lul if anything Ketoacidosis will fuck up your PH and screw up your buffers, but you should be fine as long as you're not diabetic/on a weird ketogenic diet (more fat than anything). Your protein shake will have carbs anyway so you're still getting glycolysis at least~
I don't know if tuna is enough protein... I'd check the value. A tub of whey protein costs me $15 and lasts me a few months, so it might actually be cheaper than tuna assuming tuna is like 0.80 a day and you eat one every other day.
2 years later edit: weird ketogenic diet = awesome!