People need to eat more foods high in nutrients for their own mental wellbeing, say experts.
Eating more fish could beat depression
The UK faces a mental health crisis of "monumental proportions" if their advice is not heeded, they say.
Doctors at the International Society for the Study of Fatty Acids and Lipids congress in Brighton heard evidence on omega-3s' brain benefits.
Research suggests foods rich in omega-3, like fish and eggs, can prevent depression and promote learning.
A study of 14,500 pregnant women by Dr Joseph Hibbeln from the US National Institutes of Health found those who ate fish throughout their pregnancy were less likely to suffer from depression.
Their children were also less likely to develop behavioural problems and learning difficulties.
Other researchers found children with low levels of omega-3 at the age of seven were more prone to depression as adults than those with higher levels of omega-3.
But people have lower levels omega-3 now than in previous years.
Professor Michael Crawford, from the Institute of Brain Chemistry and Human Nutrition at the University of North London and president of the congress, said there had been a complete shift in the balance of essential fatty acids in people's diets in the last 50 years.
People are eating less of the good omega-3 fatty acids, found in fish such as mackerel and salmon, that the brain needs to be healthy, he said.
Professor Tom Sanders from the Nutrition Foods and Health Research Centre at King's College said people wrongly believe all fats are bad for them.
"There is a real problem in the perception of the word fat. There are good and bad fats."
Professor Crawford said: "We are facing a mental health crisis of monumental proportions that far outstretches, in my opinion, the problem of obesity. This is a major issue."
According to the Food Standards Agency, on average, people in the UK eat a third of a portion (about 47g) of oily fish a week. Seven out of ten don't eat any at all.
It says people should eat at least two portions of fish a week, and that one should be oily.
Some oily fish contain chemicals, such as dioxins, which accumulate over time in the body and could have adverse health effects if consumed over long periods at high levels, particularly in pregnant women.
The FSA gives advice on maximum consumption levels at which the health benefits outweigh the possible risks.
Dr Ray Rice from the International Society for the Study of Fatty Acids and Lipids said: "People who eat a lot of fish are generally healthier, mentally and physically, than non-fish eaters.
"Seafood is not only rich in minerals and vitamins, it is the most important food source of the omega-3 polyunsaturated fatty acids that have a wide range of recognised health benefits," he said.
Ms Sophie Corlett from the mental health charity Mind said: "It stands to reason that what we eat affects out mental balance, and many people bear witness to the fact that changes in diet can have a huge impact on their mental health.
"We would encourage people to adopt a healthy, balanced diet incorporating ingredients such as fish oils known to boost mental well-being, as a positive alternative to over-reliance on medication."