In a large dietary survey that followed people for as long as 15 years, researchers found that around three in 50 meat eaters had cataracts, compared to about two in 50 vegans and vegetarians.

The results translated to a 30 to 40 per cent lower cataract risk among vegetarians and vegans, compared to the biggest meat eaters.

"People who don't eat meat have a significantly lower risk of developing cataracts", said Naomi Allen, an epidemiologist at the UK's University of Oxford, who co-authored the study.

According to the National Eye Institute, a cataract occurs when the lens of the eye becomes cloudy, therefore blurring vision. They're more common in older people, and more than half of Americans either have cataracts by the time they're 80 or have had surgery for them.

The study does not prove, however, that eating meat promotes cataracts. Eating a lot of vegetables may be protective, for instance, some past research has linked certain nutrients in plant foods to a reduction in the risk of cataracts. A vegetarian diet might also simply be a sign of other healthy life-style choices that contribute to the risk being reduced.

Smoking, diabetes, and exposure to bright sunlight are amongst the factors linked to an increased risk of cataracts.