Trans fats, especially common in hydrogenated vegetable oils, have been linked to long-term heart disease risk. The study found the average meal went from containing nearly 3g of trans fat to just .5g.
"It's a small step forward," said Alice Lichtenstein, a nutrition science researcher from Tufts University in Boston, who wrote an editorial accompanying the study in the Annals of Internal Medicine bit.ly/MnBiCA.
"This is just trans fat. It doesn't have any effect on calories. It doesn't mean that you can eat as much of it as you want," she told Reuters Health.
"We have to think about these changes within the context of the whole diet. This is one small change in the right direction. We need a whole lot more."
In 2006, New York City passed regulations prohibiting restaurants from serving food that contains partially hydrogenated vegetable oil and has .5g or more of trans fat per serving. Those restrictions went into effect in 2008.
To test the policy's result, researchers briefly surveyed customers leaving 168 different fast-food restaurants, belonging to 11 popular chains, the year before and the year after the restrictions were first enforced.
Those chains included McDonald's, Burger King, Subway, and Yum Brands Inc. restaurants KFC and Pizza Hut.
Based on receipts from 6,969 customers surveyed in 2007, the average fast-food meal purchased that year had 2.9g of trans fat. By 2009, that figure was 0.5g in a sample of 7,885 customers.
The number of meals without any trans fat increased from 32% of all purchases before the regulations to 59% afterward.
In addition, there was no spike in the amount of saturated fat in fast-food meals during the study period -- as some had feared -- so the total amount of "bad" fats in the average purchase dropped substantially.
Trans fat is "fully replaceable with healthier oils, so we knew that was something that could be changed," said Christine Curtis, from the New York City Department of Health and Mental Hygiene, who worked on the study.
"We were really pleased," she told Reuters Health. The study "really demonstrates that local regulation can reduce exposure to trans fat."
Curtis said the trans-fat regulation could end up leading to health benefits down the line. "It does have the potential to have a really big impact on cardiovascular disease risk," she said.