Abstract
Background Understanding the health consequences associated with exposure to risk factors is necessary to inform public health policy and practice. To systematically quantify the contributions of risk factor exposures to specific health outcomes, the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021 aims to provide comprehensive estimates of exposure levels, relative health risks, and attributable burden of disease for 88 risk factors in 204 countries and territories and 811 subnational locations, from 1990 to 2021. Methods The GBD 2021 risk factor analysis used data from 54561 total distinct sources to produce epidemiological estimates for 88 risk factors and their associated health outcomes for a total of 631 risk-outcome pairs. Pairs were included on the basis of data-driven determination of a risk-outcome association. Age-sex-location-year-specific estimates were generated at global, regional, and national levels. Our approach followed the comparative risk assessment framework predicated on a causal web of hierarchically organised, potentially combinative, modifiable risks. Relative risks (RRs) of a given outcome occurring as a function of risk factor exposure were estimated separately for each risk-outcome pair, and summary exposure values (SEVs), representing risk-weighted exposure prevalence, and theoretical minimum risk exposure levels (TMRELs) were estimated for each risk factor. These estimates were used to calculate the population attributable fraction (PAF; ie, the proportional change in health risk that would occur if exposure to a risk factor were reduced to the TMREL). The product of PAFs and disease burden associated with a given outcome, measured in disability-adjusted life-years (DALYs), yielded measures of attributable burden (ie, the proportion of total disease burden attributable to a particular risk factor or combination of risk factors). Adjustments for mediation were applied to account for relationships involving risk factors that act indirectly on outcomes via intermediate risks. Attributable burden estimates were stratified by Socio-demographic Index (SDI) quintile and presented as counts, age-standardised rates, and rankings. To complement estimates of RR and attributable burden, newly developed burden of proof risk function (BPRF) methods were applied to yield supplementary, conservative interpretations of risk-outcome associations based on the consistency of underlying evidence, accounting for unexplained heterogeneity between input data from different studies. Estimates reported represent the mean value across 500 draws from the estimate’s distribution, with 95% uncertainty intervals (UIs) calculated as the 25th and 97.5th percentile values across the draws. Findings Among the specific risk factors analysed for this study, particulate matter air pollution was the leading contributor to the global disease burden in 2021, contributing 8.0% (95% UI 6.7–9.4) of total DALYs, followed by high systolic blood pressure (SBP; 7.8% [6·4–9·2]), smoking (5.7% [4.7–6.8]), low birthweight and short gestation (5.6% [4.8–6.3]), and high fasting plasma glucose (FPG; 5.4% [4.8–6.0]). For younger demographics (ie, those aged 0–4 years and 5–14 years), risks such as low birthweight and short gestation and unsafe water, sanitation, and handwashing (WaSH) were among the leading risk factors, while for older age groups, metabolic risks such as high SBP, high body-mass index (BMI), high FPG, and high LDL cholesterol had a greater impact. From 2000 to 2021, there was an observable shift in global health challenges, marked by a decline in the number of all-age DALYs broadly attributable to behavioural risks (decrease of 20.7% [13.9–27.7]) and environmental and occupational risks (decrease of 22.0% [15.5–28.8]), coupled with a 49.4% (42.3–56.9) increase in DALYs attributable to metabolic risks, all reflecting ageing populations and changing lifestyles on a global scale. Age-standardised global DALY rates attributable to high BMI and high FPG rose considerably (15.7% [9.9–21.7] for high BMI and 7.9% [3.3–12.9] for high FPG) over this period, with exposure to these risks increasing annually at rates of 1.8% (1.6–1.9) for high BMI and 1.3% (1.1–1.5) for high FPG. By contrast, the global risk-attributable burden and exposure to many other risk factors declined, notably for risks such as child growth failure and unsafe water source, with age-standardised attributable DALYs decreasing by 71.5% (64.4–78.8) for child growth failure and 66.3% (60.2–72.0) for unsafe water source. We separated risk factors into three groups according to trajectory over time: those with a decreasing attributable burden, due largely to declining risk exposure (eg, diet high in trans-fat and household air pollution) but also to proportionally smaller child and youth populations (eg, child and maternal malnutrition); those for which the burden increased moderately in spite of declining risk exposure, due largely to population ageing (eg, smoking); and those for which the burden increased considerably due to both increasing risk exposure and population ageing (eg, ambient particulate matter air pollution, high BMI, high FPG, and high SBP). Interpretation Substantial progress has been made in reducing the global disease burden attributable to a range of risk factors, particularly those related to maternal and child health, WaSH, and household air pollution. Maintaining efforts to minimise the impact of these risk factors, especially in low SDI locations, is necessary to sustain progress. Successes in moderating the smoking-related burden by reducing risk exposure highlight the need to advance policies that reduce exposure to other leading risk factors such as ambient particulate matter air pollution and high SBP. Troubling increases in high FPG, high BMI, and other risk factors related to obesity and metabolic syndrome indicate an urgent need to identify and implement interventions.
