This review paper more or less leans towards my thoughts on metformin as a treatment to slow aging: the animal data is not great, the human data is a single study, the effect size on life span is far too small to care about, and the detrimental side effects are large in comparison to that effect size. The strategy of upregulating stress response mechanisms via drugs such as metformin is a poor strategy for long-lived species, as we clearly don't exhibit the sizable gains in life span that occur in short lived species such as mice under these circumstances. Metformin, in turn, is a low performance example of this strategy, much worse than, say, the practice of calorie restriction or mTOR inhibitors.
Metformin is sometimes proposed to be an "anti-aging" drug, based on preclinical experiments with lower-order organisms and numerous retrospective data on beneficial health outcomes for type 2 diabetics. Large prospective, placebo-controlled trials are planned, in pilot stage, or running, to find a new use (or indication) for an aging population. In 2015, Nir Barzilai met with regulators from the FDA to discuss the now famous phase III multi-site TAME (Targeting Aging with Metformin) trial. The acronym chosen and the intention behind it - namely, that aging is a "disorder" that can be treated like any other disease - was a clear provocation. The FDA's mandate is to regulate medications and devices to cure diseases or aid in their diagnosis, but aging is not (yet) an indication. Interestingly, frailty is missing from the proposed composite outcome. Other ongoing trials (e.g., NCT02570672) with metformin provide arguments that frailty may be an important endpoint. It will be interesting to compare the results with the ongoing fisetin trial (NCT03675724).
Although widely cited as evidence for the small effects of 0.1% metformin in the diet on the lifespan of older male inbred mice, earlier results obtained by researchers should be dismissed: the National Institute on Aging Interventions Testing Program could not replicate the findings regarding an extension of the lifespan with 0.1% metformin. The negative results were obtained at three different locations using genetically heterogeneous female and male mice.
The rationale for the ongoing or planned metformin trials is almost exclusively based on observations (associations) of potential benefits in a diabetic (or prediabetic) population. Its efficacy even in an at-risk cohort of aged people has not yet been proven. Metformin is associated with a higher risk of vitamin B12 and vitamin B6 deficiency, which may result in an increased risk of cognitive dysfunction. Supplementation is strongly recommended to metformin users.
Of greater concern are the results of small trials in which the effects of metformin on metabolic responses to exercise or on cardiorespiratory fitness were tested. In a placebo-controlled, double-blind, crossover trial with healthy young subjects, metformin caused a small but significant decline in maximal aerobic capacity. A double-blind, placebo-controlled landmark trial with older adults with one risk factor for type 2 diabetes investigated the effects of metformin and 12 weeks of aerobic exercise. Contrary to expectations - namely, that the effects of exercise and the drug would be additive - "metformin attenuated the increase in whole-body insulin sensitivity and abrogated the exercise-mediated increase in skeletal muscle mitochondrial respiration."