Hi ipek.Don’t know if a switch in food would at all be helpful unless your shih tzu has other issues with the food you are currently feeding other then the stains. However, I did this on DFA site with posters opinions on this kibble for what it’s worth. /forums/topic/natures-protection-superior-care-whitedog/
/forums/topic/natures-protection-superior-care-whitedog/
As long as a vet has given a clean bill of health with especially tooth decay, then hope this article can be of help to you and your fur baby.
Tear stains are usually caused by dye molecules called porphyrins. Porphyrins are iron-containing molecules, produced when the body breaks down red blood cells. Porphyrins are excreted primarily through bile and the intestinal tract, but in dogs a significant amount of porphyrin is excreted through tears, saliva and also urine.
When porphyrin containing tears or saliva sits on white fur for any length of time, stains result. These iron-related stains intensify/darken in the presence of sunlight.
All dogs produce porphyrin, but of course porphyrin staining is most noticeable on light colored dogs. If you have ever noticed a white dog who has been licking or chewing on his leg, the hair in that area will turn iron-brown in color as well.
Primarily, then, most tear stains in most dogs can be simply prevented by keeping the face meticulously free of porphyrin-containing tears. That means keeping the face hair trimmed, and wiping the face at least twice daily with a slightly damp wash cloth, to dilute out and wash away the tears.
Or, if you want to get really fancy, cleaning under the eyes with ordinary contact lens cleaning solution (containing dilute boric acid, that oxidizes the iron in the porphyrins and lightens the color) will help keep things neat and tidy.
