Dr.Mirkin is correct that the pH of the Urine is substantially determined by the acid/alkaline ash value of the food we have recently consumed.
Dr. Mirkin does not address the strain that a diet wtih less than 80% Alkaline foods, puts on the immune system.
The reason that the Saliva pH will increase as an alkaline balanced diet is utilized, is because the immune system starts working better. With less acid to process, the immune system can focus more on its job, removing aberrant cells, etc., and the Saliva pH will become closer to 7 if the Saliva pH was less than 7. There is an indirect relationship between the acid/alkaline balance of our diet and the Saliva pH.
There may factors besides diet, such as health of other organs, or the build-up of toxins, that places a strain on the immune system, as reflected in the Saliva pH.
Medical Doctors have decided that Saliva pH is a matter for Nutrionists, and do not ordinarily check the health of the immune system, by a saliva test of any otherr means.
Dr. Mirkin is apparently responding to claims made by some alkaline suplements. Dr. Mirkin is correct that the average pH balance of food most directly affects the urine. Lowered Saliva pH, below 7, may be the result of more than just taxing the Immune System with an overly acidic diet.
It should also be added, that overbalancing the diet to Alkaline, to bring up the Saliva pH, may be harmful to other organs. Overly Alkaline Diets can contribut to Kidney and Gall stones. If the Saliva pH is not coming up, check the rest of your organs. Some organs may need flushing, rinsing, detoxifying, or special supplements.