The trick, of course, is to eat from both groups, and pick the healthier foods from each. Eating too high a proportion of acid forming foods will draw minerals out of the bones; while eating a high proportion of alkalising foods often tends to create cravings for sweets, as many vegetarians will attest to.
The buffer foods are milk products and tofu (if made with calcium carbonate); they will balance either side, because they contain both calcium (alkalising) and protein (acid forming). Thus, if a diet is high in sugar, flour, and/or meats, yet low in vegetables, dairy produce will help alkalise the body because of its calcium content. Conversely, if a diet is high in alkalising fruits, vegetables, and potatoes, yet low in protein or grains, dairy products will provide supplementary acid forming protein.
For people who prefer not to use milk products, the solution is to consume instead plenty of dark green, leafy vegetables (kale, collards, mustard greens, watercress, arugula), roots (carrots, turnips, parsnips, radishes), broccoli, squash and especially chopped parsley. Parsley contains both calcium and vitamin C, as well as ergosterol, a precursor to vitamin D, which helps absorb and utilise the calcium. Although spinach is rich in calcium, it is also high in oxalic acid, which interferes with the absorption of calcium.
Acid forming foods promote a tendency toward acidosis in the bloodstream; to counteract this acidosis, calcium and other minerals will be immediately withdrawn from the bones. Thus, bone resorption (breakdown) would increase to rebalance the blood, and calcium and other minerals would be excreted in the urine. Studies done on mice at the University of Rochester School of Medicine and Dentistry (Curr Opin Nephrol Hypertens, 1993; 2: 588-96) have shown that calcium does indeed leave the bones when they are in an acid environment. In other words, metabolic acidosis stimulates bone resorption and inhibits bone formation.
An increase in alkalising foods can balance this equation, by replacing the minerals that have been "borrowed" from the bones. However, if the diet is low in vegetables and other mineral rich foods, and if this persists for year after year, the amount of calcium and other minerals needed for bone deposition would be too low, and slowly the needs of bone resorption would exceed the the amount of incoming calcium.
Both too much and too little protein can cause trouble with the bones. Vegetarians have been shown to have higher bone density than omnivores, or people who eat everything. In one study (Amer J Clin Nutri, 1972: 25: 555-8) the mean bone density of the 70 to 79 year old vegetarians was greater than that of the 50 to 59 year old omnivores. Therefore, it is considered that vegetarians have a lesser risk of osteoporosis. But another way to interpret these studies is to note the rest of the dietary context: it could mean that the omnivores eat too many sweets and not enough greens and other plant foods. A 1997 Norwegian study found that there was an elevated risk of fracture in women with a high intake of protein and a low calcium intake.
Dr T Colin Campbell of Cornell University, in collaboration with the Institute of Nutrition and Food Hygiene of the Chinese Academy of Preventative Medicine in Beijing, conducted a famous study of the dietary patterns and nutritional status of Chinese people in the early nineties (Bol Assoc Med P R, 1990; 82: 132-4). That study clearly showed that the composition of urinary acids and calcium in middle aged and elderly women is influenced considerably by their dietary intakes. The consumption of acid forming foods such as animal protein increased calcium excretion in the urine, which indicates bone mineral loss. Plant protein intake was not related to urinary calcium excretion.