Disability-adjusted life years, GBD Learning

Disability-adjusted life years, or DALYs, are an indictor for healthy years of life lost and an important health metric. This Global Burden of Disease (GBD) Learning video drills down into key findings, data, and results from the recent GBD 2023 capstone, "Burden of 375 diseases and injuries, risk-attributable burden of 88 risk factors, and healthy life expectancy in 204 countries and territories, including 660 subnational locations, 1990–2023: a systematic analysis for the Global Burden of Disease Study 2023."

Video transcript

This transcript has been lightly edited for clarity

What does the health of our world look like today? How far have we come in improving global health, and which obstacles continue to hold us back?

These questions are at the heart of the Global Burden of Disease study (GBD).

Today we’re going to take a look at the latest report from GBD 2023, a systematic analysis of 375 diseases and injuries, 88 risk factors, and healthy life expectancy across 204 countries and territories from 1990 to 2023.

As with every GBD, in GBD 2023, we reanalyzed the entire time series from 1990 to 2023. We also added five new conditions: ulcerative colitis; Crohn’s disease; thyroid diseases; other endocrine, metabolic, blood, and immune disorders; and electrocution.

We used 310,000 data sources, adding over 35,000 new data sources since GBD 2021 to make the estimates more accurate and comprehensive.

To begin, we first identify and extract data. We collect all available data on diseases and injuries from diverse sources, including population surveys, hospital records, disease registries, police records, and scientific literature.

We then process all this data to make it comparable. We adjust the data for known biases, so that observations can be compared accurately. Data reported for broad age groups or for both sexes are split into more detailed age-sex categories using estimated age patterns and sex ratios.

RegMod, a new regression modeling package, adjusts clinical data to account for readmissions, non-primary diagnoses, and outpatient care. We then scale those estimates to reflect population data.

The adjusted data are then fed into the modeling tool DisMod, which integrates information on incidence, prevalence, remission, and duration of disease to produce consistent estimates. The latest version of DisMod improves the way we capture how disease patterns change over time, including cohort effects and differences by age, sex, and location.

We then adjust for the severity of each condition and the number of conditions individuals suffer from simultaneously.

This process yields the prevalence of each cause by age, sex, year, and location. YLDs, YLLs, and finally DALYs are then calculated, along with uncertainty intervals for each estimate.

Disability-adjusted life years, or DALYs, measure the total years of healthy life lost due to disability and premature death in a population, by combining years of life lost, YLLs, and years lived with disability, YLDs.

HALE, or healthy life expectancy, measures the number of years that a population at a given age can expect to spend in good health.

We also make estimates of disease burden attributable to risk factors.

For GBD 2023, we used 59,000 data sources, adding about 16,000 new data sources and 50 new risk-outcome pairs to our risk factor analysis.

To estimate disease burden from risk factors, we need data on exposure and relative risk, as well as two custom modeling tools: ST-GPR and MR-BRT. For each risk factor, we define the “ideal” level of exposure and measure exposures above that ideal. Our resulting estimates capture how much of a cause’s burden can be explained by exposure to each risk factor.

Here’s how it works:

The first step is extracting data on relative risk to figure out how much each risk factor increases the chance of disease or death. We also gather data from surveys and scientific literature to capture how many people are exposed to each risk factor.

Next, for each risk factor, the GBD defines the “ideal” level of exposure – the level that would minimize health risks. For example, with tobacco use, the ideal level is zero exposure – that is, not smoking at all or being exposed to any form of tobacco smoke. We can then estimate how much disability and death could be avoided if everyone in the population were at the ideal level of exposure for each risk factor.

Because some risks are linked – for example, obesity can lead to high blood pressure, which can cause heart disease – the estimation process also adjusts for these overlapping effects to avoid double-counting.

Finally, using all this information, the study estimates how many deaths and DALYs are due to each risk factor, again with uncertainty intervals for each estimate.

So, what are the key results from this study?

Let’s start by looking at DALYs.

Between 2010 and 2023, the total number of global DALYs stayed steady, with about 2.8 billion DALYs in 2023. As you can see in this figure, ischemic heart disease, neonatal disorders, and stroke were the leading causes of DALYs globally in both years.

While the top four causes of DALYs remained the same between 2010 and 2023, there were still some significant shifts. As you can see, infectious diseases drop out of the top 10 and are replaced by more noncommunicable diseases. HIV/AIDS, malaria, and tuberculosis are no longer in the top 10. Diabetes, COPD [chronic obstructive pulmonary disease], and low back pain have replaced them.

This is part of a bigger trend: as fewer people die from infections and malnutrition, more people live longer with chronic diseases like heart disease and diabetes.

The age-standardized DALY rate fell by 12.6%, meaning people today experience less death and disability than they did a decade ago.

Overall burden from communicable, maternal, neonatal, and nutritional diseases (CMNNs), measured by age-standardized DALY rates, dropped by 25.8% between 2010 and 2023. As you can see in this figure, that is a continuation of significant improvements in CMNNs since 1990.

Since 2010, we’ve seen more than 40% reductions in age-standardized DALY rates for diarrheal diseases, HIV/AIDS, and tuberculosis (TB), due to improvements in water and sanitation, improved antiretroviral therapy, and improved detection efforts. However, maternal and neonatal disorders, as well as respiratory infections and TB, remain the two biggest causes of death.

While the burden from these diseases is declining, demands on the global health care system are shifting dramatically to noncommunicable diseases, or NCDs.

NCDs accounted for about two-thirds of all DALYs in 2023. Although age-standardized DALY rates for NCDs decreased slightly, the number of DALYs increased slightly between 2010 and 2023, due to growing and aging populations.

As you can see in this figure, the most significant contributors among NCDs in 2023 were ischemic heart disease, stroke, and diabetes. We also saw a sharp rise in mental health disorders since 2010, with increases in anxiety disorders and depressive disorders. Diabetes rates also increased.

Now let’s explore our estimates for risk factors.

From 2010 to 2023, the total number of DALYs attributable to metabolic risks increased by over 30%. Looking at the leading risk factors by super-region, we can see how they differ in different parts of the world. In many of the super-regions, metabolic risk factors cause the most health loss – high blood pressure, smoking, and high fasting plasma glucose are the leading risk factors in five of the seven super-regions. In two super-regions, South Asia and sub-Saharan Africa, particulate matter air pollution, an environmental risk factor, is the leading risk factor for health loss. The burden attributable to metabolic risk factors underscores the need to increase efforts to promote food security, healthier diets, physical activity, and expanded access to heart and diabetes medications, especially as populations age.

Take a look at this treemap. The darker colors indicate the fraction of each cause that is attributable to any risk factor. When you look at global DALYs attributable to risk factors, you can see that CVD [cardiovascular disease], diabetes, chronic respiratory diseases, and maternal and neonatal disorders are strongly linked to risk factors, which means that changes in some risk factors could have a big impact on health loss from these diseases.

Between 2010 and 2023, HALE rose from 61.1 years to 63.1 years. That means, on average, people around the world are living about two more years in good health than they were a little more than a decade ago. At the same time, there are massive variations between and within countries, with people in some locations living almost 30 years longer than others.

While the total number of DALYs worldwide has remained stable since 2010, age-standardized DALY rates have declined. Communicable diseases have seen dramatic drops, while NCDs are increasingly driving disease burden as populations age and grow. Notably, nearly half of all DALYs are linked to risk factors. The study also highlights stark regional differences in healthy life expectancy. Overall, people around the world are living longer and healthier lives than they were in 2010, but the shifting landscape in global health means we must work to maintain these gains in the years ahead.