Because, Metformin makes your body think it’s working out and additional exercise is difficult.

A study just published in Aging Cell reports that metformin blunts the benefits of exercise training: Metformin inhibits mitochondrial adaptations to aerobic exercise training in older adults.

Participants, average age 62, were randomized to either metformin or placebo, and undertook aerobic exercise training for 12 weeks. This isn’t a surprise, since it’s already known one way in which metformin works is by inhibition of Complex 1 in mitochondria. (Ref) By inhibiting mitochondria, the powerhouses of the cell, less ATP is formed, which activates AMPK, an energy sensor. In turn, AMPK activates many processes that contribute to better metabolism, including lower insulin and glucose and increased fat burning.

The blunting effect of metformin on exercise was not small either. Increase in VO2max in the metformin group was only about 50% that of the placebo group. The improvement in whole-body insulin sensitivity in the metformin group was zero, compared to a significant increase in the placebo group.

Another study, “Metformin and Exercise in Type 2 Diabetes”, determined the effects of metformin on the metabolic response to sub-maximal exercise, the effect of exercise (relevant to activity patterns of type 2 diabetics) on plasma metformin concentrations, and the interaction between metformin and exercise on the response to a standardized meal. There is evidence that suggests that the benefits of exercise and metformin aren’t cumulative. In a study whose results were noted in this one, the reductions in diabetic risk were similar in a lifestyle that combined metformin and lifestyle modifications to the metformin or lifestyle alone groups. And in fact, the two adjustments may have contradictory effects on diabetes. First, metformin reduces blood glucose levels. But exercise tends to increase levels of glucagon, the hormone that deals with low blood sugar. When the two are combined, glucagon concentrations become significantly higher as the body tries to compensate for the effect of metformin. Second, by increasing the heart rate, metformin has the potential to lower some patients’ selected exercise intensity, which means it could lead to the prescription of lower exercise workloads than are commonly recommended. So, the combination of exercise and metformin, both common prescriptions for diabetics, is likely less effective at lowering the glycemic response to a meal than metformin alone.

The cancer research suggests that Metformin reduces any gains in insulin sensitivity that normally would be achieved from exercise.

According to one study, physical exercise can increase insulin sensitivity by up to 54% in insulin resistant individuals, unless of course, they are taking Metformin. Metformin abolishes any increased insulin sensitivity gained by exercise. Metformin also reduces peak aerobic capacity, reducing performance and making exercise more difficult.

Metformin reduces mitochondrial ATP production in skeletal muscle by as much as 48%. Sit with that one for a moment, a 48% reduction in cell fuel. Imagine functioning at only half capacity. This would make basic activities difficult at best and exercising to lose weight a very unlikely proposition.  Moreover, despite claims to the contrary, Metformin does not appear to be an especially effective tool for weight loss, netting a reduction of only 5-10 pounds over 4-8 months. Regular exercise and a healthy diet net on average a loss of 5-10 pounds per month for most people and are significantly more effective at reducing diabetes and associated health complications without the potential side effects.

Metformin abolishes the improvement in mitochondrial respiration after aerobic exercise training. Metformin attenuates the improvement in physiological function after aerobic exercise training. The study reasoned that the cellular energy sensor AMPK would have increased activity due to the energetic stress of metformin preventing the increase in mitochondrial respiration in response to the energetic demands of exercise training.

Since cardiorespiratory fitness is one of the strongest factors for survival into old age, and since it decreases with age, the effect of metformin on this factor is concerning.

Surprisingly, a study in rats found little evidence for additive effects of combining metformin and aerobic exercise training on outcomes of obesity, type 2 diabetes, and NAFLD and actually suggest potential impairments in exercise-induced hepatic mitochondrial adaptations.

A study showed that, for individuals undergoing intensive lifestyle therapy, background metformin therapy conferred little additional benefits.