Some 65-year-olds aren't as “with it” as they once were, and some 90-year-olds are still sharp as a whip.

What accounts for the differences? Using a genetic analysis and intelligence tests given to a group of people in childhood and old age, researchers from Scotland concluded that both genes and environmental factors play a role in whether you'll maintain your level of intelligence throughout your lifespan.

Maintaining brain health into old age is key to aging well, including the ability to do everyday tasks and stay independent, according to background information in the report. Plenty of prior research has found that how smart you are in adolescence generally carries over into adulthood and old age.

"We estimated that about a quarter of the lifetime changes in intelligence test scores might be due to genetic factors," said lead study author lan Deary, PhD, a professor of differential psychology and director of the University of Edinburgh Centre for Cognitive Ageing and Cognitive Epidemiology, in Scotland. In other words, around three quarters of mental decline may be linked to factors you can change, such as how much you exercise, the quality of your diet, and how much you stimulate your brain with a variety of activities.

And yet, "some people's intelligence ages better than others," Dr. Deary and colleagues noted.

Study Details

In the study, investigators used genome-wide association data on 1,940 unrelated people in Scotland, along with information from intelligence tests participants took when they were about age 11, and then again between 54 and 68 years later, when they were 65, 70 or 79.

Genome-wide association research "involves rapidly scanning markers across the complete sets of DNA, or genomes, of many people to find genetic variations," the US National Human Genome Research Institute explains.

Dr. Deary's team looked specifically for differences in bits of DNA called SNPs (single nucleotide polymorphisms) associated with people whose intelligence either declined or stayed stable.

"We were able to make estimates of the genetic contribution to intelligence differences in childhood and old age, and the change between these times, in the same people," Dr. Deary said. "What was novel about these estimates was that they were made from actual testing of DNA, not from twin or adoption studies."

The study appears in the online edition of Nature.

Expert Perspective

S. Duke Han, PhD, an assistant professor in the department of behavioral sciences and a clinical neuropsychologist at Rush University Medical Center, in Chicago, said the study is unique in that it was able to measure intelligence in the same group of people over such a long period of time.

"What this is saying is something many researchers have accepted for a long time, that intelligence seems to be very much influenced by genetic makeup but also environmental factors, such as education," Dr. Han said.

In analyzing the data, the researchers were able to make broad inferences about how much genetics played a role in maintaining intelligence over the life span, but they weren't able to identify specific genes or gene variants that might contribute.

In general, there's a "paucity of data about genetic influences on lifetime cognitive change," with a few exceptions, the study authors noted. The APOE4 mutation, for example, is a risk factor for severe cognitive decline associated with Alzheimer's disease.

Walk to Prevent Alzheimer's

Walking just six miles weekly may prevent Alzheimer's.

Recent finding: In a study of 426 adults with or without cognitive decline, those who walked at least six miles weekly were half as likely to develop Alzheimer's disease over 13 years as non-walkers. Among those with cognitive impairment, walking five miles a week reduced cognitive decline by more than half.

Theory: Exercise improves blood flow to the brain, which helps keep neurons healthy.

To help preserve brain health: Aim to walk at least three-quarters of a mile daily.

Dietary Choline and Sharper Memory

Researchers examined dietary and memory-test data on nearly 1,400 adults (average age 61) over a three-to 10-year period.

Result: People whose diets contained the highest levels of the nutrient choline performed better on memory tests than those who consumed the least amount.

Theory: Choline is a precursor to a brain chemical called acetylcholine, which plays a key role in cognition.

Good sources of choline: Eggs, poultry, saltwater fish, liver and kidney beans.

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