As I remember, Watson's work had more to do with 'oxidative types' and how people in each type group react to macronutrients.

There's a (scanned) copy of his "Nutrition and Your Mind" available for check-out on the Internet Archive.

I'll confess to never getting past his 'oxidative types'.

That said, it seems that there is individual variation in the ability to metabolize simple carbs — and high blood sugar glucose can raise cortisol, so there’s a potential mechanism.