The many various biological properties and functions of the carotenoids are besides their ubiquitous occurrence the main reason for the importance of this class of compounds and the main aspects have been reviewed (Krinsky, 1994).

In photosynthesis the energy transfer involves the direct excitation of carotenoids by light to form the first excited singlet state, and the subsequent transfer of this excitation energy to chlorophyll to initiate the process of photosynthesis. This type of process can effectively extend the wavelength of light available to an organism for photosynthesis.

Carotenoids also play a major role in the photoprotection of cells and tissues. This ability is the result of energy transfer reactions in which the energy of triplet state sensitizers or singlet oxygen is transferred to carotenoid molecules in the ground state, forming triplet state carotenoid molecules. The energy acquired by the carotenoids is then lost as heat and the ground state carotenoid is regenerated to undergo another cycle of photoprotection.

In humans and in those animals that require vitamin A for normal growth and development the most important source is the ingestion and metabolism of carotenoids that can be converted to vitamin A, i.e. compounds with an unsubstituted beta-ring, especially beta-carotene. In Western countries the supply of vitamin A is not critical but in countries of the third world it is still a severe problem. According to an estimate of the World Health Organization (WHO), 250’000-500’000 children go blind every year due to a deficiency of vitamin A. It was demonstrated that the formation of vitamin A from beta-carotene can occur either by central or by excentric cleavage of beta-carotene.

The ability of carotenoids to act as antioxidants has been known for a long time and at the moment it is of great interest whether carotenoids behave as antioxidants in low-density lipoproteins (LDL), inasmuch as the oxidation of LDL is now considered to be an important causative agent in coronary heart disease, but the results of studying carotenoid involvement in preventing LDL oxidation remain controversial.

There have been many reports of a positive effect of dietary carotenoids on improving fertility or reproduction capacity in a number of animals, but additional evidence is still required for this proposed function of the carotenoids

There are a few example where it was demonstrated that carotenoids can alter the activity of a specific enzyme (e.g. aryl hydrocarbon hydroxylase) and this could be of importance in view of the detoxification of potential carcinogens.

It was demonstrated that various carotenoids, such as lycopene, beta-carotene, alpha-carotene, lutein and canthaxanthin can decrease the extent of malignant transformation of cells. The molecular actions occur via up-regulation of the connexin43 gene, the gene responsible for the production of one of the important components of the gap junction.

Based on epidemiological data it can be assumed that diets rich in carotenoid-containing fruits and vegetables are associated with significantly decreased risks for a variety of degenerative diseases. However in dietary epidemiology it is always difficult to pinpoint the components which may be related to the lowered risk.

Several epidemiological studies have supported the observation that a high content of blood carotenoids decrease the risk of cataract formation. This is important in view of dietary aspects, particularly of the growing elderly population.

Age-related macular degeneration (ARMD), associated with aging can lead to total blindness in otherwise healthy people. A significant reverse relationship between the incidence of ARMD and the ingestion of fruits and vegetables rich in provitamin A carotenoids was demonstrated and it was shown that there are very significant reductions in the risk of developing neovascular ARMD as a function of plasma levels of alpha-carotene, beta-carotene, cryptoxanthin and lutein/zeaxanthin.

Coronary heart disease (CHD) remains a major cause of death in Western societies. There is epidemiological evidence for an inverse association between serum levels and ischemic heart disease, but it is still unclear whether a single antioxidant plays the essential role or whether the sum of the antioxidants are responsible for preventing this disease in humans.