The study suggests that eating so-called 'energy dense foods' - which includes processed foods such as chicken nuggets, burgers and pizza - may increase the risk of cancer, regardless of a woman's weight.
The findings, published in the Journal of the Academy of Nutrition and Dietetics, revealed a link between high dietary energy density in food and obesity-related cancer in normal weight women.
The researcher said that diet is believed to play a role in cancer risk. Current research shows that an estimated 30 per cent of cancers could be prevented through changes in diet.
But while there is a proven link between obesity and certain types of cancer, less is known about how the ratio of energy to food weight, otherwise known as dietary energy density (DED) , contributes to cancer risk.
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To find out, researchers looked at DED in the diets of post-menopausal women and discovered that consuming high DED foods was tied to a 10 per cent increase in obesity-related cancer among normal weight women.
DED is a measure of food quality and the relationship of calories to nutrients. The more calories per gram of weight a food has, the higher its DED.
Whole versus processed foods
Whole foods - such as vegetables, fruits, lean protein, and beans - are considered low-DED foods because they provide a lot of nutrients using very few calories.
But processed foods, such as hamburgers and pizza, are considered high-DED foods because people need a larger amount to get their necessary nutrients.
Previous studies have shown that regularly eating foods high in DED contributes to weight gain in adults.
To gain a better understanding of how DED alone relates to cancer risk, American researchers used figures from 90,000 postmenopausal women - including their diet and any diagnosis of cancer.
They found that women who ate a diet higher in DED were 10 per cent more likely to develop an obesity-related cancer, independent of body mass index (BMI).
In fact, the study revealed that the increased risk appeared limited to women who were of a normal weight at enrollment in the programme.
Study lead investigator Professor Cynthia Thomson, of the University of Arizona, said: "The demonstrated effect in normal-weight women in relation to risk for obesity-related cancers is novel and contrary to our hypothesis.
"This finding suggests that weight management alone may not protect against obesity-related cancers should women favor a diet pattern indicative of high energy density."
Although restricting energy dense foods may play a role in weight management, the researchers found that weight gain was not solely responsible for the rise in cancer risk among normal weight women in the study.
They suspect that the higher DED in normal-weight women may cause metabolic dysregulation that is independent of body weight, which is a variable known to increase cancer risk.
The researchers said that while further study is needed to understand how DED may play a role in cancer risk for other groups such as young people and men, the information may help persuade postmenopausal women to choose low DED foods - even if they are already at a healthy BMI.
Prof Thomson added: "Among normal-weight women, higher DED may be a contributing factor for obesity-related cancers.
"Importantly, DED is a modifiable risk factor. Nutrition interventions targeting energy density as well as other diet-related cancer preventive approaches are warranted to reduce cancer burden among postmenopausal women."