The short-term effects of low-phosphate diets on fibroblast growth factor 23 (FGF23) level and the optimal amount of dietary phosphate restriction in patients undergoing hemodialysis remain unknown.This was a randomized, active-controlled trial with a crossover design that included 35 adults with ESKD undergoing thrice-weekly hemodialysis and with a serum phosphate level >5.5 mg/dl or between 3.5 and 5.5 mg/dl with regular phosphate binder use at a hemodialysis unit of tertiary teaching hospital in Taiwan. Subjects were randomized 1:1 to receive a very-low-phosphate diet, with a phosphate-to-protein ratio of 8 mg/g, or a low-phosphate diet, with a phosphate-to-protein ratio of 10 mg/g for 2 days, each with a 5-day washout during which subjects adhered to their usual diet. The primary outcome measure was mean difference in change-from-baseline intact FGF23 level between intervention groups. Secondary outcomes included difference in change-from-baseline serum phosphate, intact parathyroid hormone (PTH), and C-terminal FGF23 level between intervention groups.There was no significant difference in the mean change-from-baseline in intact FGF23 levels between the two study diets. The very-low-phosphate diet significantly lowered serum phosphate (mean difference, 0.6 mg/dl; 95% confidence interval [95% CI], 0.2 to 1.0; =0.002). There were no significant differences in change-from-baseline intact PTH and C-terminal FGF23 levels between the two study diets.Over the 2-day period, the FGF23-lowering effect of the very-low-phosphate diet is similar to that of the low-phosphate diet. The very-low-phosphate diet has an additional phosphate-lowering effect compared with the low-phosphate diet.