We depend on saliva to give us a sense of sweetness

Saliva helps us swallow food, protects our teeth and even gives us a sense of sweetness as it converts starch to glucose. Anthropologist and evolutionary biologist Nathaniel Dominy, associate professor in the Department of Anthropology at Dartmouth College, has studied saliva from around the globe. He researches how humans acquire, process and assimilate food -- and how those behaviors affect the evolution of anatomical form and function.

Lynne Rossetto Kasper: Why do our mouths water?

Nathaniel Dominy: We produce saliva -- we produce quite a lot of it for four major reasons.

The first reason is that saliva is sticky, and this helps us eat. When we're chewing food, we're trying to reduce the food to a minimum threshold of particle size. We're trying to get the food particles down to a particular size in preparation for swallowing. What the saliva does is acts like glue -- it keeps everything combined together in what we call a bolus, a big blob if you will. That bolus is what your mouth is preparing for initially swallowing. If the bolus is too large or if it's too diffuse, then you can't swallow very efficiently. Primarily the saliva acts like a glue.

It's also protective because it neutralizes very acidic foods. Acid has the effect of etching away your teeth. Saliva reduces the acidity of very acidic things like fruit. For example, lemon has a pH of about 2, so it's very acidic.

Saliva also can act like an early alarm system. Your saliva is full of these proline-rich proteins, which appear to have no function other than to bind to tannins. Tannins give us that sense of astringency. If you've ever eaten an unripe banana for example, you feel that gritty sensation on your teeth, your teeth feel a little bit roughened. That's because all those tannins are binding to the proteins. You’re sacrificing all these proteins in your saliva to give you that roughened feeling in your mouth. This is thought to be an alarm system to let you know that what you’re eating isn't exactly the most nutritious thing because those tannins are something you generally want to avoid.

The fourth thing the saliva does is it actually allows you to digest foods initially in the mouth. You tend to think of your stomach and your small intestine as being the digestive powerhouses of the body. But actually you do a significant amount of digestion in the mouth before food ever reaches your stomach.

The major digestive enzyme in saliva is called amylase. Amylase is special because it hydrolyzes -- it dissolves starch grains in your foods. Most of the food that we eat is really starchy: potatoes, rice, bread, etc. Starch has become a central aspect of all modern human diets today, and we really depend on our saliva to help initiate digestion in the mouth and also give us a sense of sweetness. The reason why these things actually taste somewhat good is because in the mouth, the saliva is converting the starch grains directly into glucose, which is something that we perceive as relatively sweet.

LRK: The human brain needs a very large amount of glucose.

ND: Our brain is very costly to maintain, energetically. We spend about 20-25 percent of our energy feeding our brain. Our brain demands more energy than almost any other tissue in the body. We need sugary foods to feed the brain. But of course today we probably indulge too much.

LRK: Different peoples across the planet eat different diets. Does their saliva test differently?

ND: In fact, yes. We've done research looking at populations around the planet that have different traditional levels of starch intake. Americans and other North Americans tend to eat a lot of starch, so our diet is relatively starch-rich because of all the agricultural products that we eat. There are other populations around the planet, for example, certain Arctic populations that are living closer to the North Pole, who tend to have very little starch in their diet. There are hunter-gatherers who live in tropical Africa -- they tend to have very little starch in their diet as well.

What we've shown is that the number of genes that allow you to produce this amylase enzyme in the saliva, they tend to vary globally and they vary as a function of the amount of starch in the diet. For example, people in North America tend to have a lot of copies of this particular gene. They produce a lot more amylase in saliva than populations that don't really need copies because they're eating much less starch.