Intake of dairy milk is associated with a greater risk of breast cancer in women, according to a new study conducted by researchers at Loma Linda University Health.
“Dairy, soy and risk of breast cancer: Those confounded milks,” published in the International Journal of Epidemiology, found that even relatively moderate amounts of dairy milk consumption can increase women’s risk of breast cancer – up to 80% depending on the amount consumed.
First author of the paper, Gary E. Fraser, MBChB, Ph.D., said the observational study gives “fairly strong evidence that either dairy milk or some other factor closely related to drinking dairy milk is a cause of breast cancer in women.
“Consuming as little as 1/4 to 1/3 cup of dairy milk per day was associated with an increased risk of breast cancer of 30%,” Fraser said.
“By drinking up to one cup per day, the associated risk went up to 50%, and for those drinking two to three cups per day, the risk increased further to 70% to 80%.”
Current U.S. Dietary guidelines recommend three cups of milk per day. “Evidence from this study suggests that people should view that recommendation with caution,” Fraser said.
Dietary intakes of nearly 53,000 North American women were evaluated for the study, all of whom were initially free of cancer and were followed for nearly eight years.
Dietary intakes were estimated from food frequency questionnaires (FFQ), also repeated 24 hour recalls, and a baseline questionnaire had questions about demographics, family history of breast cancer, physical activity, alcohol consumption, hormonal and other medication use, breast cancer screening, and reproductive and gynecological history.
By the end of the study period, there were 1,057 new breast cancer cases during follow-up. No clear associations were found between soy products and breast cancer, independent of dairy.
But, when compared to low or no milk consumption, higher intakes of dairy calories and dairy milk were associated with greater risk of breast cancer, independent of soy intake.
Fraser noted that the results had minimal variation when comparing intake of full fat versus reduced or nonfat milks; there were no important associations noted with cheese and yogurt.
“However,” he said, “dairy foods, especially milk, were associated with increased risk, and the data predicted a marked reduction in risk associated with substituting soymilk for dairy milk. This raises the possibility that dairy-alternate milks may be an optimal choice.”
A hazardous effect of dairy is consistent with the recent AHS-2 report suggesting that vegans but not lacto-ovo-vegetarians experienced less breast cancer than non-vegetarians.
Fraser said the possible reasons for these associations between breast cancer and dairy milk may be the sex hormone content of dairy milk, as the cows are of course lactating, and often about 75% of the dairy herd is pregnant.
Breast cancer in women is a hormone-responsive cancer. Further, intake of dairy and other animal proteins in some reports is also associated with higher blood levels of a hormone, insulin-like growth factor-1 (IGF-1), which is thought to promote certain cancers.
“Dairy milk does have some positive nutritional qualities,” Fraser said, “but these need to be balanced against other possible, less helpful effects. This work suggests the urgent need for further research.”
More than 20 studies have investigated the relation between meat and dairy food consumption and breast cancer risk. Of the studies during the last decade that have evaluated the association between intake of meat and breast cancer risk, four (two case-control,1,2 two prospective3,4) found no statistically significant association while four (one case-control,5 three prospective6–8) reported a direct relation.
A meta-analysis of 12 case-control and 5 cohort studies published between 1966 and 1993 reported an increased risk of breast cancer with high versus low meat intake (relative risk [RR] = 1.18, 95% CI : 1.06–1.32).
