Four days into the typhoid, I got a sudden strong craving for sugar. I've never had a sweet tooth in adult life and after a bit of thought, I had one of those eureka moments: if my body needs something, it will try telling me.
The dehydration/heatstoke spiral strikes so quickly, you really don't want to let that get beyond the first prickly feeling at the back of your neck. The only sensible approach to this is prevention. And that means regular hydration and salt to replace the daily losses.
But much the same argument applies to all the main food groups and nutrients (especially vitamin C)
One of the best indicators is the first few seconds of my trumpet practise session. Not necessarily the sound that comes out, but more the overall feeling of comfort and well-being as I settle into the routine. If anything feels wrong, I'll run through a quick checklist of what I might have missed out of the preparation - bathroom stuff, tea, fruit breakfast etc etc. They're all part of the equation, and while you can miss out for a day or two on some of them, none of them can be neglected indefinitely.
And simply knowing that everything is in place diet wise helps greatly with that relaxed mindset. I just play better knowing the fridge is well stocked with home-made sauerkraut and pickled onions