Lutein is the xanthophyll carotenoid that is the primary filter of the blue light in the retina — it is one of the two major carotenoids that constitute the macular pigment (the other is zeaxanthin), and it is the primary chromophore that absorbs the blue light (at 400-500nm, with a peak at 460nm) before it reaches the photoreceptors and the retinal pigment epithelium. The blue light is the most damaging component of the visible light spectrum — it has the shortest wavelength and the highest photon energy of all the visible light, and it generates the reactive oxygen species (ROS) in the retina through the photo-oxidation of the rhodopsin and through the activation of the pro-inflammatory signalling pathways (the NF-kappaB, the VEGF, the COX-2). The lutein in the macular pigment acts as an optical filter and as an antioxidant — it absorbs the blue light before it reaches the photoreceptors, and it scavenges the ROS that are generated by the blue light exposure. The macular pigment is the yellow spot in the centre of the retina (the macula lutea), and it is composed primarily of the lutein (approximately 60%) and the zeaxanthin (approximately 30%), which are deposited in the Henle fibres and in the photoreceptor outer segments by the retinal pigment epithelium. The density of the macular pigment is measured as the macular pigment optical density (MPOD), and it is one of the most important determinants of the visual performance and of the risk of the age-related macular degeneration (AMD). Without adequate lutein and macular pigment, the blue light reaches the photoreceptors unfiltered, the oxidative stress in the retina increases dramatically, and the retina ages at an accelerated rate — producing the visual dysfunction, the photophobia, the contrast sensitivity loss, and the increased risk of the AMD that are the hallmark of the lutein deficiency. The typical dietary lutein intake from the leafy green vegetables (spinach, kale, Swiss chard), the corn, the egg yolk, and the orange pepper is 1-3mg daily, and the therapeutic doses for the macular pigment and the visual function are 10-20mg of the lutein supplement daily — making the lutein supplementation one of the most evidence-based interventions for the protection of the macula and for the prevention of the AMD.
Lutein and the Blue Light Filtering
Lutein protects the retina from the blue light damage primarily through its optical filter function — the lutein in the macular pigment absorbs the blue light at 400-500nm with a high efficiency (the molar extinction coefficient of the lutein at 460nm is approximately 30,000 L/mol/cm), and it reduces the intensity of the blue light that reaches the photoreceptors by 40-60% at the centre of the macula. This blue light filtering is the primary mechanism of the lutein’s visual-protective effect, because the blue light is the most damaging component of the visible spectrum for the retina — it produces more ROS per photon than the red or the green light, it activates the pro-inflammatory signalling pathways in the retinal pigment epithelium and in the photoreceptors, and it accelerates the lipofuscin accumulation (the autofluorescent pigment that is one of the hallmarks of the retinal ageing and of the AMD). The lutein also protects the retina from the blue light damage through its antioxidant effects — it scavenges the singlet oxygen and the other ROS that are generated by the blue light exposure, and it reduces the oxidative stress in the photoreceptors and in the retinal pigment epithelium. The lutein also modulates the signalling pathways that are activated by the blue light — it inhibits the NF-kappaB activation, the VEGF expression, and the COX-2 expression, which are the key mediators of the inflammatory response and of the angiogenesis in the AMD.
The clinical importance of the lutein for the macular protection is underscored by the observation that the lutein supplementation increases the macular pigment optical density and improves the visual function in people with the early AMD and in the healthy adults. A meta-analysis of 12 RCTs in over 1000 participants found that the lutein supplementation at 10-20mg daily for 6-12 months significantly increased the MPOD (by 20-40%), improved the visual acuity (by 5-10 letters on the ETDRS chart), and reduced the risk of the progression to the advanced AMD (by 20-30%) — making the lutein one of the most effective interventions for the prevention and the treatment of the AMD.
Practical Application
For general lutein supplementation for the macular protection and for the visual function, the evidence-based approach is to supplement with 10-20mg of lutein daily (as the standardised lutein ester from the marigold flower, which is the most stable and the most bioavailable form of the lutein). The lutein should be taken with the meals that contain the fats (to enhance the absorption, because the lutein is a fat-soluble carotenoid), and it should be taken in combination with the zeaxanthin (at 2-5mg daily, in the 10:2 lutein-to-zeaxanthin ratio, which mirrors the ratio in the macular pigment). The lutein is generally well-tolerated with no significant adverse effects at doses up to 40mg daily, and it does not cause the vitamin A toxicity (because it is not converted to the retinol). For comprehensive macular and visual support, lutein pairs well with the zeaxanthin (which is the other major carotenoid in the macular pigment and which provides complementary blue light filtering at the very centre of the macula, where the zeaxanthin is concentrated), with the omega-3 fatty acids (which are essential for the retinal function and which have complementary anti-inflammatory effects), with the astaxanthin (which is the most potent antioxidant for the retina and which works synergistically with the lutein for the visual protection), and with the bilberry extract (which is rich in the anthocyanins and which has complementary effects on the night vision and on the ocular blood flow).
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