Future health gain from increasing physical activity in Australia, including multiple physiological effects of physical activity, and falls and injury risk: A simulation study

A simulation study projects that increasing physical activity across Australia would significantly improve population health, reduce premature deaths, and boost economic output, although the net health and economic benefits are maximized by helping the least active individuals meet guidelines rather than pushing the entire population to maximum activity levels due to diminishing returns and injury risks at extreme exertion.

The Gist

Imagine Australia's population as a massive, bustling garden. For years, gardeners (health researchers) have been trying to figure out how to make this garden bloom. They knew that watering the plants (getting people to move more) stopped the weeds of chronic diseases like heart disease, diabetes, and cancer from taking over.

But there was a catch. The gardeners were worried that if they watered too much, or if the plants grew too wild, the gardeners themselves might get hurt by the thorns, or the hoses might burst.

This new study is like a super-advanced weather forecast and garden simulation combined. The researchers asked: "What happens if we get the whole country moving? Does the garden get healthier, or do we just end up with more broken shovels?"

Here is the story of their findings, broken down into simple concepts:

1. The "Business as Usual" vs. The "Super-Active" Garden

The researchers ran a simulation for the next 20 years.

• The Current Garden (Business as Usual): Right now, Australians are doing some exercise. This is already doing a huge job. It's like having a sturdy fence that keeps the weeds at bay. The study found that our current level of activity is already saving 1.3 million years of healthy life and preventing nearly 30,000 deaths. It's also saving the government about $10 billion in medical bills.
• The "Super-Active" Garden (The Max Scenario): Then, they imagined a world where everyone became a marathon runner or a professional athlete (the maximum possible activity level).
  • The Good News: The garden would bloom even more! We'd gain another 653,000 healthy years of life and prevent nearly 10,000 more premature deaths. People would also earn more money because they'd be healthier and working more.
  • The Bad News (The Thorns): Here is the twist. When everyone pushes to the absolute limit, the "thorns" (injuries) start to hurt. More people are breaking bones, tearing muscles, or crashing bikes.

2. The "Sweet Spot" Analogy: The Goldilocks Zone

The study discovered a very important rule about exercise, similar to the story of Goldilocks and the Three Bears.

• Too Little (The Porridge is Cold): If you do nothing, the weeds (diseases) take over. You get sick, spend money on doctors, and lose years of life.
• Just Right (The Porridge is Just Right): If you are currently inactive and start doing enough to meet the basic guidelines (like a brisk walk or a light jog), you get the biggest bang for your buck.
  • Why? You get almost all the disease-fighting benefits, but you don't get hit with the heavy cost of serious injuries.
  • The Result: Moving sedentary people to the "just right" level saves the most money ($1.86 billion) and improves health the most efficiently.
• Too Much (The Porridge is Too Hot): If you push everyone to the "Super-Active" extreme (running 4200+ minutes a week), the benefits start to shrink.
  • Why? The cost of treating all the new injuries (broken legs, concussions) starts to eat up the money you saved by preventing heart disease. For men especially, the medical bills for injuries actually go up if everyone goes to the extreme level.

3. The "Mental Health" Bonus

Previous studies mostly looked at physical diseases (like heart attacks). This study added a new ingredient: Mental Health.
Think of physical activity as a "mental health vitamin." The study found that moving more is a massive booster for fighting depression and anxiety. This was a huge win, saving billions in mental health costs and adding a massive amount of healthy life years. It turns out, moving your body is one of the best ways to clear your mind, too.

4. The "Injury Tax"

Imagine a tax you have to pay for every hour you exercise.

• If you are a couch potato and start walking, the "tax" is tiny, but the "reward" (health) is huge.
• If you are already an athlete and decide to run a marathon every day, the "tax" (risk of injury) goes up steeply.
• The study found that for every extra bit of exercise you add, the reward gets smaller, and the injury risk gets bigger. Eventually, at the very top levels, the injury risk almost cancels out the extra health benefits.

The Big Takeaway

You don't need to be an Olympic athlete to get the best results. In fact, trying to be an Olympian might actually cost the health system more money because of injuries.

The best strategy for Australia (and for you):
Focus on getting the inactive people moving. If you get the people who are sitting on the couch to stand up and walk, the country wins big. We save money, we live longer, and we feel happier. But we don't need to force everyone to run marathons; the "sweet spot" is already within reach for most of us.

In short: Move more, but don't overdo it. The biggest health gains come from getting the lazy people active, not from turning the active people into super-athletes.

Technical Summary

1. Problem Statement

Previous simulation studies regarding physical activity (PA) in Australia and globally have been limited in scope. They typically account only for the prevention of major chronic diseases such as cardiovascular disease, type 2 diabetes, dementia, and specific cancers. These models often neglect:

• Mental health benefits: The causal link between PA and reduced depression/anxiety.
• Intermediate risk factors: The impact of PA on blood pressure, fasting plasma glucose, LDL-cholesterol, and bone mineral density, which subsequently affect other diseases.
• Injury risks: The negative health and economic consequences of injuries sustained during physical activity participation (e.g., fractures, cycling accidents).

There is a lack of comprehensive modeling that balances the "net" health gains of increasing PA against the offsetting risks of injury, particularly at high levels of activity. This study aims to fill that gap by estimating the long-term health and economic impacts of various PA scenarios in Australia, explicitly modeling the trade-off between disease prevention and injury incidence.

