Grossman. But it's only useful on occasion. "Get stressed day in day out and you'll literally burn that part of your brain out," he adds.

Using mobile phones



Though demonstrably lethal when stuck to the ear of someone driving through a city during rush hour, the only other confirmed threat a mobile has to health is the kicking you get when the local 14-year-olds mug you for it on your way home.

But there's good evidence that mobile phones can be good for you too. Alan Preece, who studies the biological effects of mobile-phone radiation at the Bristol Oncology Center, found that people exposed to mobile-phone radiation were 4% faster at certain mental tasks than others. "It has the effect of making you about 20 years younger," he says. Preece believes the effect is solely due to the phone heating a region of the brain called the cortex. Radiation from phones has also been shown to increase blood flow in certain regions of the brain.

Watching a great deal of TV



Yes, you'll be labelled a couch potato, but rest assured it could be worse. According to researchers at Vandebilt University in Nashville, you'll use up 20% more energy watching television than lying in bed. At a burn rate of around 100kcals for an hour's viewing, television is about as exhausting as reading a book or writing a shopping list.

Listening to loud, repetitive music



Of course it'll hamper your hearing by causing a permanent ring, or deafen you completely, but since you're going to do it anyway, you might as well know why it feels so good. "What happens when you listen to loud music is it activates a primitive acoustic sense in the ear which is inherited from our swampy ancestors," says Neil Todd of the University of Manchester. Todd reckons our ancestors' mating displays involved lots of noise and prancing about. Loud bass notes trigger the same response in our vestibular system, he says, so loud repetitive bass music stimulates the same areas of our brains that makes us think we might be about to get some. "It's the pulsing, loud bass frequencies that are particularly effective," he says. But does the fact that it gives you the horn mean it's good for you? "Anything which gives you pleasure is good. It keeps you free from stress, it keeps you happy and that's clearly healthy," he says.

Smoking



Talk to physicians and they'll tell you there are few things you can put in your mouth that are worse for you than a cigarette. But it's not all doom and gloom. Smokers are at least doing their bit to slow down the runaway obesity epidemic that is sweeping through the western world. "In many studies, you often find smokers are slimmer. We've certainly seen it in our studies," says Jodi Flaws at the University of Maryland school of medicine. "Some people think it's due to certain chemicals in cigarettes somehow making them burn more calories, but others believe it suppresses appetite. It may well be both."

Drastically upping your chances of cancer and heart disease might not be the best way to avoid obesity, but it's certainly easier than running round the block.

Scientists have also found evidence that smoking might, in some circumstances, help prevent the onset of various dementias. Many dementias go hand-in-hand with a loss of chemical receptors in the brain that just happen to be stimulated by nicotine. Smoking seems to bolster these receptors, and smokers have more of them. The theory is that smokers may then have more to lose before they start losing their minds. "It does seem that nicotine has a preventative effect, but the problem is that the other stuff in the cigarette tends to rot everything else," says Roger Bullock, a specialist in dementia and director of the Kingshill Research Centre in Swindon. So if your time is nearly up anyway, and you have somehow managed to steer a course past the Scylla and Charybdis of heart attacks and tumours, smoking might just help you retain your marbles.

Riding fast motorbikes



There are fewer sure-fire ways of decimating your life expectancy than buying a large motorbike you are clearly ill-equipped to control. But spend five minutes with a biker and you'll soon hear how invigorating it is to hare through the countryside startling the pheasants. Although the thrill of speed is often called an adrenaline rush, adrenaline has nothing to do with it. Instead, the rush comes from the release of chemicals called endorphins in the brain that act to calm your body down, essentially countering the effects of noradrenaline that gets your heart thumping. Is thrill-seeking good for you? "Endorphins are the good guys, they slow down your heart rate and make you relaxed, so if they're being released frequently, it makes sense to believe that's good for you, but we don't have any actual evidence," says Grossman.

Flying economy class



Deep-vein thrombosis may be the curse of the economy-class majority, but sometimes the cheap seats are the best place to be. In the mid-90s, the Civil Aviation Authority carried out tests to see how passengers sat in different parts of a plane faired during a typical, survivable crash-landing. They found that those in economy class often came out better because they were cushioned against some of the impact by the chair in front.

"Your body doesn't get so stretched," says Russ Williams, ex-head of flight operations policy at the CAA. "If you're in a first-class seat, there's nothing in front to stop you." It doesn't help for all kinds of crashes though. "Clearly if the wing falls off, you're going to die. Simple as that." To really up your chances of getting out in one piece, your best bet is to sit no further than three rows from an emergency exit.

Eating fatty food



Fatty food is a great supply of energy, but before you go burger-hunting, you should know that too much (and few of us have too little) will raise your cholesterol, which points you firmly in the direction of heart disease. Regardless of how lame you may feel and barring any eating disorders, it's unlikely you have too little fat to survive. "Getting enough energy to stay alive isn't usually a problem in a western industrialised society," says Ian Johnson at the Institute of Food Research. "But there are some fats the body absolute requires," he adds. Certain polyunsaturated fats are needed to help cells work properly and are especially vital for nerve cells. "It's important for pregnant women to have a certain intake as it's vital for growing the infant's brain," he says.

Drinking heavily



We've all heard about the health-enhancing properties of the odd glass of red wine, but what about the odd tequila slammer? Studies comparing wine with beer and spirits often find wine comes out best, while spirits have less of a beneficial effect. The bulk of the benefit comes from ethanol which, according to Morton Gronbaek at the Institute for Preventive Medicine in Copenhagen, reduces the tendency of blockages to form in blood vessels.

Up to 21 drinks for men and 14 drinks for women tends to protect against coronary heart disease. "The risk drops by 30-50% if you drink a little alcohol," says Gronbaek. "By not drinking alcohol, you're increasing your risk of heart disease the same amount as people who either do no exercise or have high cholesterol," he says. Other studies have proven alcohol to be a good all-rounder, helping protect against dementia, increase bone mineral density in elderly women and even lower blood pressure.

Eating salty food



Salt is great for raising your blood pressure, which sadly isn't great for anything. But wipe salt out of your diet completely and not only will your food take on an impressive blandness, you'll gradually drift into a malaise of muscle cramps, nausea and dizziness. "Ultimately it can be very serious," says Amanda Wynne at the British Dietetic Association. Salt is necessary to ensure body fluids move in and out of your body's cells only when they are supposed to. Salt is also needed to send electrical pulses along your nerves. Few people get close to suffering from salt deficiency though. "The average person has around 20 times the minimum requirement in their diet," says Wynne.

Becoming a boxer



It's no surprise that people who regularly get thumped very hard in the head occasionally die from it or suffer terrible brain damage. You can't train your brain to take that kind of abuse. But if you're good enough to dodge the head shots, or bottle out of going in the ring, boxing is only going to be good for you.

"Boxing is a marvellous form of exercise from the cardiovascular point of view. It exercises the entire body, so it's better than running or cycling," says Robert Cantu, chief medical officer at the National Centre for Catastrophic Sport Injury Research in North Carolina. "The only downside is it's not advantageous to take blows to the head." The most damaging kind of punch, says Cantu, spins the head on the neck. "Things like left hooks and right crosses are the ones to watch out for."