Members' Night Fingernail Analysis

Academy CSI: Using fingernails to reconstruct your diet

Several hundred guests submitted fingernail samples at the stable isotope laboratory in the Academy's Patrick Center for Environmental Research during Members' Night 2008, 2009, and 2010. 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 to study food webs in the environment.

Results

• 2010
• 2009
• 2008
• 2008–2010
• Explanation

Results for 2010 Members' Night

This graph shows the results from the 129 members who submitted fingernail samples during Members' Night 2010. As you can see, not all members are exactly alike. The data is scattered across the graph, which is why scientists call these “scatter diagrams.”

Although the data are scattered, there is a pattern. Most of the data for Nitrogen Isotope Values (top to bottom) are between 9 and 10. Carbon Isotope Values are scattered from left to right, but they are more concentrated on the right. (The units, δ¹⁵N‰ and δ¹³C‰, will be explained below in The Science Behind the Results.)

Results for 2009 Members' Night

A total of 115 members submitted fingernail samples during Members' Night 2009. Again the data are scattered, but the pattern is similar. Check out the combined results below to see how the two years compare.

Results for 2008 Members' Night

During our first Members' Night (in 2008), 91 members submitted fingernail samples. As with 2010 and 2009, the data are scattered, but the pattern is similar. Check out the combined results (the 2008–2010 tab) to see how the three years compare.

Combined Results for Members' Night 2008, 2009, and 2010

Data from the three years do differ, but most Nitrogen Isotope Values during each year tend to be between 9 and 10, while most of the Carbon Isotope Values are on the right.

Explanation of the Results

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. (Use the gray text and arrows as guides.)

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.

But what about the three dots in the purple boxes? It turns out that that fingernail sample came from our youngest guests during Members' Night. Find out more by reading "Mothers and Babies" below.

Mothers and Babies

Members' Night 2009 and Members' Night 2010 provided special opportunities for seeing how nitrogen and carbon isotope ratios of infants can change over time. It turns out that these depend on whether the baby is nursing from the mother or eating other food.

• Nursing
• Weaned
• Combined

Nursing Mothers and Babies

There was one mother and infant pair from Member's Night 2009 (blue dots): Mother 1 (2009) and Baby 1 (2009), while there were two pairs from Member's Night 2010 (green dots): Mother and Baby 2; and Mother and Baby 3. As you can see, all three nursing babies are at the top of the graph. They have the three highest Nitrogen Isotope Values of all the members that were tested (going from bottom to top). While the values for the nursing babies are high, those for their mothers are more like rest of the members.

Unlike with nitrogen, the Carbon Isotope Values don't differ as much between the mothers and their babies. They're almost identical between Mother 1 and Baby 1. They are somwhat lower (more to the right) for Baby 2 than for Mother 2, and somewhat higher (to the left) for Baby 3 than for Mother 3.

Weaned Mothers and Babies

In addition to results from nursing pairs of mothers and babies, we have data from two pairs with weaned infants. One pair, Mother and Baby 1, was tested when they were still nursing in 2009 and after the baby had weaned in 2010 (green dots). Baby 4 was already weaned when she was tested in 2009 (blue dots).

Things change once the baby starts eating solid foods. The Nitrogen Isotope Values for the babies are still higher than those for their mothers, but they are not nearly as high as they are for nursing babies. Click on the "Combined" tab above to find out what's going on.

All Mothers and Babies

It turns out that these values agree with those found for mother and baby pairs in scientific studies. Young, nursing babies had much higher Nitrogen Isotope Values than their mothers (Pair 1 in 2009, Pair 3 and Pair 3 in 2010). But as they switched to solid food, they became more simila (Pair 1 in 2010, Pair 4 in 2009)r. Scientists have found that the difference in Nitrogen Isotope Values between nursing babies and their mothers is typically 3 δ¹⁵N. That's similar to the difference we found between the three nursing mothers and their babies during Members' Night (Pair 1 in 2009, Pair 3 and Pair 3 in 2010).

This is also about the same difference scientists find between different trophic levels in the wild (such as the difference between a clam and the fish that eats it). One way to look at this is to think of the baby as feeding off the mother.

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, so they don't take up as much, proportionally, of the light carbon.

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 for 2008 and blue for 2009) compared to the Nitrogen and Carbon Isotope Values for different food sources. (The food source values adapted from the USGS website.)

Not surprisingly, it shows that most members eat a mixture of foods and 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

1. About 1 milligram of your fingernail is placed into a tiny tin foil capsule that is pinched closed;
2. 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.)
3. 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;
4. 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;
5. 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.)

Isotopes and 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 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

Fogel, M.L., N. Tuross, B. J. Johnson and G. H. Miller. 1997. "Biogeochemical record of ancient humans." Organic Geochemistry 5-6: 275-287.

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.

Footnote

You may have heard of ¹⁴C. It's not stable and it decays over time. Scientists have taken advantages of this property to do radiocarbon dating. (go back)