Overview of Carotenoid Function in the Skin

While only two carotenoids (lutein and zeaxanthin) are found in the human macula, all carotenoid nutrients found in human blood are also found in human skin. These are ^-carotene, a-carotene, lycopene, lutein, and zeaxanthin, with the nonpolar lycopene and carotenes having much higher concentrations in this tissue relative to the xanthophylls lutein and zeaxanthin (66,67). Unlike the eye, at least a portion of lutein and zeaxanthin in the skin appears to be esterified to long-chain fatty acids (68). There is no evidence as yet that specific binding proteins are involved in carotenoid uptake and stabilization in this tissue.

Epidemiological and experimental studies show that a high dietary intake of foods rich in carotenoids could protect against many diseases besides macular degeneration, including cancer (69-71), cardiovascular diseases, and numerous pathological conditions linked to damage by oxygen-based free radicals (72,73). In the skin, carotenoids function as antioxidants scavenging free radicals (74), singlet oxygen (75,76), and other harmful reactive oxygen species (77) that are all formed as a by-product of normal metabolism and by excessive exposure of skin to ultraviolet (UV) light or sunlight.

When increasing the amount of carotenoids in the diet or consuming carotenoid-enriched supplements, these nutrients are initially accumulated in the lipoproteins in blood (78). The concentrations can be easily increased by 100% and higher. This increase in blood carotenoids then leads to an increase of carotenoid concentrations in all organs taking up lipoproteins, including skin. It has been shown that skin carotenoid levels are strongly and significantly correlated with carotenoid levels in plasma (66). As is found in plasma, dermal carotenoid levels are lower in smokers than in nonsmokers. ^-Carotene levels in skin are known to increase with supplementation (79), and supplemental ^-carotene is used to treat patients with erythropoietic protoporphyria, a photosensitive disorder (80). Supplemental carotenoids have also been shown to delay erythema in normal healthy subjects exposed to UV light (81-83). There is limited evidence that they may be protective against skin malignancies (67), but more research is needed to confirm these findings.

As with the eye, it would be useful to know skin carotenoid levels rather than just serum levels or dietary intakes. Skin biopsies are invasive, and HPLC analysis poses special challenges since distributions can vary from body region to body region. Skin also has much lower concentrations of carotenoids, and it has considerable amounts of fibrous tissue that must be broken down prior to analysis. Peng and colleagues reported a method using enzymatic predigestion with collagenase and pronase E that yielded reliable and reproducible results on skin biopsies as small as 28 mm2 (66,84). Skin is an ideal target for noninvasive optical detection of carotenoids because it is readily accessible, is able to concentrate carotenoids, and shows a good correlation with plasma carotenoid status.

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