You can tell a person has an unhealthy diet if they look overweight, but there are other subtler giveaways. The food we eat is broken down by the body for energy, and traces of it can build up to form a long-standing record of our diet. For instance, to grow hair, the body uses amino acids from the protein we consume, preserving chemical traces of the food. Consequently, the things you can learn about someone’s life just by studying a single strand of hair might surprise you.

A new study led by researchers at the University of Utah shows that hair can be used to tell if a person is vegetarian or prefers hamburgers. What’s more, a person’s diet is strongly correlated with their socioeconomic status. So much so that researchers were able to accurately predict how much a person paid for a haircut based on the chemical analysis of their hair.

You are what you eat

For decades, Jim Ehleringer and Thure Cerling have been refining methods that assess mammal diets from their hair. Their work is based on dietary research that involves the isotopic composition of human bone, which first started in the 1970s.

Archaeologists who were keen on reconstructing ancient diets began to notice that radiocarbon dates on the remains of certain plants such as maize were offset from dates for other plant remains, although they belonged to the same site. They eventually came to the conclusion that this discrepancy is due to maize having a different photosynthetic pathway than most other plants. Consequently, maize has a different relative quantity of the isotopes  carbon-14 and carbon-13 in its tissues. Isotopes are atoms with the same number of protons but different numbers of neutrons.

Later, observations revealed that nitrogen isotope ratios can also vary between different food sources, particularly marine versus terrestrial, and stable isotope analysis for human bone quickly became a widely embraced technique among scientists.

These different isotopes present in our food not only build up in the bone but also in the hair.

In a 2008 study, Ehleringer and Cerling showed that the isotopic composition of a person’s hair could trace their travels, by virtue of the fact that water varies in oxygen and isotope ratios according to geography. Naturally, they began to wonder what they could learn from carbon and nitrogen isotopes found in hair.

“Thure Cerling and I have had a long-term interest in exploring how stable isotopes in animal tissues (hair, teeth) and foods record diets. The extension in our research to humans over the last several decades is a natural progression from our ecological studies,” Ehleringer told ZME Science.

A person who eats meat will also ingest the carbon and nitrogen isotopes of the animal feed of the livestock. Corn belongs to a group of plants with a distinct photosynthetic pathway known as C₄. Meanwhile, legumes and vegetables belong to C₃ plants. Consequently, it is possible to distinguish between meat-eaters and vegetarians, as well as the quality of food, from their hair alone.

The researchers collected hair samples from barbershops and hair salons in 65 cities across the United States. The sampling also included 29 ZIP codes in the Salt Lake Valley in order to zoom in on a particular region. All in all, samples from nearly 700 people were collected.

“One of the rewarding aspects was seeing that barbers gladly allowed us to get hair samples from trash bins, once they learned more about how hair isotopes might relate to the health of their patrons,” Ehleringer said.

The results not only reflect a person’s diet and subsequent health, the researchers also found that the carbon isotope values correlated with the cost of living for the ZIP codes. Individuals who live in areas with a lower socioeconomic status had more corn-like isotope signatures in their hair. In fact, the carbon isotopes in the hair correlated strongly with the average cost of a haircut at the sampling location.

“The results suggest that there are large differences in the amount of protein derived from plant sources versus animal sources. And these differences are associated with socioeconomic status at both local and national scales. Given that the health community has published several large studies in the last few years (cited in the publication) showing that consumption of animal-derived protein is associated with greater health risks, our easy-to-use stable isotopes in hair approach provides a means for community-scale assessments that are free of the more typical survey-based approaches. The increased health risk is likely associated with the fats contained within animal-derived protein foods. Our hope is that the health community would consider this kind of assessment in their efforts to obtain large-scale patterns and an understanding of how these patterns change over time. The analysis cost is less than $10, making it affordable for health-related studies,” Ehleringer said.

The researchers went a step further and also correlated isotope ratios with obesity rates, drawing further connections between diet, socioeconomic status, and overall health.

These results show that hair can be a reliable and objective measure for assessing a community’s health that is not biased by self-reporting.

“One of the great problems in our society is nutrition.  Thure Cerling and I have been collaborating for many years and some of our early projects on animal and human hair showed that individual diet histories could be obtained using stable isotopes in hair samples.  In North America (but not Europe) meat has a very different carbon ratio than vegetables, and nitrogen isotopes also differ (in both North America and Europe).  So our motivation was to use the tools that we have to see if we can make a contribution to this societal problem. This non-invasive approach allows those interested in overall human health to quickly obtain community-level, regional, and temporal assessments,” Ehleringer concluded.

The findings appeared today in a study published in the Proceedings of the National Academy of Sciences.