I am an extreme skeptic when it comes to a so-called alkaline diet. No doubt that most of those foods are healthy, but are there any studies that demonstrate its effectiveness?

Food cannot affect your body's pH balance. It's kept in check by homeostasis, like your body temperature. Unless you're ill or the conditions are extreme, you can go outside in warmer or cooler weather and your body temperature will remain stable. The same occurs with pH. Your stomach's pH is about 1-2. That is hundreds of times more acidic than just about anything you eat. So food has only a trace affect on this. The only thing that gets altered is your urine pH, which simply means your kidneys and the rest of your body's homeostatic system is working. If you were "too acidic" or "too alkaline", you'd be in an intensive care unit. Generally, acidosis or alkalosis can only be caused by poisoning or malfunction of the metabolic system. Diabetic ketoacidosis is an example.