The main thing is to have a balanced diet and not have too
much of any one food, since your baby needs to get a mix of nutrients. There
are a number of theories about which foods should be avoided, but as far as
normally healthy foods are concerned the one thing that many health
professionals agree should be treated with caution is oily fish. This is very
healthy in itself (mackerel, sardines etc) and it's OK to eat some of it while
feeding your baby, but only one or two portions a week becase these fish can
contain traces of mercury.

Of course you should avoid the same foods while feeding as you should avoid
normally, only more so. In other words, as few processed foods as possible
because you can't really control the fat, salt and sugar content; and generally
try to reduce sugar and salt intake and cut down on very fatty foods.

This fact
sheet may help.