Protein foods are acid-forming as they produce sulphuric and phosphoric acids in the body on breakdown, which, if there is insufficient intake of alkaline minerals to balance the acid, will build up and acidify body tissues***. This stored acidity creates all manner of disease symptoms given time. Alkaline-forming foods are thought to be only fruit and vegetables since they are rich in the minerals used to combat the acid, namely ORGANICALLY COMPLEXED sodium, potassium and calcium; they are not rich in proteins. Grains are midway or slightly acid-forming and animal proteins are at the extreme end of the acid-forming foods. Demineralised foods such as white sugar and flour are also acid in their end reaction on the body.
The problem is that the acidity of the blood MUST be kept within certain tight limits. The first (extra-cellular) line of defence against acidity is organic sodium which is obtained largely from vegetables. The second (intra-cellular) defence is potassium, obtained in abundance from fruit and vegetables, grains, seeds, and animal produce. The third is calcium obtained from vegetables, RAW dairy foods and bone broths. The fourth and last is to use proteins, by which time the body is in trouble and producing symptoms of disease.
While it seems to be true that acidified tissues are sick tissues, and if they can be made more alkaline they get healthier, the method generally followed is suspect and as I would see it has become one of the myths in alternative nutrition circles and macrobiotics.
The standard concept is to eat a predominace of fruit and vegetables as opposed to protein foods such as nuts, seeds, pulses, dairy produce, eggs, meat and fish. One of the problems encountered straight away is the level of sugar in the fruit. According to Brian Clement from the Hippocrates Institute, fruit we buy has 22 times more sugar than wild fruit due to its hybridisation for the commercial markets. This high level of sugar induces loss of alkaline minerals in the urine. See for example the work of the dentist Melvin Page who states that most people can stop further tooth decay by avoiding any sugars INCLUDING sweet fruits. In fact, far from being healthy, most fruitarians eventually suffer from blood sugar instability and tooth decay - due to LACK of minerals! Another problem is the lack of protein foods which often leaves people with inappropriate weight loss, poor energy, poor muscular development and sugar craving.
The truth is that you can take in all the minerals you want in foods or supplements and remain acid in your tissues, because taking them into your digestive system is not the same thing as absorbing and utilising them. To utilise minerals, or at least to utilise calcium and magnesium, which are two of the four major players, you need the fat soluble vitamin complexes A & D, vitamin K2 and other fat soluble factors*. Contrary to standard thinking on acid and alkaline forming foods, Weston Price discovered in the 1920's and 30's that one way to keep the body well mineralised was to take in large amounts of rich protein foods in the form of whole oily fish, which are by far richest source of the vitamin D complex, and animal organ meats of which liver is by far the richest source of the vitamin A complex. He used these in stews with vegetables, together with some whole raw milk and butter from cows fed on spring pasture, mixed with cod liver oil several times a day together with breads from freshly ground grains**. This diet could reverse dental decay and many other health problems related to a lack of mineral intake and uptake in a matter of weeks. In terms of the standard acid-alkaline balancing threory, this is a ridiculously highly acid diet, and should be producing disease, not reversing it. Proponents of the theory often ignore the fact that vegans as a whole have an poor record of dental decay, despite having on average an alkaline forming foods diet. Price was really talking about calcium as the major mineral since he was interested in teeth primarily. Calcium is ONE OF the minerals used to "buffer" the acids and eliminate them from the body.
Price obtained his knowledge from exceptionally healthy "primitive" societies around the world. Each society ate at least ten times the quantity of fat soluble activators as "civilised" nations were doing in 1930. In 1930 our grandparents were eating far more of these activators than today, with our unfounded notion that "margarines and vegetable oils are good for you" being spun by the establishment and large corporations. Vegetable oils do not contain these activators (though they can be "fortified" like some margarines are), and plant foods contain only one weak form of vitamin D (D2).
So yes, you can get acid conditions in the body by eating protein foods and insufficient vegetables, and fruits. Price noted that societies that ate less of the fat soluble "activators" had a history of tooth decay, arthritis and tuberculosis, the latter two certainly being signs of tissue acidity. However if you take sufficient quantities of fat soluble activators from fats of animal origin, whether from meat, oily fish, eggs or milk fats, you can access far more of the minerals in the diet and become more alkaline in your body tissues to the point where your bones and teeth take up more minerals. There is nothing stopping anyone taking extra minerals into the body in the form of for example, non-sweet vegetable juices to stengthen the effect by adding organic sodium and potassium. In fact if someone has already become sick they will need all the mineral acid-buffers they can get.
The last point is about our commercially produced foods. Farmed fish, battery eggs, and milk and meat from animals that do not see the light of day are bound to have appalling vitamin D levels, since animals need sunlight to produce it and fish need algae to eat, which doesn't grow in fish farms. I'm sure they are given the minimal amount to survive and produce the "product", but this is not the same as the level they need to be healthy. It seems also that organically complexed calcium is used much more readily for bone and acid buffering than a heat damaged version from pasteurised dairy products, which are of course thought of generally as a "good calcium source".
*Note that complexes of vitamins means there are several chemicals forming the whole, which is not what you get when you take most synthetic vitamin pills, and not even what you get by the action of sunlight on the skin, which produces only one of the vitamin D molecules.
**Butter and cod liver oil contain an abundance of fat soluble vitamin complexes A & D.
*** Note that with foods such as vinegar and lemon which are obviously acidic in nature, the organic acids can be broken down and eliminated easily as carbon dioxide through the lungs, so they are considered alkaline forming foods.