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Ultra-processed foods increase your risk of breast cancer

In the hierarchy of research, some types of clinical studies are considered to offer stronger evidence than others. At the top are meta-analyses, in which the data from multiple well-designed studies are pooled together. A little less strong are systematic reviews, which aim to consider all quality research related to the designated topic.

A randomized controlled trial randomly assigns participants to an experimental group or a control group, where only those in the experimental group experience the variable being studied. This is a prospective study in that the variable being studied is determined before the trial begins.

A cohort study is also prospective, but there is no intervention as in a randomized controlled trial. Instead, researchers track the participants for a designated period of time and compare characteristics of those who develop a particular disease or other outcome with those who do not. The highest quality cohort studies involve many thousands of people who are tracked sometimes for decades, with multiple check-ins from the researchers to track health changes in the participants.

Today's research article describes a case control study called the PRECAMA project (BMJ http://dx.doi.org/10.1136/bmjnph-2021-000335). Unlike cohort studies, case control studies are retrospective - they begin with those who have experienced the condition or outcome the researchers are studying and look back to see what characteristics might have contributed to the condition or outcome.

The PRECAMA project involves teams of researchers in four South American countries: Chile, Colombia, Costa Rica, and Mexico, and is coordinated by the International Agency for Research on Cancer. 525 premenopausal women between the ages of 25 and 45, who had been diagnosed with breast cancer but had not yet begun treatment, were recruited to participate in the PRECAMA project. The participants had to have resided in the same district and been seen at the same health institution for the previous three years.

In a case control study, the researchers identify people without the condition or outcome of interest who are otherwise similar to those with the condition. In this study, the authors identified 525 women who also lived in the same district for the previous three years, attended the same health care institution, and were within at least 3 years of age of the matching participant.

All 1050 participants responded to questionnaires which gathered information about general health and demographic information as well as specific variables such as number of children and whether they breastfed those children, age at menarche, and smoking status. They also filled out a country-specific dietary questionnaire that asked the participant to estimate how often, over the previous year, they had consumed the specific food, with frequencies ranging from over 6 times per day to less than once a month/never.

The authors note that compared to those without breast cancer, a higher percentage of those women who had been diagnosed with breast cancer had no children or had their first child at an older age. They also breastfed for shorter periods and more often had a history of benign breast disease. They exercised less, were more highly educated, taller, and had a lower Body Mass Index.

What the authors were especially interested in for this article was the participants' consumption of processed foods. For the purposes of their analysis, the authors classified foods using a system developed in Brazil, with four levels:

1. Unprocessed or minimally processed foods (fresh fruits and vegetables, unprocessed meat or fish)
2. Processed culinary ingredients (olive oil, butter, sugar)
3. Processed foods (canned fish, cheese, bread)
4. Ultra-processed foods (processed meats, ready-to-eat/heat foods such as hot dogs or burgers, soft drinks, packaged sweets or snacks)

The authors grouped the participants' consumption of ultra-processed foods into three increasing levels of intake while also estimating the percentage of total calories those foods contributed to the participant's diet. In addition to the dietary questionnaire, for this article the authors also measured the levels of industrial trans-fatty acids present in the blood of a subset of participants, noting that those who reported consuming more ultra-processed foods had higher blood levels of harmful trans-fatty acids.

Compared to those who consumed the lowest amount of processed foods, the researchers found that "Based on the proportion of calories intake from ultra-processed foods... we calculated that an increase of 20% in calories from ultra-processed foods was related to a 46% increase in the risk of breast cancer."

For a woman consuming 1,600 calories per day, 20% of those calories is just 320 calories. The authors noted that the greatest number of ultra-processed calories came from "industrial fruit juice beverages, industrial bread, breakfast cereals," snack foods, and margarine.

It's easy to consume 320 calories from ultra-processed foods: Minute Maid's Tropical Punch has 90 calories per 8-ounce serving, Honey Nut Cheerios has 140 calories per serving, and Nature's Own Whole Wheat Bread is 60 calories per slice (120 for a whole sandwich).

What this means for you

While case control studies offer weaker evidence than other types of studies, this should prompt you to look closely at just how much of your diet is ultra-processed foods. Beverages are an easy place to start, along with choosing the least processed breakfast cereals and healthier snacks such as fruits or vegetables rather than chips or cookies.

July 13, 2022