|Left, sun tan; center, neutral; right, carotene-rich|
According to the Science News article:
“Dr Ian Stephen, from the School of Psychology, University of Nottingham, Malaysia Campus, led the research as part of his PhD at the University of St Andrews and Bristol University. He said: "Most people think the best way to improve skin colour is to get a suntan, but our research shows that eating lots of fruit and vegetables is actually more effective.”
The article continues:
“Dr Stephen said: ‘We found that, given the choice between skin colour caused by suntan and skin colour caused by carotenoids, people preferred the carotenoid skin colour, so if you want a healthier and more attractive skin colour, you are better off eating a healthy diet with plenty of fruit and vegetables than lying in the sun.’
“Dr Stephen suggests that the study is important because evolution would favour individuals who choose to form alliances or mate with healthier individuals over unhealthy individuals.
“Professor David Perrett, who heads the Perception Lab, said: ‘This is something we share with many other species. For example, the bright yellow beaks and feathers of many birds can be thought of as adverts showing how healthy a male bird is. What's more, females of these species prefer to mate with brighter, more coloured males. But this is the first study in which this has been demonstrated in humans.’”
This research follows up on the team’s previous publication in The International Journal of Primatology, entitled “Facial Skin Coloration Affects Perceived Health of Human Faces” in which they discussed the health and fertility effects associated with carotenoid consumption and coloration in many species:
“Carotenoid supplementation is associated with improved development of the immune system in human children (Alexander et al. 1985), whereas individuals infected with HIV and malaria have reduced carotenoid levels (Friis et al. 2001). Carotenoid levels may also affect spermatogenesis in boars (Chew 1993), and women who failed to conceive during in vitro fertilization had unusually fluctuating carotenoid levels in their follicular fluid (Schweigert et al. 2003). Brightly colored carotenoid-based ornaments are displayed by many bird and fish species, the size and brightness of which reflect aspects of health and condition. Male greenfinches with brighter yellow breast feathers showed stronger humoral immune responses to a novel antigen (Saks et al. 2003). Male and female yellow-eyed penguins with more saturated yellow eye ornamentation produced more offspring per season (Massaro et al. 2003). Researchers have demonstrated mate choice based on the brightness of carotenoid ornaments in greenfinches (Eley 1991), yellow-eyed penguins (Massaro et al. 2003), and goldfinches (MacDougall and Montgomerie 2003).”
To summarize, the two studies simply show that people prefer (i.e. find more attractive) skin that has more red, yellow, and bright coloration, to skin that has blue, green, and dark coloration. Stephen et al discuss the fact that redness signifies good blood perfusion, and that red, yellow, and bright coloration associates with fertility and virility in many species. It appears that they have shown that this applies to humans as well as other species. In case you wonder, they have also shown that people with naturally dark skin – namely, African ethnics – also prefer skin with a brighter, yellowish and reddish tint.
This research reminds me of earlier work published in the PNAS entitled “Carotenoids and retinol: their possible importance in determining longevity of primate species,” (full text pdf available here ) in which Cutler showed that serum and brain tissue concentrations of carotenoids positively correlate with maximum life span potential in 13 mammalian species. That is, he found that longer-lived species have higher serum and brain tissue carotenoid concentrations than short-lived species, with humans having the highest carotenoid concentrations of the 13 species he tested. Cutler wrote:
“It is not understood what determines the serum and tissue concentrations of carotenoids, but it is likely to be based in part on the quantitative and qualitative aspects of absorption and on the activity of an enzyme found predominantly in the intestinal mucosa cells, ß-carotene 15,15'-dioxygenase (39). This enzyme is responsible for initiating the conversion of carotene to retinol, and low levels would be expected in the longer-lived species.”Indeed, humans do show lower activity of this enzyme; see below. Continuing the quote from Cutler:
"Humans unselectively absorb both carotenes and xanthophylls into their tissues, whereas the shorter-lived species absorb only the carotenes (16-18). The amount of carotenoids in the diet would of course also play an important role in determining the amount of carotenoids that are absorbed, but not the qualitative aspects. Thus, although all carotenoids are derived from the diet, the amount and type that is absorbed in serum and other tissues is clearly a species-dependent characteristic. [italics added]
“During the evolution of increased MLSP in the mammalian species, and particularly in the primates (27, 40), carotenoid concentration in serum and tissues also may have increased. This may have been facilitated by an increased absorption of both the carotenoids and xanthophyll and a decrease in the activity of intestinal ß-carotene 15,15'-dioxygenase. Thus, the nonselective absorption of the carotenoids in humans may represent an end point to this evolutionary strategy; most of the carotenoid protection attainable through the diet is now being utilized.” [italics added]
It appears that in the course of human evolution, the activity of ß-carotene 15,15'-dioxygenase has declined substantially, such that up to 45% of people do not convert ß-carotene to retinol vitamin A. Hickenbottom et al found that 45% of 11 men tested did not convert ß-carotene to vitamin A (retinol). Lin et al found the same in women. Leung et al identified gene polymorphisms contributing to this variability in carotene conversion capacity.
