*Geek Box: Carotenoids

*Geek Box: Carotenoids

Carotenoids are a dizzying group of compounds, with over 600 known in nature. Carotenoids are pigments, and the diversity in the chemical structures of the compounds is reflected in the spectrum of colour, from dark greens with chlorophyll to yellow with lutein and zeaxanthin, orange with beta-carotene, and red with lycopene. Thus, it is lycopene which gives tomatoes their distinct pigment. It is also the pigments of these compounds which confers antioxidant properties, among other mechanisms of action.

Carotenoids are synthesised by plants, algae, and certain bacteria, and cannot be synthesised endogenously in humans, although the only carotenoid with nutritional value is beta-carotene as provitamin A. The remaining carotenoids, while lacking nutritional value, like other phytochemical compounds are biologically active and exhibit a diverse range of functions. Carotenoids are highly fat-soluble, and are absorbed in much the same way as dietary fat – intestinal absorption and packaged into chylomicrons.

The bioavailability of carotenoids is strongly influenced by fat content of a meal, food processing, and type of carotenoid. Consuming a higher fat meal results in significantly greater circulation levels of lycopene and beta-carotene, compared to a very low or no-fat meals. In addition, increasing concentration through processing and the effect of heating in food processing results in significantly enhanced uptake and efficiency of absorption.

For example, while fresh tomato may contain ~2,900mcg lycopene per 100g, tomato paste may contain a range of 5,400-15,000mcg. Tomato paste may thus yield a 3.8-fold greater bioavailability for lycopene than fresh tomatoes in humans, although there is no difference for alpha-carotene or beta-carotene, which demonstrates how the carotenoids may differ in bioavailability based on structural type.

Besides provitamin A activity of beta-carotene, carotenoids act as fat-soluble antioxidants, acting as both a chain-breaking antioxidant [i.e., direct scavenging of free radicals] and also upregulate endogenous antioxidant activity. Carotenoids tend to be stored locally in tissues, and provide cells with resistance to oxidative stress and damage. There are a number of potential mechanisms of action in relation to cancer, including cell DNA repair, apoptosis of cancer cells, reduced cell proliferation, and anti-angiogenic activity.

While these remain largely mechanistic, however, carotenoid research has produced some crucial, if unheralded, progress for nutrition science, specifically “golden rice”. In the developing world, vitamin A deficiency remains the leading nutrient deficiency and cause of morbidity – specifically child blindness – and child mortality. In 2000, scientists Ingo Potrykus and Peter Beyer genetically engineered rice, a crop which lacks carotenoid synthesis capacity, to have a pathway of provitamin A [beta-carotene] synthesis. This gave the rice a yellowy pigment, hence the term “golden rice”. Rice with this provitamin A synthesis pathway were able to produce 35mcg beta-carotene per gram – and provide a potential resolution to the scourge of vitamin A deficiency in the developing world. Unfortunately, anti-GMO activists have been successful in preventing the rollout of golden rice, a tragic example of anti-science disinformation successfully derailing the alleviation of needless human suffering.