When you cut your nails you do not imagine that you are throwing away a registry of what you have eaten over the previous six months. And if you have long hair… each hair tells the story of the last few years of your life. This story can be unveiled with the help of stable isotopes. This is what researchers such as Gabriela Bielefeld Nardoto and Luiz Antonio Martinelli, from the Agriculture Nuclear Energy Center (Cena) of the University of São Paulo Paulo (USP) in Piracicaba do, when they use this technique to describe various aspects of the lives of people and other living beings.
The researchers were curious to investigate how the “supermarket culture” changed the eating habits of urban populations. To this end they collected pieces of finger nails from the outside world: the United States, Europe, the Amazon and the Southeast Region of Brazil. The results are in an article that will be published in the American Journal of Physical Anthropology in September, but is already available on the magazine’s electronic edition.
Its expectation was to discover a homogenous diet between distant regions, as a result of globalized nutrition. But this was not what came out. The analyses carried out by the group from Piracicaba showed that from, the information contained in the nail fragments, it is possible to distinguish what the person had been eating and where: in the Southeast of Brazil, in small Amazon communities, in the United States or in Europe. As well as this, Gabriela surprised herself on finding marked differences within the Amazonian region: “The population of Santarém has a diet completely different in relation to the region.” The data shows that, frequently, the foods sold in Santarém are exclusively produced in the Southeast.
The secret contained in the nails lies in stable isotopes, chemical atoms with the same number of protons, but with a different quantity of neutrons. This gives the situation that the same element – such as oxygen, hydrogen, carbon or nitrogen – can be lighter or heavier, according to the number of neutrons in its atoms. Stable isotopes, contrary to radioactive isotopes, maintain the same proportional make up over time. For nitrogen, for example, the most common isotope is 14N, which it read as “nitrogen-14.” But in nature there also exists its heavier form, 15N.
“Differences in the isotopic level of carbon and nitrogen present in people’s nails who live in different regions persist in spite of the supermarket culture” explains Gabriela. The researcher adds that a large part of the difference in isotopes of nitrogen observed between the more and lesser developed parts of America is due to the use of fertilizers, which is six times higher in the United States in relation to Brazil.
“You are what you eat, but three parts per thousand” is the slogan of the specialists in isotopic ecology. Or that is to say, if a herbivore animal has a proportion of 7‱ (parts per thousand) of 15N in relation to 14N, its predator will have 10‱. The proportions between isotopes more or less common, for various chemical elements, form the “isotopic signature” of an individual at a given moment.
“Nitrogen varies according to the trophic level and the use of fertilizers; carbon reflects the type of plant consumed, C3 or C4” the researcher explained. C4 plants are of the gramineous family, such as maize and sugarcane; the others are called C3, according to the type of photosynthesis that they carry out. The population in the southeast of Brazil have more 13C (carbon-13) in their nails due to a greater consumption of C4 plants. In the opinion of the researcher, this result reflects the feeding of cattle, which in Brazil have access to pasture. Cattle in confinement, spread throughout the United States, produce meat with a smaller proportion of 13C. In both countries, vegetarians show lower values for the two elements, in relation to omnivores of the same region. As well as this, another surprise was to verify that Brazilians do not eat more meat that North Americans.
The differences in relation to the European diet are also marked. “They present a carbon-13 signature even lower than that of the North Americans” says Gabriela. According to her, this occurs because the direct consumption of maize does not make up part of the European culture, and their sugar is made from beets instead of sugarcane.
As part of another research project, Gabriela carried out field work in the region of the city of Santarém, in the state of Pará, where she had the chance of collecting nail samples from the city’s inhabitants and of the small communities about 80 kilometers from the city. In order to manage to get the population to donate samples, the researcher had to conquer their confidence beforehand. In other Amazon regions she had not had the same reception. “People thought I was about witchcraft, or that I was going to extract their DNA and they would find their children spread throughout the world” she says.
The change in the diet of the Santarem citizens is marked. In spite of having lived close to the confluence of the Amazon and Tapajos rivers, they consume little fish. Their protein has its origin, above all, in chicken, cheaper than beef. The proportions of the isotopes in their nails are the same as the inhabitants of the southeast of Brazil, which shows that the supermarket culture has had an important impact on standardizing their diet with other urban areas of the country.
But outside of the city the situation is different. Gabriela collected samples from three communities: São Jorge, in the rainforest; Jamaraquá, close to the Tapajós river; and Socorro, on the margins of the Lago Grande lake. In these small towns, the inhabitants’ food depends upon their crops – maize, manioc and rice – and of hunting or fishing. Periodically a representative comes to the city in search of basic necessities such as beans and sugar. The isotopes indicate that only the riverside population has the habit of eating fish. The inhabitants of Socorro also fish as well as hunt. However, the São Jorge community, located some kilometers from river water, do not include fish in their diet.
Carbon and nitrogen isotopes are used to infer ancient diets, both of animals and humans. For example, it is possible to check the isotopic content of bones of mummies or fossils, and from there to have an idea of what they ate. Gabriela tells that there have only been a few studies done on contemporary human beings. Nevertheless, they are essential as a reference for interpreting historical data. The researchers from Piracicaba use questionnaires to evaluate the diet of people who give up their finger nail samples. By a comparison of the data, they can tell that the analysis of the stable isotopes is trustworthy. Mummies do not respond to questionnaires, but research Gabriela’s work is showing that it is possible to infer its diet starting from the analysis of the stable isotopes.
The most disseminated application of stable isotope analysis occurs in various branches of ecology. Their proportions in various tissues – feathers, blood, muscle – of migratory birds allows for the inference of the route covered by these animals and the types of food consumed at each location. The isotopes integrated into feathers have to do with the nutrients available when they were produced. The blood brings immediate information. Thus researchers have access to the histories of the species that do not have another way of relating them.
Only slightly eloquent species are also the plants. Rafael Oliveira, another member of Cena, wants to know how the Atlantic Rainforest plants drink water – through their roots or through their leaves. In regions of altitude there is lots of water available in the form of mist, which in according to recent research can be absorbed by the leaves. It is possible to distinguish this water from that of the soil, since the globules that form the mist have a greater proportion of the light oxygen isotopes. In this case isotopic proportions can help to reveal a little known manner of water absorption by plants.
In the manner in which technology is advancing, more and more information is being extracted from the stable isotopes present in the most diverse corners of nature. Now you know that the next time you cut your finger nails you will be throwing away part of the registration of your history.Republish