I could subtly discriminate against a fat employee, or a ginger one, or an ugly one, due to my personal thoughts, where does it end?
No, you have to draw a definitive line, and that line should be work related, direct and obvious, as otherwise people could take any form of harmelss comment and twist it using their own subjective opinion, into something sinister.
It's contextual though isn't it? If you ran a dieting group or were a personal trainer or something where the weight of people you work with was a major thing, then I'd imagine you taking the mick out of fat people would be an issue.