Hmm, I'm sure many of us have seen that documentary 'Supersize Me' where the guy eats nothing but McD's for a month. Same sort of deal, not enough nutrients because the basis of the diet has no variety. Beef, potatoes, and salt do not make a good balanced diet
and neither does flake food or any single food item, though the situation is greatly improved with the addition of other food items to vary the nutrients consumed. Selcon should not be relied on as a cure for these diseases, since the problem of a poor diet still remains.
HLLE and hole in the head are probably as reversible as metabolic bone disease in lizards and amphibians, but needs to be caught early before the effects become lethal. By the time you see physical signs, it may already be too late.