Carotenoids are a group of around 500 lipid soluble compounds produced in photosynthetic plant, bacteria, fungi and algae. Carotenoids are responsible for some of the yellow, orange and red colours seen in fruits and vegetable. β-carotene is the most widespread and most well known carotenoids, but other common dietary carotenoids include zeaxanthin, cryptoxanthin, lutein, α-carotene and lycopene. Humans are unable to synthesize carotenoids and so fruits and vegetables are the primary sources of carotenoids in the human diet. Carotenoids accumulate in the chloroplasts of photosynthetic organisms where they are used in light collection. During times of low light condition (e.g. in the shade) carotenoids help harvest light. However, when the light exposure is extreme carotenoids prevent cellular damage by dissipating excess energy as heat, using their conjugated double bonds (figure 1).
Figure 1. The molecular structure of β-carotene. Note the conjugated double bonds that are important in energy dissipation.
The most well known, and perhaps least important role for β-carotene in human nutrition is as a precursor to vitamin A. Of more importance however, is the fact that carotenoids have strong antioxidant properties because of their conjugated double bonds. Carotenoids are important in the human antioxidant defence because they are able to physically quench the singlet oxygen (1O2) molecule in cells (figure 2). Carotenoids can absorb energy from the singlet oxygen without chemical change to return excited oxygen (1O2) to its normal unexcited ground state (O2). Carotenoids then release the energy in the form of heat and thus do not need to be regenerated like other antioxidants such as vitamin C and vitamin E. Carotenoids can quench hundreds of singlet oxygen molecules directly and are thus important defence against cellular damage which can lead to degenerative disease.
1O2 + β-carotene → 3O2 + excited β-carotene → β-carotene + heat
Figure 2. Physical quenching of singlet oxygen by β-carotene. Lycopene in tomatoes is the most efficient qunecher of singlet oxygen of all the carotenoids. 1O2 = singlet oxygen, 3O2 = triplet oxygen in the ground state.
Although carotenoids are produced by bacteria, algae and fungi, these sources are not important in human nutrition. In contrast, fruits and vegetables are a major source of dietary carontenoids. Spinach is a good source of lutein, zeaxanthin and β-carotene, and α-carotene is found in carrots. The carotenoid lycopene found in tomatoes is the most highly efficient quencher of singlet oxygen amongst the carotenoids. Other food rich in carotenoids include peppers, broccoli and corn. The most common supplemental carotenoid is β-carotene and this is the form found in most multivitamins. However, eating fruits and vegetables is by far the best way to increase cellular carotenoid levels because fruits and vegetables contain a wide variety and large quantity of carotenoids. This is important as it has been demonstrated that different carotenoids have slightly different effects in the body.
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