Carotenoids and vision

In my first ever breakfast at a bed and breakfast I was surprised to find that the eggs yolks in my eggs benedict were bright orange. Qov commented that this yolk color was likely an indicator that the eggs were from free-range chickens, and according to a site Qov found the orange color probably comes from two carotenoids, lutein and zeaxanthin. Qov hinted that she'd like to know more about what these compounds are, so I thought I'd do my best.

Before I discuss lutein and zeaxanthin, we need to talk about carotenoids.

"Carotenoids are yellow, orange, and red pigments present in many commonly eaten fruits and vegetables (Astorg, 1997). More than 600 carotenoids have been identified, but most nutrition research has focused on the five carotenoids with the highest known blood concentrations in U.S. populations: a-carotene, B-carotene, lycopene, lutein, and B-cryptoxanthin." (Holden et al. 1999; note that the a here should be an alpha, and the B's should be betas)

Most readers will recognize B-carotene as the compound that causes carrots to be orange. This is another story in itself, since there's almost no reason for plant roots to store huge quantities of B-carotene that I know of, and thus orange carrots are most likely a human-selected mutation from carrots that were originally white (but I won't digress). B-carotene is well known to be important for vision ("a carrot a day keeps the optometrist away"?), primarily because it can be turned into vitamin A (trans-retinol), which in turn can become part of the molecule that converts incoming photons into interpretable signals in our retinal nerve cells.

In the cells of our retina, vitamin A (trans-retinol) is converted into 11-cis-retinal, which then combines with the protein opsin to form rhodopsin. Rhodopsin is the compound that reacts with photons in our retinal nerves to sense light. When a photon reacts with rhodopsin it converts the 11-cis-retinal in rhodopsin to all-trans-retinal (an isomer), which changes rhodopsin into bathorhodopsin, which then turns into metarhodopsin II (this last conversion is what activates transducin and starts the chain of events possibly leading to a neural signal being sent to the brain). Opsin and all-trans-retinal then dissociate, and all-trans-retinal is converted back into 11-cis-retinal by retinal isomerase, leaving the cycle at the beginning. Multiply that reaction by the number of molecules of rhodopsin in an adult retina, about 3.8 x 10^15 (Fulton et al. 1999), and you've got a small idea of what it takes for us to see.


(figure from Mathews and Van Holde 1996)

According to Holden et al. (1999), a-carotene, B-carotene, and B-cryptoxanthin can be converted into retinol and thus used directly for photoreception. However, it appears that most of the other carotenoids can't be converted into retinol (or at least aren't by our cells), and thus are not involved in the pathway described above. Lutein and zeaxanthin appear to be two of these; they're in our blood, are specifically deposited in the macular region of the retina, and likely do something related to vision, but from what I've read their role in vision is currently uncertain (Mozaffarieh et al. 2003). Mozaffarieh et al. (2003) listed some possible functions of these compounds in the eye:

• "limitation of the damaging photo-oxidative effects of blue light through its absorption
  
• reduction of the effects of light scatter and chromatic aberration on visual performance
  
• protection against the adverse effects of photochemical reactions because of the antioxidant properties of the carotenoids" (Mozaffarieh et al. 2003)

Here's where they're found naturally:

"Lutein is a common carotenoid found in most fruits and vegetables, while zeaxanthin is present only in minute quantities in most fruits and vegetables. Dietary sources of zeaxanthin are limited to greens, certain yellow/ orange fruits and vegetables such as corn, nectarines, oranges, papaya and squash. Orange pepper is recently found to have a high amount of zeaxanthin and the dried fruit of Lycium barbarum (fructus lycii) prescribed by the Chinese herbalist as a therapeutic agent for a number of eye diseases, has been shown to have a high content of zeaxanthin but negligible amount of lutein.

The highest mole percentage of both lutein and zeaxanthin can be found in egg yolk and maize. Substantial amounts of lutein are also known to be found in melon, spinach, collards, kale, and guava." (Mozaffarieh et al. 2003)

Some people are recommending taking lutein and zeaxanthin as a dietary supplement to help prevent some types of macular degeneration, though there's debate (Mozaffarieh et al. 2003) as to whether they are effective.

So now it's settled: my SO and I must regularly go back to the B&B in Victoria so we can eat more of their healthy eggs and thus have nicely protected eyes. All for our health, of course.

References:

Fulton et al. 1999. The Rhodopsin Content of Human Eyes. Investigative Ophthalmology and Visual Science 40:1878-1883. (article)

Holden et al. 1999. Carotenoid Content of U.S. Foods: An Update of the Database. Journal of Food Composition and Analysis 12, 169 – 196. (USDA page, PDF)

Mathews, C. and K. E. Van Holde. 1996. Biochemistry, 2nd edition. Benjamin/Cummings, Menlo Park, CA.

Mozaffarieh et al. 2003. The role of the carotenoids, lutein and zeaxanthin, in protecting against age-related macular degeneration: A review based on controversial evidence. Nutrition Journal 2:20. (article)