Generated Summary
This systematic analysis for the Global Burden of Disease (GBD) Study 2021 provides comprehensive estimates of risk factor exposure, relative health risks, and attributable burden of disease across 88 risk factors in 204 countries and territories and 811 subnational locations from 1990 to 2021. The study employed a comparative risk assessment framework, relying on data from 54,561 distinct sources to produce epidemiological estimates. The research focused on the impact of risk factor exposures on specific health outcomes. It used a causal web of hierarchically organized, potentially combinative, modifiable risks. Relative risks (RRs) for each risk-outcome pair were estimated separately. Summary exposure values (SEVs), representing risk-weighted exposure prevalence, and theoretical minimum risk exposure levels (TMRELs) were estimated for each risk factor, used to calculate the population attributable fraction (PAF). The GBD 2021 analysis followed a comparative risk assessment framework predicated on a causal web of hierarchically organized, potentially combinative, modifiable risks. Relative risks (RRs) of a given outcome occurring as a function of risk factor exposure were estimated separately for each risk-outcome pair. Summary exposure values (SEVs), representing risk-weighted exposure prevalence, and theoretical minimum risk exposure levels (TMRELs) were estimated for each risk factor. The product of PAFs and disease burden associated with a given outcome, measured in disability-adjusted life-years (DALYs), yielded measures of attributable burden. Furthermore, the study incorporated newly developed burden of proof risk function (BPRF) methods to yield supplementary, conservative interpretations of risk-outcome associations based on the consistency of underlying evidence.
Key Findings & Statistics
- Particulate matter air pollution was the leading contributor to the global disease burden in 2021, contributing to 8.0% (95% UI 6.7-9.4) of total DALYs.
- High systolic blood pressure (SBP; 7.8% [6·4–9·2]).
- Smoking (5.7% [4.7–6.8]).
- Low birthweight and short gestation (5.6% [4.8-6.3]).
- High fasting plasma glucose (FPG; 5.4% [4.8-6.0]).
- From 2000 to 2021, there was a decline in the number of all-age DALYs attributable to behavioural risks (decrease of 20.7% [13.9-27.7]) and environmental and occupational risks (decrease of 22.0% [15.5-28.8]).
- There was a 49.4% (42.3-56.9) increase in DALYs attributable to metabolic risks.
- Age-standardised global DALY rates attributable to high BMI and high FPG rose considerably (15.7% [9.9–21.7] for high BMI and 7.9% [3.3–12.9] for high FPG) over this period.
- By contrast, the global risk-attributable burden and exposure to many other risk factors declined, notably for risks such as child growth failure and unsafe water source, with age-standardised attributable DALYs decreasing by 71.5% (64.4-78.8) for child growth failure and 66.3% (60.2-72.0) for unsafe water source.
- For the 25 leading Level 3 risk factors in 2021, 13 (ie, more than half) were behavioural risks.
- The burden attributable to high BMI increased in the low-middle, middle, and high SDI quintiles, with the highest rates of increase (ARC increase of >1.2%) in the low-middle and middle SDI quintiles.
