Regular exercise is one of three factors that is likely to reduce the risk of dementia in old age, US experts say.

A combination of exercise, controlling blood pressure and some form of brain training may reduce mental decline in old age and reduce the risk of Alzheimer's-type dementia, US experts have concluded.

A new report released by the National Academies of Sciences, Engineering, and Medicine states there is "encouraging" evidence, although inconclusive, to support these three interventions for brain health.

"The evidence is strong enough to suggest the public should at least have access to these results to help inform their decisions about how they can invest their time and resources to maintain brain health with ageing," Alan Leshner, chair of the committee and CEO emeritus of the American Association for the Advancement of Science, said.

Additional research is needed to understand and analyse their effectiveness, the reported noted.

An earlier systematic review published in 2010 by the Agency for Healthcare Research and Quality (AHRQ) had concluded that there was insufficient evidence to make recommendations about any interventions to prevent cognitive decline and dementia.

Since then, the understanding of dementia has advanced.

Based on all the available evidence, the NASEM report concluded that three classes of interventions were supported by "encouraging but inconclusive" evidence.

These interventions are:

• Cognitive training: includes programs aimed at enhancing reasoning and problem solving, memory, and speed of processing - to delay or slow age-related cognitive decline. May or may not be computer-based.

• Blood pressure management for people with hypertension: particularly during midlife - generally ages 35 to 65 years - to delay, and slow clinical Alzheimer's-type dementia.

• Increased physical activity: to delay or slow age-related cognitive decline.