The association with red meat consumption, evaluated in seven of these studies, was stronger (RR = 1.54, 95% CI : 1.31–1.82) than that observed for total meat consumption. A statistically significant association was not observed for poultry intake (RR = 0.94, 95% CI : 0.78–1.13).9
Results on the association of non-fermented milk consumption with breast cancer have also been conflicting, with two studies observing an inverse relation,10,11 one observing a direct effect,8 and three observing no statistically significant association.3,7,12 The meta-analysis referenced above reported an increased risk of breast cancer with high versus low consumption of milk (RR =1.20, 95% CI : 1.04–1.30) and with high versus low consumption of cheese (RR = 1.20, 95% CI : 1.02–1.40).9
To provide a comprehensive summary of the relation between meat and dairy consumption and breast cancer risk, we investigated these associations and potential non-dietary effect modifiers in the Pooling Project of Prospective Studies of Diet and Cancer (Pooling Project), using the primary data from eight large prospective studies
Study inclusion
The Pooling Project has been described previously.13 We obtained the primary data from eight prospective studies3,7,14–19 (Table 1) that met the following inclusion criteria:
(1) the study initially included at least 200 incident breast cancer cases,
(2) diet assessment at baseline using a comprehensive food frequency questionnaire, and
(3) availability of a validation study of the diet assessment instrument or closely related instrument.
Follow-up was conducted via questionnaires and the inspection of medical records and/or linkage to tumour and death registries and was estimated to be more than 90% complete in all cohorts. The Nurses’ Health Study had repeated measurements of dietary intake and was divided into two cohorts – Nurses’ Health Study (a) with follow-up from 1980–1986 and Nurses’ Health Study (b) with follow-up from 1986–1996.
Following the underlying theory of survival data, blocks of person-time in different time periods are statistically independent, regardless of the extent to which they are derived from the same people.20
Therefore, pooling estimates from these two time periods is equivalent to using a single time period but takes advantage of the enhanced exposure assessment in 1986 compared to 1980. Because data regarding white meat and dairy product consumption were limited in the New York State Cohort, this study was included only in the red meat, milk product, and total dairy fluids analyses.
Dietary variables
Food intake was measured at baseline in each study by food frequency questionnaire. To account for portion size variation between and within study populations, food intake data were analysed as grams (rather than as servings) consumed per day.
For the Iowa Women’s Health Study, Nurses’ Health Study
(a), and Nurses’ Health Study
(b), the frequency data for each food item were converted to grams per day using a weight21 for the serving size listed on the food frequency questionnaire.
For the Adventist Health Study and New York State Cohort, serving sizes were not specified on the food frequency questionnaire, therefore, the most common serving size specified on the questionnaires from the other studies was used to estimate the portion consumed. For all studies, missing responses for food items were coded as zero intake.
Meat and dairy groups were defined using standard dietetic and nutritional guidelines. For analyses of meat consumption, the main groups were red meat (bacon, ground beef, roast beef, beef steak, pork, veal, lamb, blood pudding, ham, hot dogs, pâté, beef liver, chicken liver, pork liver, turkey liver, kidney, sausage, processed luncheon meats [e.g. ham, corned beef, salami, bologna]), white meat (fish fillet, canned fish, chicken, turkey, shrimp, lobster, scallops, oysters, clams), eggs (boiled, poached, fried, scrambled, omelettes), and total meat products (including all meat containing food items and eggs).
Meat sub-groups included bacon products, sausage products (blood pudding, sausage), organ products (pâté, beef liver, chicken liver, pork liver, turkey liver, kidney), processed meats (bacon, blood pudding, ham, hot dogs, sausage, processed luncheon meats), poultry (chicken, turkey), seafood (fish fillet, canned fish, shrimp, lobster, scallops, oysters, clams), fish (including canned), and shellfish (shrimp, lobster, scallops, oysters, clams).
For analyses of dairy products, because there is considerable difference in the nutrient content of 100 g of a solid versus a liquid, fluids and solids were separated whenever possible.
