Can your shades give you sunburn? WEIRD WORLD OF THE HUMAN BODY REVEALED AS MEDICAL MYSTERIES SURRENDER THEIR SECRETS.

Byline: By RORY SMITH

DID you know when the smell of someone you fancy gets your heart thumping that's your body's way of saying you will breed well together?

Or wearing shades can actually give you sunburn? The human body is a closed book to most of us but now medical expert Dr Sharon Moalem's has come up with some extraordinary answers to everyday questions...

WHY DO THE PEOPLE WE FANCY SMELL NICE?

THE scent of someone you find attractive is always a turn-on - and it turns out it's all to do with that old romantic: disease.

A pleasing smell is a sign that the object of your affection has an immune system that's different from yours.

And by combining the two, your children will have a better chance of survival than either parent before them.

DO SUNGLASSES CAUSE SUNBURN?

SKIN colour changes when you are exposed to the sun because your body is told to produce more melanin - skin pigment - by your pituitary gland.

Melanin stops your skin from absorbing ultraviolet light.

It is this ultraviolet light which has the effect of burning the skin.

But the pituitary gland only knows how much melanin to make from the signals it gets from the eye. If the eye says it's sunny, the pituitary gland goes into overdrive.

But with shades on, the gland is fooled so less melanin is produced - meaning a higher risk of burning.

DOES SUN LOTION HURT YOUR BONES?

VITAMIN D is vital for healthy bones. If we don't have enough, then adults can develop osteoporosis and children rickets.

But unlike most vitamins, we don't need to eat vitamin D - our bodies can make it by themselves when exposed to the right kind of sunlight.

Ultraviolet B rays - strongest when the sun is directly overhead - help the body convert cholesterol to vitamin D.

But suntan lotion blocks UVB meaning the body cannot create vitamin D. In Australia, the Slip-Slop-Slap suntan campaign slashed skin cancer rates - but it sent vitamin D deficiencies soaring.

CAN SUNBEDS CUT YOUR CHOLESTEROL?

SOME doctors are prescribing UVB sunbeds for sufferers of stomach complaint Crohn's disease.

Most Crohn's sufferers have a vitamin D deficiency because of the way the disease inflames the intestine and prevents the absorption of nutrients.

But by using a sunbed three times a week for six months, cholesterol is swapped for vitamin D - meaning healthy bones and a healthier heart.

WHY DO WE WEE WHEN COLD?

WHEN most of us are exposed to the cold for a long period of time, we need to go to the loo.

And according to Dr Moalem, it's a natural response to stop the body from freezing. Urinating a lot is the easiest way of keeping internal water levels low. At the same time, this increases the concentration of sugar in the water which remains in the body.

Sugar acts as an antifreeze, meaning the freezing point of the human body is much lower than it would be otherwise.

This way of controlling blood sugar levels could have been created in a genetic mutation to some of our ancestors during the sudden ice snaps thousands of years in the past. Those with the ability to control their blood sugar lived on while others froze to death and disappeared forever.

WHY DO WE GO TO WORK WITH A COLD?

ALL diseases want to reproduce and infect new hosts.

Some are carried by other creatures, like malaria-bearing mosquitoes. Others travel through water, like cholera, but the common cold needs close contact with others to spread.

If it left you too ill to move, you would stay at home and not be near people - so the disease could not find new victims.

Instead, it makes you unwell enough to have to sneeze and cough, but well enough to go out to work or school and infect other people.

HOW DO SEX DISEASES SPREAD SO EASILY?

RESEARCHERS believe sexually transmitted diseases (STDs) may influence sexual activity.

Herpes, for example, may heighten sexual feeling because it makes that area terribly itchy and makes you aware of it all the time.

And that makes you want to have more sex - which can only be a good thing for the virus.

All living things - including viruses and bacteria - have only two basic urges: to survive and reproduce.

An STD knows that to survive it has to be passed from host to host, and that means doing all it can to make you want to have sex.

WHY DO ASIANS GET DRUNK MORE EASILY THAN EUROPEANS?

WHEN our ancestors first settled in towns and cities, clean water was a major concern for survival.

In Europe, we purified water through fermentation to make beer and the alcohol made in that process killed microbes.

But in Asia, they purified by boiling their water to make tea.

So here we had a genetic need to tolerate alcohol - which was never called for in the Far East.

So Asians today do not produce the enzyme which breaks down acetaldehyde - the first by-product of alcohol - into less harmful acetate.

Acetaledehyde is 30 times more toxic than alcohol, so the build-up in the body can produce nasty results.

WHY ARE THE GREEKS HAIRIER THAN AFRICANS?

THICK hair on the arms and

legs may have been a useful defence against mosquitoes carrying malaria.

But in Africa the heat is so intense that thick body hair would not have been a wise move in evolutionary terms.

So the thickest body hair is found where malaria is common but it is not too hot - the Mediterranean.

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Taken from Survival Of The Sickest by Dr. Sharon Moalem with Jonathan Prince. Published by HarperCollins.

CAPTION(S):

CLEVER: Cold; HOT TOPIC: Be safe in the sun; ICE: Makes you go; ITCH: Herpes; THIRST AID: Beer