Accumulating evidence suggests that supplementation with the dietary macular pigment carotenoids lutein and zeaxanthin may be an important public health intervention to prevent or slow visual loss from age-related macular degeneration (AMD), a common cause of irreversible visual disability and blindness in the elderly in developed countries.1,2
Lutein and zeaxanthin are nutrients that humans obtain from dark green leafy vegetables and from orange and yellow fruits and vegetables. The typical American diet provides 1 to 2 mg of these xanthophyll carotenoids per day, and they are concentrated specifically in the primate fovea at very high levels, where they are thought to act as antioxidants and as screening pigments to ameliorate the phototoxic effects of short-wavelength visible light.3
Several very large epidemiologic studies including the Eye Disease Case Control (EDCC) study,4
a follow-up study on a subset of EDCC patients by Seddon and colleagues,5
and the original Age-Related Eye Disease Study (AREDS)6
have demonstrated a significant protective effect of high levels of dietary consumption of lutein and zeaxanthin and of high blood concentrations of these carotenoids against AMD. Although these epidemiologic studies are very important, it must be considered that nutritional surveys and blood analyses are, at best, indirect measures of the status of the tissue of interest: the macula of the human eye.7
This is because uptake and stabilization of lutein and zeaxanthin in the macula are complex, biologically regulated processes mediated by specific binding and transport proteins.8
This means that macular uptake of lutein and zeaxanthin is subject to saturation effects and is dependent on protein expression levels and binding affinities of the proteins' various isoforms. Our laboratory has identified GSTP1 as the specific binding protein for zeaxanthin in the primate macula, and we have recently identified StARD3 as the specific lutein-binding protein.9,10
Other laboratories have contributed insights into the potential carotenoid transport proteins in the gut and the eye of various organisms such as the scavenger receptor class proteins SR-BI and CD36.11–14
Epidemiology studies and biochemical investigations of nutrient roles in complex degenerative diseases of aging such as AMD are very helpful in formulating clinical recommendations, but many clinicians still demand definitive results from at least one adequately powered randomized clinical trial (RCT) before altering clinical practice. The first RCT to have a major impact on AMD treatment and prevention strategies was the original AREDS study published in 2001.15
AREDS examined the incidence of advanced AMD and severe visual loss in patients with high-risk characteristics for AMD progression such as large drusen and pigmentary changes in one or both eyes and/or advanced AMD in one eye (choroidal neovascularization or geographic atrophy). Approximately 3600 subjects followed for at least 5 years were randomized into four treatment arms: high dose zinc + antioxidants (vitamin C, vitamin E, and β-carotene); high dose zinc alone; antioxidants alone; or placebo. The AREDS group reported significant reduction of AMD progression with either zinc or antioxidants versus placebo, and even better protection when the interventions were combined.
AREDS was a crucial first step in guiding nutritional interventions against AMD, but in the interval between its inception and publication of its results, it became clear that the formulation could be potentially improved.16
Supplementation with lutein and zeaxanthin appeared to be a better choice than β-carotene because they are abundantly and specifically concentrated in the macula, whereas β-carotene is not. Moreover, there were serious concerns about elevated risk of lung cancer in smokers consuming such a high dose of β-carotene.17–19
Additionally, several epidemiologic studies indicated a potential benefit of omega-3 fatty acids typically found in fish oil (EPA and DHA).20–22
Thus, the AREDS2 study was initiated in 2006 to evaluate this updated nutritional knowledge.23
Using a similar size sample and time period as AREDS, high-risk AMD patients have been randomized to receive 10 mg lutein and 2 mg zeaxanthin (L+Z arm), 1000 mg of fish oil (EPA+DHA arm), L+Z and EPA+DHA, or placebo. All participants were also encouraged to take either an AREDS supplement or a modified AREDS supplement with reduced zinc and β-carotene. Results are expected to be released in 2013.
As with any well-designed RCT, the AREDS2 designers included as many relevant biomarkers as possible to ensure compliance with the treatment assignments and to assess physiologic response. In the AREDS2 protocol, all subjects fill out dietary surveys and have periodic pill counts, and a subset of participants has serum carotenoid levels measured by high-performance liquid chromatography (HPLC); however, as discussed earlier, these are, at best, indirect measures of response to L+Z supplementation in the tissue of interest: the subject's macula. A variety of noninvasive methods exist to quantify and image macular carotenoid pigment levels and spatial distributions,24
but due to budget and logistic limitations, they were not included in the final AREDS2 protocol. At the Moran Eye Center AREDS2 study site, we initiated an approved ancillary study to image macular pigment distributions at the initiation visit and yearly thereafter in our site's subjects, to understand more completely the time course and long-term tissue response to AREDS2 interventions and to provide insights into which AMD subjects are most likely to derive clinical benefits from carotenoid supplementation. We report here the baseline characteristics and correlations between carotenoid measurements in the subjects enrolled in our ancillary study.