Lifting raises IGF-1 after 60, lowers it before

Insulin-like growth factor 1 is one of the hormones most often invoked when people talk about resistance training, muscle maintenance and ageing. It circulates in the blood and can be measured cheaply, which has made it a popular proxy for whether a training programme is doing something anabolic. The trouble is that individual studies of resistance exercise and serum IGF-1 have pointed in opposite directions, with some reporting rises and others no change or falls. This meta-analysis set out to see whether pooling the literature resolves the contradiction.

The authors ran a systematic literature search up to May 2020 and identified 22 studies reporting the relationship between resistance exercise and serum IGF-1, covering 680 subjects in total. Effects were expressed as odds ratios with 95% confidence intervals, calculated using a continuous method with random or fixed-effect models. Results were broken out by subgroup: training programmes lasting less than 16 weeks versus more than 16 weeks, participants older than 60 versus younger than 60, and females versus males. No single training protocol was standard across the pooled studies.

Higher serum IGF-1 was seen with resistance training lasting less than 16 weeks (OR 4.03, 95% CI 2.49-5.57) and more than 16 weeks (OR 11.55, 95% CI 6.58-16.52). The largest subgroup effects were in subjects older than 60 (OR 11.88, 95% CI 9.84-13.93) and in males (OR 16.82, 95% CI 7.29-26.35), with a smaller but clear effect in females (OR 3.87, 95% CI 2.26-5.49). Against that pattern, subjects younger than 60 showed significantly lower IGF-1 (OR -4.80, 95% CI -7.74 to -1.86), a reversal the authors describe as surprising.

The pooled result supports the idea that resistance training shifts circulating IGF-1 in older adults, extending a literature that had been genuinely conflicting at the level of single studies. The age reversal complicates the simple story that lifting raises IGF-1 across the board, and no mechanism for it is offered here. The statistical approach — reporting odds ratios for what is a continuous blood measurement — is unusual and makes the effect sizes hard to translate into everyday units. What the analysis cannot tell you is whether any of these changes track with strength, muscle mass, function or longevity.

This is a reason to think that the hormonal response to resistance training is not uniform, and that age may shape it in ways the popular framing misses. It is not a reason to treat a blood IGF-1 reading as a scorecard for your training, since the analysis links exercise to the marker and not to any outcome you would feel. Anyone over 60 already has better reasons to do resistance training than a growth factor. Treat the size of these pooled figures with caution given the unconventional statistics.