The acid/alkaline diets have always puzzled me. It's one of those things that "makes sense" just thinking about it... but on closer examination seems to fall apart.
The theory is that the food you eat affects the PH balance of your blood and other fluids in your body, and the more alkaline your body is, the more it will resist disease because disease somehow can't survive in an alkaline environment. Certain foods are classified as being "alkaline foods" that cause you to become more alkaline. Followers of this type of diet will test the PH of their saliva and try to get is as alkaline as possible.
So far, I haven't found anywhere that offers concrete evidence to support any portion of that argument. I've known people who follow this type of diet and they are able to (at least temporarily) change the PH of their saliva. I've heard from several doctors, though, that the PH of your saliva doesn't actually correlate to the PH of your blood or any other body fluid. Oops!
The other major problem with this belief system is that if you look at a ton of different websites, there seems to be a lot of disagreement on what foods are actually the preferred alkaline foods, and which ones are acidic. Some say all meat will cause acidity. Some say chicken is neutral. Some say chicken will cause alkalinity. So, even if you completely believe in this type of a diet, what should you actually eat??
From looking at most of the recommended foods, it would seems that the benefits people get from this diet have to do with eating less starches, less processed carbs, less sugars, and a lot more veggies... which would obviously benefit pretty much anyone.