American health care spending, measured in trillions of dollars, boggles the mind. Last year, we spent $3.2 trillion on health care – a number so large that it can be difficult to grasp its scale.

A new study, published in the Journal of the American Medical Association reveals what patients and their insurers are spending that money on, breaking it down by 155 diseases, patient age, and category – such as pharmaceuticals, or hospitalizations. Among its findings:

Chronic – and often preventable – diseases are a huge driver of personal health spending. The three most expensive diseases in 2013: diabetes ($101 billion), the most common form of heart disease ($88 billion) and back and neck pain ($88 billion).

Yearly spending increases aren’t uniform: Over a nearly two-decade period, diabetes and low back and neck pain grew at more than 6 percent per year – much faster than overall spending. Meanwhile, heart disease spending grew at 0.2 percent.

Medical spending increases with age – with the exception of newborns. About 38 percent of personal health spending in 2013 was for people over age 65. Girls between 1 and 4 years old accounted for $2,000 in annual spending; older women 70 to 74 years of age accounted for $16,000.

The analysis provides some insight into what’s driving one particularly large statistic: Within a decade, close to a fifth of the American economy will consist of health care.

“It’s important we have a complete landscape when thinking about ways to make the health care system more efficient,” said Joseph Dieleman, an assistant professor at the Institute for Health Metrics and Evaluation at the University of Washington who led the work.

The data show that the primary drivers of health-care spending vary considerably. For example, more than half of diabetes care is spending on drugs, while only about 4 percent of spending on low back and neck pain was on pharmaceuticals. Generally, more spending is done on elderly people, but about 70 percent of the spending on low back and neck pain was on working-age adults. Such insights provide a way to find the drivers of growth in health care spending and to launch strategies to control it.

“Data like this continues to draw attention to the fact a lot of these proposals being discussed about controlling health-care costs really don’t address the underlying issue, which is rising disease prevalence,” said Ken Thorpe, a professor of health policy at Emory University who was not involved in the study but has done similar research. “You see this rise in chronic disease spending – much of it potentially preventable.”

Most of the discussion of health care in America has focused on access to insurance, but the spending breakdown shows that the biggest opportunities may come in preventing disease.

The researchers also analyzed spending on public health and prevention. In a separate editorial, Ezekiel Emanuel, a former health care adviser to President Barack Obama, pointed out that the largest public health spending was on HIV. But fewer than 7,000 Americans died due to HIV/AIDS in 2014 and it ranked 75th on the list of diseases by personal health expenditures.

“Few public health dollars focus on lifestyle conditions that ultimately contribute to the majority of chronic illnesses seen today,” Emanuel wrote. Low back and neck pain, for example, ranked low on the list of public health expenditures with $140 million in public health funding, but high on the list of health care spending.

What the data also show is that conditions that drive health care spending aren’t necessarily the ones that come to mind when people think about health care. Falls were the fifth highest cause of health spending, followed closely by depression.