The Nurses Study found no correlation between milk consumption and strong bones – how should this be interpreted?

It is important to note that the ‘Nurses study’ consists of a compilation of several pieces of research based on a single cohort. Therefore, there are several published papers based on the Nurses Study. In contrast to the original study published in 1997, the most recent study using the Nurses’ Health Study data in 2018 found that each serving of milk per day was associated with an 8% lower risk of hip fracture in men and women; with total dairy intake associated with a 6% lower risk of hip fracture per daily serving in men and women. Unlike the original study, this finding used both the Nurses’ study cohort as well as the Health Professional Follow-up Study, meaning that both genders were included; 123,000 participants were observed (compared to approx. 80,000 in the 1997 study); and follow up was over 32 years (compared to 12 years in 1997).

A number of studies have been published by the same author (Feskanich et al.) looking at milk intake and the risk of fracture using the Nurses’ Health Study, which report conflicting findings. Two earlier studies ( 1997 , 2003 ) report no reduction in fracture risk with a higher consumption of milk or other food sources of calcium. However, although these studies boast of a large number of participants followed across many years, there are a number of important limitations to the data collection; with many of these identified by the authors themselves. Firstly, the study design was observational, meaning that no intervention was applied and that only associations can be identified, without cause and effect conclusions. In fact, the study highlights in its discussion that there is “no reason to believe that dairy calcium itself was responsible for the observed increase in risk of hip fracture, rather some other characteristic of women who consume dairy foods might have contributed to the elevated risk”.