2. Methodology

The study utilized a Proportional Multistate Lifetable (PMSLT) model adapted for the Australian population, simulating a 20-year horizon (2021–2041) with 40-year projections for trend analysis.

Key Model Components:

• Population: An open cohort of the Australian population (based on 2021 ABS data) with annual births and net migration.
• Disease Pathways: The model expanded beyond standard conditions to include:
  • Directly linked conditions (CHD, stroke, diabetes, dementia, bowel/breast/uterine cancers).
  • Mental health conditions (depression and anxiety).
  • Conditions mediated by intermediate risk factors (high BP, high glucose, high LDL, low bone mineral density).
• Injury Modeling: A novel component where injury incidence was modeled as a linear function of total physical activity volume (MET-min/week). Injury rates were derived from hospitalization data (AIHW) and distributed across activity levels.
• Scenarios Modeled:
  1. Maximal Activity: 100% of the population reaches 4200+ MET-min/week.
  2. Meet Guidelines: Sedentary individuals (0–600 MET-min/week) shift to the minimum guideline level (600–1200 MET-min/week).
  3. Revert to 2018: Population shifts back to 2018 PA levels.
  4. Everyone Inactive: All individuals set to 0–600 MET-min/week (counterfactual to measure current benefits).
  5. Threshold Analysis: Iterative shifting of groups to the next highest activity level to identify diminishing returns.
• Sensitivity Analyses: Tested variations including discount rates (3%), exclusion of injury risks, exclusion of mental health/intermediate pathways, and alternative injury distribution methods (based on time vs. METs).
• Data Sources: Global Burden of Disease (GBD) 2021, Australian Burden of Disease Study (ABDS) 2024, AIHW cost data, and UK Biobank data for risk factor distributions.

3. Key Contributions

• Comprehensive Pathway Modeling: This is the first study to simultaneously model the full spectrum of PA benefits (including mental health and intermediate risk factors) alongside the specific risks of participation injuries.
• Net Health Gain Calculation: It provides a "net" estimate of Health Adjusted Life Years (HALYs) and economic impact, explicitly quantifying how injury risks erode the benefits of high-intensity PA.
• Identification of Diminishing Returns: The study identifies a specific threshold (4200+ MET-min/week) where the marginal health costs of injuries begin to outweigh the marginal benefits of disease prevention.
• Policy-Relevant Scenarios: It compares extreme theoretical scenarios (maximal activity) with pragmatic policy targets (meeting guidelines), offering evidence that targeting the sedentary population yields better economic returns than pushing the entire population to maximal levels.

4. Key Results

Health Outcomes (20-Year Horizon, Undiscounted):

• Current Status: The existing level of PA in Australia (BAU) currently contributes to 1.3 million HALYs, prevents 28,700 deaths (all ages), and saves AUD$9.8 billion in health spending compared to a scenario where everyone is inactive.
• Maximal Activity Scenario (4200+ MET-min/week):
  • HALYs: Gain of 653,000 (0.16% increase).
  • Deaths: Reduction of 9,720 premature deaths (before age 75).
  • Economic Impact:
    • Income: Increase of AUD$16.8 billion.
    • Health Spending: Net decrease of AUD$748 million. However, this masks a significant divergence: spending decreases for females (-$2.06B) but increases for males (+$1.33B) due to higher injury costs.
• Meeting Guidelines (Sedentary to Active):
  • Shifting sedentary individuals to meet guidelines reduces health spending by AUD$1.86 billion. This is a more efficient economic outcome than the "Maximal Activity" scenario, which incurs high injury costs.
• The Injury Trade-off:
  • In the "Maximal Activity" scenario, injuries increase health spending by AUD$16.9 billion, which offsets the AUD$17.3 billion saved from avoided diseases.
  • Threshold Effect: For every additional 600 MET-min/week, net health gains diminish. Above 4,200 MET-min/week, the health costs from injuries outweigh the disease reduction benefits, resulting in a net negative or negligible gain in HALYs per person.
• Mental Health Impact: Mental health improvements were the largest driver of HALY gains and cost savings, contributing significantly more than cardiovascular disease prevention.

5. Significance and Implications

• Policy Optimization: The findings suggest that public health interventions should prioritize moving sedentary individuals to meet minimum guidelines rather than pushing the entire population to maximal activity levels. The latter incurs disproportionate injury costs that erode economic and health benefits.
• Injury Prevention is Critical: As PA levels rise, injury prevention strategies become as important as promoting activity itself. The study highlights that without injury mitigation, the economic case for extreme PA levels weakens.
• Holistic Health Economics: By including mental health and intermediate risk factors, the study demonstrates that the economic value of PA is significantly higher than previously estimated in standard models (which often exclude these factors).
• Future Research Directions: The authors call for more granular data on injury risks specific to activity types (e.g., cycling vs. swimming) and intensities to refine models. They also note that the model currently excludes secondary prevention benefits (e.g., reduced severity in existing disease), suggesting actual benefits may be even higher.

In conclusion, increasing physical activity in Australia yields substantial health and economic gains, but these gains are non-linear. The "sweet spot" for policy lies in eliminating inactivity and meeting guidelines, while pushing beyond this threshold requires careful management of injury risks to maintain net positive outcomes.