A new study suggests that even low levels of physical activity could protect the brain from Alzheimer’s disease – but not in the way scientists expected.

The researchers tracked almost 300 older adults with early brain signs of Alzheimer’s for nine to 11 years using pedometers. They found that physical activity didn’t reduce the toxic amyloid plaques that most Alzheimer’s treatments now target.

Instead, in people who already had these plaques, physical activity reduced the accumulation of misfolded tau proteins in specific brain areas. These proteins appear later in Alzheimer’s disease and are more closely linked to cognitive and functional decline. These signs of dementia were reduced by almost half in more active participants.

Benefits appeared at just 3,000 steps – roughly half an hour of walking at a moderate pace. The optimal range was 5,000 to 7,500 steps daily, after which the effect plateaued. More steps didn’t necessarily mean greater protection, which suggests a realistic target for older, sedentary people, rather than the often-cited 10,000 steps.

The study had limitations, however. It involved a fairly small group of mostly white, well-educated people in the US, and it didn’t take other lifestyle or health factors into account. Also, there may be other protective mechanisms of walking at play. But it does support other research suggesting that being physically active may lower the risk of dementia.

A UK study of 1,139 people over 50 found that those who were moderately to vigorously active had a 34-50% reduction in dementia risk when followed over eight to ten years. Among those who developed dementia, staying active reduced their memory decline, particularly in older women.

A larger 2022 UK study tracked 78,430 people for seven years using wrist accelerometers. It found a 25% reduction in dementia risk with just 3,800 steps daily, rising to 50% at 9,800 steps.

However, people who walked more also had better cardiovascular health – lower cholesterol, better sleep and blood pressure and reduced diabetes risk. Since these heart and stroke risk factors also increase dementia risk, the picture is complex.

Healthy habits often go together. People who exercise are more likely to eat well, not smoke, look after their heart health and have fewer financial stresses. This makes it hard to know which factor is having the biggest effect. The researchers tried to account for this, but because these habits are so closely linked, it’s difficult to say that exercise alone is responsible.

However, there is a strong case for this as there are multiple ways exercise might support the brain: improving cardiovascular health, increasing blood flow and boosting chemicals that promote brain-cell connections.

One such substance is irisin, a hormone produced by muscles that acts on almost all faulty brain mechanisms associated with Alzheimer’s, including inflammation. This and other chemicals, such as BDNF, associated with exercise, provide plausible biological pathways for how physical activity might directly influence brain health beyond its cardiovascular benefits.

But the relationship might work in reverse, too. People may become less active because of early Alzheimer’s symptoms. Those with hearing problems, for instance – itself a dementia risk factor – often report barriers that make them stop being active before other dementia symptoms appear.