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Estimates of renal net acid excretion and bone health

Department of Nutrition and Health
Research Institute of Child Nutrition
Heinstuck 11
Dortmund 44225
Germany
E-mail: remer{at}fke-do.de

Dear Sir:

The recent Journal article by New et al (1) concerning positive associations of indexes of bone health with lower dietary acidity in a large group of premenopausal and perimenopausal women appears to provide further evidence of a relevant link between acid-base status and bone health. The authors estimated renal net acid excretion (NAE) as an index of net endogenous noncarbonic acid production by using a simple algorithm proposed by Frassetto et al (2) that includes daily dietary potassium and protein intakes. New et al calculated an estimated average energy-corrected NAE of 0.161 mEq · d^–1 · 8.29 MJ^–1 for the whole study population. However, with the use of the mean values for protein (82.5 g/d) and potassium (3395 mg/d) intakes provided by New et al, the Frassetto algorithm

(1)

yields 5 mEq · d^–1 · 8.29 MJ^–1, ie, a value >30-fold that reported by New et al. It is interesting that the latter value corresponds closely to the individually energy-adjusted renal NAE values that we, on the one hand, analyzed in 24-h urine samples and, on the other hand, estimated from dietary records in 16–18-y-old adolescents participating in the Dortmund Nutritional and Anthropometric Longitudinally Designed Study (3; range of means: 5.1–5.6 mEq · d^–1 · MJ^–1). Thus, it appears that New et al may not have converted potassium intake (mg/d) to mEq/d as is required for use of the algorithm of Frassetto et al (2). In this case, the corrected overall mean estimate as well as the (corrected) estimates used for grouping the data set into quartiles should be reported to the interested reader to allow better understanding of the NAE values at which a possible influence on bone might become apparent (4).

Another important question raised by New et al concerns possible consequences of long-term low intakes of protein and dietary acid. The use of the minimum values for protein (20 g/d) and potassium (1475 mg/d) intakes (as reported for the study population; 1) in the formula of Frassetto et al results in an NAE estimate of 2.2 mEq · d^–1 · 8.29 MJ^–1. Should potassium intake increase, the estimate would be even smaller. In this context, it is of interest to learn what percentage of the women with a low protein intake [<45 g/d (reference nutrient intake for protein)] and with consequently lower dietary acid loads did show bone mineral densities (BMD) or "areal BMDs" above the median value at the bone sites studied in this population. Several recent studies (eg, 5) suggested that low dietary protein intakes might exert detrimental effects on skeletal health.

Finally, it should be taken into consideration that the compensatory effect of a base (eg, potassium bicarbonate) on an acid that is metabolically released (eg, sulfuric acid) is additive (1 mEq sulfate –1 mEq potassium) and not multiplicative. Additional dietary components (eg, phosphorus, magnesium, and calcium) are also involved, and they can be relatively easily included in an additive estimation model (the potential renal acid load model), which—according to our own results (3)—can provide a better fit with the analyzed NAE in 24-h urine samples than can the ratio of protein to potassium.

REFERENCES

1. New SA, MacDonald HM, Campbell MK, et al. Lower estimates of net endogenous noncarbonic acid production are positively associated with indexes of bone health in premenopausal and perimenopausal women. Am J Clin Nutr 2004;79:131–8.[Abstract/Free Full Text]
2. Frassetto LA, Todd KM, Morris RC Jr, Sebastian A. Estimation of net endogenous noncarbonic acid production in humans from diet potassium and protein contents. Am J Clin Nutr 1998;68:576–83.[Abstract]
3. Remer T, Dimitriou T, Manz F. Dietary potential renal acid load and renal net acid excretion in healthy, free-living children and adolescents. Am J Clin Nutr 2003;77:1255–60.[Abstract/Free Full Text]
4. Remer T, Manz F. High meat diet, acid-base status and calcium retention. J Nutr 2003;133:3239 (letter).[Free Full Text]
5. Kerstetter JE, O'Brien KO, Insogna KL. Dietary protein, calcium metabolism, and skeletal homeostasis revisited. Am J Clin Nutr 2003;78(suppl):584S–92S.[Abstract/Free Full Text]

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