Therefore, the main dairy groups were total dairy fluids (whole cream, whipped cream, custard or pudding, ice cream, skim milk, 0.5% milk, 1% milk, 2% milk, whole milk, evaporated milk, buttermilk, sherbet, ice milk, sour cream, lite yoghurt, regular yoghurt, yoghurt dressing), and total dairy solids (butter, high fat cheese, low fat cheese, hard cheese, cottage cheese, ricotta cheese, cream cheese, other cheese). Dairy sub-groups included cheese products (high fat, low fat, hard, other), milk products (skim, 0.5%, 1%, 2%, whole, buttermilk, evaporated milk), yoghurt products (lite, regular, dressing), high fat dairy fluids (whole cream, whipped cream, ice cream, 2% milk, whole milk, sour cream, buttermilk, evaporated milk), high fat dairy solids (butter, hard cheese, high fat cheese, cottage cheese, ricotta cheese, cream cheese, other cheese), low fat dairy fluids (skim milk, 0.5% milk, 1% milk, sherbet, ice milk, lite yoghurt, regular yoghurt, yoghurt dressing, custard or pudding), fermented dairy fluids (buttermilk, sour cream, lite yoghurt, regular yoghurt, yoghurt dressing), fermented dairy solids (high fat cheese, low fat cheese, hard cheese, cottage cheese, ricotta cheese, cream cheese, other cheese), high fat non-fermented dairy fluids (whole cream, whipped cream, ice cream, 2% milk, whole milk, evaporated milk), and low fat non-fermented dairy fluids (skim milk, 0.5% milk, 1% milk, sherbet, ice milk, custard or pudding). A low fat dairy solids sub-group was not created, because only one dairy product—low fat cheese—fitted into this group.
Associations were also evaluated for individual meat and dairy items for which at least five studies provided data.
Results
In the eight prospective studies included in these analyses, the median total meat intake among the controls ranged from 25 g per day in the Adventist Health Study (where many cohort members are practising vegetarians due to religious guidelines with 8% reporting zero red or white meat intake and 18% reporting total meat intake of ≤5 g per day) to 215 g per day in Nurses’ Health Study (a) (Table 1).
For reference, 1 quarter-pound hamburger weighs 114 g, 1/2 chicken breast weighs 86 g, and 1 can of tuna weighs 170 g.28 Median egg intake (1 egg weighs 50 g28) among the controls varied from 5 g per day in the Sweden Mammography Cohort to 22 g per day in the Nurses’ Health Study (a). Variability among studies in dairy product consumption among the controls ranged from 203 to 262 g per day of dairy fluids (8 oz of milk weighs 244 g, 6 oz of yoghurt weighs 170 g) and from 23 to 34 g per day of dairy solids (1 oz of cheese weighs 28 g).28
When modelled as continuous variables, no significant associations were observed between total meat, red meat, white meat, total dairy fluids, and total dairy solid intakes and risk of breast cancer (Table 2). Statistically significant direct associations within the Nurses’ Health Study (b) were observed for the effects of total meat (RR = 1.13 per 100 g per day increment, 95% CI : 1.03–1.21) and white meat (RR = 1.14 per 100 g per day increment, 95% CI : 1.03–1.26).
For the effect of total dairy fluids, a statistically significant inverse association was found within the Canadian National Breast Screening Study (RR = 0.93 per 100 g per day increment, 95% CI : 0.87–0.99).
There were no other statistically significant study-specific results. There was little evidence of confounding of the unadjusted results by the breast cancer risk factors included in our multivariate models (age-adjusted model data not shown) nor were the results altered by exclusion of cases that occurred in the first year of follow-up (data not shown). There was no evidence of effect modification by menopausal status.
When intakes of total meat, red meat, white meat, total dairy fluids, and total dairy solids were modelled as quartiles, no trends were observed in the associations of these groups with breast cancer risk (Table 3). In the opposite direction of the hypothesized deleterious association between red meat consumption and breast cancer risk9, six of the nine study-specific point estimates comparing quartile 4 versus quartile 1 of red meat consumption were below the null (Figure 1).
