The study suggested long-term vitamin C intake lowered the risk of cataracts, but was inconclusive.

Vitamin C Sources

Citrus fruits Tomatoes Asparagus Broccoli Sweet potatoes Peppers Greens

The women received eye examinations for the signs of several different forms of cataracts.

They also gave detailed information about their diet, and any vitamin supplements they were taking.

In all, 34% had "cortical opacities", a common form of cataracts involving a particular type of lens cell.

Inconclusive

And 39% of these had other forms of cataracts, mostly at an early stage with no loss of vision.

For women younger than 60 years, a high intake of vitamin C (362mg) was linked to a 57% lower risk of developing cortical opacities.

The researchers also found that the use of vitamin C supplements for at least 10 years was associated with a 60% reduction in the risk of cataracts, compared to women who took no supplements.

A UK expert, Dr Chris Bates, from the Medical Research Council Human Nutrition Centre at Cambridge University, said that the study added to the weight of evidence that vitamin C was important to eye health.

Vitamin C could help eyesight

He said: "The reason we're interested in vitamin C is that we know there is a lot of it in the eye - and that people who have cataracts often have low concentrations of it in the eye.

"However, we haven't yet got enough evidence to show that this effect is all down to vitamin C, or whether there are other confounding factors."

The women studied are part of the wider "Nurses Health Study", a group of nurses in the Boston area whose diet and health information has been systematically followed up every other year since 1976.