These data indicate that in the course of our evolution, humans have shifted from use of ß-carotene as a precursor for vitamin A, to dependence upon animal sources of retinol (vitamin A), such as liver, exerting a negative pressure on the retention of activity of ß-carotene 15,15'-dioxygenase. This simultaneously gave us opportunity to use carotenoids and xanthopylls for other purposes (as they were no longer needed for retinol production). As noted by Cutler, we humans absorb these non-selectively, suggesting that the human organism has found uses for most carotenoids. It appears that this shift facilitated an increase in lifespan, perhaps by virtue of the carotenoids now being available for use primarily as free-radical scavengers and protection against genetic malfunctions leading to cancer, etc.
As an example, consider the photoprotective effects of carotenoids. You may have noticed that green leaves sit out in the sun all day long without any sunscreen, yet do not become cancerous regularly. Aside from being a sign of health, the storage of carotenoids in the skin protects the skin from ultraviolet radiation thereby retarding the aging process. For more support for this assertion, see this, this, this, this, this, and these:
These data appear to provide evidence similar to something I have noted in my experience, namely that a higher skin content of carotenoids may reduce both the tendency to burn and the need and production of melanin in response to UV radiation. When a youth and living in Ohio, I got sun burned many times in the summer sun there. At the time, my diet had no where near the carotenoid levels of my current diet. Now I live in Arizona, and I often go out in the blazing summer sun around noon. In the 10 years I have lived in Arizona, I have never gotten a sun burn anything like I had when a child. I might turn a little red on my shoulders for 24-36 hours, but I don’t blister and peel, despite the higher intensity of the Arizona sun compared to Ohio.
Carotenoid Complexion and Sun Tan Not Mutually Exclusive
While Stephen et al may appear to consider sun tans and carotenoid complexion as mutually exclusive, as if one can only have one or the other, I would not agree with this. Sun exposure is necessary for vitamin D production, not to be avoided.
However, in my experience, copious consumption of carotenoid-rich food increases skin content of carotenoids to a level that leads to reduction of sun burning AND a reduced melanin response. In all of the more than 20 years that I have eaten a high carotenoid content diet, my sun tan appears more orange-yellow than brown.
If you don’t believe that carotenoids can modify your response to sun exposure, both in terms of burning and melanin response, don’t look for more data. I already gave you a dozen references above. The next step is to try it yourself. In my experience, even fair skinned (e.g. Scandinavian) individuals can increase their sun tolerance simply by increasing their intake of carotenoids. Again, the best data comes from your own experience. Try it.
I believe that traditional consumption of high carotenoid content foods help explain why our ancestors could spend most of their time outdoors without suffering the type of malignant skin damage found among modern people who spend less time outdoors but consume lower amounts of carotenoids. This is likely an aspect of dietary influence on skin cancer incidence. Modern people may consume lower amounts of carotenoids than our ancestors, making modern man's skin more susceptible to sun damage despite less total sun exposure.
I further wonder if by eating both fatty meat and cooked or fermented carotenoid-rich vegetables (more about the cooked and fermented below), humans were able to reduce the use carotenoids for vitamin A and increase their use for photoprotection, which in turn reduced the need for body hair to protect the skin from UV radiation, as in other species. In other words, I wonder if our unique approach to omnivory made it possible for us to shed most of our body hair and still withstand the African homeland sun.
Carotenoid Absorption and Sources
These authors emphasized eating fruits and vegetables for carotenoids. However, they did not mention several important facts discussed at length in Carotenoids: Nutrition and Health :
- Carotenoids are fat-soluble so we must consume fats with carotenoid-rich foods to optimize carotenoid absorption. [p.136]
- Fiber present in fruits and vegetables reduces carotenoid absorption:
“Not only purified fibre, but also fruit and vegetables as sources of this fibre, cause reductions in carotenoid bioavailability. Dietary fibre, lignin, and resistant proteins found in green leafy vegetables inhibited the release of ß-carotene and lutein  and citrus pectin reduced the plasma ß-carotene responses .” [p.138]
- By rupturing plant cells in which carotenoids are stored, cooking and/or pureeing carotenoid-rich vegetables dramatically increases bioavailability of carotenoids.
