4.6 Article

Risks of misinterpretation of biomarker measurements in spot urine adjusted for creatinine-A problem especially for studies comparing plant based with omnivorous diets

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ELSEVIER GMBH
DOI: 10.1016/j.ijheh.2023.114142

Keywords

Creatinine; Specific gravity; Urine; Diet; Vegans; Biomarker; Meat

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This study compared the concentration of a specific metabolite in the urine of vegans and omnivores, and found that the difference disappeared when adjusted for creatinine. However, after adjusting for creatinine, the concentration in omnivores was found to be 26% lower than in vegans. This may be due to the consumption of meat and fish as exogenous sources of creatinine.
Biomarker measurements in spot urine are often adjusted for creatinine to control for dilution resulting from individual hydration. We here report on results of a study involving age- and sex-matched vegans and omnivores (n = 36 each). The daily urinary excretion of 2,3-dihydroxypropylmercapturic acid (DHPMA, a diet-independent endogenous C3-metabolite used as an example compound) was found not to be different in vegans and omnivores (median 433 mu g/24 h each), however, creatinine-adjusted levels were 26% lower in omnivores (median 285 mu g/g creatinine) than in vegans (median 383 mu g/g creatinine, p = 0.003). This difference results from the higher urinary excretion of creatinine in the omnivores compared to vegans (median 1.51 vs. 1.21 g/24 h, p = 0.009). Linear regression showed - besides the fat-free mass - a significant impact of the factor diet (vegans vs. omnivores). This may be due to the consumption of meat and fish as exogenous sources of creatinine. A literature search revealed broad evidence for this interpretation, as creatinine is formed from creatine during heating of meat and fish. Accordingly, consumption leads to temporary increase of serum/plasma creatinine and urinary creatinine excretion, resulting in higher levels in omnivores compared to vegans/vegetarians. An adjustment of the urinary DHPMA concentrations using specific gravity revealed 13% lower values in omnivores (median 225 mu g/L) than in vegans (median 260 mu g/L, p = 0.07). Compared to creatinine-adjustment, adjustment for specific gravity introduces a smaller but still obvious difference between omnivores and vegans. Especially with respect to future studies comparing vegans, vegetarians and omnivores, researchers should be aware of the risks of severe misinterpretations if biomarker measurements in spot urine are adjusted for creatinine.

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