Article Date: 12/1/2012

NUTRITION
nutrition

“Carrots Are Good for Your Eyes”

The truth behind a widely held belief.

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JEFFREY ANSHEL, O.D., F.A.A.O.

When mentioning nutrition for eye health, carrots come to mind. So, are carrots really good for our eyes?

That red-orange pigment

Carrots are an excellent source of beta-carotene, which is a strongly-colored, red-orange pigment abundant in plants, fruits and vegetables.

Plant carotenoids are the primary dietary source of pro-vitamin A worldwide, with beta-carotene the most well-known pro-vitamin A carotenoid. Vietnamese “Gac” (Momordica Cochinchinensis Spreng) and crude palm oil are particularly rich beta-carotene sources, as are mangoes, papayas, yams and green leafy vegetables, such as spinach, kale, sweet potato leaves and sweet gourd leaves.

Absorption and conversion

Carotenoid absorption is restricted to the duodenum and dependent on the class B scavenger receptor (SR-B1) membrane protein, which is also responsible for vitamin E (alpha-tocopherol) absorption. One molecule of beta-carotene can be cleaved by a specific intestinal enzyme into two molecules of vitamin A.

The absorption efficiency of beta-carotene is estimated between 9% to 22%. The absorption and conversion of carotenoids may depend on the beta-carotene’s form (cooked vs. raw vegetables; in a supplement), fat and oil intake at the same time and the individual’s current vitamin A and beta-carotene levels. The conversion of beta-carotene to vitamin A is likely the reason for “carrots are good for your eyes.” It seems to follow that since the retina needs vitamin A in the form of retinol, which is necessary for both scotopic vision and color vision, beta-carotene leads to better sight. It’s not that simple.

Not so fast

The conversion of beta-carotene to vitamin A depends on the liver stores of vitamin A. If the liver has adequate vitamin A in storage (it is fat soluble), there is no physiological need to convert any beta-carotene to vitamin A. The body maintains this balance, and excess beta-carotene is simply stored in fat tissue beneath the skin. If you measure carotenoid antioxidant levels in the skin, you may get high readings, but that is irrelevant to ocular levels.

The process of beta-carotene conversion decreases with age.1,2 Fortunately, being a fat-soluble vitamin, there is less depletion of vitamin A, so levels are usually maintained as one ages. Also, the enzyme that converts beta-carotene to vitamin A can degenerate with age. One study shows it’s converted less efficiently in women vs. men.3

Another issue with excess beta-carotene: Being a carotenoid, it competes with other carotenoids for transport space on lipoproteins (both LDL and HDL). Remember that lutein and zeaxanthin must get transported to the macula to provide adequate protection to this tissue. If excess beta-carotene is ingested, it might impede ocular carotenoid transport and leave the eye under-protected.

Beta-carotene dangers

Supplemental beta-carotene increases lung cancer risk in smokers, says the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) study. Also, a higher incidence of lung cancer was seen in those who took 50mg of synthetic beta-carotene, plus 25,000IU of pre-formed vitamin A retinol, says The Beta-Carotene and Retinol Efficacy Trial (CARET).

Many scientists speculate the press on these two studies is an example of a misinformed rush to judgment, since their conclusions were released without appropriate supporting data. The ATBC study had not been published in a medical journal when the press addressed that the study participants were instructed to stop taking the beta-carotene study formulation in 1993. Nor had the study conclusions been reviewed by scientific peers at that time — a major breach of scientific protocol. The ATBC study was eventually published in April 1994’s The New England Journal of Medicine

The amount of supplemental vitamin A and beta-carotene in both studies was beyond the safe upper limits (UL) for daily consumption established by the Food and Nutrition Board at The Institute of Medicine.

The Dietary Reference Intake UL for the combination of preformed vitamin A as retinol and pro-formed vitamin A as retinol activity carotenoids (primarily beta-carotene) is 10,000 IU or approximately 3,000 retinol equivalents per day.

New data on beta-carotene from the ATBC study suggest smokers who eat a wide variety of dietary antioxidants, including reasonable amounts of beta-carotene, actually have a substantially lower risk of developing lung cancer. Thus, antioxidants may exert their protective effects through interactions with other vitamins and phytochemicals in the same food sources.

Not a substitute

Beta-carotene is not an adequate substitute for vitamin A, and the best form of vitamin A is retinyl palmitate. A daily dose of roughly 2,500IU is adequate for a patient older than 50. Oh, and eating some carrots is still a good idea to stay healthy. OM

1. Promislow JH, Goodman-Gruen D, Slymen DJ, Barrett-Connor E. Retinol intake and bone mineral density in the elderly: The Rancho Bernardo Study. J Bone Min Res 2002 Aug;17(8):1349-1358.

2. Feskanich D, Singh V, Willett WC, Colditz Ga.et al. Vitamin A intake and hip fractures among postmenopausal women. JAMA 2002; Jan 2;287(1):47-54.

3. Lin Y, Dueker SR, Burri BJ, et al. Variability of the conversion of beta-carotene to vitamin A in women measured by using a double-tracer study design. American J Clin Nutr. 2000 Jun;71(6): 1545-54.

DR. ANSHEL HAS WRITTEN NUMEROUS ARTICLES AND BOOKS REGARDING NUTRITIONAL INFLUENCES ON VISION AND COMPUTER VISION CONCERNS. HE IS THE FOUNDER OF CORPORATE VISION CONSULTING AND IS PRESIDENT OF THE OCULAR NUTRITION SOCIETY. E-MAIL HIM AT EYEXAM@COX.NET, OR SEND COMMENTS TO OPTOMETRIC MANAGEMENT.COM.



Optometric Management, Volume: 47 , Issue: December 2012, page(s): 49 51