It looks like there's a really simple way for doctors to diagnose whether young Caucasian men are getting enough vegetables: it's detectable in the colour of their skin.
What causes it is a class of organic pigments called carotenoids found in a variety of fruit and vegetables.
You've probably already seen the power of carotenoids in the colour of other animals that eat them - mostly birds. Goldfinches and their yellow colouring, cardinals and their red, and flamingoes and their pink are all the result of carotenoids.
So it is that carotenoids can also affect the pigment of human skin. Excessive consumption of carotene can turn the outer layer of the skin orange, resulting in a jaundice-like, or orange-tinged appearance.
This problem is known as carotenoderma, and you'd have to eat a lot of high-carotenoid foods over an extended period of time. According to a 2003 literature review, it takes 4-7 weeks for visible changes to appear.
But no matter the amount of vegetables you're eating, there's a detectable change in skin pigmentation, not necessarily with the naked eye but non-invasive spectroscopy - and this could be useful to health practitioners.
The study focused on young Caucasian men, because the pigmentation changes would be more easily detected in lighter skin, and because young men in Australia eat the least fruit and vegetables.
The researchers examined the skin colour of 30 Caucasian men aged between 18 and 30, living in Perth on Australia's west coast.
"The aim of our study was to determine whether there was a connection between fruit and vegetable consumption, carotenoid intake and yellow skin colour in young Caucasian men, as Australian men are typically known to consume less fruit and vegetables than women," said dietician Georgia Bixley from Curtin University in Australia.
"We were able to find this connection through a process called reflectance spectroscopy (RS), an emerging technique which measures the colour and intensity of reflected light on the skin pigments."
An earlier Australian study had already been conducted on a group of over 100 Caucasian women, published in 2015. It found that yellowness in the skin was associated with a higher fruit and vegetable intake.
Just as the researchers expected - higher yellowness in the skin correlated with a diet higher in fruits and vegetables. They found that the forehead, biceps, palms of the hands, and soles of the feet were the best predictors.
Sun exposure can interfere with this measurement system, as UV light breaks down carotenoids in the skin as well as increasing the effects of melanin exposure, which effects colour. But of the ideal sites, the forehead's thin outer skin layer means it retains a greater amount of carotenoid.
While we do know that darker skin colours can be affected by carotenoderma, further research across other populations and ethnicities could help to extend our understanding of the relationship between carotenoids and colouration effects on human skin.
But in a very practical sense, the study could help to diagnose dietary issues, and promote healthy eating, the researchers said.
"Our research could play a key role in identifying people who have a low consumption of fruit and vegetables, and better understand their higher risk of chronic diseases," said nutrition scientist Karin Clark, also of Curtin University.
"By conducting further research into this connection, it could open up the possibility of being able to predict someone's fruit and vegetable intake from their skin colour, rather than relying on them to remember every meal they eat."
The research has been published in the Journal of Nutrition & Intermediary Metabolism .