A recent study published in The Lancet estimates relative health risks, exposure levels, and burden of disease as part of the Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2021.

Study: Global burden and strength of evidence for 88 risk factors in 204 countries and 811 subnational locations, 1990–2021: a systematic analysis for the Global Burden of Disease Study 2021. Image Credit: Chinnapong / Shutterstock.com

The coronavirus disease 2019 (COVID-19) pandemic highlighted significant health disparities at the individual and geographic levels due to multiple risk factors and differences in healthcare systems worldwide.

Carefully conducted risk factor meta-analyses can inform public policies on emerging or persisting health challenges, as well as identify notable areas of public health progress. To produce data for these analyses, the GBD estimates relative health risk by exposure, exposure to risk factors, and attributable disease burden for multiple risk factors.

Several other research networks, such as the NCD Risk Factor Collaboration (NCD-RisC), have provided valuable population- and multi-country-level data on specific risk factors. However, only the GBD conducts systematic analyses of several risk factors in 204 countries and territories worldwide. For example, between 1990 and 2021, the GBD analyzed 88 risk factors in 204 countries and 811 subnational locations. 

The current study summarizes the methods adopted in GBD 2021. It presents estimates of risk factor exposures for 88 risk factors and their combinations and the relationships of these risk factors with health outcomes. Data were obtained from 54,561 distinct sources to produce epidemiological estimates, with estimates obtained for 631 risk-outcome pairs.

The risk-outcome association was data-driven, and estimates specific to sex, age, location, and year were calculated at the regional, national, and global levels. For a given risk factor, relative risks (RRs) of a specific outcome were estimated. 

Summary exposure values (SEVs) measured the risk-weighted exposure prevalence. Additionally, for each risk factor, theoretical minimum risk exposure levels (TMRELs) were used to calculate the population attributable fraction (PAF). The attributable burden of a risk factor was the product of the PAF and disease burden expressed in units of disability-adjusted life years (DALYs).

Particulate matter air pollution was identified as the main contributor to the global disease burden in 2021, as it contributed 8% to the total DALY. The following key contributor was high systolic blood pressure (SBP), which accounted for 7.8% of total DALYs. Smoking, low birth weight, short gestation, and high fasting plasma glucose (FPG) contributed 5.7%, 5.6%, and 5.4% to total DALYs, respectively.

Unsafe water, low birth weight, short gestation, handwashing, and sanitation were leading risk factors for individuals between zero and four and those between five and 14. For older cohorts, high body-mass index (BMI), FPG, SBP, and low-density lipoprotein (LDL) cholesterol levels were key risk factors. 

A notable shift in global health challenges was observed between 2000 and 2021. During this period, all-age DALYs declined, which could be attributed to a 20.7% reduction in behavioral risks and a 22% reduction in environmental and occupational risks. This was accompanied by an almost 50% increase in DALYs driven by high metabolic risks.

Global DALY rates, standardized by age, rose during this period, possibly due to high BMI and FPG at 15.7% and 7.9%, respectively. Comparatively, exposure to many other risk factors and risk-attributable burden declined, even for notable risks such as unsafe water and child growth failure. More specifically, age-standardized attributable DALYs declined by 66.3% for unsafe water and 71.5% for child growth failure.

The current study identified several risk factors for which sufficient actions have not been taken. Linking disease burden to risk factors is essential, as it can inform prioritization with limited resources.

A key limitation of the GBD 2021 was the omission of several potentially important risk factors. For example, the significant impact of the COVID-19 pandemic was not formally integrated or quantified. 

Another notable limitation of the study was variable data quality and inconsistent availability. Data unavailability made it difficult to estimate RRs, given the prevalence of significant heterogeneities across a range of socioeconomic factors.

In the future, the GBD should expand the scope of risk factors, particularly for outcomes that significantly contribute to disease burden, such as mental health disorders and musculoskeletal disorders. Mental health disorders represent 5.4% of global DALYs; however, only 8% of mental disorders have been attributed to risk factors. Likewise, musculoskeletal disorders account for 5.6% of the global burden; however, in the current GBD, only 20.5% of this burden was linked to risk factors.