Drinking beetroot juice with a specific amount of nitrate doesn’t make the body use less oxygen when older adults with a certain type of heart failure exercise lightly — so if they feel less tired, it’s probably because their blood flows better or their heart doesn’t have to work as hard, not because their muscles are using energy more efficiently.

In older adults diagnosed with HFpEF, participants would consume either beetroot juice providing exactly 6.1 mmol inorganic nitrate or a matched placebo on separate days in random order, with VO2 measured during standardized submaximal exercise (e.g., 60% of peak workload on a cycle ergometer) using indirect calorimetry. Primary outcome: difference in VO2 between nitrate and placebo conditions. Duration: two 24-hour washout periods between conditions.

In older adults with HFpEF, daily consumption of beetroot juice (6.1 mmol nitrate) for 4 weeks would be monitored for changes in VO2 during standardized submaximal exercise, with pre- and post-intervention measurements. Endurance (e.g., time to fatigue at fixed workload) would be tracked alongside VO2 to assess dissociation between metabolic efficiency and performance.

Among older adults with HFpEF consuming beetroot juice (6.1 mmol nitrate daily for 4 weeks), individuals showing improved endurance without reduced VO2 would be compared to those with no endurance improvement, to determine whether perfusion or afterload changes (e.g., via Doppler echocardiography or arterial stiffness) differ between groups.

In aged rodents with induced HFpEF (e.g., via hypertension and metabolic syndrome), beetroot juice (dose-equivalent to 6.1 mmol nitrate) would be administered to assess whether VO2 during treadmill exercise is unchanged while endurance improves, and whether perfusion (muscle capillary density) or afterload (aortic stiffness) mediates this effect.

All published RCTs testing beetroot juice (≥6 mmol nitrate) on VO2 during submaximal exercise in HFpEF patients would be pooled to determine overall effect size and heterogeneity, with subgroup analysis for endurance outcomes and hemodynamic mediators.