The difference in median intake between quartiles 4 and 1 ranged from 83 g per day in the Sweden Mammography Cohort to 209 in Nurses’ Health Study (a) for total meat intake, from 61 in the Sweden Mammography Cohort to 156 in the Nurses’ Health Study (a) for red meat intake, and from 35 in the Adventist Health Study to 128 in the Nurses’ Health Study (a) for white meat intake. The difference in median intake between quartiles 4 and 1 ranged from 366 g per day in the Sweden Mammography Cohort to 627 in the Adventist Health Study for total dairy fluids, while the difference ranged from 49 in the Iowa Women’s Health Study to 84 in the Adventist Health Study for total dairy solids.
In the continuous analyses (Table 4) for the meat sub-group and specific animal product consumption, the risk of breast cancer was directly related only to egg consumption with risk increasing 22% per 100-g per day (approximately two eggs) increase in daily intake (RR = 1.22, 95% CI : 1.03–1.45). Only the estimates from the Iowa Women’s Health Study (RR = 1.44, 95% CI : 1.00–2.08) and Nurses’ Health Study (b) (RR = 1.67, 95% CI : 1.20–2.32) were statistically significant. There was no evidence of effect modification by menopausal status for any of these associations (data not shown).
We evaluated the association with egg consumption after controlling for energy-adjusted cholesterol intake. While the pooled relative risk for cholesterol consumption in this updated dataset was 1.03 (95% CI : 1.00–1.07) for a 100-mg per day increment. When egg consumption was included in the model, the relative risk for cholesterol was attenuated (RR = 1.00, 95% CI : 0.95–1.05), but no material change in the relative risk for egg intake was observed (data not shown). In addition, when we controlled for saturated, polyunsaturated, and monounsaturated fat intakes, the effect estimate for egg consumption was unchanged (data not shown).
However, in the categorical analyses, a J-shaped association was observed with risk decreasing for >0–<14 g per day intake (RR = 0.93, 95% CI : 0.82–1.05), 14–<25 g per day intake (RR = 0.94, 95% CI : 0.82–1.09), and 25–<50 g per day intake (RR = 0.98, 95% CI : 0.80–1.21), and increasing for intakes of ≥50 g per day (RR = 1.07, 95% CI : 0.90–1.28) when compared to zero intake.
When meat sub-groups (organ products, processed meats, bacon products, sausage products, poultry, seafood, and fish) were modelled as quartiles, no trends were observed in the associations of these groups with breast cancer risk (data not shown).
No statistically significant associations were observed between dairy sub-group or specific dairy product intakes and the risk of breast cancer in the continuous analyses (Table 5). There was no evidence of effect modification by menopausal status (data not shown). When the dairy sub-groups (milk products, low fat dairy fluids, low fat non-fermented dairy fluids, high fat dairy fluids, high fat non-fermented dairy fluids, fermented dairy fluids, yoghurt products, high fat dairy solids, fermented dairy solids, cheese products) were modelled as quartiles, no trends were observed in the associations of these groups with breast cancer risk (data not shown).
We investigated whether several non-dietary breast cancer risk factors modified the association between intakes of the main meat and dairy groups and breast cancer risk. Of the 84 interactions tested, two significant pooled interactions were observed: (1) age at first birth and red meat consumption (P-value, test of effect modification = 0.03), and (2) age at menarche and total meat consumption (P-value, test of effect modification = 0.03). The relative risk for a 100-g per day increment in red meat consumption was 1.08 (95% CI : 0.94–1.24) for women who gave birth at age ≤20 and 0.84 (95% CI : 0.70– 1.01) for women who gave birth after age 30. The relation decreased monotonically across the four categories of age at first birth. The relative risk for a 100-g per day increment in total meat consumption was 0.96 (95% CI : 0.89–1.05) for women who began menstruating before age 12 and 1.08 (95% CI : 0.96– 1.20) for women who began menstruating at age ≥15. This association was also monotonic across the five categories of age at menarche, although the change in the relative risks across categories was small.
More information: Gary E Fraser et al, Dairy, soy, and risk of breast cancer: those confounded milks, International Journal of Epidemiology (2020). DOI: 10.1093/ije/dyaa007
