Blood and urine acid-base status of premenopausal omnivorous and vegetarian women

The effect of long-term differences in diet composition on whole-body acid-base status was examined in thirty-three young healthy females. The volunteers were recruited from two separate groups matched approximately for age, height and weight; one group regularly ate meat (omnivores; n 20) and one group did not (vegetarians; n 13). All subjects completed a 7d weighed intake of food, and from their dietary records, total energy, carbohydrate (CHO), fat and protein content were estimated using computer-based food composition tables. During this week they reported to the laboratory on two occasions, following an overnight fast and-separated by at least 48 h. Arterialized venous blood samples were obtained on each visit and these were analysed for blood acid-base status. Haemoglobin and packed cell volume, serum total cholesterol and HDL-cholesterol, serum albumin and total protein were also determined. Two 24 h urine collections were completed; the volume was recorded and samples were analysed for pH, titratable acid and Mg and Ca concentration. Total energy intake of the omnivores was greater (P = 0.0003) than that of the vegetarian group. Dietary intake of CHO (P = 0.024), fat (P = 0.0054) and protein (P = 0.0002) were higher in the omnivorous group than in the vegetarians. There were no differences between the two groups with respect to blood CO2 partial pressure, plasma HCO3(-) and blood base excess, but blood pH was slightly higher in the omnivores (P = 0.064). Measures of urine acid-base status suggested a lower pH in the omnivore group, but this difference was not statistically significant; a greater titratable acid output was observed with the omnivorous group compared with the vegetarians (48.9 (SE 20.3) v 35.3 (se 23.3) mEq/24 h; P = 0.018). Although the dietary intake of Ca was not different between the two groups, urinary Ca excretion of the omnivores was significantly higher (3.87 (SD 1.34) v. 3.22 (SD 1.24) mmol/24 h) than that of the vegetarians (P = 0.014). It is suggested that the higher protein intake of the omnivores resulted in an increase in urinary total acid excretion, which may explain the higher rate of Ca excretion.