I forget what the term for this body type is, but basically; there is a type of person who's body turns food into calories extremely well. This is beneficial to the brain, as it is a huge consumer of calories.

The downside to this body type is that the brain is always very active; it always has lots of calories to draw from. Makes for great programmers and problem solvers, but also leads to poor attention span and insomnia.

So my theory is that marijuana is a dampener on the brain for these people. It doesn't improve insulin control, it's just that people who like to use marijuana tend to have fantastic insulin control. The logic being good insulin control = noisy brain = marijuana use.

I don't think you could prove this, you'd need some way to quantify noise in the brain.