Metformin is one of the most widely used medications in the world for type 2 diabetes. Doctors have relied on it for decades because it lowers blood sugar, is affordable, and is generally safe. Some people also believe metformin has anti-aging properties and may offer protection against cancer or cognitive decline, with some data support. But new research from Rutgers University suggests that metformin might come with an unexpected downside. It may reduce some of the most important benefits people gain from regular exercise.
What Metformin Is Used For
About one in nine adults worldwide has diabetes, and the vast majority have type 2 diabetes. Treatment focuses on healthy lifestyle habits and medications that improve insulin sensitivity and blood sugar control. Metformin has been the first-line treatment since the FDA approved it in 1994. It improves the way the body handles glucose and reduces the risk of complications such as heart disease, kidney damage, and nerve problems.
Because exercise also boosts insulin sensitivity and helps control blood sugar, doctors commonly recommend both together. The assumption has always been that the benefits would add up. But that assumption is now under serious review.
The new findings come from a study led by Steven Malin, PhD, a professor in the Department of Kinesiology and Health at Rutgers. His team published the research in The Journal of Clinical Endocrinology and Metabolism. According to Malin, the medical community has long believed that taking metformin while exercising would give people extra protection. Instead, he said, “most evidence shows metformin blunts exercise benefits.”
The researchers enrolled 72 adults who were at high risk for metabolic syndrome. These participants had traits like high blood pressure, high blood sugar, or excess abdominal fat, all of which increase the odds of developing type 2 diabetes.
They were divided into four groups:
- High intensity exercise with placebo
- High intensity exercise with metformin
- Low intensity exercise with placebo
- Low intensity exercise with metformin
Participants trained for 16 weeks. Throughout the study, scientists measured vascular insulin sensitivity, which is the ability of blood vessels to respond to insulin and widen to deliver oxygen, nutrients, and glucose to tissues after eating.
The Key Findings
Exercise alone improved vascular insulin sensitivity, which means blood vessels became better at helping the body move glucose into muscles for energy. But when participants took metformin, these improvements were noticeably reduced.
Malin explained why this matters. Improved blood vessel function helps deliver glucose to tissues. If this process is weakened, muscles cannot use sugar as effectively, especially after meals when insulin rises. Poor blood vessel function is tied to high blood sugar, hypertension, and increased risk of heart disease.
Other results were just as concerning. Participants taking metformin gained less aerobic fitness, had weaker improvements in fasting glucose, and showed fewer reductions in inflammation. Malin noted that these changes affect daily life. Lower fitness is linked to frailty, decreased mobility, and greater risk of heart disease and early mortality.
He also pointed out that people taking metformin often report that exercise feels harder, something his team is currently analyzing in more detail.
Why Metformin May Interfere With Exercise
The exact reason for this interference is still being studied. Malin said metformin works partly by affecting mitochondrial processes. While this helps reduce oxidative stress and regulate blood sugar, it may also block the cellular changes that exercise normally triggers, such as better mitochondrial efficiency and improved aerobic performance.
This study was one of the first to look closely at how metformin affects blood vessels in both major arteries and small capillaries during different levels of training. The results suggest that the interaction between medication and exercise is more complicated than expected.
David Cutler, MD, a family medicine physician in California who was not involved in the study, urged caution in interpreting the results. He said that although metformin may reduce vascular insulin sensitivity, it still delivers major benefits by lowering blood sugar and reducing the risk of serious diabetic complications.
Cutler explained that the slight drop in exercise gains must be weighed against metformin’s strong track record of preventing heart attack, stroke, kidney failure, and other dangers of diabetes. He said the finding should not alarm people who depend on metformin, but it may be important for people taking the drug for unproven reasons like anti-aging.
He added that more studies are needed in non-diabetics to determine whether the tradeoff is worth it.
The Rutgers team emphasized that no one should stop taking metformin or give up exercising. Instead, the study shows that doctors need to pay closer attention to how medications interact with lifestyle changes. For the nearly 35 million Americans with type 2 diabetes, long-term treatment strategies depend on both medication and exercise working together.
Malin said researchers need to figure out how to recommend exercise while ensuring metformin continues to help rather than hinder. He also noted that other medications may interact with exercise in similar ways, meaning clinical guidelines may need updating.
A Tiny Crack in the Reputation of a Useful Medication
Metformin remains a cornerstone of diabetes treatment. But these findings highlight a potential blind spot. A drug known for lowering blood sugar may also weaken the body’s ability to benefit from the most natural diabetes therapy of all: physical activity. As more people experiment with metformin for anti-aging or disease prevention, the question becomes even more important.
For now, the message is simple. Metformin is valuable and often life-saving, but it is not perfect. And people who take it may need closer monitoring to make sure they get the full benefits of the exercise that doctors recommend.
