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SUMMARY Recent findings suggest that supplemental calcium could lower the abnormally high proliferation rate found in the colonic mucosa of subjects at high risk for colon cancer. In this double blind controlled study, this effect in volunteers previously operated upon for a colorectal adenocarcinoma was tested. Thirty subjects were randomised to receive either elemental calcium 1200 mg/day or a placebo. Mucosal proliferation was measured with tritiated thymidine labelling before and after the 30 day intervention period. Diets, faecal pH and the concentration of calcium and bile acids in the aqueous phase of feaces were also measured. Labelling index did not differ significantly in the two groups before intervention (placebo 4 0(2 4) v calcium 4 9(2 9), but the difference approached significance afterwards (4 4(2-4) v 6 5(3 4), p=006). Individual changes occurring with intervention were tabulated and comparison of the means for the groups was not significant (A=0-3 vs A=1-8, p=011). Calcium concentration, faecal pH and deoxycholic acid concentration increased in the calcium group (p=0.02, 0 005 and 0 004 respectively). Calcium does not show any effect in decreasing colonic mucosal proliferation in this high risk group for colon cancer; it may increase faecal pH and the production of deoxycholic acid in the colon.Colorectal cancer remains a leading cause of cancer related deaths in North America and no major change in incidence has been reported over the last decade. Environmental factors are suspected of playing an aetiological role in this disease; in particular, high fat consumption has been under scrutiny. -One proposed mechanism explaining the effect of fat is an increase in bile acid secretion' which subsequently leads to a rise in concentration of potentially toxic secondary bile acids in the colonic lumen.7 This hypothesis is supported by animal experiments showing that these compounds can induce mucosal cell damage, increase proliferation rates"3 and enhance carcinogenesis by acting as tumour promoters"' and by field studies showing that high risk populations for colon cancer have higher concentrations of bile acids in their stools.31112

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Measurement of the Ca2+ in spot urine samples at a standard pH provides a more discriminative test than total Ca for the presence of urinary tract stones. The pH dependence of the Ca2+ may have important consequences in the formation of urinary tract stones. The poor correlation between measured and computed values of Ca2+ emphasizes the importance of accurate measurement of this variable.

Objective To assess the role of pH in determining the formers, at 7.67 (0.62) and 7.21 (0.54), respectively, and nearer to their voided pH of 5.92 (0.70) and 6.26 solubility of Ca2+ in urine from normal subjects and stone formers and to determine the conditions which (0.71), respectively. The lower pH n in stone-formers could not be explained by altered concentrations cause the formation of crystalline products. Materials and methods Urine, collected from normal or binding properties of Ca2+-binding ligands. pH n increased after diluting the urine and the value of pH n subjects and urinary tract stone-formers, was used in vitro. The concentration of ionized calcium, [Ca2+], was strongly dependent on the [Ca2+] at a standardized pH. and pH were measured with ion-selective electrodes and the formation of crystalline products measured asConclusions Crystalline precipitates appear in urine at a critical pH which is closer to the voided pH in stonethe change in turbidity of the specimen. The composition of the crystalline products was analysed by infraformers than in normal subjects and might explain the greater propensity of this group to form stones. red spectroscopy. Results The [Ca2+ ] decreased with increasing pH andThe value of pH n is critically dependent on the urinary [Ca2+] and manoeuvres which reduced its concenshowed a biphasic relationship with a critical pH (pH n ) at which Ca-containing precipitates appeared.tration would reduce the tendency to form stones.