I feel like putting that title statement on my website. I am the first to admit and get in line to say a good diet can in fact "prevent" many diseases and certainly should be considered as a component of preventative medicine and disease treatment. I'll even go so far as to say that it should be considered in every clinical case, but it is not a Silver Bullet and it won't work overnight.
The most frustrating cases I work on include allergies, behavior problems/concerns and gut issues. These are very frustrating because if your pet has one, you want it gone and the cause or causes can be multi-faceted and difficult to identify.
Allergies: I have a dog with allergies and it's terrible. She licks her feet (which are now brown stained), scratches and chews on herself. I am a pretty good nutritionist but I CANNOT make her environmental allergies go away. Although I have tried, I simply can't and believe me, I HAVE tried. My dogs are always the "guinea pigs" for all the things you see and read on the internet. Everything from coenzyme Q10, to local honey to coconut oil, Trinkit has evaluated it. Whether her issues are genetic or related to over-vaccination in her youth before she came to us, I cannot say (I do have my suspicions). We have been successful managing her diet related allergies but the environmental allergies are a battle that even a good raw diet cannot win. I have a wonderful graduate student currently evaluating some health factors in dogs fed raw diets and the dogs on study have genetic health concerns including allergies. The dogs have never been fed raw diets previously (they are adults) and while their allergies have improved, they HAVE NOT gone away. Unless you know for certain what foods trigger the response, diet may help but it won't necessarily fix allergy issues.
Behavior Issues: This one is a major frustration of mine. Mostly because as a nutritionist and a fairly savvy dog trainer, I don't know in most cases what the roots of the behavior issues actually are. There are nutrients such as thiamine, B6 and trypophan that are precursors or regulators of neurotransmitters. These can affect mood. Through the Holidays, most of us have experienced the amazing influence of tryptophan from turkey consumption. It's a GREAT thing. Tryptophan is a biochemical precursor of Serotonin which is associated with feelings of well-being. For that very reason, turkey often is a great addition for dogs with behavior issues. It can also be useful to use "Cool" foods along with manipulations of protein and carbohydrate in the diet; however, these changed WILL NOT FIX the issue because you'll need to include some good training, exercise, and behavior modification in that plan as well.
Gut issues: As with the above issues, I cannot determine if loose stools or vomiting are results of something genetic, acute, a major underlying inflammatory condition, or stress. I hesitate to even mention the last trigger because we all get so weird about it. The last thing any owner wants to believe is that they may be causing their pet stress. My husband refers to this as "SOD" or Spastic Owner Disease and he can in fact quickly identify these owners. He has a long history of working in the zoological field; therefore, has a pretty good handle on the long term impact of low grade stress on animals. That's a very hard thing to stomach for most owners. I'm not sure how well I'd take it if someone told me that my dogs were living under chronic stress that was causing low grade inflammation, resulting in inflammatory diseases down the road. Is that possible?? ABSOLUTELY. There is so much research documenting the effects of long term, low grade stress on inflammatory responses in the gut. Please do NOT take this as my saying every dog's inflammatory gut issues are related to the owner, that's not what I'm saying at all. BUT, I am saying it's possible and we should all be considering the stresses and pressures we place on our dogs. Dogs that get soft stools every time they travel or compete are the ones that make me nervous. If you have one of those dogs, I urge you to consider altering the diet before you travel and compete. Adding a digestive enzyme and a good probiotic before you leave can help this process. Realize there is likely inflammation in the gut during these times which will open up opportunities for other issues to wreck havoc on the gut. Rather than look for simple treatments and cures, we should always try to address the root cause if we can.
Diet and nutrition can be an amazing strength within any treatment and preventative plan. A solid diagnosis from a veterinarian along with any medical treatments, plus a well balanced diet, can do wonders toward treating and curing diseases. I love it most when I get to work as a team with a client and their vet...it WORKS so much better that way. Diet and nutrition are amazing and that's why I do what I do, but I will never underestimate the value of teamwork while working through health concerns.