Letter to the Editor
Paleolithic diet, sweet potato eaters, and potential renal acid load
Thomas Remer and Friedrich Manz
Department of Nutrition and Health Research Institute of Child Nutrition
Heinstuck 11 Dortmund 44225 Germany E-mail: [log in to unmask]
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):
(1)
As a result, 36 mEq/d is yielded for sweet potato eaters [young adult males
weighing 53 kg, 1.55 cm tall, and with a body surface area of 1.5 m2; (7)],
which together with their hippuric acid output amounts to 67 mEq total OA
excretion/d.
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:
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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.
REFERENCES
1. Sebastian A, Frassetto LA, Sellmeyer DE, Merriam RL, Morris RC Jr.
Estimation of the net acid load of the diet of ancestral preagricultural
Homo sapiens and their hominid ancestors. Am J Clin Nutr
2002;76:1308-16.[Abstract/Free Full Text]
2. Eaton SB, Konner M. Paleolithic nutrition. A consideration of its
nature and current implications. N Engl J Med 1985;312:283-9.[Medline]
3. Remer T, Manz F. Estimation of the renal net acid excretion by adults
consuming diets containing variable amounts of protein. Am J Clin Nutr
1994;59:1356-61.[Abstract]
4. Remer T, Manz F. Potential renal acid load of foods and its influence
on urine pH. J Am Diet Assoc 1995;95:791-7.[Medline]
5. Vinson JA, Su X, Zubik L, Bose P. Phenol antioxidant quantity and
quality in foods: fruits. J Agric Food Chem 2001;49:5315-21.[Medline]
6. Zuo Y, Wang C, Zhan J. Separation, characterization, and quantitation
of benzoic and phenolic antioxidants in American cranberry fruit by GC-MS. J
Agric Food Chem 2002;50:3789-94.[Medline]
7. Oomen HA. Nitrogen compounds and electrolytes in the urine of new
Guinean sweet potato eaters-a study of normal values. Trop Geogr Med
1967;19:31-47.[Medline]
8. Souci SW, Fachmann W, Kraut H. Food composition and nutrition tables.
6th ed. Stuttgart, Germany: Medpharm Scientific Publishers, 2000.
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