More than half take up eating oily fish (53%) and 46% eat more fruit and vegetables.
He suggested that people should eat more oily fish or take fish-oil supplements.
Vitamin D is naturally found in few foods other than oily fish.
Study leader Dr Alan Dangour said claims about the benefits of oily fish in warding off dementia in older people seemed to have been oversold.
Smaller, less oily fish don't take as well to this preparation.
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The survey also looked at oily fish consumption, which has increased from about a quarter of a portion to a third of a portion per week.
The lack of sunshine and a diet low in oily fish are thought to be the main factors in Scots having a low level of the vitamin.
Experts advise eating a couple of portions of fish a week, with at least one being an oily fish, because there are proven benefits on the heart.
Dr Mozaffarian said the cooking process had either destroyed all of the good omega-3 or the fried fish were lean white fish like cod rather than oily fish.
And as I wrote recently, oily fish like salmon contain omega-3 fatty acids, which appear to carry health benefits, especially when compared to the fats contained in other meats.
They say consuming 250 milligrams of omega-3 fatty acids a day -- t he equivilant of 6 ounces a week of wild salmon or other oily fish -- could prevent heart disease.
Of course, these findings only suggest that oily fish, or the lack of it, could be to blame - many other items have either left or joined the national diet during that period.
While the American Heart Association recommends oily fish as the preferred source of omega-3 fatty acids, supplements are a convenient source for people who don't like fish or are unable to consume the advised dosage.
Omega-3s are found in oily fish such as salmon, as well as in walnuts and kiwi fruit, and there is a strong negative correlation between the extent to which a country consumes fish and its levels of clinical depression.
Standing above him, Edward Sherman Hoar, his sole companion, holds aloft a string of fish and examines the oily glistening of inanimate scales.
The Food Standard Agency's advice is to eat two portions of fish a week, one of which should be oily.
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