FormalPara Key Points

  • The intake and biological fate of carotenoids are influenced by a complex array of environmental and genetic factors.

  • This chapter focuses on the impact of gender and body composition on carotenoid bioprocessing.

  • Colorful bird and fish species provide the most spectacular examples of gender-specific patterns in carotenoid metabolism and accumulation; such gender differences in humans are more subtle.

  • Women tend to consume more dietary carotenoids and have higher circulating carotenoid concentrations than men.

  • Some studies suggest that estrogen status may modulate carotenoid utilization in humans, but these observations require further clarification.

  • A higher body mass index has been correlated to lower serum carotenoid concentrations. This effect is probably mediated by several mechanisms.

Introduction

Carotenoid intake, absorption, transport, and storage in humans as well as other animals are influenced by a wide range of host-related factors. These include, but are certainly not limited to, expression levels of metabolic enzymes such as carotenoid monooxygenase 1 and 2, serum lipoprotein homeostasis, and fatty tissue prevalence. Such factors are impacted by environmental modulators including the type of diet consumed and individual genetics. Thus, the biological fate of carotenoids is complex and multifaceted.

The focus of this chapter is on the impact of gender and body composition on carotenoid bioprocessing. The discussion is initiated using some dramatic examples of gender-specific utilization of carotenoids for coloration in some male birds and fish. Much is known about the mechanisms that govern these processes, some of which may be informative when considering the potential gender differences in human carotenoid bioprocessing. In addition, gender-specific carotenoid metabolism within these species has been useful in deciphering some of the physiological roles of carotenoids, notably their impact on the immune system.

Next, epidemiological studies in humans are examined that have provided some correlative evidence for gender-specific patterns in carotenoid intake, circulating concentrations, tissue distribution, and utilization. While detailed mechanistic insights into their etiology are lacking, some differences may be due to the specific biochemical effects of sex hormones as suggested by studies demonstrating a modulation of carotenoid levels in women during different phases of the menstrual cycle. Finally, the importance of body composition in shaping carotenoid metabolism and biodistribution patterns in humans is examined.

Carotenoids as Gender-Specific Colorants in Birds and Fish

The prevalence of carotenoids in nature is nothing short of prolific. Their first biological use probably occurred early in evolution by primitive bacteria to stabilize cell membranes [1]. Upon the emergence of photosynthesis, carotenoids became further utilized as both light-harvesting and antioxidant pigments. Within eukaryotes and multicellular animals, carotenoids serve as lipophilic antioxidants but are also a key component of the colorful pigmentation found in everything from sponges and crustaceans to many vertebrates including fish, amphibians, reptiles, and birds. Some of these species utilize dramatic, gender-specific colorations as a central component of reproductive signaling. Because carotenoids are an essential part of these pigments, distinct gender differences in carotenoid bioprocessing have evolved. For example, during mating season, avian males may sequester dietary carotenoids for sexual display whereas females and their offspring maintain less conspicuous coloration patterns, perhaps to avoid notice by potential predators (Fig. 8.1). Thus, birds serve as especially transparent examples of gender-specific carotenoid metabolism in the animal kingdom.

Fig. 8.1
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Gender differences in feather carotenoid accumulation in the American Goldfinch lead to a much brighter yellow hue in males compared with females in the spring and summer months (top photograph by Leslie Morrison, reprinted with permission). During winter, males and females are nearly indistinguishable from each other (bottom photograph by Robert Dever, reprinted with permission)

