Renal net acid excretion capacity is comparable in prepubescence, adolescence, and young adulthood but falls with aging.

OBJECTIVES

To evaluate whether renal net acid excretion capacity (NAEC) varies across different age groups and, specifically, whether it falls in elderly people.

DESIGN

Cross-sectional observational study.

SETTING

Community-based.

PARTICIPANTS

Young participants were from the DOrtmund Nutritional and Anthropometric Longitudinally Designed Study, Dortmund, Germany; elderly participants were from Gothenburg, Sweden.

MEASUREMENTS

Twenty-four-hour urine pH, net acid excretion (NAE), urinary phosphorus, total nitrogen excretion, and anthropometric data were measured in healthy elderly people (aged 55-75; n=85), young adults (aged 18-22; n=117), adolescents (aged 13-14; n=112), and prepubescent children (aged 6-7; n=217). NAEC was determined as 24-hour NAE adjusted for urine pH using the residual method.

RESULTS

In elderly participants 24-hour urinary pH (5.9+/-0.53) was lower (P<.05) and NAE (60+/-27 mEq/d) higher (P<.05) than in the three other groups. In a regression model adjusted for age, sex, and body surface area, NAEC showed a clear decrease with age, with highest values in prepubescents and lowest in elderly participants. However, NAEC remained significantly lower only in elderly participants (P<.001) after the inclusion of total nitrogen excretion, a protein intake index, which was included because protein intake is known to modulate renal function. NAEC was approximately 8 mEq/d lower in healthy elderly participants than in young adults.

CONCLUSION

The capacity to excrete net endogenous acid does not vary markedly from childhood to young adulthood but falls significantly with age, implying that elderly people may require higher daily alkalizing mineral intake to compensate for renal function losses.