The typical Western diet is considered to be acidic in its nature. This is because it results in the production of systemic acidity because it contains a high concentration of foods that are acid precursors. Foods are classified as acid forming or alkali forming depending on whether they generate or consume protons. Foods that generate protons are acid forming and can change the acid-base composition of the plasma in favour of acidity. Likewise, foods that consume protons can shift the acid-base composition of the blood towards alkalinity. Generally, grains, alcoholic beverages, meat and cheese lower the pH of the plasma and acidify the systemic circulation. In contrast, fruits and vegetables tend to have the opposite effect and this may explain some of the health benefits of incorporating fruits and vegetables to the diet.
The average adult human of a Western diet has been estimated to produce ~ 1mEq / kg body weight of acid per day. The pH of extracellular fluid in humans in maintained at between pH of 7.35 and 7.45 and H+ ion concentration must be maintained between 0.035 and 0.045 mEq / l. To maintain the narrow window of correct pH the body can use buffers, alter exhalation of CO2 or use the renal system. Acidity is relevant to bane health because one of the buffer the body can use to decrease the pH of an acidic systemic circulation is calcium that is released from its storage site in bone. It has been shown for example that feeding of ammonium chloride results in a decrease in bone substance and the formation of osteoporosis in rats.
Because osteoporosis develops very slowly over a long period of time, the rise in the level of acid in the plasma would only have to be small. However, a life-long need to use the calcium in bone to buffer a slightly acidic plasma caused by acid precursor diet choices has been previously calculated to be adequate to cause serious bone depletion. If 2 mEq of calcium per kg per day is required to buffer the ~ 1mEq / kg per day in the average Western diet, then over a period of 10 years this could lead to a loss of total body calcium of 15 % (assuming 1 kg of total calcium in the skeleton)1. The short term feeding of potassium bicarbonate has been shown to have a beneficial effect on the calcium and phosphorus balance and urinary excretion of acid in postmenopausal women (table 1).
Table 1. Short-term effects (18d) of potassium bicarbonate supplementation on calcium and phosphorus balance in postmenopausal women1.
1New, S. A. 2003. Intake of fruit and vegetables: implications for bone health. Proceedings of the National Society. 62: 889-899