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Urinary Excretion of 5-L-Oxoproline (Pyroglutamic Acid) Is Increased in Normal Adults Consuming Vegetarian or Low Protein Diets^1,2,

Institute of Human Nutrition, University of Southampton, Southampton SO16 7PX, United Kingdom

A method for measuring 5-L-oxoproline in urine, which involves isolation by short-column chromatography, acid hydrolysis to glutamic acid and enzymic assay of glutamic acid, was used to measure the rate of excretion in normal adults, aged 20 to 45 y. There was no difference in the daily excretion between omnivorous males (217 µmol/d) and females (195 µmol/d). In vegetarian males, urinary 5-L-oxoproline (404 µmol/d) was significantly greater than in vegetarian females (267 µmol/d, P = 0.013). Compared with omnivorous males or females, excretion of 5-L-oxoproline was significantly greater in vegetarian males (P < 0.0001) and females (P = 0.005). When normal adults consumed a diet in which the protein content was controlled at either 4.0 or 6.2 g N/d for 5 d, there was a significant increase in urinary 5-L-oxoproline on d 5, compared with either d 1 or 4. There was a significant inverse linear relationship between the increased urinary 5-L-oxoproline on the fifth dietary day and the nitrogen content of the diet. On the basis of this relationship, when the urinary excretion of 5-L-oxoproline (320 µmol/d) for vegetarians was predicted from an estimate of their dietary intake of nitrogen, the estimate was, on average, close to the measured value (345 µmol/d). As a matter of course, vegetarians excrete more 5-L-oxoproline in urine than do omnivores, and we speculated that this difference might be accounted for by differences in dietary nitrogen and the endogenous capacity for de novo synthesis of glycine.

KEY WORDS: • nitrogen • amino acid • glycine • protein • glutathione • humans

¹ Supported in part by grants from Nestle Nutrition Research Fund, Rank Prize Funds, and the Biotechnology and Biological Sciences Research Council, U.K.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 26 February 1996. Revision accepted 23 July 1996.