- The burden attributable to high FPG increased across almost all SDI strata, with the highest rates of increase (ARC increase of 0.7-1.2%) in the low-middle SDI quintile.
- The burden attributable to high SBP and high LDL cholesterol decreased over time across SDI strata, and rates of decline were highest in the high-middle and high SDI quintiles (ARC decreases of 2.2-3.2%).
- From 2000 to 2021, reductions in age-standardised DALY rates attributable to child and maternal malnutrition were broadly evident in Latin America and the Caribbean, central Asia, and parts of east Asia and southeast Asia.
- In 2021, 1190 million (95% UI 1090–1330) global DALYs (41.4% of 2880 million DALYs) were attributable to all GBD 2021 risk factors combined.
Other Important Findings
- The study found that particulate matter air pollution was the leading contributor to global disease burden in 2021.
- From 2000 to 2021, there was an observable shift in global health challenges marked by a decline in the number of all-age DALYs broadly attributable to behavioural risks and environmental and occupational risks.
- There was a 49.4% increase in DALYs attributable to metabolic risks.
- For the 25 leading Level 3 risk factors in 2021, 13 were behavioral risks, five were metabolic risks and seven were environmental or occupational risk factors.
- At the global level, the attributable burden was highest for behavioral risks followed by metabolic risks then environmental and occupational risks.
- For most metabolic risks, exposure increased between 2000 and 2021.
- In 2021, 12 risk-outcome pairs were identified as having very strong (five-star) relationships.
Limitations Noted in the Document
- The GBD 2021 risk factor findings are limited by several considerations, including the omission of various potentially consequential risk factors and covariates, most notably the impact of the COVID-19 pandemic.
- The study is limited by the inconsistent availability and variable quality of data to estimate relative risks (RRs) and risk exposure, with considerable disparities in risk exposure across socioeconomic factors that are further exacerbated in areas affected by violence and conflict.
- The study assumes that RRs as a function of exposure are universal; however, the analysis did not make this assumption for all risk functions.
- The study’s assumption that RRs as a function of exposure are universal, and that RR functions apply to all locations and time periods, may not be accurate, particularly in cases with high between-study heterogeneity.
Conclusion
The GBD 2021 study identified that particulate matter air pollution, high SBP, smoking, low birthweight and short gestation, high FPG, and high BMI were the largest contributors to the disease burden in 2021. These findings suggest that focusing on these risk factors could reduce the overall disease burden. Sustained efforts are needed to minimize the impact of these risk factors, especially in low SDI locations. The observed increase in metabolic risks highlights the urgent need to identify and implement interventions related to obesity and metabolic syndrome. Moreover, the BPRF analysis provides a useful way to assess the evidence of risk and the strength of a risk-outcome association, which can be used to develop actions. The study also identifies a group of specific risk factors for which there is consistent evidence of strong risk-outcome relationships, highlighting the need for accelerated policy action. The decomposition analysis revealed that although global SEVs for household air pollution decreased substantially, the burden increased for ambient particulate matter pollution, emphasizing the need for interventions on both fronts. Efforts should focus on exposure reduction and should be adapted to the population and location. Maintaining and expanding efforts to minimize the impact of leading risk factors, particularly those related to maternal and child health and unsafe water, sanitation, and handwashing, is necessary to sustain progress. Although the study focused on the overall contribution of a given risk factor, it also examined trends over time in risk-attributable burden. The analysis revealed decreases in risk exposure and burden for many risk factors. Although the study indicates that the risk attributable to many risk factors has decreased, it also highlights that there have been increases for numerous other risk factors. The study concludes that these findings suggest areas of concern and interaction between risk factor exposure and demographics. The study uses BPRF methods to identify risk factors that substantially contribute to the overall burden, and whose associated risk-outcome relationships are supported by compelling evidence to provide policy makers with a more informed approach to risk mitigation. Such risks include particulate matter air pollution, high SBP, smoking, high LDL cholesterol, high FPG, high alcohol use, and child growth failure, for which the evidence is strong. As such, the findings underscore the need for sustained efforts to mitigate the impact of these factors through evidence-based policies and interventions.