Metformin may blunt exercise gains

Metformin is the standard first drug for high blood sugar. Exercise is the standard lifestyle prescription. So the obvious assumption has been simple — do both, get more benefit. But that neat equation is looking shakier. A Rutgers-led trial, now published in the April 2026 issue of *The Journal of Clinical Endocrinology & Metabolism*, adds to a growing pile of evidence that metformin can blunt some of the very adaptations exercise is supposed to build. ### What actually changed? The new piece here is not “metformin has side effects.” It is more specific than that. Researchers tested whether metformin changes how blood vessels respond to insulin after training — basically, whether exercise still helps the vascular system open up and deliver blood, oxygen, and nutrients the way it should. The answer was yes for exercise alone, but less so when metformin was added. (academic.oup.com) ### Who was studied? This was a double-blind, placebo-controlled trial in adults at risk for metabolic syndrome. Participants were split into four groups for 16 weeks: low-intensity exercise plus placebo, low-intensity exercise plus metformin, high-intensity exercise plus placebo, and high-intensity exercise plus metformin. The metformin dose was 2,000 mg per day. That matters because this was not a vague observational signal — it was a controlled head-to-head test. (academic.oup.com) ### What got blunted? A few things. The placebo groups improved aerobic fitness, measured by VO2 max. The metformin groups did not show the same gain. Exercise alone also improved insulin-stimulated blood vessel function and microvascular blood flow, while metformin dampened those changes. Researchers also saw weaker improvements in fasting glucose and inflammatory markers like endothelin-1 and TNF-α when metformin was in the mix. (academic.oup.com) ### Why do blood vessels matter here? Because insulin does more than move glucose into cells. It also helps blood vessels dilate, which increases delivery of glucose and nutrients to muscle after meals. Think of it like opening more lanes on the highway right when traffic hits. If exercise improves that response, blood sugar control gets easier. If metformin mutes that adaptation, some of exercise’s payoff may never fully arrive. (academic.oup.com) ### Is this a brand-new idea? Not really. This study builds on earlier work showing metformin can interfere with exercise-related gains in insulin sensitivity and cardiorespiratory fitness, especially in older adults. A 2025 physiology paper also tied metformin to suppressed mitochondrial and transcriptional responses to exercise, which gives the field a plausible mechanism instead of just a weird clinical pattern. (rutgers.edu) ### Does this mean metformin is bad? No — and that is the part people can easily overread. Metformin still lowers blood sugar and remains a first-line drug for type 2 diabetes. The issue is narrower: when you pair it with exercise, the combination may not be as additive as clinicians long assumed. “Helpful drug” and “may blunt training adaptations” can both be true at once. (onlinelibrary.wiley.com) ### Should people stop taking it before workouts? That would be way too big a leap from the evidence. This research raises questions about timing, patient selection, and what outcome you care about most, but it does not establish a simple rule like “pause metformin on training days.” It also does not mean exercise stops working. Even in this literature, the concern is reduced benefit, not zero benefit. (academic.oup.com) ### So what is the real takeaway? Basically, the old one-plus-one story looks too tidy. For people using metformin, exercise is still worth doing — very much so. But clinicians may need to set expectations differently and think harder about when the drug helps most, when it may trade off against training adaptations, and which patients actually need both at the same time. (rutgers.edu) (academic.oup.com)