More than a third of dementia cases could be prevented with lifestyle changes such as controlling high blood pressure and getting more exercise, according to a major report published Thursday in the Lancet.

Alzheimer’s disease is the most common form of dementia, a set of symptoms generally characterized by memory loss and cognitive decline that affect about 14 percent of Americans over age 71. Scientists are still untangling what causes dementia, but in this new report, a team of international researchers identifies nine key risk factors that together account for about 35 percent of cases.

“People shouldn’t feel helpless,” says Lon Schneider, M.D., a professor of psychiatry, neurology, and gerontology at the Keck School of Medicine at the University of Southern California and one of the authors of the new report.

There are steps people can take to reduce their risk of dementia, but researchers say they need to be undertaken over the course of one's life.

The takeaway message, says David Knopman, M.D., a professor of neurology at the Mayo Clinic School of Medicine and the chair of the Alzheimer's Association Medical and Scientific Advisory Council, is that a lifestyle that’s good for your general health offers some protection against dementia as well. “If you take care of your heart, it may take care of your brain, too,” he says.

According to the new Lancet report, the nine factors they have identified as risks for dementia are all potentially changeable. 

Early Life (Up to 18 Years Old)

1. Limited education. People with very little formal education, which the researchers define as stopping before secondary school, are at an increased risk of developing dementia.

The theory behind this is that more years of education build up what’s called “cognitive reserve,” making your brain less vulnerable to cognitive decline since it can still function at a high level even when it’s not running at full throttle. 

In later life, “ongoing education might continue to increase cognitive reserve,” the researchers write.

Mid-Life (45 to 65 Years Old)

2. Hypertension. Most patients with dementia also show signs of problems with the blood vessels in their brain, and dementia and high blood pressure are closely linked.

A June report from the National Academies of Science, Engineering, and Medicine, in conjunction with the National Institute on Aging, noted that the evidence was “encouraging but inconclusive” that managing blood pressure in mid-life might help prevent dementia.

3. Obesity. While the new report does not make any specific dietary recommendations, the researchers did point to obesity as a key risk factor in mid-life.

People who are obese, especially those who develop a hormonal condition called insulin resistance, may have higher levels of inflammation throughout their bodies and problems with hormone production in the brain that can lead to impaired cognition.

But the relationship between weight and dementia risk is complex. A study of nearly 2 million people in the United Kingdom, published in the Lancet two years ago, found that being underweight in mid-life was also associated with an increased risk of dementia.

4. Hearing loss. Hearing loss affects nearly a third of people over 55, the researchers write, but the idea that it may be a risk factor for dementia is relatively new. While scientists are not yet sure why hearing problems have been linked to cognitive decline, there are at least two theories.

One is that the extra effort required when people are struggling to hear may take away from the cognitive resources needed to successfully encode and preserve memories. The other is that hearing loss can lead to social disengagement, depression, or brain atrophy, all of which may accelerate cognitive decline.

There’s not yet direct evidence that hearing aids help forestall dementia, but that’s the hope.

Later Life (65+ Years Old)

5. Smoking. The link between smoking and dementia may be related to the many ill effects smoking has on cardiovascular health. But cigarette smoke also contains substances that are toxic to the brain, further compounding the risk.

6. Depression. While there are definite links between depression and dementia, scientists are still trying to determine whether it’s truly a risk factor for dementia or just an early symptom of it.

But according to the Lancet researchers, depression in later life may well make people more vulnerable to dementia, since it can increase stress hormone levels, adversely affect the growth and survival of brain cells, and shrink the volume of the hippocampus, a part of the brain involved in memory.

7. Physical inactivity. “Older adults who exercise are more likely to maintain cognition than those who do not exercise,” the researchers write. While there’s only limited evidence that shows physical activity may help prevent cognitive decline, research on thousands of people suggests it seems to have some protective effects.

The National Academies of Sciences report recommends exercise “to delay or slow age-related cognitive decline,” and other studies have suggested that tai chi, longer exercise sessions (at least 45 minutes at a time), and resistance training may all have particular benefits for the brain.

8. Social isolation. Loneliness and a lack of physical contact, especially later in life, are linked to a constellation of health problems, including heart disease, high blood pressure, and depression. So it’s no surprise social isolation is linked to an increased risk of dementia as well.

And spending less time interacting with others may also lead to cognitive inactivity, the researchers write, which could speed cognitive decline. Previous studies have also shown that older adults with a busy schedule score better on tests of memory.

9. Diabetes. Diabetes is a potential risk for many of the same reasons as obesity, and controlling diabetes may also help act as a brake against early cognitive impairment progressing into full-blown dementia, according to the report.

And according to the Alzheimer’s Association [PDF], studies of thousands of people have shown that cognitive abilities decline more quickly and more severely in aging people with uncontrolled diabetes, who also show signs of memory deficits.