A grandparent's mental decline or a great uncle's waning memory may indicate you, too, have greater risk for Alzheimer's disease -- especially if closer relatives have the condition, a new study says.

Alzheimer's in both a first-degree relative (parents, siblings) and a second-degree relative (grandparent, aunt, uncle, nieces or nephews) doubles your risk of the brain-destroying disorder, researchers found. But if you have one first-degree relative and two second-degree relatives with the disease, your risk increases by 21 times.

"Family history is a very powerful piece of information that can accurately predict someone's risk of Alzheimer's disease, and this study shows there are more people at risk than we know," said the study's lead author, Lisa Cannon-Albright. She's a professor of epidemiology at the University of Utah School of Medicine.

Family history is a known risk factor for Alzheimer's disease, a progressive disease that causes difficulty in thinking and memory. Previous research has focused on close family relations, the researchers noted.

The current study used a database that includes family records of Utah pioneers dating back to the 1800s. That information is also linked to health-related registries. The researchers were able to link information from the database with death certificates for more than 270,000 people.

In this group, almost 4,500 people had Alzheimer's disease when they died, the study team found.

Having one parent or sibling with Alzheimer's upped the risk of developing the disease by 73 percent. Two such first-degree relatives boosted the risk four times higher, the study said.

Having two second-degree relatives and no first-degree relatives raised the risk of Alzheimer's by 25 percent. Two third-degree relatives (great grandparents, great aunt or uncle) upped the odds of the memory-robbing disorder by 17 percent. And the more distant relatives that had the disease, the higher a person's risk.

When Alzheimer's occurred in close relatives and second-degree relatives, the risk jumped significantly.

The study authors acknowledged some limitations to the study. One is that Alzheimer's disease often isn't listed as a cause of death, so the prevalence of disease may be underestimated.

Rebecca Edelmayer is director of scientific engagement for the Alzheimer's Association. She pointed out that the study's population is unique, so the findings might not translate to a more diverse group of people.

"Family history is a risk factor for Alzheimer's disease, so that finding is not unexpected. But this is an important study that adds to our information and really starts to look at the relative risk outside of the first-degree population," said Edelmayer, who wasn't involved in the research.

She said it's important for people to have a discussion about their family history with their doctor.

While you can't change your family history, Edelmayer said there are modifiable risk factors, too. Make sure you keep your brain active (cognitive stimulation), eat well and exercise regularly to reduce your risk of Alzheimer's disease, she suggested.

Cannon-Albright said that even if knowing your excess risk doesn't motivate you to make a change in your lifestyle, it's still good information to have.

"Family history information can identify who's high-risk and low-risk, and maybe a medical system could screen those at high risk more aggressively. The information could also be used in clinical trials to stratify groups by risk. There are lots of ways family history information can help," she said.

Edelmayer agreed.

"Understanding your medical history is an important thing. You should have conversations with your family and your physician about your family history," she recommended.

The study was published March 13 in Neurology.

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