The compounds, carotenoids, are what give fruit and vegetables and egg yolks their colour.

The researchers say they may not really help you see in the dark, but brightly coloured yellow and green vegetables do have sight-enhancing properties.

In a study of post-menopausal women, researchers from the University of Wisconsin found that a high consumption of these vegetables was associated with a reduced risk of age-related macular degeneration (AMD).

At the beginning of the study, participants filled out a questionnaire to evaluate what their diets were like 15 years before the beginning of the study.

Blood samples were taken to assess levels of carotenoids and color photographs of the retina were used to determine the presence and progression of AMD.

AMD is the leading cause of blindness in older people in developed countries, and occurs when the region at the back of the retina that produces the sharpest vision deteriorates over time.

The condition is incurable but research has suggested that lutein and zeaxanthin may help combat AMD by absorbing damaging blue light, preventing attack by destructive groups of atoms called free radicals, and strengthening eye cell membranes.

The team led by Dr. Suzen Moeller studied women aged 50 to 79 who had either high or low dietary intakes of lutein and zeaxanthin and found that higher intakes over 15 years were associated with a reduced risk of intermediate-stage AMD in women under 75.

The study is published in the Archives of Ophthalmology.

Professor Barnett suggests that if every calorie rich soft drink and fruit drink cost more people would consume less of it whereas if fruit and vegetable production was subsidised and cost less people would consume more of it and a healthier diet would result.

He says the problems associated with obesity, such as diabetes and cardiovascular disease, are going to increase rapidly.

Professor Benjamin Senauer from the University of Minnesota who has compared lifestyles in the U.S., which has high obesity rates with Japan, which has low rates, says the average Japanese household spends almost a quarter of its income on food compared to under 14% in the U.S.

Senauer says that while a direct tax on food in the U.S. to reduce obesity would not be politically acceptable, agricultural subsidies which resulted in cheap food could be effective.

Professor Senauer says other factors such as exercise, also play an important role and he points out that Japanese cities are based on efficient public transport and walking while the average American commutes to work, drives to the supermarket and does as little walking as possible.