A new study says that people who are physically active during their 40's and 50's don't really have any lower risk of developing dementia later in life.

The researchers analyzed the effect of exercise on the onset of dementia in more than 1,000 adults. Most of the participants were middle-aged manual-class men.

They then matched data regarding the participants' levels of physical activity when they were 48 to 60 years old to their cognitive functions measured 16 years later. Researchers did not find any relation between physical activity- leisure or work related -in mid-life to the risk of dementia later in life.

Previous studies had suggested that people who exercise during mid-life can cut risk of dementia.

According to researchers, long follow-up period of this study (16 years) may be the reason that they found no evidence that supports association between physical activity and dementia risk.

"Although our results show no protective effect of mid-life physical activity for cognitive decline in later life, there is good evidence that physical activity prevents other health problems. It is important that the health benefits of physical activity continue to be promoted by the public health community. However our study suggests that the protective effects on dementia and cognitive impairment may be overly optimistic," said Dr. Gemma Morgan, lead researcher from Bristol’s School of Social and Community Medicine.

According to researchers the studies published on physical activity and dementia are all positive in nature.

"This study represents an important addition to the literature on physical activity and cognitive disease as few studies have data with more than 15-years follow-up. It is possible that previous studies have shown protective effects due to a tendency for the literature to preferentially publish positive studies and the phenomenon known as 'reverse causation'," said Professor Yoav Ben-Shlomo from Bristol’s School of Social and Community Medicine.

Dementia, much like many other diseases that emerge during old-age do not happen suddenly, rather they can go undetected for many years.

"Subjects with very early disease, before any clinical diagnosis, reduce physical activity levels as a secondary effect of the disease rather than physical activity itself being protective. We are discovering that the pathology leading to dementia may have a long latency period before diagnosis so this is a possibility," Ben-Shlomo added.