Aim

Spot-tests of urine pH are claimed to be an accessible biomarker of net acid excretion (NAE), and as such, they may be able to determine changes in an individual's intake of acid- or base-forming foods. To test this hypothesis, we aimed to determine if spot-tests of urine pH could index NAE and relay the consumption of a fruit and vegetable (F&V) concentrate whilst determining this concentrate's capacity to modulate NAE.

Methods

In a double blind, placebo-controlled, cross-over trial, healthy adults (n = 13) were allocated by simple randomisation to receive a F&V concentrate or placebo for three days each, with diet standardised throughout. Measurements of 24-hour NAE, 24-hour urine pH and spot-tests of urine pH were taken throughout the study.

Results

The 24-hour urine pH predicted 24-hour NAE (P = <0.0001). However, spot-tested urine pH displayed prediction intervals too wide to infer 24-hour NAE and inconsistent ability to reflect concentrate ingestion, despite 24-hour NAE and 24-hour urine pH decreasing (-25.8 mEq, 95% CI -44.3 to -7.4, P = 0.01, d = 0.94) and increasing (+0.51, 95% CI 0.25-0.79, P = 0.002, d = 1.3), respectively, following supplementation.

Conclusions

Spot-tests of urine pH are not a valid dietary biomarker of daily NAE and were unable to reliably track changes, despite a F&V concentrate clearly modulating the daily rate of NAE.

Aim

The gold standard of measurement for net endogenous acid production (NEAP) is net acid excretion (NAE), a test that is not readily available, and consequently, estimative equations by Remer and Manz and Frassetto et al. are often used. These equations rely on nutrient databases and it is recommended that their validity be assessed using a country's database before their application in research in that country. We sought to delineate the accuracy and precision of these estimation equations using the Australian food database.

Methods

In a double blind, randomised, cross-over fashion, healthy participants (n = 13) residing in regional Australia were exposed to varying net acid loads while they collected weighted food diaries and 24-hour urine samples for measurement of NAE.

Results

In comparison to the Frassetto et al. equations (equation one bias = -57.1 mEq/day, equation two bias = -32.8 mEq/day), only the Remer and Manz equation was accurate (bias = -5.4 mEq/day); however, all equations were imprecise.

Conclusions

Using the Australian database, the performance of these equations to predict NEAP appears equal to other databases; however, caveats apply in their application. For future research, the equation by Remer and Manz is preferential for group estimates. None of the equations are recommended for individual estimates.