We’ve long known that leafy greens are particularly beneficial to patients at risk of, or living with, macular degeneration. New research out of the University of Queensland has found that sweetcorn and orange capsicum can also work wonders.
It was surprising to me, as a plant physiologist, to discover that humans and other primates accumulate two pigments in our macula that are synthesised predominantly in plants. Those ‘macular’ pigments are called zeaxanthin and lutein, and are orange- and yellow-coloured, respectively. Furthermore, they are deposited in our macula, where they are thought to protect our photoreceptors from blue-light induced oxidative damage. In fact, they are accumulated to such a concentration, that where they are deposited is known as the macula lutea, which translates to ‘yellow spot’.
orange capsicum… has, by far and above, more zeaxanthin than any other fruit or vegetable you will find in the marketplace
Studies have found that AMD is commonly associated with a low concentration of these pigments in the macula, and that consuming food that is high in zeaxanthin and lutein will increase macular pigment optical density (MPOD). Humans can’t manufacture lutein or zeaxanthin, so they have to be obtained through what we eat, or through an artificial supplement.
AN INTERCEPT FOR BLUE LIGHT?
The most common foods containing macular pigments are dark green leafy vegetables for lutein, and sweetcorn or orange capsicums for zeaxanthin. Foods containing lutein seem to be much more common in our diet than those containing zeaxanthin.
Having a mix of both lutein and zeaxanthin may be important, since each pigment is deposited in a slightly different location within the macula. While lutein is more abundant in the macula periphery, zeaxanthin is actually most abundant in the central macula, where our photoreceptors are at their highest concentration. The reason for this is not clearly known, but it may have to do with the angle at which the molecule sits in the cell membranes to intercept blue light before it damages the photoreceptors.
Lutein is a yellow pigment, while zeaxanthin is orange in colour. Together, they absorb a range of potentially damaging blue light frequencies. Since they absorb blue light from the visible light spectrum, they appear as ‘complementary’ yellow and orange colours to us when we view them.
With this in mind you’d be forgiven for thinking that any yellow or orange fruit or vegetable contains lutein and zeaxanthin. However, it is not so simple. This is partly because some pigments mask the yellow colour of lutein, and some non-macular orange pigments are a similar colour to zeaxanthin.
The reason, for example, that dark green, leafy vegetables are recommended as a good source of lutein, is because lutein helps protect the green chlorophyll, present in the plant’s leaves, from excess light damage. Consequently, any green-coloured vegetables or fruit is also likely to contain high amounts of lutein, even though they appear green in colour.
Unfortunately, the same cannot be said for zeaxanthin which is relatively rare in our diet. One of the few vegetables containing a significant amount of this macular pigment is yellow sweetcorn.
As part of a plant breeding biofortification program, we decided to try and increase the level of zeaxanthin in sweetcorn, so that consumers could get more of this macular pigment without having to eat more. In the end, we bred a new sweetcorn that had 10 times the amount of zeaxanthin found in normal yellow sweetcorn.
While lutein is more abundant in the macula periphery, zeaxanthin is actually most abundant in the central macula
As you might expect, increasing the level of an orange pigment, such as zeaxanthin, also changed the colour of the sweetcorn. The resulting colour was more of a golden colour than an orange colour, and so we called it ‘Supergold’. We were very cautious to avoid changing the colour too drastically, and most consumers we tested were quite happy with the resulting colour, even more so once they found out it was high in zeaxanthin with potential benefits for AMD. This product is currently being commercialised, and should reach supermarket shelves within a few years.
Another vegetable that is unexpectedly high in zeaxanthin is orange capsicum. This vegetable has, by far and above, more zeaxanthin than any other fruit or vegetable you will find in the marketplace. Most orange capsicums will be a great source of zeaxanthin, with a single fruit of some varieties containing the equivalent amount of zeaxanthin as that found in a bottle of AMD supplement tablets. Sadly, red, green and yellow capsicums contain much less zeaxanthin than their orangecoloured relative.
Increasing consumption of vegetables high in lutein and zeaxanthin is one way to increase macular pigment density, but encouraging their uptake into your body during digestion is also of benefit. As both lutein and zeaxanthin are oil- or fatsoluble pigments, consumption of these vegetables in combination with an oilbased salad dressing, or a dob of butter/ margarine on sweetcorn, should assist in improving uptake.
It’s important to know that just because a fruit or vegetable is orange in colour, it may not necessarily contain zeaxanthin. Another orange compound called betacarotene is abundant in many vegetables, such as carrots, pumpkin, and orange sweet potatoes. Beta-carotene is important in its own right, being converted into Vitamin A (retinol), which is important for night blindness. The downside however, is that beta-carotene may compete with the uptake of lutein and zeaxanthin during digestion, as they are both transported in the body on the same transporter ‘carrier-protein’.
The question as to whether we should eat vegetables high in beta-carotene at the same time as we are eating those high in macular carotenoids remains to be answered. And, of course, human genetics comes into it as well. Some people happen to be better at converting beta-carotene into retinol. As retinol does not compete with macular pigments for transport within the body, it seems that people who are better at converting beta-carotene to retinol, so that there is less beta-carotene present, are also better at accumulating zeaxanthin and lutein in their macula.
In the second Age Related Eye Disease Study (AREDS2) completed in 2013, it was found that replacing beta-carotene with lutein and zeaxanthin in the AMD supplement resulted in an 18% improvement in slowing the progression of AMD, particularly neovascular (wet) AMD. The study also found that lutein and zeaxanthin contributed to a 26% improvement in slowing the progression of AMD in participants with a low initial level of macular pigment optical density. It makes sense that taking lutein and zeaxanthin is beneficial for those who are lacking them. It also makes sense that if a person already has sufficient macular pigment in their eyes, then adding more lutein and zeaxanthin is like trying to overfill an already full glass of water. Checking your patients’ macular pigment density can therefore, provide them with valuable information about whether or not they should consider modifying their diet.
Associate Professor Tim O’Hare is a Principal Research Fellow at the Centre for Nutrition and Food Sciences, Queensland Alliance for Agriculture and Food Innovation at The University of Queensland. Fruit and vegetable research was in part funded by the ‘Naturally Nutritious’ project (HN15001).