Exercising for at least 2.5 hours per week may delay cognitive decline in people carrying a genetic mutation that causes Alzheimer's disease, according to a study.

Autosomal dominant Alzheimer's disease (ADAD) is a rare genetically-driven form of the disease in which the development of dementia at a relatively young age is inevitable, said researchers from the University Hospital of Tubingen in Germany.

The study, published in the journal Alzheimer's & Dementia, shows a significant relationship between physical activity, cognition, functional status and Alzheimer's disease pathology even in individuals with genetically-driven ADAD.

The officially recommended physical activity duration of 150 minutes per week was associated with significantly better cognition and less Alzheimer's disease pathology in ADAD.

A physically active lifestyle is achievable and may play an important role in delaying the development and progression of ADAD, researchers said.

They analysed data generated from 275 individuals (average age 38.4) who carry a genetic mutation for ADAD and are participating in an international observational study of individuals and families with ADAD led by researchers at Washington University School of Medicine in St Louis in the US.

Researchers aimed to determine if at least 150 minutes of physical activity (walking, running, swimming, aerobics, etc) per week -- the current recommendation by the World Health Organization and the American College of Sports Medicine -- would produce cognitive benefits for the study participants.

As many as 156 were classified as high physical activity individuals (150 minutes physical activity/week) and 68 as low physical activity individuals (150 minutes physical activity/week).

Researchers found individuals who engaged in more physical activity scored better on the Mini-Mental State Examination (MMSE) and the Clinical Dementia Rating Sum of Boxes (CDR-SOB), which are well-accepted standard measures of cognition and function.

Similarly, individuals who exercised more had lower levels of Alzheimer's disease biomarkers in cerebrospinal fluid, including lower tau, a protein that builds up in the brains of people living with Alzheimer's disease, researchers said.

"A physically active lifestyle is achievable and may play an important role in delaying the development and progression of ADAD. Individuals at genetic risk for dementia should therefore be counselled to pursue a physically active lifestyle," they said.