In 1991 the Polish government cut subsidies for saturated fats - i.e. from dairy and animal sources. Researchers examined how this affected diet in the general population, and whether there were any changes to death rates from heart disease.
The study found that by 2002, deaths from coronary heart disease had dropped by over a third in the 45-64 age group - a 38% drop for men and 42% for women.
Over a similar period (to 1999), people were consuming 7% less saturated fat, while consumption of polyunsaturated fats had risen by 57%.
The sharp drop in deaths cannot simply be explained by the effect of any polyunsaturated fat, say the authors, but is likely to be related to the type consumed. Rapeseed and to a lesser extent soya bean oil made up most of the rising numbers of polyunsaturated fats available in Poland in the 1990s - both of which contain omega 3 and omega 6 fatty acids, also thought to reduce heart problems.
The researchers also looked at changes in smoking trends and fruit consumption after 1991, to see if these could account for the drop in mortality rates.
They found that the fall in numbers of people smoking - which would also result in fewer deaths from heart disease - did not match the dramatic drop in death rates after 1991. And although consumption of imported fruit rose during the 1990s - from 2.8kg/year per person in 1990 to 10.4kg/year by 1999 - the increase was not enough to influence death rates by more than 1 or 2%.
These results concur with other studies which show that partly substituting polyunsaturated fats for saturated fats in the diet, while maintaining a low intake of trans fatty acids, can reduce deaths from heart disease.