Scientists have pinpointed a reason why sickle cell disease tends to hit men harder than women.

Sickle cell disease is a genetic condition in which the body produces deformed red blood cells - shaped like a sickle rather than round, as they should be.

It's tempting to speculate that women with sickle cell do better because of oestrogen Dr Mark Gladwin

They do not carry oxygen around the body as effectively as they should, and clog up the blood vessels.

These blockages cause painful episodes known as sickle cell crises.

Men with sickle cell disease tend to experience more crises after puberty.

They also die earlier - on average at the age of 42 - compared with women with the disease, whose average life expectancy is 48.

Key chemical

US scientists believe the reason could be a gender difference in production of a chemical called nitric oxide.

The compound helps to reduce the risk of sickle cell blockages by dilating the blood vessels, giving the misshapen cells more room to pass through.

It also acts both to keep the blood vessels healthy, and to reduce clotting by keeping the component parts of blood from sticking together.

The researchers found that nitric oxide is up to two times more available in women than men with sickle cell disorder.

Blood flow

They compared blood flow in the forearms of 21 sickle cell patients with 18 who were free from the disease before and after administering a series of drugs known to affect vessel dilation and blood flow.

The tests showed that men appear to have problems both making nitric oxide and with destroying the chemical they do have too quickly.

When the men were given a substance containing nitric oxide, their blood flow increased by 86%, while women's blood flow increased 171%.

Scientists believe the key to the difference between the sexes could be the female sex hormone oestrogen, which has been shown to stimulate the production of nitric oxide.

Mystery

Researcher Dr Mark Gladwin, of the US National Institutes of Health Clinical Center, said: "One of the great mysteries of sickle cell disease is that everyone has the same mutation for the most part, yet others have extremely severe disease and early death and some patients have hardly any complications or pain.

"It's tempting to speculate that women with sickle cell do better because of oestrogen."

The researchers now plan to test the effect of a class of drugs called statins on sickle cell patients.

Statins are used to treat people with heart problems, as they help to lower levels of potentially harmful cholesterol in the blood.

But they have also been shown to boost nitric oxide levels by increasing its production and the length of time it survives in the body.

Sickle cell disease affects mostly Afro-Caribbean and Asian people.

The research is published in Circulation: Journal of the American Heart Association.