Nutrition, what we eat on a daily basis, affects many aspects of our health. Eating is something that we all do and our choices in an industrialized society are many. It is recognized that many choices are not healthy for us and there is a fairly well supported theory that a person's diet may affect their development of myopia. The possibility is that by eating the correct foods one may reduce the risk of becoming myopic.
Nutrition is certainly important for many ocular structures, from proper tear production to protect against dry eye to support of the functioning of the macula to help prevent macular degeneration. You can read about ocular nutrition at the Ocular Nutrition Society web site.
The first thing to understand is that there are no direct studies that implicate diet with myopia progression. It has become obvious through many studies that the urbanization of modern society creates something called a myopigenic environment. By that it is meant that industrialized cultures around the world are creating myopia. One of the major differences between native cultures and modern cultures is diet and so theories postulate how diet may be responsible for myopic progression since diet has been shown to affect many disease processes. Until about 10,000 years ago agriculture was essentially non-existent. For over a million years, our genetic ancestors were classified as hunter-gatherer and they ate what they could kill or find growing. Good vision was essential and myopic individuals would have found themselves at a significant survival disadvantage. Any genes for myopia would have been presumably selectively eliminated. In studies of every current native culture, unaffected as much as possible by contact with modern society, the risk of becoming myopic is very low.
These same native people, within one or two generations of adapting modern lifestyles find that they are becoming myopic at rates that rival or exceed other industrialized populations. It is not purely genetics that is creating the myopia because genes do not adapt that quickly unless one postulates a dormant myopic gene or genes that are triggered by something in the modern environment. The issue is also not increased reading or near work as a society industrializes. Educated people in rural environments have much lower rates of myopia than their urbanized educated counterparts. Illiterate people in urbanized environments have significantly increased rates of myopia in at least one study. There is something about urbanization that is creating the myopigenic environment that so much of the world now lives in.
With the advent of agriculture and the increased consumption of cereal grains along with the more recent (1970s) advent of refined sugars such as high fructose corn syrup, we now have a situation where 36% of the total energy in the typical US diet is supplied by foods for which we have very little genetic adaptation. These newer foods are termed high-glycemic-load carbohydrates due to their composition. One hypothesis is that consumption of these foods has the potential to create insulin resistance in the body, leading to many disease processes. You can find a link to an abstract about it here: An evolutionary analysis of the etiology and pathogenesis of juvenile-onset myopia. The next entry in the research listing is a printed version of a slide presentation lecture on the same topic.
According to this theory, the "correct" diet is one that is closer to that consumed by the hunter-gatherer diet that we are genetically adapted to. The diet is low in sugar, salt, fruit juices, dairy products and cereal grains such as wheat, rice and corn. Thus no breakfast cereals, breads, pasta, cakes, cookies, pastry, or any sugar except occasionally honey. No added salt. No potatoes, yams, beans, soy products, peanuts, cashews, lentils. Basically, meat, fruit, vegetables and tree nuts. While such a diet is followed by some in industrialized cultures, it is obviously not a mainstream diet. However, the incidence of obesity (also increasing as myopia is increasing) would indicate that our diets are totally out of sync with our ability to process the food.
You can read one person's interpretation of this diet on this site: Ben Balzer's Paleolithic Diet Weblog
While the data for implicating diet as a risk factor for myopia development is relatively persuasive, there have been no diet-control studies done for myopia development. No one has followed a diverse group of children over time, tracking their individual diets and seeing who develops myopia. The only thing we have so far are population statistics of large groups of people. These statistics certainly indicate an area for future research but the question arises whether the data is strong enough to recommend an actual change in diet.
I think the reasonable approach at this time is to modify your diet in the direction indicated by the theory, especially in areas where it aligns with other known health benefits. Research certainly indicates that obesity is a problem in the United States and diet modifications are proven to reduce the risk of many diseases that are made worse by too much body fat. The elimination of many carbohydrates from the diet would help reduce a person's weight and could also reduce their risk of developing myopia. Many people who are not over-weight also obtain a significant portion of their daily caloric intake from carbohydrates that are relatively new to the human diet. Reducing their consumption may reduce the incidence of myopia.
Diet modifications are often not psychologically easy. It takes a change in lifestyle to adapt to a significantly different diet. Processed carbohydrates are a cheap source of calories and are heavily advertised by the food industry because they are profitable. Advertisements are for sugar and starch, not fruit and vegetables. If meat is advertised, it comes with a baked potato, garlic bread and cake or pie for dessert. It takes a concerted effort to move past this effort to derail good dietary choices.
Some people need specific diets for health reasons. The inability to digest milk (lactose intolerance) is a fact for much of the world's population and many have significant allergies to items such as nuts. Certainly such diet restrictions need to be followed.
In the end, since myopia is not caused by a single factor, diet changes may not work to prevent it. While not proven to help myopia, the evidence is persuasive that it could help and a reduced carbohydrate diet has other health benefits.