Introduction

Carotenoids are predominantly produced by photosynthetic plants, algae, bacteria, and some fungi (Britton et al., 1995; Weedon, 1971). The functions of carotenoids in photosynthesis are: 1) to intercept and quench the excited triplet-state chlorophyll molecule preventing the generation of singlet oxygen and 2) to serve as

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accessory pigments absorbing light in the blue region of the visible light spectrum where chlorophyll is an inefficient absorber (Goodwin, 1980; Krinsky, 1971). In higher plants, carotenoids function in the zeaxanthin cycle, which is responsible for regulating the extent that blue light is converted into chemical energy by the photo-synthetic process (Demmig-Adams et al., 1996). A host of other essential or subsidiary functions are known or postulated for carotenoids in plants, including the important functions of leaf, flower, and fruit coloration (Goodwin, 1980). The process of evolution has refined photosynthesis for a period of more than one billion years during which a wide diversity of naturally occurring carotenoids have developed specialized roles in the many plant and algal species. Nearly 1000 structurally distinct carotenoids that are natural products have been isolated from living systems (Straub, 1987). The total global biosynthesis of carotenoids is estimated to be in excess of 100 million tons per year (Britton et al., 1995). Higher animals are unable to synthesize carotenoids but, nevertheless, have developed a dependence on these compounds for a range of functions.

Primarily because of the limited number of carotenoids that are present in the dominant food plants, higher animals and humans consume only a small fraction of the known natural carotenoid structural types (Khachik et al., 1991). In human serum there are as many as 50 carotenoids, but only half a dozen or so are normally present in percentages exceeding 5-10% (Khachik et al., 1995; 1992). These include a- and P-carotene, lycopene, P-cryptoxanthin, lutein, and zeaxanthin. Figure 12.1 shows the typical composition of the carotenoids extracted from human serum in the U.S. These carotenoids are all fairly abundant in varying quantities in the green and yellow vegetables, and red and orange fruits (Klaui and Bauernfeind, 1981). The hydrocarbon carotenoids, a- and P-carotene, and lycopene have received intense scrutiny because of their abundance and potential or proven functions in animal physiological processes, including provitamin-A status. It is the xanthophylls, oxycarotenoids, that are the focus of our attention.

P-Cryptoxanthin, lutein, and zeaxanthin are the major, although not exclusive, oxycarotenoid components of human serum. With the exception of P-cryptoxanthin, these are not provitamin A carotenoids because of the functional oxygen groups present in their structures. There is a keen interest in defining what their functional significance may be. It is important to understand to what extent these compounds individually or collectively contribute to optimal human health, and whether they do so by uniquely serving one or more vital physiological functions within one or more tissues. We include in our discussion the naturally abundant oxycarotenoid astaxanthin, and while it is a minor or perhaps more accurately an occasional component in the human diet, it has been identified as an especially effective antioxidant (Martin et al., 1999).

A question that naturally arises as part of this discussion is: What criteria establish when a dietary component, an oxycarotenoid in this instance, serves an essential functional role within a biological system? We might also ask: What criteria establish when a nonessential dietary component confers benefits that are significant to optimal health? Clearly, there are simplistic answers to each question, but it is

equally clear that an in-depth understanding of the human physiology of the oxy-carotenoids will be essential for the development of comprehensive answers as they apply to each specific carotenoid.

Important criteria which determine whether carotenoids individually or collectively are required by humans for normal, optimal health can be itemized Table 12.1. An essential dietary component must be universally present in all human populations. This is a surprisingly high hurdle to overcome and the conclusive data needed are not currently available for all human populations, much less for all carotenoids. The American dietary intake of carotenoids is not necessarily representative of that of other human populations. Therefore we cannot conclude that the carotenoids characteristically observed in human serum in the U.S. will be universally seen in all populations throughout the world. The seasonal availability of vegetable sources of carotenoids and the diversity of diets around the world add to the challenges of such investigations. Even if it is accepted that a dietary component is universally available in the human diet, there remains a considerable amount of physiological evidence required to establish unambiguously that it has an essential, functional role. The ability of humans to accumulate a carotenoid from the dietary sources at concentrations that are significant to a proposed function in a specific tissue, or cell types within a given tissue, must be established. Accumulation of a carotenoid in specific identifiable tissues, or cell types, by active transport is strongly supportive of the argument that the carotenoid serves a functional role, but passive nonspecific accumulation does not argue against the possibility of required function. That a given carotenoid is

TABLE 12.1

Evidence Needed to Establish a Functional Requirement for Carotenoids in Humans Must Satisfy Three Criteria

  1. The carotenoid must be universally available in the diet of all human populations.
  2. The carotenoid must be accumulated (either by active or passive transport) at concentrations consistent with the proposed function.
  3. Evidence of active transport
  4. Identification of regional sites of accumulation within specific tissues or subcellular organelles
  5. Evidence must exist that a pathological condition is associated with a deficiency of the carotenoid.

essential ultimately must be established by the associated pathology or dysfunctional condition that occurs in its absence.

Whether or not any of the oxycarotenoids found in humans unambiguously meets these requirements remains to be established. As we briefly review the role of the four title carotenoids, we should look for not only evidence that they may play an essential, functional role, but also evidence that they may play a more vague nonspecific, yet nevertheless functional and advantageous, role in human physiology.

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