Nutrient-Rich Eating For Eye Health
One of the things that is easy to lose sight of in nutrition is the importance of nutrient-rich eating for the basic healthy ongoing functioning of our bodies. Chronic health conditions such as heart disease, diabetes and obesity are such prevalent problems that we often overlook the fact that the basic functions of our body can only be carried out when the appropriate nutrients are available.
Good vision depends on the tiny blood capillaries that supply the retina (at the back of the eye) and other eye parts with nutrients and oxygen. The most common cause of irreversible blindness in older ages is age related macular degeneration (AMD) which affects central vision. Eyes need certain nutrients to remain healthy but whether we should get these from supplements or foods is where the controversy lies.
A growing body of evidence suggests that inflammation and immune system health may play a role in development of AMD 390. Against a background of observational studies suggesting people who eat a diet rich in antioxidant vitamins and minerals may be less likely to develop AMD, a 2012 Cochrane review concluded evidence is accumulating to show taking supplements of vitamin E, beta-carotene, vitamin C or lutein and zeaxanthin will not help prevent AMD 391. However, another associated Cochrane review found that in people who already have AMD, there is some evidence that antioxidant vitamin and mineral supplements may help delay progression, 392 which more recent studies have also supported 393,394.
Some studies 344,395 have suggested that foods rich in certain nutrients have a protective effect that is not necessarily seen when supplements are used. Whole natural foods contain these identified nutrients in smaller amounts than supplements, but it may be the proportions or the complex combinations that are important. The presence of many other phytochemicals, both known and unknown, which are not found in specific supplements, may also form part of the puzzle. Once again it seems likely to be the whole food package that counts, not isolated nutrients.
Which are the key nutrients under investigation?
A recent review 396 concludes that there is clinical evidence for potential benefits from vitamin C, beta-carotene, vitamin E and zinc, as well as emerging epidemiological and clinical data for the carotenoids lutein and zeaxanthin and for omega-3 fatty acids.
The carotenoid pigments, found in colourful vegetables and fruitssuch as beta carotene, lutein and zeaxanthin, are found within the eye’s macular (the area in the centre of the retina responsible for central vision). One review concluded there is increasing evidence that consumption of fruits and vegetables rich in carotenoids, such as lutein and zeaxanthin, may decrease the incidence of AMD 395. A recent meta-analysisfound eating foods rich in carotenoids diet may help prevent late-onset AMD, though not early-onset; it also found a significant inverse association between the risk of neovascular AMD (a subset of AMD) and the intake of these carotenoids in the diet 397.
Vitamins A, C and E
These three antioxidant vitamins are found in the retina and work together in a highly integrated system. They are key to both function (vitamin A in photopigment) and also protection (vitamin E and C together with lutein and zeaxanthin help retard light-induced oxidative damage)398. These food-based vitamins, along with zinc (below), may also help prevent the development of cataracts 399.
Zinc, a trace mineral, is found in high concentrations in the retina. Zinc is thought to play many roles thereincluding: interacting with vitamin A, modifying photoreceptor plasma membranes, regulating the light-rhodopsin reaction, modulating synaptic transmission and also antioxidant actions 400.
Low Glycaemic index
The glycaemic index (GI) of foods is a measure of the rate of blood sugar rise after consumption. A meal with a high GI typically results in hyperglycaemia in the two hours immediately after consumption 401. Epidemiological evidence suggests that GI is associated with AMD risk, 401 and is also supported by mechanistic theories 401,402.
Evidence that long-chain omega-3 fats play a role in AMD development is emerging; although there are currently no published large-scale clinical trials, the authors of a recent review reported that observational study results are strong and existing studies showing benefits are highly consistent. 396 A very recent prospective population study reports a reduced risk of AMD with increasing levels of plasma omega-3s 403,401.
Avocados and nutrients for eye health
Ensuring a diet rich in the vitamins, minerals and phytochemicals discussed above may be an important strategy in maintaining eye health. Avocado not only contains the carotenoids lutein and zeaxanthin 405,406,407,408,409.
Sources of zinc include animal foods, such as meat and seafood, along with plant foods such as nuts and legumes. Low GI foods include wholegrain breads and cereals, legumes, dairy and most fruits. Long-chain omega-3s are mainly found in fish and seafood.
Author: Lisa Yates, Consultant Dietitian Adv APD
Lisa Yates is an Advanced Accredited Practising Dietitian with 16 years experience in nutrition, communications, clinical practice, as well as strategy development and implementation.