Nutrition Discussion Forum

Abstract

Higher calcium urinary loss induced by a calcium sulphate-rich mineral water intake than by milk in young womenComments by Arnaud I did not intend to revisit the discussion of the publication of Brandolini et al.1 on Ca urinary excretion attributed to the sulfate content of water but in their answers 2 to my comments 3 , they said that 'I accept their experimental results' and that 'I do not contest the difference in calciuria between milk and sulfate-rich water'. I never wrote that I agree with their results and even more with their conclusions but questioned how is it possible to evaluate a 20 mg difference in daily Ca urinary excretion from a study with subjects under uncontrolled dietary intakes, unbalanced experimental study design and without analytical results on acid -base balance and sulfate to support their acidogenic hypothesis of sulfate. I did not find any convincing explanations in their answers but I want to raise several points of disagreement, which are either repeated or new.First, the acidifying mechanism of ingested free sulfate and sulfate produced from sulfur amino acids after protein ingestion are two different processes. In urine, the excretion of SO 4 22 reflects the oxidation of sulfur amino acids methionine, cysteine or cystine of dietary or endogenous proteins and is accompanied by the generation of 2 mEq H þ per mmol sulfur oxidised 4 -7 . In contrast, when calcium sulfate or calcium chloride is ingested at levels to allow for equivalent absorption of sulfate and their cations, there is no 'net acid' intake 8 . The acid effect of ingested CaCl 2 is due to a much greater absorption of Cl 2 than Ca 9 . Similarly, calcium sulfate given intravenously is neutral. When ingested, fractional sulfate absorption is higher that that of Ca and the type of anionic exchange determines its effect on the acid -base balance 10 . The acid load in that case is metabolically different from sulfate derived from absorbed amino acids and endogenous protein, as protons released during sulfur oxidation must be added to sulfate excreted in urine. Thus we cannot say that there exists 'a commonly accepted consensus to attribute acidifying property of sulfate' when sulfate originates from inorganic salts or from organic compounds.Second, I disagree with the claim that 'it is well known that sulfate is well absorbed and excreted in urine because this anion cannot be metabolised or retained'. In our study 11 cited by Brandolini et al.1 , it was shown that 7 % of sulfate from a water containing 1479 mg per litre was incorporated and in urine and stool between 30 and 60 % was in the form of conjugates or bound to organic compounds. There are hundreds of sulfur-containing compounds in the human body 12 and sulfated oligosaccharides have important biological roles, their unique structure contributing to recognition by a receptor 13 . Proteochondroitin sulfate plays a major role in the mechanical support of cartilage; its functions are dependent on the high charge of the sulfate and any decrease in the sulfation mig...