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Nutritional Epidemiology

Urinary Potassium Is as Reliable as Urinary Nitrogen for Use as a Recovery Biomarker in Dietary Studies of Free Living Individuals¹

Nataa Tasevska, Shirley A. Runswick and Sheila A. Bingham²

Medical Research Council Dunn Human Nutrition Unit, Cambridge, CB2 2XY, UK

² To whom correspondence should be addressed. E-mail: sab{at}mrc-dunn.cam.ac.uk.

Twenty-four-hour urinary nitrogen (UN) is commonly used to validate dietary assessment methods. Potassium is more widespread in food than nitrogen, but the role of 24-h urinary potassium (UK) as a biomarker has been less studied and characterized. To investigate the performance of UK as a recovery biomarker compared with UN in subjects consuming their normal diet, 7 males and 6 females consumed known amounts of food based on their habitual varying diet (assessed beforehand from 4 consecutive 7-d food diaries) for 30 d in a metabolic suite. All daily urine samples and dietary duplicates were collected, and N and K were measured. Stool K was determined in 5-d pooled samples. Thirty-day mean analyzed K intake was 121.3 ± 25.1 (mean ± SD) mmol/d. Overall, 77 ± 6.7% of K in the diet was excreted in urine and 18 ± 5% in stool. Dietary K was correlated with UK (r = 0.89; P < 0.001). UN was 77.7 ± 6.6% of N intake and was correlated with N in the diet (r = 0.87; P < 0.001). When 16 d of intake and 8 d of urine-collection measurements were randomly selected from the 30-d measurements, correlations were significant for both K (r = 0.86; P < 0.001) and N (r = 0.92; P < 0.001). The high correlations between UK and K in the diet show that UK is a reliable recovery biomarker for use in studies of dietary measurement error. Factors for use of urinary N as a recovery biomarker are also confirmed.

KEY WORDS: • dietary biomarkers • diet • urine potassium • stool potassium • urine nitrogen

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