Scientists put fruit flies on meth, and observed the chemical results. What they found was a link between meth, sugar, and cancer cells.
There are a surprising number of tests in which scientists expose bugs to drugs. Many of those test are, I suspect, the scientists just being jerks - such as when they exposed a bee colony to cocaine to see if that messed things up (it did) or when they exposed spiders to marijuana to see if that messed up their webs (it did). Sometimes, though, scientists actually find out something useful when they drug insects. A recent study of fruit flies on meth showed important changes that meth makes in a organism's cells.
For one thing, meth alters the process that cells use to fuel themselves - changing until it resembles the fueling process of cancer cells. Healthy cells use oxygen to break down stored energy. The process is slow, but efficient. Cancer cells, however, use glycolysis. This is a process that splits glucose fast, and splits it independent of the presence of oxygen. Glycolysis is quick, but wasteful, and so requires a great deal of ready glucose to keep the cell going.
Researchers found that fruit flies which consumed trehalose, a sugar that insects metabolize, lived longer than fruit flies that did not. People on meth crave sugary drinks and snacks. It could still be a psychological reaction to the drug, but this research indicates that the need for sugar is a biological response to the meth, and the body's way of keeping itself alive when its healthy biological processes are altered.