“In healthy women, feeding heat-processed and pureed carrots and spinach cause serum ß-carotene to be three times higher than when the same dietary level of ß-carotene was consume in the raw food sources . In a population of women at risk for breast cancer, serum concentrations of lutein and a-carotene, but not of ß-carotene, ß-cryptoxanthin, or lycopene, were higher in women consuming vegetable juice, rather thatn cooked or raw vegetables .” [p.134]
- Fake-fats (sucrose-polyesters) inhibit carotenoid absorption. [p.136]
- Statin drugs and plant sterols reduce carotenoid absorption. [p.137]
Some people may suggest that animal fats from grass-finished animals supply substantial amounts of highly bioavailable carotenoids.
- Duckett et al found that meat from grass-finished cattle contains 54% more ß-carotene than meat from corn-finished animals.
- Mother Earth News reported that pastured chickens produce eggs that contain 7 times as much ß-carotene as conventional eggs.
This photo from my collection shows the difference in color between an egg yolk from a chicken fed a supplemented grain concentrate (purchased at a Wild Oats natural food store) and one from a chicken raised on pasture (purchased from A Bar H Farm), a difference caused by carotenoid concentration:
|Egg yolks: L, from grain-fed hen; R, from pastured hen.|
However, these numbers and photos may mislead. According to Duckett et al, conventional beef and grass-finished supply only 29 and 44 mcg ß-carotene per 100 g serving, respectively. One pound of grass-finished beef thus provides about 200 mcg of ß-carotene. The Mother Earth News study found that one egg from a pastured hen supplies 79 mcg of ß-carotene.
In comparison, 100 g of cooked carrots contains 8332 mcg of ß-carotene, more than 40 times what we could get from an entire pound of grass-finished beef. Hedren et al produced data on absorption of carotenes from carrots suggesting a maximum extraction of about 3% of the ß-carotene if we eat it raw, and 39% if we cook the carrots to a soft texture and consume them with fat.
The raw supplies less than the cooked, because in raw vegetables, all of the carotenoids lie inside the plant cell walls, which consist of cellulose, and we can't digest cellulose. Cooking explodes the cells, allowing the juice to flow out for utilization.
Thus, 100 g of raw carrot would deliver about 250 mcg of ß-carotene, 25% more than a whole pound of grass-finished beef and about 3 times as much as an egg yolk from a pastured egg. One hundred grams of carrot cooked soft with fat would provide 3250 mcg of ß-carotene, more than 16 times what one would get from a whole pound of grass-finished beef, and 40 times what we could get from one egg yolk from a pastured hen.
|Carotenoid-rich Primal Food: Beef with carrots, squash, and greens.|
Since it appears that people prefer carotenoid complexions (which in my experience can manifest in concert with a sun "tan"), and people with high blood levels of carotenoids have better health than those without, and this also occurs across ethnic groups as well as in other primate species, and high carotenoid levels of plasma and brain correlate with longevity across mammalian species, this strongly supports the idea that humans evolved to attain long lifespans on diets containing high amounts of both vitamin A and carotenoids, i.e. dark green leafy and deep orange or red vegetables or fruits such as kale, collards, spinach, carrots, winter squashes, and tomatoes, and not exclusively carnivorous diets.
Ethnographic data (see Guts and Grease: The Diet of Native Americans ) even largely carnivorous tribes of humans such as Inuit and Blackfoot recognized the value of deep green plant foods and got some carotenoids by consuming the partially fermented grasses found in the foreguts of ruminants such as buffalo and caribou. As reported by Enig and Fallon in "Guts and Grease," according to John (Fire) Lame Deer, the eating of guts in his tribe had evolved into a contest:
Again, I suggest you to test this in your own experience. Dark green leafy vegetables and carrots have negligible carbohydrate content yet deliver large amounts of carotenoids. If you are sun sensitive, or want to see how increasing the carotenoid content of your skin affects your sun tan or untanned appearance, all you have to do is eat more high carotenoid foods to find out. It takes about 10 weeks of high carotenoid consumption to produce a change."In the old days we used to eat the guts of the buffalo, making a contest of it, two fellows getting hold of a long piece of intestines from opposite ends, starting chewing toward the middle, seeing who can get there first; that's eating. Those buffalo guts, full of half-fermented, half-digested grass and herbs, you did not need any pills and vitamins when you swallowed those." 
1. John (fire) Lame Deer and Richard Erdoes, Lame Deer Seeker of Visions, Simon and Schuster, 1972, p. 122.