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The underlying mechanisms that govern gender-specific patterns in carotenoid sequestration in birds and fish have been extensively examined. The overarching theme that emerges is the crucial role of sex hormones, including testosterone and estradiol, in regulating carotenoid utilization by particular physiological systems and accumulation in specific physiological compartments. In Arctic Char, a salmonid fish species, sexual maturation coincides with selective accumulation of carotenoids in the skin and gonads [2]. Implants of 11-ketotestosterone in this species led to a reduction of astaxanthin and canthaxanthin concentrations in the plasma and fillets and increased levels of these carotenoids in liver and skin directly demonstrating the ability of this particular sex hormone to regulate carotenoid metabolism [3]. Similar processes are at work in avian species. Treatment of male and female diamond doves with testosterone and dihydrotestosterone increased the hue and size of their characteristic red eye ring which contains high levels of lutein esters [4]. Treatment with estradiol had no effect but did increase circulating lipoprotein levels. This suggested that carotenoid sequestration in the eye ring may be conferred by androgen-dependent upregulation of carotenoid-specific cellular transporters in the eye-ring tissue rather than increased overall circulation of carotenoids by lipoproteins. In male zebra finches, testosterone levels were positively linked to increased beak coloration [5]; however, in contrast to diamond doves, testosterone was also associated with lipoprotein upregulation concomitant with increased serum levels of carotenoids. Female zebra finches also exhibited increased beak color upon testosterone administration but without upregulation of serum lipoproteins suggestive of a mechanism of carotenoid sequestration more similar to diamond doves [6]. Collectively, these studies demonstrate that, while sex hormones play a major role in regulating gender-dependent carotenoid pigmentation, the exact interaction is species, gender, age, and tissue specific.

Animals with distinct gender differences in carotenoid bioprocessing also serve to illustrate the role of carotenoids in certain physiological processes, notably the immune system. In general, higher testosterone levels correlate to decreased immune function; however, this trend appears to be somewhat abrogated in species that utilize carotenoids to develop colorful traits. In one study, nonbreeding male zebra finches had higher circulating carotenoid levels than females and mounted a stronger humoral immune response when challenged with phytohemagglutinin [7]. Both male and female finches supplemented with dietary carotenoids showed an improved humoral immune response compared with non-supplemented birds. Another study in carotenoid-supplemented nestling great tits (Parus major) noted that the carotenoids commonly used for coloration in that species (lutein and zeaxanthin) were not as immunoenhancing as carotenoids not utilized for coloration (β-carotene) suggesting that individual carotenoids have specific immunological effects [8]. Thus, while male birds with a higher degree of coloration may be more sexually attractive to prospective females, it remains unclear if increased coloration correlates to increased immunocompetence.

Carotenoid-Related Gender Differences in Humans

Gender clearly plays a dramatic role in carotenoid bioprocessing in many species of wildlife; however, the story is more subtle in humans. Whereas in our own species carotenoid-based coloration is not relevant, gender-related differences with regard to carotenoid intake and bioprocessing have been detected (Fig. 8.2).

Fig. 8.2
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Gender-specific trends in human carotenoid intake and metabolism. On average, women consume more dietary carotenoids and have higher serum carotenoid concentrations than men. Some evidence suggests that estrogen status in women may affect carotenoid utilization and health benefits. There is currently no evidence for the existence of gender differences in human carotenoid absorption and clearance

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Many epidemiological studies have been undertaken over the past 30 years to clarify the overall relationship between carotenoids and health and to develop a more thorough understanding of factors that impact carotenoid intake, circulation, and utilization in various subsets of the population. In cohorts including men and women, gender comparisons have been extrapolated. Such studies comprise the current understanding of carotenoid-related gender differences in humans; however, it is important to exercise some caution in interpreting such correlative data. First, total carotenoid intake is often measured using dietary questionnaires and thus can be prone to inaccuracies and/or assumptions even under the best study circumstances. Second, measurement of serum carotenoids which typically include β-carotene, α-carotene, β-cryptoxanthin, lutein, zeaxanthin, and lycopene is notoriously difficult due to methodological variation and carotenoid degradation during sample processing, even when state-of-the-art analytical technology is utilized. Third, the inherently high level of human genetic and environmental diversity can confound results or trends emerging from different carotenoid-based studies.

