Does metformin block muscle growth from strength training?

The claim checked: Metformin stops strength training from building muscle Hear it at 52:55

What do the studies say?

Leaning for

In the one trial built to test it, adults 65 and over took metformin or a placebo while strength training. The metformin group gained 0.41% lean mass, against 1.95% on placebo. Two more reports from that trial and a small trial in people with prediabetes found a partial or mixed effect.

Computed from 4 studies that test this point, each judged by two readers, in full.

The study behind the claim tested something else. See what it tested

What was said on the podcast?

“So if you're taking metformin and you do strength training versus taking a placebo and do strength training after twelve weeks, the metformin group won't have built muscle and the other group will.”

Mark Hyman, The Dr. Hyman Show · I've Had 6 Back Surgeries And Almost Died: Here's What 40 Years of Medicine Actually Taught Me · 52:55 on YouTube

The study behind the claim The retelling overstates it

What did the study actually test?

On tape: strength training on metformin or placebo, twelve weeks, no muscle gain on metformin. The closest trial is MASTERS: 14 weeks of training, adults 65 and over. Gains on metformin were smaller, not absent on every measure: CT thigh muscle area still rose. Metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults: A randomized, double-blind, placebo-controlled, multicenter trial: The MASTERS trial. Aging Cell, 2019. Randomized, double-blind, placebo-controlled trial (MASTERS): 14 weeks of progressive resistance training on metformin or placebo, Healthy adults aged 65 and over.

What was saidWhat the study did

What was given Matches Taking metformin while strength training Metformin during 14 weeks of progressive resistance training

Compared with Matches Placebo plus strength training Placebo plus the same training

What was measured Matches Metformin group won't have built muscle Lean mass and thigh muscle by DXA and CT: gains blunted on metformin, and CT thigh muscle area still rose

Who took part Differs Anyone training to keep muscle Healthy adults aged 65 and over

What it found

“CT scan showed that increases in thigh muscle area (p = .005) and density (p = .020) were greater in placebo versus metformin.”

PMID 31557380

How the authors extend it

“However, these gains were blunted by metformin administration.”

PMID 31557380

The study was funded by National Institute on Aging, Grant/Award Number: R01AG046920 and National Center for Advancing Translational Sciences, Grant/Award Numbers: UL1TR001998, UL1TR003096. Two readers agreed on the label, and the paper was read in full.

The point checked Leaning for

In adults doing strength training, metformin stops muscle gain: the metformin group builds no muscle while the placebo group does

Agree 1

controlled trial · 2019 · landmark · cited on tape

“However, metformin prevented gains in lean mass with PRT (between groups p = .003); placebo gained 1.95% ( SD 2.69) lean mass ( p < .001), while the 0.41% change ( SD 2.25) in metformin did not reach significance ( p = .218) (Figure 2 b; Appendix S2 : Table 2 ).”

Metformin blunts muscle hypertrophy in response to progressive resistance exercise training in older adults: A randomized, double-blind, placebo-controlled, multicenter trial: The MASTERS trial. Aging Cell
Who: 109 healthy, nondiabetic, community-dwelling adults aged 65 and over (median age 69.3, 56% female) in Kentucky and Alabama; 94 completed
PMID 31557380

Partly agree 3

other primary study · 2022

“In contrast, statin users randomized to metformin demonstrated a significantly higher increase in fiber cross-sectional area (fCSA) compared to metformin alone (MET vs. MET + STAT, p = 0.002), with significant growth occurring in both fiber types.”

Potential Benefits of Combined Statin and Metformin Therapy on Resistance Training Response in Older Individuals. Front Physiol
Who: 94 healthy older adults aged 65 or more who completed the MASTERS trial, split by statin use: placebo only 34, statin only 14, metformin only 29, metformin plus statin 17
PMID 35492586

trial or observed group · 2012

“Metformin increased FFM in adolescent girls treated during puberty ( 23 ), but in the current study, FFM was unaffected by metformin and slightly higher after training alone.”

Independent and combined effects of exercise training and metformin on insulin sensitivity in individuals with prediabetes. Diabetes Care
Who: 32 otherwise healthy, nonsmoking men and women with impaired glucose tolerance (prediabetes) from the local community, Amherst, Massachusetts
PMID 22040838

trial or observed group · 2021

“Our primary outcomes report from the MASTERS trial showed that, compared to placebo, metformin blunts hypertrophy, strength, and normal muscle density gains following PRT [54].”

Associations of muscle lipid content with physical function and resistance training outcomes in older adults: altered responses with metformin. Geroscience
Who: 94 healthy, moderately active older adults aged >=65 (mean 70.4 y, 54% female, 98% Caucasian) completing progressive resistance training in the MASTERS trial
PMID 33462708

Disagree 0

None.