There is no current non-invasive way to measure blood glucose in the medical industry - if there was, every diabetic would already be using it and we'd be using it in hospitals. We just aren't. Most of the devices floating about now are hit & miss, not very accurate and require calibrating with an actual blood sample.
I'm not saying it's impossible that Apple might come up with it, but it's pretty damn close to impossible. Plus, I doubt Apple is going to attempt to go down the road of making the Apple Watch a 'medical grade' device - slightly more hoops to jump over there (FDA in the US for a start)... for good reason. If it's not accurate, it will cost lives.
People who think blood glucose can be detected the same way the pulse is detected on the watch (using differential absorption of light), clearly have no understanding of physiology, physics or biochemistry... current monitors make a lot of assumptions.
And for non-diabetics, what the hell are you going to do with the data - the signal to noise ratio shrinks even further, yet we think we're all 'well informed'. The Dunning-Kruger effect on display.