My inbox has been inundated with questions about this topic for a long time, and even more so lately. Yes, there can be a dietary component to the problem, and in fact diet may be the "cure" (cure is a word that I use with caution), but there's so much hype about tear and saliva stains that we need to get to some facts first. Make yourself comfortable. This is a bit longer than usual.
So, what's causing the red stains? Molecules containing iron from the body's normal breakdown of red blood cells are called porphyrins. Dogs excrete a large amount of this via tears and saliva, and they become more visible on white fur, of course. They also get darker in sunlight. There is no dietary link to this. All dogs produce and excrete porphyrins, but diet can change things for many dogs, and that information is coming. I need to walk you through the entire issue, including "red yeast" before getting to the dietary component, so bear with me.
Just in case you've read about "red yeast" called Ptyrosporin being the underlying cause of these stains, let me clarify. First, there is no such thing as Ptyrosporin. Someone misspelled Pityrosporum (I wrote about this years ago, so if it sounds familiar, I'm sorry to repeat myself, but this info is critical to understanding the possible yeast connection to staining of fur) and the internet being what it is, it's been repeated over and over again despite it not being factual. Way back when, what we now call Malassezia (the most common yeast in dogs) was called Pityrosporum (not ptyrosporin). So, with one misspelled word, "red yeast" became a battle cry based on well, nothing. It's important to understand that yeast can indeed be an issue though, and here's how:
Yeast lives on healthy skin quite happily without our ever realizing it until it overgrows. The cause of that overgrowth can be anything from allergies, immune system challenges of many sorts, and more. There is always an underlying problem in these cases, so we need to deal with that problem in order to get the yeast to calm down to normal numbers. The area around the eyes and nose are more difficult to keep clean in certain breeds simply because of the shape of the face. Keeping the face very clean can prevent a yeast infection, so it's important to distinguish between this problem and the simpler cosmetic issue of tear stains. As regards staining, wash the area morning and evening religiously, and many people see best success with making the stains look paler by also using contact lens cleaner with boric acid on the stained fur.
Ok, now about diet!
Check your drinking water. If it's high in iron, it could be adding to the problem. Unsure? Use filtered water (not bottled water!)
Both food and environmental allergies can cause tearing. You can sometimes understand the dog's pollen allergies (of course, testing for them via a veterinarian is best) by keeping track of specific pollen and the count by doing a search, or watching the weather channel. Keep a log. It's surprising how much this can tell you. So, what can you do about pollen allergies? Consider oral allergy syndrome when you decide on a diet, use an air cleaner, hepa filter, dust daily.
An elimination diet can work wonders as can a switch from commercial to home-prepared diets. I'm including raw diets when I mention commercial diets because feeding something made by anyone other than yourself means you can't gain control, and that is what it takes - control - to get to the bottom of things. Stop feeding a diet that includes many foods because any of them can be the culprit. Keep it simple! Use the steps for an elimination diet.
Supplements you may not consider to be part of the diet nevertheless are exactly that. If it's being ingested, it's dietary. So, if your dog is taking something for arthritis that contains fish, you are feeding fish protein...see where I'm going with this? You have be diligent and consider every single thing your dog consumes.
I've been fortunate to see success with many clients for the staining problem, but would be lying if I said it was 100% of the time. Sometimes diet is not the answer. Sometimes the issue is bacterial, or best managed by a veterinarian. In all cases, having your dog's eyes checked before starting a diet change makes good sense. You don't want to waste time on dietary changes while unknowingly allowing a serious problem to escalate.
"It's hard not to immediately fall in love with a dog who has a good sense of humor.” Kate DiCamillo