Women who consumed diets high in inflammatory foods, such as red meat, processed fats and sugar, had a 12% increase in their risk of developing breast cancer compared with women who consumed anti-inflammatory diets, according to a sweeping new study of more than 350,000 women.

The study also found that the effect was more pronounced in premenopausal women, suggesting that diet may play a key role in preventing the development of breast cancer in younger women. The findings, which are among the first to find a direct link between dietary factors and breast cancer, are being presented Monday at NUTRITION 2021 Live Online, a conference run by the American Society for Nutrition for thousands of researchers and other professionals.

"Following more anti-inflammatory diets could help prevent breast cancer," said first author Carlota Castro-Espin, a Ph.D. student at Institut Català d'Oncologia.

The Global Cancer Observatory reported that in 2020, breast cancer surpassed lung cancer as the most diagnosed cancer around the world, with an estimated 2.3 million cases. In the United States, the Centers for Disease Control and Prevention reports that breast cancer is the second most common form of cancer among women, trailing slightly behind certain forms of skin cancer.

However, while most skin cancer is caught early, giving patients a high survivability rate, the overall chance a woman will die from breast cancer is one in 39, as reported by the American Cancer Society. And while the death rate for older women has gone down in the last several decades due to better screening procedures, these rates remain steady in younger women.

To date, dietary factors have been linked only indirectly to breast cancer. For example, obesity, which is heavily influenced by diet, is a known risk factor for breast cancer. And studies have shown that avoiding weight gain and alcohol consumption as an adult can reduce the risk of developing breast cancer.

Chronic inflammation, a long-term, nonspecific immune response by the body that can often go unnoticed for years, has also been shown to reduce the likelihood of surviving breast cancer. Markers of chronic inflammation have also been shown to persist in breast cancer survivors.

However, direct evidence connecting specific aspects of diet, including inflammatory foods, to breast cancer is scarce, prompting the researchers to adopt a new, more practical approach to their study.

"No single dietary component, apart from alcohol, has been found to be a cause of breast cancer with convincing degree of evidence," Castro-Espin said. "Thus, the examination of diet as a whole can be more readily translated into dietary guidelines."

The researchers turned to data from EPIC, the European Prospective Investigation into Cancer and Nutrition, one of the largest cohort cancer studies in the world, which tracked cancer-relevant data for over half a million people across Europe from 1992 to 1999.

Because EPIC includes data from both men and women with a variety of different types of cancer, the first step was homing in on a subset of this massive pool of information. The researchers ended up using data about the overall diet of 351,284 women, focusing on the big picture rather than specific components.

"People consume food, not nutrients," Castro-Espin said. "Thus, examining overall dietary patterns, rather than single components of diets, can lead to more accurate conclusions when analyzing associations with a health outcome such as breast cancer."

The researchers used this dietary data to establish a score for each woman's diet measuring the level of inflammatory foods consumed, allowing them to quantify the effect of diet on cancer risk.

In addition to their main finding, that a highly inflammatory diet resulted in a 12% increase in the likelihood of developing breast cancer, the researchers also found that that this relationship was mathematically dependent on the inflammatory diet score, rather than just a general correlation.

For each increase in score by one standard deviation, a statistical unit of measurement that compares data points to a calculated average, there was a 4% increase in risk. And while Castro-Espin maintains that the findings will "require further confirmation," such a direct mathematical relationship suggests that the relationship between diet and cancer risk is causal.

The next step for the researchers will be getting their findings peer-reviewed, as they have just been accepted for publication in the European Journal of Epidemiology, where it should appear within the next few months. The team is also working on using the same dataset to explore how inflammatory diets affect breast cancer survival rates.

More broadly, the researchers hope their findings will encourage women, particularly those with a family history of breast cancer, to consider their diet more carefully and incorporate anti-inflammatory foods, such as nuts, fatty fish, fruits and vegetables.

"This finding is particularly relevant for breast cancer prevention, since diet together with physical activity and weight control are key modifiable lifestyle factors, and breast cancer is [among] the leading cause[s] of cancer death worldwide," Castro-Espin said.

The abstract, "Inflammatory potential of the diet and risk of breast cancer in the European Investigation into Cancer and Nutrition (EPIC) study," to be presented June 7 at NUTRITION 2021 Live Online, was authored by Carlota Castro-Espin, Antonio Agudo and Catalina Bonet, Institut Català d'Oncologia and Hospitalet de Llobregat; Elisabete Weiderpass and Laure Dossus, World Health Organization; Elio Ribol, Imperial College London; Paula Jakszyn, Hospitalet de Llobregat; and co-authors, EPIC.