Finally, while the observed correlations between carotenoids and gender are in some cases compelling, information on the potential mechanisms driving them is generally quite limited. The reported modulation of carotenoid levels in women during different phases of the menstrual cycle does hint at a possible regulatory role for sex hormones [9, 10]. In addition, men and women naturally differ in their body composition. This, combined with observations that body mass index (BMI) and waist circumference impact carotenoid biodistribution patterns, makes it a potentially relevant factor when considering the underlying etiology of any carotenoid-based gender difference.

Gender and Carotenoid Intake

One consistently reported carotenoid-related gender difference in humans has been that of intake. Multiple human studies suggest that women, on average, consume greater amounts of carotenoids than men. In one study examining the impact of various physiologic and lifestyle factors on serum carotenoid concentrations in a randomized cohort of Americans over the age of 43 years, men reported consuming 17% less β-carotene, 18% less α-carotene, 30% less β-cryptoxanthin, and 25% less lutein  +  zeaxanthin compared with women through a National Cancer Institute Diet History Questionnaire [11]. Lycopene consumption was similar between men and women. A nearly identical gender trend in carotenoid consumption was detected in a cohort of older Americans (age 67–93 years) based on their answers to a Willett 126-item food frequency questionnaire [12]. In a cohort of nearly 600 Dutch men and women (age 20–59 years), men reported consuming, on average, 10% less vegetables and 20% less fruit compared with women [13] via a semiquantitative food frequency questionnaire, and this translated into a lower calculated intake of β- and α-carotene for the males. In addition to the diversity of dietary questionnaires utilized in studies, each relied on separate databases to estimate the total carotenoid content of the diet. This particular gender trend thus has additional strength because of its detection using multiple methodologies.

In contrast to the aforementioned investigations, there have been some studies where gender differences in carotenoid intake were not apparent. While comparing the effectiveness of two types of food questionnaires, no difference in carotenoid intake could be detected between 162 healthy men and women recruited by the Arizona Cancer Center [14]. In a comprehensive study comprising over 36,000 people from ten different European countries, men and women reported consuming similar amounts of β-carotene; however, region-specific analysis of the overall cohort did reveal differences in male and female β-carotene intake in some areas [15].

In summary, women may consume more dietary carotenoids than men, but variation exists within particular population subgroups due to the types of food available and the dietary customs in particular regions. Regiospecific changes in overall food consumption patterns over time are likely to modulate this presumably dynamic carotenoid-related gender trend.

Gender and Carotenoid Serum Concentrations

Differences in the consumption of carotenoids by men and women also correlate with differences in their circulating carotenoid concentrations. In studies where carotenoid intake by men was significantly lower than that of women [11–13], circulating serum levels of the less-ingested carotenoids in men were also lower. Fitting with this trend, a recent large-scale investigation in >15,000 US adults (>20 years old) found that men comprised only 41% of individuals with serum α-carotene concentrations of 6–8 μg/dL and 35% of those with >9 μg/dL circulating α-carotene [16]. Likewise, in a study where no gender differences in carotenoid intake were detected [14], similar serum carotenoid concentrations in men and women were observed. These results imply that intake is an important predictor of serum carotenoid concentrations. Furthermore, with few notable exceptions, the reported correlation coefficients between carotenoid intake and circulating concentrations in men and women have been similar. This suggests, albeit indirectly, that the overall absorption and clearance of carotenoids do not significantly differ between the two genders. This assertion, however, is far from proven as, to date, virtually no mechanistic studies have been undertaken to directly investigate the role of gender on carotenoid bioavailability and clearance in humans. If such trends are to be detected, it will require targeted studies with high statistical power to overcome the wide variability inherent to carotenoid metabolism.

