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January 4, 2011

Diet DNA

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When starchy staples like potatoes, pasta and pizza make your mouth water, consider this: a gene that lets our saliva digest those complex carbs may have helped set us apart from the apes. This ScienCentral News video explains the finding has implications for human health.

Past the Pasta

We may not think of pasta as brain food, but our ability to digest it may have helped make us smart, by literally fueling the evolution of our big brains. The evidence is from a collection of spit and DNA samples from around the world.

Anthropologist Nate Dominy, of the University of California, Santa Cruz and his colleagues studied a gene for an enzyme in saliva that digests starchy foods. They started by sampling graduate students and found that those with more of the enzyme, called amylase, have more copies of the gene in their DNA.

Counting genes. This individual has a total of 14 copies of the amylase gene.
They used a technique called Fiber FISH to visualize the gene copies by attaching differently colored fluorescent molecules to the beginning and end of the gene. "We used a green probe for the beginning and a red probe for the end… we can actually photograph the genes themselves on the strand of DNA, and what's beautiful is it looks like a string of pearls," says Dominy. "And we can then count the number of red and green probes because that's the start and the finish of the gene… and then you can very easily just count them, one, two, three, four and so on."

"It was exciting that we found this very nice positive correlation," Dominy says. But he adds, "students of European descent are not representative of the globe."

So they traveled around the globe collecting saliva and DNA from populations with different diets.

"Spit is one of those things that people are happy to volunteer and we never had any problems," Dominy says. "We may get a few quizzical looks, but otherwise, people were happy to participate."

As they wrote in the journal Nature Genetics, they found more copies of the gene on average in populations with high-starch diets, and fewer copies in groups with low-starch diets.

Dominy with saliva sample for genetic test.
"We can actually show that for a gene -- and we know its function, we know exactly what it does-- we can show thatevolution has favored more and more copies of this gene for the people that need it because they consume so much starch," says Dominy.

That's surprising because it shows evolution doesn't always change our bodies' recipes -- it can just tweak the amounts of some ingredients. It's also important because there are other human genes that occur in varying numbers of copies.

"We're finding that genes that have more and less copies are having a profound effect on the way the body works," he says.

He thinks that in the case of this gene, the ability to get extra calories from starches may have helped humans grow our big brains (and here's another theory). While our closest cousins, chimpanzees, only have two copies of the amylase gene, humans can have as many as 15 copies.

"Meat is relatively rare. It's hard to come by as a food resource, so maybe our human ancestors started digging up these roots and tubers and such, and these are starchy food items which would provide a lot of sugar for our big brains," Dominy explains. "Unfortunately, there's [been] no evidence for it because these plant tissues are perishable-- they don’t preserve in the archaeological record-- and the tools you would use to dig them up, they're made of wood and they themselves would not preserve in the archaeological record."

"So, for us, this particular gene was sort of like the smoking gun. We could identify if natural selection has operated on this gene because it helps you digest starch. And this would imply, then, that here has been positive selection for eating starch in human evolution."

Starchy foods like bread are part of our staple diet today.
But in today's high-carb world, is that still an advantage?

"It's possible that those individuals with more copies, they may have had advantages 50-thousand years ago when times were tough and they had to resort to eating starchy foods," he says. "But in today's diet where we're eating a lot of refined sugars, it may make them susceptible to things like Type 2 diabetes."

That's a question for biomedical researchers, not genetic anthropologists, to noodle.

In fact, Dominy says, copy-number variability is now becoming a hot topic for health researchers. "Now the exciting thing is to look at copies and at people with particular medical conditions to see if there's a relationship."

This research was published in Nature Genetics, September 9, 2007 and funded by the L.S.B. Leakey Foundation, Wenner-Gren Foundation, Department of Pathology, Brigham & Women's Hospital, the National Institutes of Health and the Wellcome Trust.

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