Being a slob puts you at risk of mental health problems, experts have warned.
Lack of exercise could lead to dementia in later life
A lack of physical activity leads to depression and dementia, evidence presented at the British Nutrition Foundation conference shows.
It comes as new research from the University of Bristol found that being active cuts the risk of Alzheimer's disease by around a third.
Currently only 35% of men and 24% of women reach the recommended weekly amount of physical activity.
Professor Nanette Mutrie, an expert in exercise and sport psychology at the University of Strathclyde, told the conference that mental health was not a trivial issue.
"It's only recently that people have begun to see the link between physical activity and mental health.
"It's important for increasing people's self esteem, general mood, coping with stress and even sleeping better.
"And we now have very strong evidence that physical activity can prevent depression."
She said inactive people had twice the risk of becoming depressed and there was also very good evidence that exercise is a useful treatment for depression.
Researchers at the University of Bristol carried out an analysis of 17 trials looking at the effects of physical activity on dementia and Alzheimer's disease.
They found that in both men and women physical activity was associated with a 30-40% drop in the risk of Alzheimer's.
It is unclear why there is such a great effect but it could be associated with benefits to the vascular system as well as release of chemicals in the brain.
Professor Mutrie added: "It could be a simple case of use it or lose it.
"It is estimated that over 700,000 people in the UK currently suffer from dementia and more research is needed to determine how this condition can be prevented."
Professor Judy Buttriss, director general at the BNF, said with people living longer the implications of such studies were "enormous".
"There has already, justifiably, been a lot of emphasis on good nutrition but we must also find ways of helping people to be more physically active to ensure that they maintain health and quality of life in later years."
Department of Health figures show the majority of adults do not do the recommended 30 minutes of moderate activity at least five times a week.
Children are also leading increasingly inactive lives.
Around 30% of boys and almost 40% of girls fail to reach the recommended hour of moderate intensity activity per day.
Professor Chris Riddoch, expert in sport and exercise science at the University of Bath, said: "We have half a Century of evidence showing active people have lower levels of disease.
"We also have a very good handle on how much exercise people should take."
But he added efforts to get people to be more active had not been very successful to date.