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London - The eyes are the window to your soul? Maybe, but new research is highlighting how the colour of your eyes could hold clues to your health.
The curious links between eye colour and health have fascinated scientists for years, leading to a body of research that suggests the colour affects how much you feel pain, your chance of getting diabetes, how quick your mind is - and even how well you can throw a Frisbee or handle your drink.
The latest piece of evidence in this scientific jigsaw has been added by scientists at the University of Pittsburgh. They found Caucasian women with light-coloured eyes - blue or green - appear to tolerate pain and distress better than those with brown or hazel eyes.
Inna Belfer, a professor of anaesthesiology at Pittsburgh University, says the study of 58 pregnant women found those with light-coloured eyes seemed to experience less pain while giving birth.
Furthermore, the women with lighter eyes suffered less anxiety after the birth, lower rates of depression and fewer negative thoughts, Professor Belfer told the annual scientific meeting of the American Pain Society.
While more work is needed to find the reasons for this difference, the researchers said a genetic link seemed likely.
Until recently, it was thought there was one form of a gene for blue eyes and another one for darker eyes.
“But that is absolutely rubbish,” says Dr Jari Louhelainen, a senior lecturer in biomolecular sciences at Liverpool John Moores University. “What we know now is that eye colour is based on 12 to 13 individual variations in people’s genes.
“These genetic markers give us a good chance of telling someone’s eye colour. We can predict eye colour with between 70 percent and 90 per cent accuracy,” says Dr Louhelainen, who is investigating how genes determine eye colour to help forensic scientists identify murder and accident victims from their skeletal remains.
When it comes to explaining why eye colour might determine our health, the explanation is that genes behind the colour can multi-task. “These genes do other things in the body,” says Dr Louhelainen. “One of them, NCX-4, which is linked to darker eyes, controls many proteins, of which one has recently been linked to pain.”
Other genes that are linked to melanin, the pigment that makes eyes darker, have also been connected to pain. The inherited liver condition Dubin-Johnson syndrome, which causes brown pigment in liver cells, is linked to a build-up of melanin. Symptoms include pain and fatigue.
Melanin may also make brown-eyed people more susceptible to alcohol. When psychologists at Georgia State University in Atlanta surveyed more than 12 000 men and women, they found those with light eyes consumed significantly more alcohol than those with dark eyes. The reason brown-eyed people may drink less - and also be less likely to be alcoholics - is because they need less alcohol to become intoxicated.
Melanin not only determines eye darkness, it’s also an insulator for the electrical connections between brain cells. The more melanin in the brain, the more efficiently, sensitively and faster the brain can work, the researchers reported in the journal Personality and Individual Differences.
This apparent advantage in processing speed may also explain why brown-eyed people’s reaction times are faster than those with blue eyes, and why they are generally better at throwing a Frisbee at a target and hitting a ball with a racquet, according to experiments reported in the journal Perceptual and Motor Skills.
Similar findings emerged in experiments on more than 100 volunteers by Dr Anthony Fallone, a psychologist at Edinburgh University, in 1991.
He concluded that dark-eyed students performed significantly better in several time-limited intelligence tests, where speed of thinking gave a definite advantage. Dr Fallone, who is retired - and has dark eyes - says: “The eye is so closely linked neurologically to the brain that you might call it the only part of our brain you can see from the outside. It seems to hold vital clues to our brain function.”
And that is not the end of the story. It appears blue-eyed people’s slower reactions may mean they think in a more considered, strategic way.
In tests at the University of Louisville, blue-eyed students proved more successful in activities requiring them to plan and structure their time, such as golf, cross-country running - and studying for exams.
Blue eyes are a new development in human evolution. Researchers from Copenhagen University say the gene mutation required for this arose by chance somewhere around the Black Sea coast in a single individual as recently as 6 000 to 10 000 years ago and is responsible for all blue-eyed people today.
The genetic change does not “make” blue in the iris; rather it switches off the mechanism that produces brown melanin pigment. “Originally, we all had brown eyes,” says Dr Hans Eiberg, a geneticist at the university. One theory is that cold weather and dark skies helped the colour change become widespread.
The lack of melanin also made people’s skin lighter, and fair skin is better at making vitamin D, vital in northern climes as there is less sunlight, which the body needs to produce the vitamin.
Blue eyes also filter out less light, so can see a little better in weaker sunshine.
However, increased sensitivity to light makes blue-eyed people more likely to develop age-related macular degeneration (AMD), when light-sensitive cells in the macula at the back of the eye begin to die out, leading to sight loss, says Francesca Marchetti, of the London College of Optometrists. The same is believed to be true for green-eyed people.
Type 1 diabetes is another risk associated with blue eyes. An Italian study in 2011 confirmed earlier reports that blue-eyed people suffer a disproportionately high level of this condition.
The researchers found that among people with type 1 diabetes in Lazio and Sardinia, 21 percent had blue eyes, compared with nine percent of the population.
Three gene changes associated with light eyes were suspected of being involved in the development of the condition.
Blue eyes may also be linked with hearing problems - several studies have found fair-eyed workers are more likely to suffer hearing loss in loud factories.
Researchers at the National Hospital for Neurology and Neurosurgery in London have suggested the melanin pigment may help protect nerves in the brain from noise-induced damage.
Intriguingly, the colour of your eyes could also have a bearing on your personality. In 1977, researchers in the Journal of Social Psychology noted blue-eyed people were far less likely to make disclosures about themselves when talking to counsellors. Further studies have supported this.
In 2006, German psychologists found that blue-eyed children are more likely to be inhibited in their behaviour than dark-eyed children. Other research has found blue-eyed children to be more wary of new things.
And in 2010, Czech researchers discovered that men with brown eyes are seen as more dominant than their blue-eyed peers. They suggested we associate blue eyes with babyhood, so blue-eyed adults may appear more childlike. (Permanent eye colour is not set until a baby is at least nine months old because the human body takes time to produce the melanin that darkens eyes. In some children it can take up to 18 months.)
But if this sounds like a social handicap, don’t worry just yet. While there appear to be genetic links between eye colour and our health and personality, Dr Louhelainen stresses that none has yet been proven conclusively - and some results could be down to statistical flukes.
“Because of the multiple genes involved, it is a very complex picture,” he says. - Daily Mail