Featured article in the Fall 2018 Issue of Nutrition Close-Up; written by Jirayu Tanprasertsuk, MS and Emily Mohn, PhD
Autumn is the perfect time to observe carotenoids in nature.
Carotenoids are a group of plant pigments that are responsible for the beautiful yellow, orange, and red foliage during this season. We also find carotenoids in many of the colorful fruits and vegetables we eat. Although there are more than 600 naturally occurring carotenoids, only six of them are common in the American diet. Most of us are familiar with beta-carotene, responsible for the orange color of carrots and sweet potatoes, and lycopene, responsible for the red color of tomatoes. Lutein is a carotenoid found mostly in green leafy vegetables like kale and spinach, but is also found in eggs and avocados. The amount of lutein in selected food sources is provided in Table 1.
Because carotenoids are fat-soluble, the intestine absorbs them better when they are consumed with fat. For example, adding avocado or oil to a salad significantly increases lutein absorption. Another good example is egg yolk. Because of its fat content, egg yolk is a highly bioavailable source of lutein.
Among all major dietary carotenoids, lutein and zeaxanthin (found in the same food sources but in about 1/5 the amount as lutein) are uniquely important for visual health. Only these two carotenoids are selectively taken up into the macula – the central area at the back of the eye – where they comprise macular pigment (MP). MP provides the sharp central vision we need for activities like reading and driving.
MP is often viewed as an intrinsic pair of sunglasses that screens out damaging blue light from the sun and electronic devices. Just like UV rays damage the skin, blue light exposure over a lifetime causes damage to the macula and may lead to age-related macular degeneration (AMD) and blindness in older individuals. The density of MP reflects the amount of lutein (and zeaxanthin) in the macula. MP density can be measured simply, quickly, and non-invasively in humans. Many studies that have measured MP density have found that adults with higher MP density have a lower risk for AMD.1 For younger individuals (18-40 years old), increased MP density improves contrast sensitivity, reduces glare, and enhances visual performance during activities in low ambient illumination, like driving at night.
While lutein’s role in vision has been investigated for decades, an additional role beyond the eye has emerged in more recent years. In 2008, a clinical trial studying lutein and visual health surprisingly found that adults (60-80 years old) supplemented with lutein (with or without DHA – a fatty acid found in fatty fish like salmon) improved their cognitive performance by the end of the trial.2 This finding sparked a wave of investigations into lutein’s novel role in cognition.
Analysis of post-mortem brain samples from older adults found that lutein is the most predominant carotenoid in the brain, despite being consumed less than other carotenoids. This suggests that, like the macula, the brain preferentially takes up lutein from the diet, presumably for a specific purpose. Moreover, older individuals who are more cognitively intact tend to have higher brain lutein levels.3
Since the eyes are anatomically an extended system of the brain, researchers wondered whether MP density may be a biomarker of brain lutein levels. Indeed, an investigation into matched macula and brain tissues found that MP density was related to brain lutein concentrations.4 Many studies have since demonstrated that older individuals with higher MP density, which reflects higher brain lutein content, have better cognition across different domains like memory, language, and learning.1
Why is lutein important for cognition? The answer remains unclear, but may be related to the anti-oxidative and anti-inflammatory properties of lutein. Current studies investigating where lutein localizes in the brain may shed additional light on its mechanism of action.
Since 2008, several intervention studies using lutein-rich foods or supplements have yielded promising results for preventing or delaying cognitive decline in the elderly.5–7 An unofficial recommended intake for lutein, based on evidences regarding visual function, is currently set at 6 mg/d. However, the averaged US intake is only 1-2 mg/d.1
Advanced age is a major risk factor for both AMD and age-related cognitive impairment. As the American population ages, lutein may play a critical role in preventing both diseases at a national scale. Given that lutein can only be obtained through consumption, it is important to incorporate various sources of lutein in our diet to keep our eyes and brain healthy as we age.
Table 1. Lutein/zeaxanthin content in selected food sources1
|Food||Amount (mg)/ serving unit|
|Egg, hard-boiled||0.2/one large|
|Spinach, cooked||6.7/half cup|
|Spinach, raw||4.5/half cup|
|Kale, cooked||10.3/half cup|
Jirayu Tanprasertsuk, MS is a PhD candidate at the Friedman School of Nutrition Science and Policy and a research assistant at the USDA Human Nutrition Research Center on Aging at Tufts University. His research focuses on investigating the relationship between fat-soluble nutrients, including carotenoids, and cognitive health.
Emily Mohn, PhD is finishing her postdoctoral position at the USDA Human Nutrition Research Center on Aging at Tufts University where she studies the transport of dietary carotenoids in the circulation and their effects on cognition. Emily recently joined the RNA Therapeutics Institute at UMass Medical School as a Scientific Writer.