NUTRITION AND EYE HEALTH
Original article from Medscape
How important is nutrition in protecting eye health?
Cumulative oxidative damage has been implicated in the pathogenesis of age-related macular changes.
Emerging scientific data suggest that high concentrations of key nutrients are critical to protecting the eye against oxidative stress and maintaining optimal function. Ensuring proper nutrition is particularly important during aging, to offset age-related decline of key nutrients.
Oxidative stress and the eye
The eye is uniquely conducive to the generation of reactive oxygen intermediates and therefore highly susceptible to oxidative stress.
The retina is particularly vulnerable. The retina has a rich oxygen supply, with an oxygen tension almost as high as that of arterial blood. Oxygen consumption in the retina is greater than in any other tissue.
Half of the lipid bilayer of photoreceptor rod outer segment membranes is composed of polyunsaturated fatty acids, which are highly susceptible to lipid peroxidation.
The retina is also subject to high blue light exposure and associated photooxidation.
The antioxidant system in the eye includes antioxidant enzymes (for example, catalase and superoxide dismutase), vitamins C and E, zinc, and the macular pigment.
Omega-3 fatty acids in the retina also participate in the oxidative stress response and are important in protecting retinal function.
The macular pigment is composed of the dietary carotenoids lutein, zeaxanthin, and meso-zeaxanthin, a metabolite of lutein.
These carotenoids protect against oxidative stress in two ways:
- By filtering potentially damaging blue light, thereby reducing photooxidation
- By quenching reactive oxygen intermediates
In addition to its antioxidant roles, it has been suggested that macular pigment might help minimize chromatic aberration, enhance contrast, and contribute to visual acuity. Thus the macular pigment may have functions both in protecting the retina and in vision.
The omega-3 fatty acid docosahexaenoic acid (DHA) has a well-established role in retinal function. DHA is found at high concentrations in rod outer segments of the retina.
As photoreceptor outer segments are continuously shed, adequate DHA may be critical to replenish levels.
DHA is a precursor of NPD1 (neuroprotectin D1), a neuroprotective mediator synthesized by retinal pigment epithelium (RPE) cells as part of the cellular response to oxidative stress.10 Eicosapentaenoic acid (EPA) is another important long-chain polyunsaturated omega-3 fatty acid in that it serves as a substrate for DHA biosynthesis and also may have anti-inflammatory effects.
Factors associated with decline in the protective antioxidant system:
- Aging
Aging is associated with a decline in macular pigment. A variety of possible factors may contribute to this decline, including changes in dietary intake of lutein and zeaxanthin, age-related changes in the absorption or distribution of the macular carotenoids, and/or excessive depletion due to an age-related increase in oxidative stress.
- Age-related decline in macular pigment optical density (MPOD)
In addition to a reduction in macular pigment, the aging eye experiences decline in other key components of the response to oxidative stress. Research has shown a reduction in zinc and in the activity of the antioxidant enzyme catalase in older RPE cells.
Levels of vitamin C and E in the body also drop significantly with age.
- Smoking
History of smoking is associated with reduced macular pigment (as evidenced by lower MPOD), perhaps due to a higher oxidant load that could increase turnover of carotenoids.
- Genetics
Some variation between individuals in macular pigment may be attributable to hereditary factors. Blue to gray iris color is associated with lower MPOD than dark iris color.
One explanation posited for this association is that light irises transmit more light than dark irises, thus the oxidative burden may be greater.
Apolipoprotein E (ApoE) profile may influence MPOD through an effect on serum transport and/or retinal capture of carotenoids.
Factors associated with lower macular pigment optical density:
• Aging
• Genetics
• Diet – Blue to gray iris color
• Smoking – ApoE genotype
Strategies to increase macular pigment
Several studies of adopting a diet rich in lutein- and zeaxanthin-containing foods have demonstrated a positive effect on macular pigment.
Similarly, supplementation with lutein and zeaxanthin has been demonstrated to significantly increase macular pigment.
Source: http://www.maculacenter.com/News/lutein.htm
Increase in macular pigment in an individual receiving lutein supplementation27. In this study, lutein was dosed at 5 mg, 10 mg, and 20 mg daily. All groups receiving lutein showed an increase in MPOD. Graph depicts the response in a subject dosed at 10 mg/day.
Ensuring adequate nutrient intake
Intake of an adequate amount of essential eye nutrients, including vitamins C and E, zinc, macular carotenoids, and omega-3 fatty acids, is fundamental to eye health. These nutrients are not synthesized by the body and must be obtained from diet or by supplementation.
A variety of foods can be sources of lutein, zeaxanthin, and omega-3 fatty acids.
Dietary sources of key eye nutrients
- Lutein
- Zeaxanthin
- Omega-3 fatty acids
- Leafy green vegetables
- Other colored vegetables (eg, squash, sweet pepper, sweet corn, peas)
- Eggs
- Vegetable oils (canola, soy, flaxseed)
- Nuts
- Oily fish
- Organ meat
Unfortunately, many Americans do not obtain adequate nutrition through diet alone.
The foods associated with high levels of lutein and zeaxanthin are among the food groups showing the greatest gap in attainment of recommended intake.
More than 95% of adults have intakes of dark green vegetables, orange vegetables, and legumes that are below the minimum recommended amount.34 Zinc intake is suboptimal, particularly in older people, with 20%-25% of the population aged 60 years or older not obtaining adequate daily zinc intake.
On any given day, 75% of the US population does not consume any of the longer-chain omega-3 fatty acids, DHA or EPA.30 Most omega-3 fatty acid intake is in the form of alpha-linolenic acid, of which only a small (≤15%) amount is converted to DHA and EPA.33 The major dietary source for DHA and EPA is fish; however, consumers need to be careful about which fish they eat and how often.
Dietary supplementation
For many individuals, dietary supplementation may be an option to help provide adequate nutrient intake. Conventional multivitamins contain little (0.25 mg) or no lutein.
However, nutritional supplements specifically formulated for eye health, with more lutein (5 mg), zeaxanthin, DHA, EPA, vitamins C and E, plus zinc, are available. The DHA and EPA in these nutritional supplements for the eye are sourced from smaller species of fish, such as anchovies, that do not accumulate toxins.
Over half of adults use at least one dietary supplement, suggesting that awareness of the benefits of supplementation is high.
If patients are not asking about multivitamins for eye health, it may not be due to lack of interest. It may reflect a lack of knowledge about what options exist.
You would have to eat 4¼ cups of fresh broccoli every day to obtain a daily intake equivalent to 5 mg lutein.
Dr. Pinna says:
This article dramatically illustrates the confusing information the world is receiving concerning vitamins and health. Just recently the world was told that increased use of vitamins is associated with higher mortality rates. This article clearly states that vitamins are necessary for eye health.
Who do you believe?
All of our science supports vitamins as necessary for survival. The studies concerning increased mortality and vitamin use do not necessarily contradict this basic fact.
People who use additional vitamins daily may be unhealthy and searching for a cure. The use of additional vitamins or supplements and increased mortality is an association, not a cause and effect.
Bear this in mind when you consider the use of vitamins and supplements.
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