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Set Your Sights on Carrots—and Greens

As a child, you probably heard that carrots are good for your eyes. This may be because carrots (and other orange and yellow vegetables and fruits) are abundant in beta-carotene, which is a provitamin A carotenoid, meaning it is converted to vitamin A in the body. Vitamin A is important for eye health, especially for night vision as it helps to produce a pigment called rhodopsin in the retina, which helps the eye detect low levels of light and allows us to see at night. As such, vitamin A deficiency is known to cause night blindness.1,2

However, beta-carotene is not the only carotenoid that contributes to healthy vision. Out of about 600 known carotenoids, twenty have been found circulating in human blood and only two are found in the eye. They are lutein and zeaxanthin, which cannot be synthesized by the body and are primarily found in green leafy vegetables. Once consumed, these 2 carotenoids accumulate in the macula, the inner portion of the retina, which has a high concentration of photoreceptor (or light receptor) cells. The typical amount of lutein and zeaxanthin in the macula (called “macular pigment”) is quite low among Americans, due to low intake of leafy greens. The retina is the most metabolically active tissue in the body, and lutein and zeaxanthin provide antioxidant protection. Furthermore, macular pigment reduces glare and enhances contrast and visual acuity, and acts as a filter to protect the macula from blue light damage.3-6 Blue light is a part of visible light (and sunlight), and electronic devices and energy-efficient lighting increase our exposure to it, especially in the evenings.

The idea that leafy greens benefit vision began to gain momentum about twenty years ago in research on age-related macular degeneration (AMD), a disease in which the photoreceptors in the macula are progressively damaged or lost, causing impaired vision. AMD is the leading cause of blindness worldwide.7 In 1994, a study on AMD found that higher total carotenoid intake was associated with lower risk of the disease, and lutein and zeaxanthin were the specific carotenoids most strongly associated with decreased risk. When looking at foods, higher intake of spinach and collard greens (rich sources of lutein and zeaxanthin) were also associated with decreased risk.5,8 More studies followed, many reporting that higher lutein and zeaxanthin intake was linked to lower AMD risk.7,9 Supplementation trials in AMD patients also reported increases in macular pigment (more lutein and zeaxanthin in the macula) and improvement in visual performance.10-12

These results sparked the marketing of eye health supplements containing lutein and zeaxanthin. However, previous findings on isolated carotenoid supplements should urge us to be cautious; several trials of beta-carotene supplements have reported an increased risk of lung cancer, cardiovascular disease, or overall mortality.12-14 This is an unacceptable risk for a nutrient we can easily get from foods, which have no risk. These nutritional benefits are also enhanced by accompanying phytonutrients in green vegetables that have further benefits to the entire body.

The typical American diet is dangerously low in leafy greens, and the average adult’s intake of lutein + zeaxanthin from foods is a meager 1.5 mg per day.15 Just a single cup of cooked spinach or kale contains more than 20 mg of lutein + zeaxanthin, and collards more than 14 mg; commercial vision supplements commonly contain 10-20 mg of lutein plus 2 mg or less zeaxanthin. So, a healthful diet actually supplies more of these beneficial nutrients for the eye than supplements do, and of course leafy greens have several advantages over supplements, in particular a huge variety of additional carotenoids and other beneficial nutrients, with no risk of excess.


References:

1. Oregon State University. Linus Pauling Institute Micronutrient Information Center. Vitamin A. http://lpi.oregonstate.edu/infocenter/vitamins/vitaminA/. Accessed
2. WHO: Global prevalence of vitamin A deficiency in populations at risk 1995–2005. WHO Global Database on Vitamin A Deficiency. Geneva: World Health Organization; 2009.
3. Stringham JM, Bovier ER, Wong JC, et al: The influence of dietary lutein and zeaxanthin on visual performance. J Food Sci 2010;75:R24-29.
4. Abdel-Aal el SM, Akhtar H, Zaheer K, et al: Dietary sources of lutein and zeaxanthin carotenoids and their role in eye health. Nutrients 2013;5:1169-1185.
5. Koushan K, Rusovici R, Li W, et al: The role of lutein in eye-related disease. Nutrients 2013;5:1823-1839.
6. Widomska J, Subczynski WK: Why has Nature Chosen Lutein and Zeaxanthin to Protect the Retina? J Clin Exp Ophthalmol 2014;5:326.
7. Schleicher M, Weikel K, Garber C, et al: Diminishing risk for age-related macular degeneration with nutrition: a current view. Nutrients 2013;5:2405-2456.
8. Seddon JM, Ajani UA, Sperduto RD, et al: Dietary carotenoids, vitamins A, C, and E, and advanced age-related macular degeneration. Eye Disease Case-Control Study Group. JAMA 1994;272:1413-1420.
9. Ma L, Dou HL, Wu YQ, et al: Lutein and zeaxanthin intake and the risk of age-related macular degeneration: a systematic review and meta-analysis. Br J Nutr 2012;107:350-359.
10. Ma L, Yan SF, Huang YM, et al: Effect of lutein and zeaxanthin on macular pigment and visual function in patients with early age-related macular degeneration. Ophthalmology 2012;119:2290-2297.
11. Richer SP, Stiles W, Graham-Hoffman K, et al: Randomized, double-blind, placebo-controlled study of zeaxanthin and visual function in patients with atrophic age-related macular degeneration: the Zeaxanthin and Visual Function Study (ZVF) FDA IND #78, 973. Optometry 2011;82:667-680 e666.
12. Age-Related Eye Disease Study 2 Research G, Chew EY, Clemons TE, et al: Secondary analyses of the effects of lutein/zeaxanthin on age-related macular degeneration progression: AREDS2 report No. 3. JAMA Ophthalmol 2014;132:142-149.
13. Omenn GS, Goodman GE, Thornquist MD, et al: Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease. N Engl J Med 1996;334:1150-1155.
14. Bjelakovic G, Nikolova D, Gluud LL, et al: Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev 2008:CD007176.
15. What We Eat in America, NHANES 2009-2010, individuals 2 years and over (excluding breast-fed children), day 1 dietary intake data, weighted. United States Department of Agriculture. Agricultural Research Service.

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