The pigment you can eat

Two substances do nearly all the colouring in skin: melanin in the epidermis, haemoglobin in the blood below it. There is a third, it is much smaller, and it is the only one of the three anyone can move on purpose. Carotenoids are the yellow-to-orange pigments of carrot, sweet potato, peppers and dark leaves, and a fraction of what is eaten ends up deposited in the skin of the person who ate it.

That was measured on people who had been given nothing. Alaluf and colleagues took twenty-two lightly pigmented adults on ordinary unsupplemented diets, measured skin carotenoid concentration spectrophotometrically at several body sites, and set it beside tristimulus colour readings. Carotenoid level tracked b* — yellowness — at .85 on the back, .85 on the forehead, .78 on the palm and .75 on the inner forearm. It did not track lightness. It did not track redness. Carotenoid is a yellowness and nothing else, which is a much narrower claim than the word "glow" is usually asked to carry.

It also moves. Whitehead, Re, Xiao, Ozakinci and Perrett followed thirty-five people for six weeks, recording diet and skin colour at the start, at three weeks and at six. Changes in fruit and vegetable intake correlated with changes in skin redness and yellowness — and, the part that makes it evidence rather than coincidence, the shape of the reflectance change matched the absorption of carotenoids and not of melanin. So it was not a tan. They then asked the perceptual question, how much change is enough to see, and got about 2.9 extra portions a day to shift apparent health and about 3.3 for attractiveness.

Thirty-five light-skinned Britons is a thin base for anyone reading this in Seoul. The replication that matters more was run in Malaysia: Tan, Graf, Mitra and Stephen randomly assigned eighty-one Malaysian Chinese students to a carotenoid-rich fruit smoothie or to mineral water, daily, for six weeks. Yellowness rose sharply and redness slightly, both at p < .001, and both were still raised two weeks after the drinking stopped. The pigment behaves the same way in skin that carries more melanin. Whether the colour it makes is as visible against that melanin is a separate question, and a harder one.

Offered the two ways of going yellower, people have picked this one. Lefevre and Perrett gave observers faces transformed to be high or low in carotenoid colouration and high or low in melanin colouration; both pigments were preferred to their low versions, and carotenoid was preferred over melanin when the amount of colouration was matched. Stephen, Coetzee and Perrett had already found the same asymmetry from the other direction — asked to make a face look as healthy as possible, participants added more carotenoid colour than melanin colour — and reported that the preference for increased lightness and yellowness held in UK Caucasian and Black South African samples alike.

Now the contested part, and it is contested hard. Foo, Simmons and Rhodes built a comprehensive model of what predicts facial attractiveness — masculinity, femininity, symmetry, averageness, adiposity, colour — and found skin colour predicting attractiveness in neither sex; yellowness predicted perceived health in men only. Their own summary was that colour may play a limited role. Sharper still, and closer to home: Lu, Yang, Xiao, Pointer, Li and Wuerger used eighty calibrated, unmanipulated photographs of Chinese and western European faces, rated by twenty-two Chinese and twenty-two European observers. Lightness predicted attractiveness, health and youth — but only when Chinese observers judged Chinese faces. It was the European observers who linked yellowness to attractiveness in Chinese faces. Redness predicted nothing good anywhere, and in European faces it went with looking older. Their conclusion, in shape: both groups use colour, less than had been thought, and not in the same way as each other.

Two of the studies that built the case had an interested funder and said so on the page. The 2002 carotenoid measurements came out of Unilever Research; the six-week diet study was supported by Unilever's American research arm and declares it. That does not make the numbers wrong — the Malaysian trial carries no such tie and found the same direction — but it does explain why this particular corner of skin science is unusually well populated.

And the shortcut has been tested, at a scale nothing else here approaches, with a result nobody wanted. The Beta-Carotene and Retinol Efficacy Trial gave 18,314 smokers, former smokers and asbestos-exposed workers thirty milligrams of beta carotene and 25,000 international units of retinol a day. The relative risk of lung cancer in the treated group came out at 1.28, and the trial was stopped twenty-one months early. Nobody in the skin-colour literature is recommending a capsule. The difference between the smoothie and the capsule is not a detail.

What survives is small and real. There is a third pigment in skin, it is dietary, it is a yellowness, it moves within four to six weeks and it fades back, and it has been shown to move in a South-East Asian sample and not only a European one. Whether that yellowness reads as health, as beauty, or as neither depends on the face it sits on and the eyes in front of it — and the one study that put Chinese and European observers side by side got two different answers. A colour reading is about which colours go next to a face. This is one of the very few things on the list that changes the face.