Eating fish helps keep your brain young, claim scientists.
They found diets lacking an essential nutrient in oily fish may hasten brain shrinkage and mental decline. People eating the least amount of omega-3 fatty acids had less brain mass – equivalent to about two years of chronological ageing.
Low levels of omega-3 fatty acids were also associated with poor test scores for visual memory, problem solving, multi-tasking and abstract thinking.
It is thought that omega-3 fatty acids in fish oils may reduce inflammation of the brain and play a part in brain development and nerve cell regeneration.
Britons are currently advised to eat fish at least twice a week, including one portion of oily fish.
In a new study, brain scans carried out on 1,575 people with an average age of 67 showed a greater rate of brain shrinkage in those who lacked docosahexaenoic acid (DHA).
DHA is an omega-3 fatty acid that is thought to help nerve cells communicate with each other. The richest source of the nutrient is oily fish, such as herring, mackerel and sardines.
Dr Zaldy Tan, an Alzheimer's researcher from the University of California at Los Angeles, who led the US research reported in the journal Neurology, said: 'People with lower blood levels of omega-3 fatty acids had lower brain volumes that were equivalent to about two years of structural brain ageing.'
The study involved magnetic resonance imaging (MRI) brain scans and tests for mental function and omega-3 fatty acids in red blood cells.
Participants whose DHA levels were among the bottom 25 per cent had smaller brains than those with higher DHA levels.
Low levels of all omega-3 fatty acids were also associated with poor test scores for visual memory, problem solving, multi-tasking and abstract thinking.
The best dietary source of omega 3 fatty acids is oily fish because the human body cannot produce omega-3 fatty acids.
There has been an explosion in the number of foods fortified with omega-3 oils, such as chickens, margarine, eggs, milk and bread, but they contain only small amounts.
Types of fish that contain high levels include tuna, salmon, mackerel, herring, sardines, and anchovies.
White fish is also a healthy food including cod, haddock and plaice although it contains lower levels of essential fatty acids.
Fish oil supplements are recommended as protection against heart attacks and sudden death, with regular fish eaters a third more likely to survive a heart attack.
Omega 3 fats work in several ways to reduce heart attack risk by cutting blood fats, reducing the chances of a blood clot and blocking dangerous heart rhythms that might otherwise prove fatal.
Dr Marie Janson, Director of Development at Alzheimer's Research UK, the UK's leading dementia research charity, said: 'There has been a lot of research into the effects of omega 3, and this study will add to that debate.
'One strength of this study is that it used blood samples to measure people's dietary intake of omega 3, rather than relying on answers to questionnaires to assess the link between omega 3 and cognition.
'However this research does not tell us whether the people studied got worse or better over time.
'We would need to see large-scale, long-term studies before we can know whether a diet high in omega 3 can protect against dementia, and people shouldn't fill their freezers with oily fish just yet.
'The best evidence for reducing your risk of dementia is to eat a healthy, balanced diet, take regular exercise and keep your blood pressure and cholesterol in check.
'With 820,000 people affected by dementia in the UK, and a rapidly ageing population, we urgently need to find ways to prevent and treat the condition – that means we must invest in research.'