Blood sugar is very closely linked to self-control, including suppression of emotion. While this may appear to be a different thing, it isn't: when you include feedback loops and association spirals, a transient, weak emotional distraction can become deep and overwhelming if normal modes of suppression fail.

See here, here and here.

Anecdotally: I'm not diabetic that I know of, but my mood is highly dependent on how well and how recently I've eaten. I get very irritable and can break down into tears easily if I'm more than four hours past due.

Low moods and despair are both made of atoms. The quality of atomness doesn't vanish as bad feelings get worse. The quote is suggesting that keeping this fact in mind may be therapeutically valuable -- you're probably less likely to despair if you understand that your despair has a knowable, physical, potentially correctable basis.