Rather than focusing on individual nutrients, serum concentrations of nutrients or biomarkers of fruit and vegetable intake, as other studies have done with mixed results, the Harvard researchers, led by Dr William Christen, decided to look at the development of cataract in relation to specific foods or food groups, such as fruit and vegetables.
"This approach enables an assessment of the combined effects of antioxidant nutrients together with the effects of other components in the diet, such as other micronutrients, phytochemicals and fiber," they wrote in the American Journal for Clinical Nutrition (2005; 81: 1417-22).
A cataract is when the crystalline lens of the eye, which enables humans to focus on objects at different distances, becomes clouded, resulting in blurring vision. It can lead to blindness if left untreated and the only available treatment method is surgical removal of the crystalline lens.
Cataract interferes with the vision of around one in 49 Americans - 2.02 percent of the population - and removal is the most common surgical procedure carried out in the country.
For the purpose of the Harvard study, cataracts that were responsible for a decrease in best-corrected visual acuity to 20/30, in the opinion of an ophthalmologist or optometrist, were counted. The specialist also had to verify that the cataract was age-related in origin.
The study was carried out using data supplied by 39,876 apparently healthy female health professionals aged over 45, who were involved in the Women's Health Study.
The participants were asked to complete a semi-quantitive food frequency questionnaire that asked how often, on average, they had eaten each of 29 different vegetables and 15 fruits over the last year. Responses were converted to average daily intakes for each item for each participant.
Once women with existing cataract at baseline and those whose daily calorie intake was less than 600 or more than 3500 were excluded, 35,724 participants were deemed eligible for the study.
Over a ten-year period, a total of 2,067 cases of incident cataract were confirmed amongst the participants, and 1,315 cases of removal. Women in the highest quintile of fruit and vegetable consumption were seen to have a 15 to 20 percent reduced risk of developing cataract, compared with those in the lowest quintile.
The researchers considered this reduced risk to be only sight, and said that additional observational studies and dietary supplement trials may identify specific components in the diet that reduce cataract formation.
But they wrote: "The possible beneficial effects of fruit and vegetables on the risk of many chronic diseases, including cataract, have a strong biological basis and warrant the continued recommendation to increase total intakes of fruit and vegetables."
A study published last year in the Archives of Ophthalmology (122:883-9214) also established a link between fruit and vegetable consumption and a reduced risk of age-related macular degeneration (AMD), a degenerative eye condition that is the leading cause of vision loss in over-55s in the United States, affecting more than 10 million people.
AMD affects the macula of the eye, the central part of the retina that is responsible for most fine vision, leaving sufferers with only peripheral sight.
Much attention has been given in recent years to the supplement approach to preventing AMD, in particular to powerful carotenoid antioxidants lutein, zeaxanthin and astaxanthin.
Last year Bausch & Lomb launched its Ocuvite PreserVision Soft Gels Lutein Formula and multivitamin makers are also getting in on the act - such as Wyeth Healthcare, which recently added lutein to its market-leading adult product, Centrum.
Lutein and zeaxanthin occur naturally in the macula, where they absorb harmful blue light and protect tissues from the products of lipid oxidation. Astaxanthin, derived from microalgae, is very similar in structure to lutein.
The new Dietary Guidelines of American released in January upped the recommended daily intake of fruit and vegetables from five portions to between five and nine. Yet the average American manages only three.
As well as the benefits conferred on vision, a diet rich in fruit and vegetables has also been shown to aid gastrointestinal health, reduce the risk of cancer and cardiovascular disease, and help prevent low blood and high cholesterol.