Is redder, more oxygenated, skin judged as healthier, more desirable? Does the ability to detect a healthy cardiovascular system influence our choices of potential mates? Was our retinal system under evolutionary pressure to detect the oxygenation level of skin? Is the evolutionary importance of the signals presented by healthy oxygenated skin one of the foundations of the cosmetic industry?
Good skin blood flow and oxygenation is dependent on the underlying health of the cardiovascular, hormonal, and circulatory system. The detection of good blood flow in the skin is particularly noticeable in the face, and prominent in the cheeks. Hence, the general healthy look of rosy red cheeks we have all observed, and heard about. Evolution, being what it is, could it be that rosy red cheeks are used as a predictor in primates for a health, and hence a good mate choice?
Stephen et al., 2009 examined this exact issue. In their introduction the point out the various research that indicates the link between skin redness color and underlying ‘vascular’ health:
In humans too, skin redness caused by skin vasodilation and
vascularisation has connections to physiological status including
health. Additionally, blood oxygenation state is related to health
status and affects skin colour. In women, increased sex hormone levels
are associated with increased skin vascularisation  and vasodilatory
response , which arterializes the blood in the skin . The
cutaneous vasodilator system becomes more responsive with physical
training , but is impaired in type 2 diabetes  and hypertension
. Increased blood oxygenation is associated with increased
aerobic fitness  whereas increased blood deoxygenation is
associated with hypoxia and can lead to cyanosis (blue tinted skin),
which is indicative of coronary and respiratory illness .
Hence, it appears there is some validity to the idea of a healthy glow. Now in general the color red, not only in skin color but in any object, is associated with being vibrant and lively. But more particularly do humans pick up and judge people with redder skin as healthier, and more desirable?
The researchers hypothesis was that increased blood color in the face will be judged as healthier and oxygenated blood color would be considered healthier than deoxygenated blood color.
(side note: human oxygenated blood is 98-99% saturated with oxygen, deoxygenated blood is around 75% oxygen saturated)
First the scientist wanted to test if oxygenated skin blood color is judged as healthier than deoxygenated blood color. Subjects were presented with faces with varying degrees of oxygenated or deoxygenated color. These Caucasian faces (or later different ethnic faces), which were presented, had been transformed to match ‘redness’ of empirically collected data from oxygenated vs non-oxygenated faces. The subjects then were asked to alter the images (via photoshop like manipulation) along two axis - they could add ‘oxygenated’ color, or ‘deoxygenated’ color and their instructions were to ‘make the face as healthy as possible’.
The subjects were not told that these two manipulations were oxygenated and deoxygenated color.
The paler the faces the greater the increase in redness (both oxygenated and deoxygenated) was added by the subjects to make the faces look healthy. If the faces were already quite red (oxygenated) then less redness was added by the subjects. Hence, the faces which appear to have the least skin blood perfusion (pale) the greatest degree of redness was added to optimize the appearance of health.
While face pictures of both sexes were altered to increase redness, more oxygenated color was added to female faces and more deoxygeneated color added to male faces (in comparison). The authors suggest that females faces are more ’sensitive’ to blood oxygenation color - but I am not sure what they really mean.
Overall, oxygenated blood color adding was judged as improving the look of healthiness compared to added deoxygenated blood color adding. This was consistent with the researchers hypothesis - but also makes sense in the general concept of a healthier look is judged by degree and amount of oxygenated blood is reaching the face (sign of a healthy cardiovascular system).
In an additional component of the study, the researchers found that participant ethnicity, or face ethnicity, did not result in any differences in the amount of color changes added by participants to optimize a healthy appearance. However, there was an interaction effect, with African participants adding more redness to African faces than the other faces. The authors did not offer much of an explanation for this finding.
The researchers conclude from their study:
The healthy appearance of faces is enhanced by increased blood
colouration in this study, suggesting that participants interpret skin
blood colouration as a cue to underlying health. This is consistent
with the established relationship between skin blood perfusion and
physiological status. Increased vasodilation and vascularisation of
the skin leads to increases in skin blood colour. These processes are
enhanced by increased levels of sex hormones in women , and
by physical training . Skin blood flow is reduced in patients
with hypertension , type 2 diabetes , senescence  and
in smokers .
Beyond the idea that us humans use skin color to detect the underyling health of our fellow humans, the authors suggest their might be various levels of evolution going on:
Indeed, it has been suggested that the maximum sensitivities of the medium and long wavelength cones in the retinas of routinely trichromatic primates (a group which includes humans) are ideally suited for identifying small changes in blood perfusion and oxygenation in the skin of conspecifics . The results of the current study suggest that the ability to perceive health cues provided by skin blood perfusion and oxygenation may be an additional advantage of trichromatic colour vision in primates.
(43: Changizi MA, Zhang Q, Shimojo S (2006) Bare skin, blood and the evolution of primate colour vision. Biol Lett 2: 217–221.)
This part seems like a stretch for me, that our retinas specifically evolved to detect healthy mates. Though I appreciate beauty as much as everybody else it seems to me there were many other important evolutionary pressures in retinal evolution (to find and catch prey, and avoid being prey as just a few important examples) other than to detect beautiful-health. But if further research indicates that skin color detection was so important that it fundamentally affected the evolution of our retinal system - well then praise beauty.
Out of curiosity I took a look at the abstract they cited (Changizi MA, Zhang Q, Shimojo S (2006)):
We investigate the hypothesis that colour vision in primates was selected for discriminating the
spectral modulations on the skin of conspecifics, presumably for the purpose of discriminating
emotional states, socio-sexual signals and threat displays. Here we show that, consistent with this hypothesis, there are two dimensions of skin spectral modulations, and trichromats but not
dichromats are sensitive to each. Furthermore, the M and L cone maximum sensitivities for routine trichromats are optimized for discriminating variations in blood oxygen saturation, one of the two blood-related dimensions determining skin reflectance. We also show that, consistent with the hypothesis, trichromat primates tend to be bare faced.
Okay, they have examined it at a reasonable level - though I am sure others in the field could come up with alternative points of discussion. I will be dig deeper - all in the name of evolution and beauty.
(strange thought: maybe you could see if individual differences in the ability to detect degrees of face coloration is related to specific ‘coloration’ gene polymorphisms. If true it would scare me a bit).
Stephen et al., give their summary paragraph at the end of the paper:
In the current study, we show that colour associated with skin blood
perfusion and oxygenation affects the healthy appearance of
human faces. Attractiveness, thought to signal underlying health
[60,61], and strongly related to perceived health  is a major
factor in human mate choice, particularly by men . It is likely,
therefore, that the enhanced health appearance associated with
increased skin blood colour and oxygenation colour has consequences
for attractiveness and mate choice.
Take home message:
All of this really comes down to humans may use skin color/tone - rosy red cheeks - as a measurement of blood flow/oxygenation which is an indicator of general health. We would judge the rosy red cheeked faces as more desirable. Then this evolutionary desirability of health gets transformed in our higher language as beautiful / handsome, and a host of other similar words. But most biological signals are subject to manipulation. For how long have humans pinched their cheeks to bring about a rosy complexion? Moving onward to various natural substances that could be rubbed lightly on the cheeks. And from this a whole industry of ‘blush’ makeup was founded, and still flourish probably to the tune of multiple millions of dollars every year. Well instead of spending all that money on blush one could do things to increase your vascular health such as exercise. Instead of just signaling you are healthy, you could actually be healthy.
Is a healthy oxygenated skin color obtained via a healthy lifestyle not only a health hack but also a date hack