Science Supporting Dairy's Role in Bone Health
We review a lot of research and help translate it for fellow health and wellness professionals and educators working with the public. As we do this, it is important to frame up each new study within the broader body of science on a given topic. Here’s how a recent study fits within the totality of the evidence on bone health.
A paper published last fall in the British Medical Journal was an observational study based on two large groups of Swedish men and women, and results associated drinking three or more glasses of milk per day with greater risk of mortality and higher risk of fractures. Studies of this nature show correlations vs. causation and add to the growing discovery.
The impact of milk, and the nutrients it provides, on bone has been well documented by an extensive body of peer-reviewed research, including randomized controlled trials (the gold standard of scientific research) — more details can be found in this research summary.
The authors of this study hypothesized that since milk contains D-galactose (when lactose is digested it is split into its component sugars glucose and galactose), which has been shown to increase oxidative stress and aging in animal studies, that milk consumption will be associated with oxidative stress, higher mortality and increased risk of fracture.
When you look at this study through the lens of the total body of evidence and standards of practice, there are considerations when communicating the results:
The study hypothesis was based on animal studies, which cannot be generalized to human outcomes. There have been no studies in humans that have demonstrated this effect. Although the study found that drinking milk was associated with two markers of oxidative stress evaluated, the study did not demonstrate that lactose or D-galactose was the cause.
The researchers only examined two biomarkers of inflammation and did not factor in others that have shown favorable effects of dairy food (i.e., milk cheese and yogurt) consumption on inflammation (Nettleton JA, 2006; Qureshi NM, 2009; Esmaillzadeh A, 2010; Zemel MB, 2008;Panagiotakos DB, 2010; Zemel MB, 2010; Stancliffe RA, 2011).
The researchers used a food frequency questionnaire (FFQ) administered only once to most subjects to assess food consumption. Relying on a single FFQ does not account for any changes in eating patterns over time that may affect disease outcomes.
The authors did not provide an overview of the totality of the research which shows a favorable association of milk and dairy food consumption on reducing the risk of cardiovascular disease, type 2 diabetes, hypertension and improving bone health in children and adolescents.
A large body of research led to the 2010 Dietary Guidelines for Americans recommendation for three servings of low-fat and fat-free milk or equivalent milk products daily as part of a healthy diet. More recently, the 2015 Dietary Guidelines Advisory Committee Scientific Report noted that dairy foods are excellent sources of nutrients of public health concern, including vitamin D, calcium, and potassium, and consumption of dairy foods provides numerous health benefits including a link to lower risk of diabetes, metabolic syndrome, cardiovascular disease and obesity.
The researchers of this observational study acknowledged that the results should be “interpreted cautiously,” since this type of study can only show an association and cannot prove cause and effect. And further studies are needed before any dietary recommendations based on this study are made.
In the bigger picture of bone health, it is also important to note measures such as bone mineral density (BMD) and bone mineral content (BMC) are better indicators of bone health than fracture risk. Studies evaluating BMD and BMC support the positive benefits of milk/dairy foods on bone health (see research summary above).
I hope this information is helpful to you as you help your clients interpret research within the broader context of the evidence.
You may find this article, Interpreting Science in a Social Media World, helpful, as well as these time-tested guidelines, Improving Public Understanding: Guidelines for Communicating Emerging Science on Nutrition, Food Safety and Health.