The hypothesis behind the alkaline diet is that foods high in sodium, protein and phosphates send your body out of its "normal" alkaline pH levels. Proponents claim the diet will improve your bone health, prevent or treat cancer, and keep you happy and healthy.
The claims of bone health or cancer cures appear to be over-reaching. Scientific research hasn't uncovered any solid evidence that the foods you eat cause your body to become "acidified." In fact, your body normally has a whole range of pH. Your stomach has a very acidic environment so you can digest proteins and minerals, but your blood is slightly alkaline. A woman's vagina is on the acidic side while the pH of the intestinal tract should be neutral or a little on the alkaline side.
As long as you're healthy, your body regulates the various pH levels of your body nicely, and while certain diseases such as kidney disease and diabetes might cause problems in pH regulation, there's no scientific evidence supporting the idea that foods you eat make your body more acidic.
The alkaline diet promotes an increased intake of fruits and vegetables and avoids overly processed foods that are high in sodium and saturated fat. That's good because the typical Western diet is low in fruits and vegetables and has much more sodium than is necessary. In fact, if there is any benefit to this diet, it's probably due to the inclusion of nutrient-dense fruits and vegetables, because the pH affects are immaterial. Avoiding sodium is also beneficial. The Institute of Medicine recommends no more than 1,500 to 2,400 milligrams each day for most adults, but most people consume two or three times that amount.
As far as fad diets go, this one isn't as potentially harmful as some, but it does eliminate some nutritious foods. Following this diet requires you to eliminate milk and dairy products (even low- and non-fat dairy), which are excellent sources of dietary protein and calcium. That's unfortunate (and unfounded) because research studies indicate consuming these foods doesn't have an acidifying effect on the body, nor does it mess up calcium metabolism. Eating meat doesn't change the pH of your body either.
What About Alkaline Supplements?Most "alkalinizing" supplements contain minerals like calcium, magnesium and potassium. These are important minerals' and if you don't eat a balanced diet, you may not be getting enough. However, you can find these minerals in most any daily multi-vitamin and mineral supplement.
Fenton TR, Lyon AW. "Milk and acid-base balance: proposed hypothesis versus scientific evidence." J Am Coll Nutr. 2011 Oct;30(5 Suppl 1):471S-5S.
Fenton TR, Tough SC, Lyon AW, Eliasziw M, Hanley DA. "Causal assessment of dietary acid load and bone disease: a systematic review & meta-analysis applying Hill's epidemiologic criteria for causality." Nutr J. 2011 Apr 30;10:41. doi: 10.1186/1475-2891-10-41.
Institute of Medicine of the National Academies. "Dietary Reference Intakes: Electrolytes and Water." Accessed January 21, 2013. http://www.iom.edu/~/media/Files/Activity%20Files/Nutrition/DRIs/DRI_Electrolytes_Water.pdf.
Schwalfenberg GK. "The alkaline diet: is there evidence that an alkaline pH diet benefits health?" J Environ Public Health. 2012;2012:727630.