Teen girls who eat an “inflammatory diet” may be at higher risk for breast cancer later in life, according to a study released earlier this year.
The Link Between Inflammation and Cancer
Inflammation is a normal, necessary occurrence that helps injured tissue to heal. An inflammatory response is triggered when chemicals are released by the damaged tissues. The National Cancer Institute explains that, in response, white blood cells make substances that cause cells to divide and grow to rebuild damaged tissue. Once this process is complete, the inflammatory process subsides.
However, with chronic inflammation, the inflammatory process may begin even if there are no damaged tissues. Chronic inflammation may be the result of infections that don’t go away, abnormal immune reactions to normal tissues, or conditions such as obesity. This prolonged inflammation can eventually lead to damaged DNA that leads to cancer. 
For the recent study, researchers looked at women who, in high school, consumed diets thought to increase inflammation levels in the body. These women were found to be more likely to develop breast cancer before entering menopause compared with women who ate a different type of diet during their high school years. 
An increased risk of breast cancer was also found in women who ate an inflammatory diet in their 20s, 30s, and 40s.
Foods that Cause Inflammation
So, what exactly is an “inflammatory diet”? It’s a pattern of eating that includes few vegetables, lots of sugar-sweetened and diet soft drinks, refined sugars and carbohydrates, red and processed meats, and margarine. Eating these foods has been linked to higher levels of markers of inflammation in the body.
Read: 4 Must-Have Foods for Reducing Inflammation Naturally
Karin B. Michels, an epidemiology professor at the University of California, Los Angeles Fielding School of Public Health, says:
“Our results suggest that a habitual diet that promotes chronic inflammation when consumed during adolescence or early adulthood may indeed increase the risk of breast cancer in younger women before menopause.” 
The risk of breast cancer appeared to increase by more than 1/3 among teen girls who consumed a poor diet. 
Researchers tracked the data from over 45,000 women enrolled in the Nurses’ Health Study II. The participants filled out food frequency questionnaires about what they ate as teens and young adults, and they were followed for up to 22 years.
Each of the diets was assigned an “inflammatory score,” based on a method that links diet with established inflammatory markers in the blood.
Women in the five groups with the highest inflammatory scores during their teen years were found to have a 35% higher risk of breast cancer than those with the lowest scores. Participants in the highest inflammatory score group during their early adult years had a 41% higher risk of developing the disease.
No link was found between the women’s inflammatory scores and overall breast cancer incidence, or with postmenopausal breast cancer, however.
During the 22-year, follow-up period, 870 of the participants who completed the high school diet questionnaire were diagnosed with breast cancer before menopause, while 490 women were diagnosed with breast cancer post-menopause.
The study was limited in that it can’t definitively prove that eating a poor diet during teen years causes breast cancer. In order to show that, the team would’ve had to have randomly assigned large numbers of teens to eat varying diets and then observe what happened to them for 20 years. That would obviously be unethical and likely unworkable. 
There’s also the possibility that teens who consumed healthier diets had other lifelong healthy habits.
Read: A Mediterranean Diet Could Cut Your Risk of Breast Cancer
While 30% of breast cancer cases are caused by genes, food is something people can control. Consuming less refined flour, sugar, and red meats and eating more vegetables can help lower the risk of other conditions as well, including obesity (which is linked to cancer), diabetes, and gastrointestinal disorders.
 The National Cancer Institute
 Live Science
 NBC News