Alzheimer's disease may be the third major cause of death in the U.S., after heart disease and cancer, according to a new study in the journal Neurology.

The study shows that the number of deaths caused by Alzheimer's is five to six times higher than what the Centers for Disease Control and Prevention (CDC) reports, which is based on data from death certificates. According to the CDC, Alzheimer's is the sixth-leading cause of death in the U.S.

"It is up to the discretion of the person filling out the death certificate to list Alzheimer's as an underlying cause," said Bryan James, a professor at Rush University Medical Center in Chicago. Death certificates have a place to name the immediate cause of death, and then usually have room to list up to three other underlying causes of death, he said.

Alzheimer's might not be listed as an underlying cause on a death certificate because there are more immediate underlying causes, or because the person filling out the death certificate does not know that the person who died suffered from Alzheimer's, James said. And, sometimes, even the family or the person who died was not aware that he or she had Alzheimer's, because they had never been diagnosed. [Top 10 Leading Causes of Death]

"Many people do not conceive of Alzheimer's disease as a fatal disease, but it actually is," he said.

"It is a much larger killer than people recognize," James told Live Science.

In the study, the researchers focused on a group of 2,566 people ages 65 and older, some of whom developed Alzheimer's disease. Over an eight-year period, 1,090 people in the study died, and more than one-third of the deaths were associated with Alzheimer's.

This rate of deaths could mean that more than 500,000 people in the U.S. died with Alzheimer's disease in 2010, according to the study.

Alzheimer's starts in the brain, with the loss of memory, James said, and, eventually it leads to a person's inability to function independently. Eventually, the disease affects the parts of the brain that control basic body functions, for instance, breathing, swallowing or the heart rate. Alzheimer's may ultimately lead to complications such as pneumonia or one's inability to eat, which may eventually lead to a person's death.

"Trying to identify a single cause of death for older people may not accurately reflect the process of dying for most elderly people," James said, as it is often a "chain of events" that leads to a person's death.

Knowing how many people die from Alzheimer's is important in understanding the disease's burden on society, he said.

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This story was originally written for LiveScience and has been republished with permission here. Copyright 2014 LiveScience, a TechMediaNetwork company.