Gender- and Carotenoid-Related Health Benefits

It is well known that increased intake of carotenoid-containing fruits and vegetables is associated with a lower risk of many age-related diseases. Much research has centered around β-carotene; however, other major carotenoids, notably α-carotene [16], are now being investigated for their potential health benefits (see Chap. 11). While such effects apply to both men and women, there is evidence that the health-imparting properties of carotenoids may be modulated via gender-specific mechanisms. A study of over 2,000 individuals found that low dietary intake of lycopene was associated with an increased risk of rectal cancer in women (odds ratio of 1.5–1.7), but not men (odds ratio of 0.9) [17]. Women participants were then further divided into estrogen-positive (premenopausal) and estrogen-negative (postmenopausal with no hormone replacement therapy) groups. In the estrogen-negative cohort, lower β-carotene and lycopene intake were associated with a two- and threefold higher risk, respectively, of rectal cancer compared with women in the estrogen-positive cohort. Consistent with these results, data collected from a representative sample of >2,500 US adults over the age of 65 years revealed a strong association between low total serum carotenoid concentrations and all-cause mortality in women, but not men [18]. These two studies suggest that increased carotenoid intake may be especially important in postmenopausal women for disease prevention. They also imply a potentially important, but presently unclarified, mechanistic role for estrogen in the regulation of carotenoid bioactivity.

Carotenoids and the Menstrual Cycle

Further evidence that estrogen and other sex hormones modulate carotenoid bioprocessing comes from observations that circulating carotenoid concentrations fluctuate during different phases of the menstrual cycle. In a carefully designed and well-controlled study [9], serum carotenoid concentrations were measured at menses, early and late follicular, and midluteal phases in 12 women placed on a carotenoid-controlled diet for two complete menstrual cycles. Individual carotenoids were found to vary in distinct patterns throughout the cycle, but all were at their lowest concentrations during the menses phase. β-Carotene concentrations increased 10% and peaked during the late-follicular phase, while α-carotene did not fluctuate. Lutein/zeaxanthin, anhydrolutein, and lycopene increased 10%, 30%, and 12%, respectively, and peaked during the luteal phase. Serum retinol concentrations were also increased during the luteal phase. In a separate analysis [10], carotenoid concentrations within each of the major lipoprotein fractions were measured during menstrual phases. All carotenoids were found to primarily reside within the LDL fraction. From the early to late-follicular phase, α- and β-carotene concentrations increased approximately 10% in the LDL fraction, yet were not lower in any other fraction. From the late-follicular to the luteal phase, carotenoid concentrations decreased in the LDL fraction and increased slightly in the VLDL  ±  IDL fraction. Some fluctuation of carotenoid concentrations was also detected in the HDL2 fraction.

Collectively, these data suggest that carotenoid transport and distribution are affected or even regulated by the natural fluctuations in estradiol, luteinizing hormone, and progesterone associated with the menstrual cycle; however, the physiological ramifications of these changes remain unclear. Carotenoids are known to be absorbed into the reproductive tissue including the ovaries in cats [19] and dogs [20] and may have some biological role, perhaps as a source of vitamin A, during particular phases of the female reproductive cycle. Further studies will be necessary to verify and decipher the implications of these trends.

Carotenoids and Body Composition

The body composition of an individual, specifically the amount and distribution of adipose tissue, plays a critical role in the distribution and possibly metabolism of carotenoids. Whereas the underlying mechanisms driving gender-related carotenoid trends are mostly unknown, some biochemical and physiological details have been identified that may at least partially explain the effects of body composition on carotenoid biological concentrations. It is worth noting that, due to the natural differences of the body compositions of men and women, the impacts of gender and body composition on carotenoid bioprocessing are intrinsically linked.

