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Department of Nutrition and Health Research Institute of Child Nutrition Heinstuck 11 Dortmund 44225 Germany E-mail: remer{at}fke-do.de
Dear Sir:
In a recent article in the Journal, Sebastian et al (1) provided a detailed analysis of the probable effect of ancestral preagricultural diets on systemic acid load (net endogenous acid production, or NEAP) and compared this with the average acid load of contemporary diets. The NEAP was calculated for retrojected preagricultural diets for which compositions were suggested by Eaton and Konner (2). Current food tables served to estimate the respective nutrient content. Final computation was based on an existing calculation model (3, 4) that was modified to more accurately estimate those food-dependent acid loads that lead to endogenous production (and renal excretion) of sulfate and organic acids (OAs).
This modified approach, which considers individual dietary sulfur-containing amino acids (instead of average protein content) and dietary determinants of OA production, offers an important improvement to existing estimation models for net acid excretion. However, we do not fully agree with Sebastian et al, who argue that food-dependent endogenous OA production can be sufficiently predicted (with specific formulas) from the same nutrients (sodium, potassium, calcium, magnesium, chlorine, and phosphorus) that are needed to estimate the major food-dependent component (apart from sulfur-containing amino acids) of NEAP or of potential renal acid load (PRAL).
It is highly probable that the renal excretion of different OAs is dependent on diet. Aromatic organic acids are a dietary component, not mentioned by Sebastian et al, that may have a particularly strong effect. For example, phenolic and benzoic acids, which are found in considerable amounts especially in fruit (5, 6), are metabolically inactivated (detoxified) and excreted (mainly via the kidney) as acids, largely in the form of hippuric acid.
Interestingly, in the highlands of New Guinea, some Papuan tribes consume a low-protein vegetarian diet consisting predominantly of sweet potatoes. These sweet potato eaters excrete extremely high amounts of hippuric acid (31 mmol/d on average compared with 4 mmol/d in European control subjects), which adds substantially to their basal (not primarily food-dependent) urinary OA excretion (7). Basal OA excretion can be estimated from average anthropometric data as follows (3, 4):
The reported data (7) on average daily food intake and 24-h urinary excretion of sodium (7 mmol/d), chloride (4 mmol/d), potassium (180 mmol/d), and total nitrogen (2.6 g/d) allowed us to estimate the NEAP and PRAL of the sweet potato eaters. Urinary excretion rates not given in the original article (7) were calculated (3, 4) from the corresponding daily intakes of magnesium (443 mg/d), calcium (728 mg/d), and phosphorus (936 mg/d). They were obtained from the reported food consumption by using food tables (8) and yielded values of 12, 9, and 34 mEq/d, respectively. Urinary sulfate output (11 mEq/d) was estimated from protein degradation, ie, from total nitrogen excretion (see above), corresponding to an absorbed amount of 16.3 g protein/d. The nutrient-dependent PRAL (sulfate + phosphate + chloride - sodium - potassium - magnesium - calcium) was then calculated as -159 mEq/d. Because the NEAP corresponds to PRAL + OA, an average overall endogenous acid production of -92 mEq/d was finally yielded. This NEAP, directly calculated for "modern" stone age farmers by using measured (ie, hard) data for the intake and renal excretion of nutrients, is nearly identical to the average NEAP (-88 mEq/d) found by Sebastian et al for 159 retrojected preagricultural diets. However, the protein intake of the sweet potato eaters was very low (22 g protein/d, as estimated from urinary nitrogen output under the assumption of 75% net absorption), whereas protein intakes of 200 g are assumed for most ancestral diets (1, 9).
If the protein intake of sweet potato eaters was to isoenergetically increase by only 100 g/d (with protein replacing carbohydrates), the NEAP would increase (ie, net base production would fall) to -43 mEq/d. Therefore, it appears to us that the average net base production of -88 mEq/d (ie, the absolute figure) calculated by Sebastian et al may be too high for Stone Age persons with high protein intakes. This is also confirmed if the average PRAL and NEAP are calculated from the average nutrient intakes of Stone Age persons as recently published by Eaton and Eaton (9). Using their figures on daily nutrient intakes, we estimated a negative PRAL of -39 mEq/d, leading to an NEAP of 22 mEq/d, which is markedly lower than current net acid excretion (64 mEq) in the United States (Table 1).
View this table:
TABLE 1 . Potential renal acid load (PRAL) and net endogenous acid production (NEAP) in the Paleolithic Age and today
Taken together, we also conclude that the average Paleolithic diet principally led to net base production (yielding a negative PRAL), but was possibly less alkaline than suggested by Sebastian et al. One of several uncertainties in this respect is obviously the intake of those OAs not metabolically combusted but renally excreted, eg, phenolic acid, which is excreted in the form of hippuric acid. Reasons for the historical shift from negative to positive PRAL are not only the displacement of alkali-rich plant foods in the ancestral diet by cereal grains and nutrient-poor foods in the temporary diet but also the modern processing and preparation of foods, which lead to considerable losses of base-forming nutrients such as potassium and magnesium.
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