
Clinical depression affects your brain in a way that makes them function abnormally. My earlier analogue to a brain tumor was not very much off. If some of my body parts is not working normally I would automatically consider it not normal, thus being an illness. How are brains different in this regard? If they're not functioning as they should, they are ill, it's not that difficult to understand.
Eating too much sugar affects brain in a way that healthy body can handle without hindering brain functions. Hyperactivity caused by sugar is not abnormal, and it is a self-induced situation where the brain have a lot of energi at their disposal but they start to get tired and need sleep, thus creating normally harmless situation called "sugar intoxication". The brain have the power, but they start to need rest, and especially children jsut don't understand always how tired they are, especially if the body is not tired. That can cause erratic behaviour, but that passes; itäs not an illness from same reasons being drunk is not an illness as such.
Diabetes is whole another case, in that the too high/low blood sugar can of course cause the brains to function abnormally or indeed to stop them altogether. But that's not the point here, is it? Your straw mans are getting more and more ridiculous. Try something better next time.
WTF, is this some kind of a chat room? seven new replies while I was writing this? Dear Spaghetti Monster give strength...

Anyway, you can also say that there are no bone fractures, there are just people who have too weak bones for their actions. People should be strongly enough built to tolerate being hit by cars. Also, there is no flu; jsut a bunch of people susceptible to a viral infection in respiratory organs.
Of course, that is the definition to flu.

Just like mental illnesses are defined to be severe enough disruptions from normal levels in cerebral activity. If you want to make new definitions, fine. But if you do, remember to mention it and define the terms you prefer to use.