Studies of healthy adults randomised to receive dietary manipulation (based on the acid-ash hypothesis) of acid-base intake through foods or supplemental salts to change urine net acid excretion were eligible for inclusion in the review. The eligible outcome was calcium balance, defined as calcium intake minus excretion. Interventions had to comply with recommendations from the Institute of Medicine Panel on Calcium and Related Nutrients for calcium studies. Participants with chronic conditions (such as renal diseases, diabetic ketoacidosis and any other condition that could alter calcium excretion) were excluded.
Most study participants were women. All included studies manipulated the amount and/or type of protein and compared adequate intakes with higher intakes. The calcium balance methodology for each study was reported in the paper. Urine net acid excretion (mEq/day), urine calcium (mmol/day), calcium balance (mmol/day) and a marker of bone metabolism (percentage change of N-terminal telopeptides) were measured.
The authors did not state how many reviewers selected the studies.