Fresh vegetables bought "out of season" may have much higher levels of nitrates - linked by some to ill-health - than frozen alternatives.
The government recommends five portions a day
Researchers say that fresh vegetables which have been imported from a distant country may also have fewer nutrients.
UK supermarkets offer seasonal vegetables such as broccoli all year round.
However, a survey by the Austrian Consumers' Association suggests that the extra price paid by shoppers may not always be worthwhile in health terms.
The longer food is stored, the more nutrients degrade
Spokesman, Food Standards Agency
It said that some winter crops tested were higher in nitrates - which can be converted into nitrites in the body.
Although there is no strong evidence linking these to ill-health, some experts still believe that our consumption of nitrates should be kept in check.
Conrad Brunnhofer, the researcher involved in the study, told BBC News Online that nitrate levels were more than seven times higher in some fresh vegetable samples compared with frozen.
He said: "There is more than one reason why I would prefer frozen vegetables to fresh vegetables out of season.
"The problem of nitrates is one of these."
Previous research from the association has pointed to a reduction in vitamins in vegetables imported during the winter months.
Any vegetable will start to lose nutrients from the moment it is harvested.
However, while the freezing process can take place within hours of the vegetables being picked, it may be some time before fresh vegetables reach supermarkets in the UK.
We import many of our vegetables from other parts of Europe, or even other continents, meaning a long journey in a refrigerated container.
The association found that vegetables such as broccoli lost significant amounts of nutrients when imported "fresh", and ended up less nutritious than their frozen equivalent.
The UK government is trying to encourage fruit and vegetable consumption as a way of cutting rates of cancer, coronary heart disease and stroke.
The current recommendation is five portions of fruit or vegetables a day.
A spokesman for the Food Standards Agency told the BBC that it did not matter for these purposes whether the portion was fresh or frozen vegetables.
She said: "The longer fresh fruit and vegetables are stored, the more nutrients degrade.
"The fresher the food, the better."