Mixing of Apples and Oranges in Milk Research: A Cohort Analysis of Non-Fermented Milk Intake and All-Cause Mortality

Mortality in relation to type of milk intake is unclear. We present mortality rates by intake of non-fermented milk fat content type and examine the degree of bias when other fat content types of non-fermented milk are kept in the reference category. For this purpose, we used a longitudinal cohort consisting of 61,433 women who had been administered food frequency questionnaires in 1987-1990 and in 1997, and analyzed time to death. Non-fermented milk consumption was divided into low <= 0.5%, medium 1.5%, or high fat 3%. For each specific type of milk, the first analysis (A) is restricted to those who consumed less than one serving per day of the other milk subtypes. In the second analysis (B), everyone is retained, i.e., leading to a reference category contaminated with other milk consumers. During follow-up, 22,391 women died. Highest (>= 3 glasses/day) vs. lowest consumption category of milk (<1 glass/day) with 0.5% fat content was associated with a multivariable hazard ratio (HR) of 1.71 (95%CI 1.57-1.86) in analysis A, whereas the same comparison with a contaminated reference category in analysis B provided a HR of 1.34 (95%CI 1.24-1.45),p-value for homogeneity <0.0001. The corresponding HRs for 1.5% fat milk were: 1.82 (95%CI 1.63-2.04) and 1.38 (95%CI 1.25-1.51), and for 3% fat milk 1.95 (95%CI 1.77-2.15) and 1.40 (95%CI 1.29-1.52). HR for >= 3 glasses/day of total milk was 1.95 (95%CI 1.84-2.06). We observe a higher mortality in women with high milk consumption, irrespective of milk fat content. A contaminated reference group substantially attenuates the actual estimates.