You are what you drink: local water leaves an isotopic mark in your hair.Digital VisionThe tap water that you drink leaves a
‘signature’ in your hair that can provide a history of where you’ve
lived, according to researchers. Using these imprints to trace people’s
past movements may eventually become a common tool for anthropologists
and law-enforcement officials.
Tell-tale isotope signatures of what we eat and
drink are found throughout our bodies in bones, fingernails and hair.
Tests of bones in archaeological sites can show whether a person lived
and ate locally for years or migrated from elsewhere1,
and have been used in forensics to help determine where an unknown
victim came from. But hair has been of particular interest because it
grows steadily: depending on its length, a strand can yield several
years of chronicled information about a person’s whereabouts.
“Bone represents a long-term average and it’s
constantly being remoulded,” says study author James Ehleringer, a
biologist at the University of Utah in Salt Lake City. “Hair is
produced sequentially and once it’s produced it’s never modified.”
That feature, says Ehleringer, allows
researchers to trace a person’s geographic history — to an extent.
While the technique cannot pinpoint specific locations in which a
person has lived, it can narrow the possibilities. The results will be
published this week in Proceedings of the National Academy of Sciences 2.
Elementary analysis
Isotope ratios of rainwater vary around the United States with distance from shore and temperature.courtesy of PNAS/National Academy of SciencesMost tap water comes from local precipitation,
and the ‘isotopic signature’ — the ratio of heavy to light elements —
of local rainwater is heavily influenced by geography. In general,
heavier isotopes like deuterium (hydrogen with one extra neutron) and
oxygen-18 (oxygen with two extra neutrons) tend to be ‘rained out’ of a
cloud first, as the heavier isotopes condense more easily. So, as rain
gets carried inland from where it is created over the oceans, the heavy
isotopes tend to rain down first on the coastal areas. Inland areas
tend to have lower isotope ratios, as do colder climes.
Local water is the primary ingredient in much of
the liquid we drink, including soft drinks, which tend to be bottled
regionally in the United States. Of course people also consume a lot of
non-local food and drink, but Ehleringer and his colleagues started
with the assumption that most people have the same ‘global diet’, and
looked to see if they could detect the remaining signal from local
water in the hair.
They collected hair clippings from 65
barber-shops across the United States. They compared the hydrogen and
oxygen isotopic ratios in the hair with tap water from each location,
and found that they matched.
The team also collected tap water samples from
431 other locations around the United States to develop a map of
deuterium and oxygen-18 levels. Texas and Florida, being warm and close
to the coasts, have the highest relative levels of both isotopes.
Crossing borders
A centimetre length of hair corresponds to
roughly a month of growth, the team reports. To test the ability to
produce a history of travel, they chopped up lengths of a single strand
of hair and found isotope ratios that were consistent with a recent
move from Beijing, China to Salt Lake City. For hair to register a new
location, a person must live in a new place for three or more months,
they say.
The technique has already been used to assist
law enforcement, says Ehleringer. In once case, the researchers were
able to help confirm that a murder victim had travelled around western
states including Utah, Montana and Wyoming over two years.
The technique "will never be able to give you a
precise location", says Ehleringer, although it can help exclude
potential areas of residence, or help confirm other evidence of a
person’s whereabouts.
On a diet
More work may be needed to weed out
complications in the analysis before it becomes an established tool.
Individual metabolism and diet may influence isotope ratios in hair,
for example. “There is quite a lot of biological noise from one
individual to another and that has to be taken into account,” says
Robert Hedges of the University of Oxford, UK. "I suspect that they
might get two different lines if they looked at American meat-eaters
and compared them with American vegetarians."
Ehleringer admits that their assumption that
most people share a ‘global supermarket’ diet might not withstand
additional testing.
Co-author Thure Cerling, also based at the
University of Utah, sees additional potential in using the technique to
study animal migration. The long hairs on the tails of African
elephants, Cerling says, may contain up to two years of history.
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References
- Price, T. D. , Knipper, C. , Grupe, G. & Smrcka, V. Eur. J. Arch. 7, 9–40 (2004).
- Ehleringer, J. R. et al. Proc. Natl Acad. Sci. USA advanced online publication, doi: 10.1073/pnas.0712228105 (2008).
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