Carotenoids: The pigments of life

Actually we are all very familiar with the carotenoids. Higher plants produce them abundantly, but this abundance first becomes visible to us in the brilliant yellow and red leaves of autumn.

Moreover, the yellow, orange, red and even pink coloring of many fruits and vegetables are also due to carotenoids; for example the orange color of carrots (beta-carotene), the red of tomatoes (lycopene) and the yellow of corn (zeaxanthin). Many yellow flowers also contain the carotenoid lutein. However, albeit important, color is not the only role these intriguing pigments play in nature. Their wide-range of biological functions also brings with it the promise of health benefits, benefits which are closely linked with their properties of protecting cells and tissues from the harmful effects of various environmental agents generated  in today's world.

Humans are unable to synthesize carotenoids, but by consuming yellow and green vegetables and fruits, we ingest large amounts of these biologically important compounds. To date, about 600 naturally occurring carotenoids have been identified by researchers, and of these 50 have been located in food, while 6 major carotenoids have been found in substantial quantities in human plasma.

Figure 7: Selection of carotenoids for the eye. 600 carotenoids have been identified in nature, 50 are contained in the food chain, while 6 major carotenoids are located in human plasma. However, only two - lutein and zeaxanthin - are found in the human eye. This means that the eye specifically takes up the macular carotenoids from plasma.

Two of these carotenoids, namely lutein and zeaxanthin, are found exclusively in the macula lutea of the eye; often being referred to as the macular carotenoids. Since the absorption maximum for lutein and zeaxanthin lies within the blue-spectrum of visible light, they appear yellow, which is the complementary color of blue. The dual function of lutein and zeaxanthin, namely, the absorption and filtering out of harmful near-to-UV-blue light, while at the same time acting as potent antioxidants has remained throughout the course of evolution as crucial factors in the development of the human eye. These two important functions of lutein and zeaxanthin, though, give us clues as to how they might benefit eye health.

Figure 8: The carotenoids of the eye. Chemical structures of lutein and zeaxanthin.

Hammond et al. [7] have pointed out that the wide individual variability of lutein and zeaxanthin concentrations in the human fovea could have an effect on visual performance. The authors reviewed evidence that these two carotenoids protect against light and oxygen damage, while at the same time they improve visual acuity by absorbing short-wave light.

Visual acuity is classically defined as the ability of the eye to distinguish between the details of an object. How sharp an object is viewed essentially depends on the resolution of details, and this ability can vary from individual to individual. Easily scattered short-wave light is able to pass through the lens to fall on the retina. Since near-to-UV blue light is scattered, it disrupts resolution of the object details of an image at the moment it reaches the retina. Zeaxanthin and lutein - also known as the macular pigment - may improve visual acuity by absorbing or screening out unfocused near-to-UV blue light before it can disturb image processing by the retina’s photoreceptors. Preliminary scientific evidence  suggests that there is a large individual variation in the amount of zeaxanthin and lutein contained in the macula of the eye. This suggests that by increasing the content of these macular carotenoids in the yellow spot, human visual acuity may also be improved.

The highest concentration of zeaxanthin is at the foveal center of the macula , whereas lutein is located closer to the periphery. This high concentration of zeaxanthin at the fovea’s center is thought to offer an excellent shield against the damage of critical photoreceptors of the retina, i.e. the rods and the cones, by absorbing scattered near-to-UV blue light, thereby protecting visual acuity. Since the incidence of harmful near-to-UV blue light is most intense at the center of the fovea, zeaxanthin may play a role in the health of our eyes.

Lutein and zeaxanthin may also function as antioxidants in the macula by quenching or neutralizing damaging reactive free radicals. Free radicals  arise from normal biochemical reactions in the body or through exposure to toxic agents from the environment such as air pollutants or cigarette smoke. Moreover, the eye is exposed to the simultaneous presence of near-to-UV blue light and molecular oxygen, which facilitates the generation of reactive oxygen species. Photo-oxidative damage can therefore occur in the macula if the free radicals are not inactivated. In laboratory experiments, zeaxanthin has been shown to be a more potent antioxidant than lutein, possibly due to its chemical structure.

Since humans are unable to synthesize lutein and zeaxanthin, they have to consume foods, which contain these special carotenoids. Many fruits and dark green leafy vegetables are a dietary source. Spinach, one of the most popular vegetables in the Western diet, contains about 7.5 mg of lutein per 100grams of spinach, followed by  broccoli (2 mg/100g).

The main sources of zeaxanthin are sweet corn (0.5mg/100g), persimmon  (0.5mg/100g) and spinach (0.3mg/100g).

Figure 9: Foods rich in lutein. The highest concentrations of lutein are found in spinach and broccoli.

Figure 10: Foods rich in zeaxanthin. According to the United States Department of Agriculture (USDA), sweet corn, persimmons and raw spinach contain the highest concentrations of zeaxanthin.

Based upon the preliminary findings of  several epidemiological studies, a daily intake of 1mg zeaxanthin plus 5 mg lutein per day is recommended. These amounts can be obtained through the following sources:

Healthy diet rich in fruits and vegetables containing  lutein and zeaxanthin

Dietary supplements

Fortified foods or beverages

The average dietary intake of zeaxanthin and lutein, as reported by food surveys, is 0.2 to 2.1 mg  per day, respectively. By switching to a diet rich in fruits and vegetables containing zeaxanthin and lutein, these amounts may be increased to 0.3 mg and 2.3 mg for zeaxanthin and lutein, respectively. Nevertheless, a big dietary gap is still present.

The lifestyles in the Western world have changed dramatically over the last years, and in many cases people are not willing to change their eating habits. Therefore it’s good to know that DSM Nutritional Products offers its customers the opportunity to add both lutein and zeaxanthin to their dietary supplement products.

Zeaxanthin and lutein can also be used to fortify certain food products in order to reach a large population. These products may be referred to as functional foods, and the health-benefiting nutrients contained in them are often broadly referred to as nutraceuticals.

References

7. Hammond, Jr. BR, Wooten BR, Curran-Celentano J (2001) Carotenoids in the retina and lens: possible acute and chronic effects on human vision performance. Arch. Biochem. Biophys. 385: 41-46

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