Effect of dietary protein supplementation on blood pressure: a randomized, controlled trial.

Randomized controlled crossover trial in adults with prehypertension or stage 1 hypertension enrolled in New Orleans, Louisiana, and Jackson, Mississippi. After a 2-week run-in, 352 participants were randomized in a fixed 1:1:1 allocation to 3 intervention sequences; end-of-phase BP data were available for 284 soy phases, 286 milk phases, and 287 carbohydrate phases.

Participants received either soy protein powder or milk protein powder, 40 g/day orally in water or juice, split twice daily in the morning and evening for 8 weeks per phase. A carbohydrate control phase served as the comparator in the crossover design; the three products were provided in identical packets and had comparable sodium, potassium, and calcium content.

Soy and milk protein both lowered systolic blood pressure versus carbohydrate control, with no significant difference between the two proteins. The net change versus carbohydrate control was -2.0 mm Hg (95% CI -3.2 to -0.7; P = 0.002) for soy and -2.3 mm Hg (95% CI -3.7 to -1.0; P = 0.0007) for milk. Overall systolic BP change during the intervention phases was -1.5 (95% CI -2.4 to -0.6) for soy and -1.8 (95% CI -2.7 to -1.0) for milk, while carbohydrate changed 0.5 (95% CI -0.4 to 1.3); P = 0.0007. Diastolic BP changes were not significant. The authors concluded that partially replacing carbohydrate with soy or milk protein may help prevent or treat hypertension.

Each intervention phase was only 8 weeks, so long-term blood pressure effects are unknown. BP reductions were modest, diastolic outcomes were null, and end-of-phase data were not available for all randomized participants. The crossover design also raises potential concerns about phase and carryover effects, although no major between-protein difference was seen.