About 600 METs-minutes a week, and walking counts

Design

Hierarchical Bayesian network meta-analysis with dose-response modelling, CINeMA certainty rating

Population

124 randomised trials, 18,429 adults aged 40 and over

Primary outcome

Change in bone mineral density at lumbar spine, femoral neck and total hip

Effect

Combined aerobic-resistance 0.013 g/cm² (95% CrI 0.010 to 0.016) at lumbar spine; benefit plateaus near 600 METs-min/week

One hundred and twenty-four randomised trials and 18,429 adults aged 40 and over were combined in a Bayesian network meta-analysis to answer which exercise, and how much, protects bone. For lumbar spine density, brisk walking or jogging and combined aerobic-resistance training came out ahead, each by about 0.013 g/cm². For the femoral neck, walking or jogging and mind-body exercise led; for total hip, walking or jogging alone.

The dose-response curve is the useful part, and it is an inverted U rather than a line: benefit accumulates to a point and then flattens or falls away. A minimal clinically important difference appeared at roughly 400 METs-minutes per week for the lumbar spine and around 600 for the femoral neck and total hip — about two to three hours of brisk walking a week.

Fractures were analysed separately in 26 trials. Mixed aerobic exercise gave an odds ratio of 0.29 and mind-body exercise 0.58; other modalities were neutral. Certainty was low to moderate throughout, which is why this is a prescription to give with a dose rather than a guarantee to make.

• Prescribe a dose: two to three hours a week of brisk walking, or the equivalent in combined aerobic and resistance work.
• Say that more is not better beyond that range; the curve turns over.
• For a patient who will not jog, mind-body exercise showed both a femoral neck and a fracture signal.
• Bone mineral density is the primary outcome here; fracture data come from a quarter of the trials.
• Pair the prescription with vitamin D and calcium status and a falls assessment, which this analysis did not address.

Why it matters

It replaces 'stay active' with a number a patient can act on and a clinician can review.

Don't overread it

Bone density is the measured outcome; the fracture reduction rests on a minority of the trials at low certainty.

The statistics, in plain English

Credible intervals from a Bayesian analysis read like confidence intervals: 0.013 g/cm² (0.010 to 0.016) for combined training is a small but consistent gain, and roughly a one to two per cent density change rather than anything dramatic. The fracture estimate for mixed aerobic exercise (OR 0.29, 0.11 to 0.78) comes from 26 trials and few events, and low certainty means the true effect may sit well away from that estimate. A non-linear dose response also means the single best dose is inferred from the curve's shape, not measured directly.

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