Body Composition and Carotenoid Concentrations

The most notable and well-documented physiological effect of body composition with regard to carotenoids centers on their physiological concentrations. Multiple observations suggest a strong, inverse relationship between adiposity and carotenoid concentrations in both serum and the adipose tissue itself. In a population-based sample of 400 individuals, increased BMI was associated with a decrease in serum α-carotene, β-carotene, and β-cryptoxanthin [11]. In a European cohort of over 1,000 middle-aged and elderly participants, a 5 kg/m2 increase in BMI and a 10 cm increase in waist circumference were predictive of a 15–30% decrease in adipose tissue carotenoid concentrations [21]. In a small cohort of healthy men and women, higher BMI, percent body fat, and fat mass correlated to lower plasma carotenoid concentrations in women aged 60–80 years [22]. BMI, percent body fat, waist circumference, and waist-to-hip ratio were correlated to lower serum β-carotene concentrations in 276 women, but the association was less apparent in men [23]. In a multiracial cohort, serum carotenoids were 22% lower in individuals with a BMI >30 kg/m2 compared to those with a BMI <22 [24]. Two investigations correlated obesity to lower serum concentrations of all carotenoids, except lycopene in children and adolescents [25, 26].

The consistency of these associations among population cohorts that span both age and ethnicity adds to their strength. Lower carotenoid concentrations in obese individuals may be a contributing factor to increased risk for cardiovascular disease and cancer. Individuals with the lowest serum concentrations of α-carotene, the increased presence of which is linked to a lower risk of death, also have the highest BMI [16]. Decreased β-carotene concentrations in adipose tissue were also correlated to an increased risk of myocardial infarction in a Costa Rican cohort [27].

Potential Mechanisms

Multiple factors likely contribute to the lower carotenoid concentrations present in individuals with more body fat (Fig. 8.3). First, it is possible that obese individuals simply ingest less carotenoids per kg body weight. Two studies in particular found that individuals with a high BMI did not report consuming more carotenoids than individuals with a low BMI [11, 24]. Thus, in obese individuals, the physiological carotenoid pool may simply be more dilute.

Fig. 8.3
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Potential mechanisms contributing to lower serum carotenoid concentrations in individuals with a higher BMI

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A second explanation for the lower carotenoid concentrations in humans with a higher BMI may relate to the increased overall levels of oxidative stress that are associated with obesity, which, in turn, could result in carotenoid oxidation and depletion [28]. Higher serum carotenoid concentrations have been associated with lower levels of the oxidative stress markers 8-hydroxy-2′-deoxyguanosine [29] and malondialdehyde-thiobarbituric acid [30], but the biological significance of carotenoid depletion by excessive free radicals in obese individuals is not clear.

Finally, adipocytes not only readily absorb but also utilize and metabolize carotenoids. More body fat may thus lead to increased partitioning of carotenoids out of serum and into adipocytes concomitant with a higher rate of carotenoid clearance. Carotenoid metabolism in adipocytes is at least partially accomplished via the carotenoid monooxygenases. For example, β-carotene metabolism results in formation of two molecules of retinal by carotenoid monooxygenase 1, which can be converted to retinoic acid [31], or multiple asymmetric β-apocarotenals by carotenoid monooxygenase 2, some of which have been shown to regulate adipogenesis [32]. Other major carotenoids, including α-carotene and β-cryptoxanthin, are likely to produce additional novel metabolites that may have unique biological activities. The characterization of such metabolites and their effects in adipose tissue is an active area of research.

Conclusions

The biological fate of ingested carotenoids is influenced by a wide range of variables, both environmental and genetic. The most dramatic examples of gender-specific carotenoid utilization occur in bird and fish species where carotenoid-based coloration patterns serve as sexual attractants. Sex hormones are primarily responsible for regulating these gender-specific effects.

In humans, increased dietary consumption of carotenoids by women compared with men has been documented. This phenomenon is more pronounced in some regions than others. Gender differences in carotenoid intake are likely the major cause of the higher serum carotenoid concentrations typically detected in women. The apparent health benefits of carotenoids may be manifested through mechanisms affected by gender-specific hormones.

Finally, high BMI and overall adiposity correlate strongly with low carotenoid concentrations in the serum and adipose tissue. This may be due to the overall dilution of the physiological carotenoid pool in obese individuals, increased clearance of carotenoids via free radical oxidation, or increased absorption and metabolism of carotenoids within adipocytes, but other mechanisms are likely involved. Future studies may clarify these processes and determine their biological significance.