This does sounds like an underlying medical problem, and possibly the dog is suffering from pain. Sometimes it takes detailed blood tests, and even X-rays of the hips/spin, eye tests etc to ascertain what type of condition is causing this type of aggression.

Although I think one of the other respondents was dead right in saying the dog could be ill, I do not agree with their additional comment that the food incident is a sign of dominance and asserting the alpha position. Food bowl guarding can be treated by placing a little kibble in the bowl and then adding more, and more and more until the meal has been consumed so that the dog thinks "I like it when people are near my food bowl".. However, please be aware that snapping around food bowls can again have medical causes, e.g food allergies can trigger this type of behaviour, so can problems like liver disease, or even systemic portal shunt in the terrier type breeds. So please do have you veterinary check this dog out THOROUGHLY! Also it is best to see a veterinary behavioural specialist, or a properly qualified behaviourist for something like this as opposed to a dog trainer who has chosen to call themselves a behaviourist because it is the in thing to do. Beware 'cause there are plenty of those out there.

Fransetter