The research, carried out for safefood by a team from the University of Ulster, found that 'low fat' and 'reduced fat' products such as semi-skimmed milk and low fat coleslaw may actually contribute to people putting on weight.
Researchers say it is because people assume that foods with 'healthier' labels are lower in calories - and react by eating larger portions.
Over 180 adults with a range of body weights took part in the study for the all-Ireland food safety body.
It aimed to discover what people estimated a reasonable portion size of 'healthier' food was - and its calorific content - compared to 'standard' food.
Dr Cliodhna Foley-Nolan, of safefood, said: "There has been a huge increase in the number of food products with nutrition and health claims sold over the last 20 years, but we also know that the population's weight has continued to increase.
"The research was commissioned to explore people's understanding of these products and shows that these foods are viewed by some consumers as a license to overeat, despite the fact that in many cases the fat removed from the 'healthier' product is replaced by other ingredients, such as sugar, and the calorie savings are small.
The safefood Director of Human Health & Nutrition continued: "Northern Ireland Consumers really need to relook at their portion sizes, as any benefit they might get from these 'healthier' processed foods could be undone by just how much of them they are eating."