Because we are renting an apartment here in Chiang Mai, we provide our own breakfast and sometimes lunch. We are quite close to a Tesco (Lotus) where we buy butter and bread, items that are not available in Thai markets. For fruit and veg we found a little market that opens everyday till quite late. It sells meat, fruit, vegetables, fish, herbs and spices. Some of the fish and meat is pre cooked. The majority of Thai people do not have cooking facilities where they live, so buy food that is already prepared.
Another food market sells food that is already cooked and can be eaten there or taken home with you. Some of it was high in protein but the appearance was not very appealing.. Make your own mind up.