Exercise Dose for Better FMD: What Studies Can Actually Say

Exercise can improve flow-mediated dilation (FMD), a measure of how well an artery widens in response to increased blood flow. The frustrating part is that research still cannot name one perfect weekly workout prescription for maximizing it. The useful part is simpler: sustained moderate-to-vigorous aerobic exercise improves FMD in several populations; in heart failure with reduced ejection fraction (HFrEF), high-intensity interval training (HIIT) outperformed moderate-intensity continuous training (MICT); and training more than twice weekly appears to matter more than stretching each workout longer.

FMD is a surrogate measure of endothelial function, not a direct measure of heart attacks, strokes, lifespan, or how energetic you feel on Tuesday afternoon. It is still biologically relevant. Increased blood flow creates shear stress along the arterial wall, which can stimulate endothelial nitric oxide synthase and increase nitric oxide availability. Nitric oxide helps blood vessels relax. That mechanism is plausible and well described, but FMD improvements do not prove that exercise caused a specific clinical outcome through that pathway.

What the heart failure evidence supports

The best-defined finding comes from a 2023 systematic review and meta-analysis by Fuertes-Kenneally et al. in people with heart failure. Aerobic-based cardiac rehabilitation improved FMD in HFrEF compared with usual care. Across the included studies, HIIT produced larger FMD gains than moderate-intensity training.

That comparison matters because it tests intensity, rather than merely comparing exercisers with people who received usual care. The proposed explanation is that repeated high-intensity intervals create larger bursts of blood flow and shear stress, potentially producing a larger endothelial nitric oxide signal in people whose baseline endothelial function is impaired. The evidence supports the outcome difference in HFrEF; it does not prove that shear stress alone caused it.

The same analysis found its most consistent dose signal in frequency. Programs with more than two sessions per week produced larger FMD responses and systemic vascular adaptations, including changes in non-exercised limbs, than programs performed two or fewer times weekly. This comes from between-study moderator analysis, not a randomized trial that assigned otherwise identical patients to three versus five weekly sessions. Treat it as a useful clue, not a precise threshold with magical properties.

A separate 2023 meta-analysis of HIIT in cardiovascular disease supported an advantage for HIIT on vascular outcomes in cardiovascular populations, and a 2026 network meta-analysis extended the comparison across broader cardiovascular disease groups. Network meta-analysis can compare interventions that individual trials did not directly place head-to-head, but its estimates inherit differences in patient populations, exercise prescriptions, and FMD protocols from the underlying trials.

Healthy adults over 45: aerobic training works, but the dose remains fuzzy

For healthy middle-aged and older adults, a 2022 meta-analysis by You et al. pooled nine randomized controlled trials with 221 participants aged 45 and older. Every included study reported a positive FMD outcome after aerobic exercise. Programs at both moderate and vigorous intensity improved FMD, while the vigorous-intensity subgroup was numerically better.

That is encouraging, but it is not a license to declare vigorous exercise universally superior. The review included small groups, roughly 8 to 22 participants per arm, and most participants were perimenopausal or postmenopausal women. The studies also prescribed intensity using different measures, including percentage of maximum heart rate, percentage of maximum oxygen uptake, perceived exertion, and metabolic equivalents. Those methods do not map neatly onto one another.

The most defensible duration conclusion is that chronic programs lasting at least eight weeks produced statistically significant FMD gains in this evidence base. Two examples show how varied the prescriptions were: one eight-week trial enrolled 22 adults with a mean age of 63 years and used 40 to 50 minutes of exercise at 70% to 75% of maximum heart rate, six or seven days per week. Another enrolled 30 adults with a mean age of 79 years and used exercise at 88% of maximum heart rate for 9.5 weeks, twice weekly. Both sit under the broad label of aerobic training, but they are plainly different programs.

