Exercise cut fasting insulin across 58 trials in women

Physical activity is thought to lower breast cancer risk partly through the insulin and insulin-like growth factor signalling system, a set of hormones and binding proteins that also governs blood sugar control. Before that pathway can be taken seriously, the first link has to be demonstrated: does exercise actually shift insulin and IGF markers in women? Those same markers matter to anyone interested in metabolic health, independent of cancer. This review set out to quantify that first step rather than assume it.

The authors ran a systematic review searching for randomised controlled trials, Mendelian randomisation studies and prospective cohort studies that examined the effect of physical activity on insulin and IGF signalling in adult women, including IGFs, their binding proteins and markers of insulin resistance. Fifty-eight randomised trials met the inclusion criteria; no observational or Mendelian randomisation studies did. Results were combined in meta-analyses to generate pooled effect estimates. Risk of bias was assessed formally, and the GRADE system was used to rate the overall certainty of the evidence for each outcome.

Compared with control conditions, physical activity interventions reduced fasting insulin, the Homeostatic Model Assessment for Insulin Resistance and fasting glucose. Physical activity also increased IGF-1, but there was no clear effect on IGFBP-3 or on the ratio of IGF-1 to IGFBP-3. Crucially, when certainty was graded, strong evidence was established only for fasting insulin and insulin resistance: the other results rest on weaker footing. The authors note that the effect of physical activity on C-peptide and HbA1c in women still needs further research.

The findings support the biological plausibility of the first segment of the proposed physical activity to insulin/IGF signalling to breast cancer pathway, without testing the pathway's later steps. That only randomised trials qualified is a strength for causal inference but also shows how thin the observational and genetic evidence base is in this specific area. The IGF-1 increase is a reminder that the signalling system does not move in one simple direction. Whether these marker changes translate into altered disease risk remains an open question this review cannot answer.

This is solid, trial-based reason to think that becoming more physically active improves insulin-related measures in women, which is one of the better-evidenced metabolic returns on training. It is not evidence that exercise changes cancer risk through this route; that link is a hypothesis the review only partly probes. The mixed hormone picture, insulin markers improving while IGF-1 rises, is a useful corrective to the idea that one biomarker tells the whole story. For anyone tracking fasting glucose or insulin resistance, activity is among the levers with the strongest supporting evidence here.