Sight is generally the most valued sense. Most of the information useful for our daily lives comes through it. However, most people are unaware of the importance of diet for the health of our eyes. At most, we hear repeatedly: “Carrots have vitamin A” and “Eating carrots is good for our eyesight.” But how much of it is true? Is this all the information we have?
The complexities of vitamin A
It is true that vitamin A is necessary for vision, but it is also necessary for growth, the integrity of epithelium, reproduction and the immune system. It is, like all vitamins, essential for life.
Since our body cannot synthesize it, it depends on the foods we eat. When we calculate how much vitamin A we consume through our diet, we consider retinol, which is the most potent compound, i.e. already formed vitamin A. We also take into account some carotenoids, which are plant pigments (for example beta-carotene or beta-cryptoxanthin) that can be converted into vitamin A in the body (they are provitamin A).
We obtain retinol from foods of animal origin. It is found in cod liver, meat, eggs, dairy products, etc. On the other hand, foods of plant origin contain this vitamin in the form of provitamin A. This difference is important because excessive consumption of retinol can cause poisoning. However, this is not the case when vitamin A is obtained from plant sources, since the body controls the conversion of provitamin A into retinol.
What other food components does the eye need?
A lack of this compound (retinol) in the eye can cause vitamin A deficiency disease, which causes vision loss and, in children, early death.
But the presence of vitamins C and E, some carotenoids (such as lutein and zeaxanthin), minerals (such as zinc, selenium, copper) and fatty acids (such as DHA-docosahexaenoic acid) also stand out in the eyes.
It has been almost three decades since the first major epidemiological study was published linking a high intake of fruits and vegetables (rich in lutein) with a lower risk of age-related macular degeneration (AMD), the main cause of blindness in people over 65 years of age. Since then, many others have confirmed this relationship.
We now know that some chronic or degenerative eye diseases (such as cataracts, glaucoma or AMD) are influenced by a wide variety of factors, including diet. In fact, some of the components of the foods we eat:
- They prevent deficiency disease (lack of intake of the corresponding nutrient).
- They reduce the risk of degenerative diseases.
- They improve visual function.
Lutein, a carotenoid with a key presence in the retina
In addition to vitamin A, there is another essential element: lutein, which belongs to the group of carotenoids, fat-soluble pigments that give color to numerous plant foods and some animal foods (such as salmon).
Carotenoids have various biological activities of interest. For example, some of them have the ability to transform into retinol. In addition, they have antioxidant activity and act as a blue light filter to protect the retina and skin, among others.
Of all the carotenoids we take in through our diet (more than fifty), only six are normally found in the blood and two of them are concentrated in the retina. The latter are lutein and zeaxanthin. They are found in the macula, where they form the so-called “macular pigment”.
The density of this macular pigment can be measured and is related to better visual function (visual acuity, less discomfort from glare, better contrast sensitivity). In recent years, it has also been considered as a marker of the concentration of these carotenoids in the brain, which in turn is associated with cognitive functions.
Could eating more carotenoids slow the progression of age-related macular degeneration (AMD)?
Numerous studies have sought to determine the effect of increasing the amount of various food components on eye health, either through food or with food supplements.
The results were encouraging and a study was started on a large number of people with age-related macular degeneration. These people took, for five years, a mixture of antioxidant compounds (beta-carotene, vitamins C and E, zinc and copper) in high quantities.
The results showed that the risk of AMD progressing was reduced and visual acuity was improved. Although these compounds cannot cure the disease, they have important benefits, such as delaying vision loss and improving quality of life.
Years later, the study was repeated, replacing beta-carotene with lutein and zeaxanthin in the formula. Beta-carotene was eliminated because it had been used in studies (in high quantities) on smokers and caused an increased risk of lung cancer.
The results of this new study showed that people with advanced AMD who took lutein and zeaxanthin also had a reduced risk of their disease progressing and improved visual function.
Taking lutein is not the same as eating foods that contain it.
Given these findings, we wonder whether the amount of lutein and zeaxanthin we consume in food is similar to that used in AMD studies. Lutein and zeaxanthin are mainly provided by fruits and vegetables. They are also used as food colorants and can be consumed in the form of food supplements.
The carotenoids most widely present in foods are beta-carotene (especially in red or orange vegetables and fruits) and lutein (especially in green leafy vegetables).
Although it is easy to get significantly more lutein through a varied diet rich in green vegetables (up to around 3 mg lutein per day), the amounts reported in the study mentioned above (10 and 2 mg per day of lutein and zeaxanthin, respectively) would not be possible for people with AMD. Therefore, these people would have to take them in the form of food supplements.
It is not always necessary to take supplements
The benefits for the general population (such as avoiding deficiency disease and improving visual function) are obtained through adequate intake of nutrients and other food components that are important for eye health. For example, vitamins A, C, E, zinc, lutein and zeaxanthin and omega-3 fatty acids.
Therefore, fruits and vegetables should be consumed in quantity and in a variety of colors, since they provide us with almost all of these components, except for omega-3, for which fish is a good source.
However, when people have a retinal disease (mainly AMD), the dietary intake is insufficient to slow the progression of the disease and they must resort to additional dietary supplements. These supplements are useful for people with a high risk of developing AMD.
These people, in addition to following the previous recommendation for the general population, should consider the use of supplements with antioxidant compounds and zinc (with the composition of the AREDS 2 formula), always following the recommendations of a specialist. However, it is not known whether these extra supplements are useful in people who do not have AMD in preventing the onset of the disease, since it has not yet been studied.
Finally, people who consume diets with few vegetables and fruits (and who do not want or are unable to change their habits) can also benefit from the use of food supplements. In these cases, the amount of compounds present in these supplements should be lower than those present in the “AREDS formulas” and similar to those that can be obtained through a varied diet. Although before reaching this point, we must assess the importance of eating a balanced diet.
Begoña Olmedilla Alonso, Scientific researcher, Institute of Food Science and Technology and Nutrition (ICTAN – CSIC)
This article was originally published on The Conversation. Read the original.
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