Thursday March 8 2012
Fruit and veg: the research linking it to good skin was limited
Fruit and vegetables are not only nutritious but could also make you “cuter”, according to the Daily Mail. Apparently, eating more foods such as carrots, broccoli, squash and spinach enhances attractiveness and gives the skin a healthy glow within six weeks.
These rather fruity claims are based on a small experimental study that investigated whether people could improve their complexion by eating the yellow-red carotenoid pigments found in many fruits and vegetables. In the first phase of the study, 35 people completed dietary questionnaires and had their skin colour recorded over a six-week period. The researchers found that a modest increase in self-reported fruit and vegetable consumption over the period was related to an increase in skin colouration (yellowness and redness). In the second part of the study, the researchers enrolled 24 young students and asked for their subjective opinions of the attractiveness of computer-created, colour-manipulated images, which the researchers said reflected different degrees of fruit and vegetable consumption.
No conclusions can be drawn from this small, short study, which has numerous limitations. To more reliably assess whether fruit and vegetable consumption caused skin colour change, the researchers could have performed a trial asking people to eat different diets and looked at the results it had. Also, the researchers did not account for other factors that might affect complexion, such as exposure to daylight and exercise, and can provide no evidence that the diet caused the observed skin colour over the short six-week period. In the second part of the study, the subjective opinions of 24 people cannot be interpreted as a universal measure of attractiveness.
Regardless of whether or not fruit and vegetables have a beneficial effect on the complexion, a healthy, balanced diet has many other known health benefits.
Where did the story come from?
The study was carried out by researchers from the University of St Andrews and was funded by the Economic and Social Research Council and by Unilever Research and Development USA. Unilever is a large food manufacturer. The research was published in the peer-reviewed scientific journal PLoS One.
This apparent “good news” study was widely reported in the papers, for the most part uncritically. The BBC included comments from independent experts, one of whom pointed out that the study did not take account of food preparation technique and another who said the effects of daylight could not be ruled out. The Mail’s report, which said that people who increased their fruit and vegetable intake became more attractive, and even “cuter” in an online version of the story, was misleading because it attempted to combine the results of two separate experiments.
What kind of research was this?
This experimental study set out to examine the effects of fruit and vegetable intake on skin colour, and specifically to find out how much dietary intake needs to change and for how long to have a perceptible change on skin colour. A second part of the study looked at the minimum colour change required to make skin look healthier and more attractive.
The authors say that a recent cross-sectional study has associated higher fruit and vegetable consumption with human skin colour (yellowness), mainly because of the presence of carotenoids, the yellow-red organic pigments abundant in many fruit and vegetables. Carotenoids are described as high in antioxidants, which the authors say may be beneficial for skin health. They say that the accumulation of carotenoids imparts colour to the skin, but that it is not known how much is needed to give the skin a healthy colour. In evolutionary terms, a healthy skin colour, they argue, indicates suitability as a mate and is therefore of benefit in sexual selection.
What did the research involve?
In the first experiment, the researchers monitored the fruit and vegetable intake of 35 individuals (21 women and 14 men) over a period of six weeks. Participants were undergraduate students, mostly of Caucasian origin. None of them wore facial makeup or reported recent sunbathing or use of self-tanning products. Researchers recorded their diet and skin colour at an initial session and in two follow-up sessions at three and six weeks, between March and June 2010
Students completed a validated food-frequency questionnaire to establish their daily fruit and vegetable intake, from which researchers worked out a daily average. The participants reported consuming an average of 3.41 fruit and veg portions daily over the three sessions. Skin colour and “reflectance” (the amount of light reflected off the skin) were recorded using a specialist device called a spectrophotometer. Measurement of skin colour included three separate components: skin lightness and degrees of yellowness and redness. Skin colour on seven body locations was recorded: the left cheek, right cheek, forehead, part of forearm, outer bicep, shoulder and palm.
The researchers analysed whether there were any associations between changes in diet and changes in skin colour over this period. They also conducted a further analysis to investigate whether changes in skin colour associated with dietary changes were caused by the absorption of carotenoids or melanin, a skin pigment that gives the skin its colour and which also protects against UV rays.
In a second experiment involving 24 students (19 women and 5 men), the researchers investigated the effects of skin colour changes on perception of health and attractiveness, using what they call “psychophysics”. For this, they took close-up photos of two white women and two men, taking various precautions to reduce the possibility of reflection from external light and ensuring each image was colour calibrated.
They then digitally created two on-screen face-shaped colour masks, which they say represented the average skin colour of 15 high and 15 low fruit and vegetable consumers, as derived from a previous study. The skin areas of the photos were manipulated to create a row of 22 images for each face, with the middle one showing the original face and those either side varying in their colour tone. The full set of 22 images represented a total range of colour equivalent to a change of plus or minus 5.55 fruit and vegetable portions a day.
The 24 students were asked to view the images and, in three separate tasks, to choose the face that appeared more yellow, healthier or more attractive.
What were the basic results?
In the first experiment, the researchers found that changes in fruit and vegetable consumption over a period of six weeks were significantly correlated with changes in skin “redness and yellowness” over the same period, across all seven measured regions of the body.
However, when they considered only the three facial areas (left cheek, right cheek, forehead), researchers found no significant association between changes in fruit and vegetable intake and changes of redness or lightness in complexion. There was only a marginal association between dietary changes and an increase in facial yellowness. They also found that the changes in skin “reflectance” were significantly associated with the absorption of carotenoids and not melanin (which means they could be attributed to compounds found in fruit and vegetables rather than to the skin’s natural pigment).
In the second experiment, they found that modest dietary changes are required to enhance apparent health (2.91 more portions of fruit and veg a day) and attractiveness (3.30 more portions a day).
How did the researchers interpret the results?
The researchers concluded that increased fruit and vegetable consumption confers “measurable and perceptibly beneficial effects” on Caucasian skin appearance within six weeks. This effect, they say, could potentially be used as a “motivational tool” in dietary intervention.
This small study attempted to look at how fruit and veg intake affected attractiveness and skin tone in two experiments. The first found that self-reported increases in fruit and vegetables over a period of six weeks were associated with changes in complexion. The second asked people to rate the attractiveness of digital facial images manipulated to reflect different levels of fruit and vegetable consumption.
Despite all the prominent, positive news coverage this research received, no conclusions should be drawn from this experimental study, which has numerous limitations. There is no reason why the first part of this study could not have been conducted using a randomised controlled design that assigned people different diets and then followed them over time to assess skin colour change. This would have been relatively simple and would probably have given much more reliable results. Instead, the study asked just 35 students to report their dietary intake over the course of the six weeks while having their skin colour assessed. The study had a small number of participants, and the results cannot prove that the diet caused the change over this short period. For example, changes in complexion could be related to other factors including exercise, exposure to daylight and even sleep.
In the second experiment, researchers tried to link this association between complexion and diet by asking 24 people to give their subjective perceptions of the health and attractiveness of computerised facial images that had been manipulated to show varying complexions, apparently related to fruit and vegetable intake. Again, no significance should be attached to these results. Judgements of attractiveness are related to many factors.
Overall, this widely reported research cannot tell us very much about whether fruit and vegetables have a beneficial effect on complexion, although there are plenty of other good reasons to eat fruit and vegetables.