Members' Night 2008 Fingernail Analysis
Academy CSI: Using fingernails to reconstruct your diet
More than 100 guests during Members' Night 2008 submitted fingernail samples at the stable isotope laboratory in the Academy's Patrick Center for Environmental Research. We collected these samples because the ratios of carbon and nitrogen isotopes in your fingernails reveal information about what you've eaten a few weeks to a month ago. Scientists in the Biogeochemistry Section of the Patrick Center also use isotope ratios study food webs in the environment.
Results
This graph shows the combined results of the fingernail analyses. As a general guide, dots closer to the top of the graph represent members who have diets richer in fish, while those in the middle eat more meat (such as beef or chicken) and those near the bottom eat little to no meat. Fingernail samples located on the right side of the graph are from members who eat—directly or indirectly—more corn or sugar than wheat or rice.
Now let's looks a seven individuals. The blue dots (1 & 2) represent two members who eat about the same percentage of wheat or rice as they do corn or sugar, but Member 1 has eaten more meat (or fish) than Member 2. Members 3 and 4 (orange dots) eat more meat or fish that most members, but Member 3 is much higher on the chart, which indicates a diet richer in fish.
The green dots (5, 6, & 7) represent members who eat about the same amount of meat but they differ in the importance of wheat or rice vs. corn or sugar. Corn syrup and sugar are major ingredients in many processed foods, while most of the beef we eat is corn-fed. Chances are, Member 7 eats a lot of processed foods and/or corn-fed beef.
The Science Behind the Results
All living things—including you—have two isotopes of nitrogen, 14N and 15N. They're chemically identical, but 15N has an extra neutron. This makes it heavier. Think of 14N as "light nitrogen" and 15N and "heavy nitrogen."
As it turns out, animals—including humans—that eat other animals accumulate more of the heavy nitrogen (15N) than do animals that only eat plants. This means that if you eat more fish or meat, you'll have accumulated more of the heavy nitrogen. If you eat less fish or meat, you'll accumulate less of it. Scientists can analyze your fingernails to test how much heavy nitrogen you have accumulated.
Comparing the different isotopes of carbon can also tell us something about your diet. But instead of how much fish or meat you eat, carbon isotopes tell us something about the types of plants you consume.
Carbon has two common isotopes in nature. The extra neutron in 13C makes it heavier than 12C. Again, think of 12C as "light carbon" and 13C as "heavy carbon." All green plants get their carbon from CO2 (carbon dioxide) through photosynthesis, but they take up more of the light carbon isotope in CO2 than they do of the heavy carbon. In addition, some plants (including wheat, rice, soybeans, and potatoes) use a type of photosynthesis (C3) that takes up even more of the light carbon than do plants, that use another type (C4) of photosynthesis. Corn and sugarcane use C4 photosynthesis.
Scientists use the ratio between the heavy and light isotopes (15N/14N and 13C/12C) known as their delta (δ) value. A higher δ value indicates there is more of the heavy isotope, while more negative δ value indicates there is less. These δ values are measured in parts per thousand (‰).
This new graph shows the same members' fingernail data (in red) compared to the Nitrogen and Carbon Isotope Values for different food sources. (Food source values adapted from the USGS website.) It shows that many members eat a mixture of corn-fed meat with lesser amounts of non-corn-fed meat. In general, if you eat more fruits, vegetables, and grains—and meat fed with these foods, your fingernail will plot farther to the left. In contrast, eating more corn, sugar, and corn-fed meat will result in fingernails that plot farther to the right. People, who eat more animal protein, particularly marine fish, will plot higher up on the graph, whereas those who eat less meat will plot lower.
How It's Done
- About 1 milligram of your fingernail is placed into a tiny tin foil capsule that is pinched closed;
- The capsule is loaded into the automated sampler located on top of our Elemental Analyzer. (The photo shows Academy scientist Paula Zelanko placing samples into this instrument.)
- The fingernail sample drops into a furnace within the Elemental Analyzer that's heated to 900° C. Here, the sample is instantaneously combusted into CO2, N2, and other (unwanted) gases;
- This gas mixture is passed through columns containing chemicals that remove the unwanted gases. The remaining CO2 and N2 is then separated in a gas chromatograph;
- The separated gases of CO2 and N2 are put into the mass spectrometer (shown in the bottom left corner of the photo) where the isotope ratios of N2 (15N/14N) and CO2 (13C/12C) are measured. A chromatograph is generated and stored on the computer. (One such chromatograph is shown below.)
Food Web Reconstruction
Food webs (or chains) tell us how ecosystems function and support themselves. Some food webs are simple (plant -> herbivore -> predator) and easy to reconstruct. At the Patrick Center, however, we study freshwater tidal marshes and other aquatic environments that are much more complex. They typically have many species that occupy the same trophic levels or feed on a variety of food types. This makes it more difficult to reconstruct their food webs.
One way to reconstruct a food web is to do gut analysis, where you inspect the gut contents of animals, like fish, within the food web. Gut analysis only shows you what that animal ate yesterday or today. It doesn't’t show you what it has eaten over the past weeks or months. Also, the animal has to be killed in order to look inside its gut.
Another way to reconstruct food webs is to use nitrogen and carbon stable isotopes. This technique tells us what the animal has eaten for the past few months, and does not require the animal to die in order to be analyzed.
For More Info
- Websites
- wwwrcamnl.wr.usgs.gov/isoig/projects/fingernails/
- ethomas.web.wesleyan.edu/ees123/carboniso.htm
- Scientific Publications
- Fraser, I., W. Meier-Augenstein and R. M. Kalin. 2006. "The role of stable isotopes in human identification: a longitudinal study into the variability of isotopic signals in human hair and nails". Rapid Communications of in Mass Spectrometry. 20: 1109-1116.
- Nardoto, G.B, S. Silva, C. Kendall, J.R. Ehleringer, L.A. Chesson, E.S.B. Ferraz, M.Z. Moreira, J. Ometto and L.A. Martinelli. 2006. "Geographical Patterns of Human Diet Derived from Stable-Isotope Analysis of Fingernails". American Journal of Physical Anthropology131:137–146.
- Michener, R.M. and K. Lajtha, eds. 2007. Stable Isotopes in Ecology and Environmental Science 2nd Edition. Blackwell Scientific: Oxford.