Longer is not automatically better. The review noted that FMD did not rise monotonically throughout eight to 12 weeks in all included trials. No identified randomized trial isolated session duration or frequency while holding the rest of the program constant. Current research therefore supports an aerobic habit, not a finely calibrated FMD optimization algorithm.

Why HIIT should not become a universal answer

HIIT has stronger support than MICT for FMD in HFrEF, but that hierarchy does not travel cleanly to every group with metabolic or cardiovascular risk. A 2023 systematic review in type 2 diabetes found that HIIT can improve cardiorespiratory fitness and endothelial function, yet did not find consistent evidence that it beats MICT specifically for endothelial outcomes. A 2025 randomized trial comparing two aerobic intensities in type 2 diabetes also reported variable cardiometabolic responses rather than a uniform higher-intensity advantage.

Those findings do not negate the HFrEF result. They limit its generalization. Type 2 diabetes and HFrEF can both involve endothelial dysfunction, but they differ in medications, disease biology, functional capacity, training tolerance, and the studies available for review. Shared biology does not guarantee an identical exercise dose-response curve.

Other populations provide supportive but indirect evidence. A 2023 meta-analysis found HIIT superior to moderate-intensity interval training for endothelial function in overweight or obese adults, while a 2024 randomized trial in 44 adolescent boys with obesity found HIIT improved vascular measures more than MICT after eight weeks. Neither result establishes an optimal program for older adults or heart failure patients. Different age groups, comparators, and baseline health make that leap too long.

HFpEF is still a blank spot for FMD prescriptions

Do not apply the HFrEF FMD prescription to heart failure with preserved ejection fraction (HFpEF) as if the trials had tested it. The 2023 heart failure review identified no exercise cardiac rehabilitation trials reporting FMD outcomes in HFpEF. The 2023 Gevaert et al. substudy examined vascular repair markers, including endothelial progenitor cells and microparticles, after training in HFpEF. The OptimEx substudy compared HIIT, MICT, and guideline-based exercise using HDL function rather than FMD. These studies show active investigation of vascular biology in HFpEF; they do not provide an FMD dose-response prescription.

How much confidence should you place in FMD gains?

Moderate confidence is reasonable for the broad claim that aerobic exercise improves FMD in HFrEF and in healthy adults over 45. Confidence drops when the question becomes more granular: exact weekly volume, ideal session length, the best intensity for every diagnosis, or the amount of FMD change that produces meaningful clinical benefit.

• Both primary meta-analyses reported substantial between-study heterogeneity in sensitivity analyses, with I² values above 60%.
• Studies mixed brachial and radial artery FMD measurements, even though those vessels differ in baseline diameter, sympathetic innervation, and shear-stress profiles.
• The healthy-aging review had a 100% positive-study record, which raises the possibility that null studies were unpublished or selectively reported.
• Several trials had unclear or high risk of performance or detection bias.
• One study in the healthy-aging pool measured an acute response after a single 30-minute session in 15 menopausal women. That transient response is not equivalent to vascular adaptation after weeks of training.

Acute studies deserve their own category. A 2025 randomized crossover trial in adults with hypertension compared one HIIT session with one MICT session and found complex post-exercise endothelial responses. Such work can help explain immediate physiology, but it cannot determine the best chronic training dose.

A practical reading of the evidence

For people with HFrEF, aerobic cardiac rehabilitation has evidence for improving FMD versus usual care. When a clinician considers HIIT medically appropriate, the available meta-analytic evidence favors it over MICT for FMD. Training more than twice weekly is the clearest dose signal, although the underlying evidence does not prove that frequency alone caused the difference.

For healthy adults in midlife and later life, choose an aerobic program you can sustain for at least eight weeks at moderate or vigorous intensity. Vigorous work may deliver somewhat larger FMD gains, but the available evidence does not justify overriding safety, tolerance, medication effects, orthopedic limits, or adherence to chase that numerical edge.

Exercise has a credible case for improving this endothelial marker. The research has not earned the right to prescribe a single FMD-maximizing dose.