You absolutely can, but having a needle in you 24/7 isn't the most convenient thing, not to mention the cost when it's not covered by your health plan.

There's one theory which is that the most effective form of weight loss might be simply to always keep blood sugar below a certain level. Still just a theory, though.

It's possible that sometime in the next decade the Apple Watch might be able to monitor glucose levels non-invasively. However, it's extremely difficult technologically, so it's not clear Apple will succeed. (They seem to be working on it though.)