One of the functions of the vertebrate skeleton is the buffering of the blood with alkali minerals stored in bone. As blood pH drops, osteoclast resorptive activity is increased and the deposition of alkaline minerals in bone by osteoblasts is reduced. This releases hydroxyl ions (OH–) into solution to buffer acidic blood at the expense of skeletal health. The pH of arterial blood is close to 7.40, but this drops to around 7.36 in venous blood because of mixing with the metabolic waste from the interstitial fluid. The acidity of this metabolic waste is determined to a large extent by the food choices made in the diet. This is because some foods are metabolised to acid-forming compounds and some are metabolised to base-forming compounds. Evidence also suggests that chronic acidified blood can lead to the propagation of certain disease, perhaps cancer.
Foods rich in potassium and bicarbonate are thought to increase the pH of the blood and thus may be beneficial in the prevention of disease. The potential renal acid load (PRAL) is a description of the acid produced by food at the kidneys after metabolism. The PRAL for various food is shown in figure 1. Commonly eaten foods that rate highly (negative) on the PRAL score are generally fruits and vegetables, because they are good sources of both bicarbonate and potassium. Tea, coffee and wine also have negative PRAL scores. This may go some way to explain the health benefits of fruits, vegetables, tea and wine in the diet, as demonstrated in numerous epidemiological studies. Modern diets however, incorporate more grains and meat which are poor sources of bicarbonate and potassium and as such decrease the pH of the blood.
Figure 1. PRAL values of a variety of foods and food groups1.
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