A large, retrospective cohort study adds to growing body of evidence that fiber intake provides a protective benefit for young women.
Farvid MS, Eliassen AH, Cho E, Liao X, Chen WY, Willett WC. Dietary fiber intake in young adults and breast cancer risk. Pediatrics. 2016;137(3):1-11.
This was a retrospective study that utilized data generated by the Nurses’ Health Study to determine multivariable-adjusted Cox proportional hazards modeling relative risks (RRs) for breast cancer based on how much dietary fiber was consumed during adolescence.
The Nurses’ Health Study II (NHSII) is an ongoing prospective cohort of 116,430 female registered nurses aged 25 to 42 when enrolled in 1991. Among the 90,534 premenopausal women who completed a dietary questionnaire in 1991, 2833 cases of invasive breast cancer were documented during 20 years of follow-up. In 1998, 44,263 of these women also completed a questionnaire about their diets during high school. Among this subgroup, 1,118 cases of breast cancer were documented.
Total fiber, insoluble fiber, and soluble fiber intake during adolescence and early adulthood
Diagnosis with breast cancer
Among all women, total dietary fiber intake during early adulthood was associated with a significant 19% decrease in risk for breast cancer (BC). The RR for the highest vs lowest quintile was 0.81 (95% confidence interval [CI]: 0.72-0.91; P for trend=0.002). Higher intakes of soluble fiber were associated with a 14% lower risk (RR: 0.86; 95% CI: 0.77-0.97; P=0.02) and insoluble fiber with a 20% decreased risk (RR: 0.80; 95% CI: 0.71-0.90; P<0.001). Total dietary fiber during adolescence was also associated with lower BC risk (RR: 0.84; 95% CI: 0.70-1.01; P=0.04). Data on both adolescent and adult fiber intake were available for 41,092 women. When their average fiber intake was calculated for both periods, the RR comparing highest with lowest quintiles was 0.75 (95% CI: 0.62-0.91; P for trend=0.004).
In recent years it has become apparent that the progression to breast cancer starts early in life. Exposures during childhood and adolescence affect a woman’s long-term risk of breast cancer. We have seen a surge of studies that examine diet during pre-adolescence and adolescence that suggest that efforts at prevention may be more effective if begun in adolescence rather than in adulthood.1 Earlier studies suggested that caloric restriction, poor food quality, high total fat intake, and high alcohol intake might increase risk but these studies were methodologically limited and the results inconsistent. The newer studies have hopefully overcome these weaknesses and provide useful guidance.
Breast cancer prevention must start earlier rather than later in life.
The risk of abnormal cellular changes is highest when breast tissue is growing most rapidly—between the time a young woman begins her menstrual periods and her first full-term pregnancy. Interventions to prevent breast cancer are most effective when started at an early age and continued during the course of a woman’s life.2
This current paper under discussion written by Maryam Farvid PhD and colleagues at Harvard’s T. H. Chan School of Public Health is the latest in a recent series on adolescent diet and adult breast cancer risk.
Liu et al reported in 2014 that dietary fiber, vegetable protein, and nuts provide a protective benefit against breast cancer. A food frequency questionnaire was used in this Canadian cohort to compare diet followed between the ages of 10 and 15 and the risk of diagnosis of breast cancer as an adult. Some 2,865 breast cancer cases were compared to 3,299 control cases and estimated odds ratios (ORs) and 95% confidence intervals (CIs) were determined through logistic regression. Inverse associations were found between intakes of dietary fiber, vegetable protein, vegetable fat, and nuts during adolescence with breast cancer risk, an association that persisted after controlling for adult intakes. In other words, eating dietary fiber, vegetable protein, vegetable fat, and nuts during adolescence was associated with reduced breast cancer risk. The ORs (95% CI) for the highest vs the lowest quintile of intake were 0.66 (0.55-0.78; P for trend<0.0001) for fiber, 0.80 (0.68-0.95; P for trend=0.01) for vegetable protein, 0.74 (0.63-0.87; P for trend=0.002) for vegetable fat, and 0.76 (0.61-0.95 for ≥1 serving/day vs <1 serving/month intake; P for trend=0.04) for nuts. Again in simpler words, eating lots of fiber, vegetable protein, vegetable fat, and nuts lowered risk of breast cancer by 34%, 20%, 26%, and 24% respectively.3
These decreases in risk for breast cancer reported in this Liu paper are consistent with what Maryam Farvid reports in the current paper.
A July 2015 study, also written by Farvid et al, failed to find a significant association between a high carbohydrate or a high glycemic diet during adolescence and breast cancer risk. Surprisingly, the study found that diets high in GI, GL, insulin index, and insulin load during adolescence or early adulthood were not associated with an increased risk of breast cancer.4 Carbohydrate consumption may be less of a problem than we might have suspected.
These results are also surprising because they seem to contradict a 2013 paper by Minicozzi et al, which found that high fasting blood glucose and obesity significantly and independently increased the risk of breast cancer death in hormone receptor-positive disease,5 results that suggest glycemic loads should play a role.
The same team of researchers, Farvid et al, reported in April 2015 that meat consumption during adolescence was significantly associated with higher premenopausal breast cancer risk. When comparing the highest vs lowest quintile of consumption, high meat eaters had a 43% increase in relative risk (RR: 1.43; 95% CI: 1.05-1.94; P for trend=0.007). This association did not hold in postmenopausal breast cancer. Poultry was associated with lower risk of breast cancer overall. For each serving per day, the RR was 0.76 (95% CI: 0.60-0.97). Replacement of one serving per day of red meat with one serving of a combination of poultry, fish, beans, or nuts was associated with a 15% lower risk of breast cancer overall (RR: 0.85; 95% CI: 0.74-0.96) and a 23% lower risk of premenopausal breast cancer (RR: 0.77; 95% CI: 0.64-0.92).6
As a result of these recent studies, we are accumulating knowledge that allows us to more accurately describe how a young woman should eat to lower her risk of developing breast cancer later in life. Her goal should be a diet that emphasizes dietary fiber, vegetable protein, vegetable fat, nuts, and poultry and decreases meat consumption by substituting fish, poultry, or beans. Lowering glycemic load may or may not be useful.