It is increasingly being recognised that the acid base balance of the blood is an important determinant of fitness and health. This is evidenced by the observation that alkalinisation of blood is beneficial to athletes in allowing increased ability to undertake high intensity exercise before the point of fatigue is reached. In addition, chronic depression of blood pH appear detrimental to a number of physiological parameters, particularly bone loss. Acidification of the blood occurs through the consumption of acid forming proteins, particularly those containing sulphurous amino acids. Methionine and cysteine both contain sulphur, and the sulphur component is able to form sulphate (dissociated sulphuric acid) during metabolism, and this can lower the pH of the blood. Similarly, dietary phosphorus can be metabolised to phosphate (dissociated phosphoric acid). It is now accepted that dietary manipulation is effective at altering the acid load of the blood and this can have a pronounced effect on health.
One way of measuring the ‘acidity’ of a diet is to measure the potential renal acid load (PRAL). The PRAL estimates the amount of acid produced from the endogenous metabolism of food and takes into account factors such as the intestinal absorption rates, the minerals in the food (which can have an alkalising or acidifying effect), the amount of sulphurous amino acids as well as the sulphate produced metabolically. Studies have shown that the PRAL is an effective estimate of the renal net acid excretion (NAE) and can be used to estimate the acid forming nature of diets. Infant formulas that contain high amounts of acid forming protein are known to cause growth retardation and catabolic effects, because they are devoid of alkalising foods that can help balance the PRAL. A number of studies have therefore investigated the acid forming nature of diets consumed by children in order to assess the detrimental effects of the pH of the blood, in order to reach conclusions of the nature of acid forming diets.
For example, one group of researchers1 investigated the possibility of using food diaries to estimate the net renal acid excretion in healthy children. To achieve this they analysed the food diaries of the children who were aged between 8 and 18 years, and also measured the acidity of the urine from provided samples. As had been shown previously, it was possible to estimate the net acid excretion from the components of the food diaries and this correlated closely to the measured acidity of the urine. In addition, the researchers showed that a significant association between the estimated and measured net acid excretion could be obtained by analysis of just 4 food components. In this regard, estimates of the total protein, phosphorus, potassium and magnesium in the diets of the children gave almost exactly the same estimate of net acid excretion as more complex methods. While the protein and phosphorus increased the net acid excretion of the urine, the magnesium and potassium decreased the net acid excretion.
Dr Robert Barrington’s Comments: These results suggest that it is possible to estimate the net acid excretion of the urine, and by extrapolation the acid forming nature of diets, from analysis of 4 foods in the diet. Estimates from these four foods (protein, phosphorus, magnesium and potassium) yield almost the same result as more complex methods of estimating net acid excretion. Protein and phosphorus are metabolised to sulphate and phosphate, respectively, and this would expected to increase the PRAL estimate of the diet considerably through their ability to acidify the blood. In contrast, potassium and magnesium are metabolised to potassium and magnesium salts, respectively, and these are able to neutralise acids within the blood because they can act as buffers. The high content of potassium and magnesium in plants foods explains the alkalising effects of high plant food diets. They may also explain the association between consumption of magnesium and bone health. The high protein and phosphorus content of meat and soft drinks, respectively, may also explain their detrimental effects on bone health.
RdB
