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How come termites can eat wood, but people can?t?

Q. How can termites eat wood? Could people, theoretically, at least, eat wood? What is it about wood that makes it hard to digest? If you made really fine sawdust, could you eat that?

KW, Boston

A. To understand what termites do when they eat wood, and how this differs from what we do, one first has to know a little bit about sugars.

Sugars are compounds of one part each of carbon and oxygen and two parts hydrogen. There are lots of kinds, but the most biologically useful one is glucose, an excellent source of energy, which your body needs to live. Plants often store glucose in long chains of molecules attached one to the other, and this can happen in two ways: in one way you get starch, and in the other way, you get cellulose -- the main solid component of wood. In other words, there is only a very small chemical difference between starch and wood. (I'm simplifying just a little here as there are many sorts of starches, but this is basically correct.)

We humans are able to produce enzymes called amylases, which break down starch into glucose molecules that we can use for energy. We don't have the right enzymes to break down cellulose, however, so if we eat wood, however finely ground, it will just pass through us undigested. It might offer some benefit as ''roughage" or ''fiber" but would not provide any food value. Personally, I wouldn't recommend the experiment. Termites have bacteria and protozoa in their guts that make enzymes that break down wood, or cellulose, into glucose molecules, so these little guys can eat wood usefully. Some termites can even make the necessary enzymes themselves.

Dr. Knowledge answers your questions about science each week. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. Include your initials and hometown.

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