expert opinion

The science behind dietary recommendations for dairy

May 15, 2014

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Connie M. Weaver, PhD, Professor and Head of the Department of Foods & Nutrition at Purdue University, West Lafayette, Indiana, USA.

“The contribution of dairy products to providing recommended calcium intakes has largely driven the recommendation for dairy. Since the agricultural revolution when energy sources shifted from plant foods relatively high in calcium in the diets of hunter-gatherers to cereal crops with low calcium content, the major source of dietary calcium has been milk. In addition to calcium, dairy products provide many other nutrients. They are a good source of high-quality protein, potassiummagnesium, phosphorus, zinc, selenium, vitamin A, vitamins B1 (thiamin), B2 (riboflavin) and B12, as well as vitamin D (when fortified). Not all dairy products are equal sources of nutrients. Dairy intake recommen- dations vary from region to region. Some countries, such as the United Kingdom, provide general recom- mendations to consume milk and other dairy products daily, but most countries have quantitative recom- mendations that usually range from 2 to 3 servings or cups of milk or yogurt or sometimes the equivalent serving of cheese. The 2010 Dietary Guidelines for Americans specify low-fat dairy products because of concern over the high prevalence of obesity. The amount of milk and milk products (and other food groups) is determined by the Dietary Guidelines for Americans Advisory Committee (DGAC) on the basis of 1) intakes of the food groups needed to achieve the Dietary Reference Intakes (DRIs) for essential nutrients without exceeding energy needs and 2) the evidence for the relation of intake of food groups and relevant health outcomes (1). Twelve food intake patterns developed for various energy and nutrient needs of different age and sex groups were created to meet the DRIs for that subpopulation and guidance from evidence-based reviews. For some nutrients, the Recommended Dietary Allowance (RDA) was used, and for others with insufficient evidence to determine the RDA, the Adequate Intake (AI) for a healthy population was used.

The intent of food guidance is to be flexible to accommodate the diversity of culture and preferences of the population. For most food groups, there is a large choice of items within the category. However, for the milk and milk products food group, most foods within the category stem from a similar raw ingredient, i.e., milk from a domesticated animal supply. The food guides include alternative sources of protein and calcium and guidance for those with milk protein allergy or lactose intolerance. However, few people who avoid dairy products achieve recommended intakes of several shortfall nutrients, such as calcium, potassium, magne- sium, riboflavin, and vitamin D. Milk products, along with fruit, vegetables, and whole grains, were identified by the 2010 DGAC as foods that need to be increased to meet nutrient needs and for improved health (2). The best and most economical source of the limiting nutrients calcium, potassium and magnesium is dairy (3). A number of studies have indicated that milk intake is a marker for dietary quality because of its nutrient contributions (3, 4), while a recent study found that yogurt is also a marker of dietary quality (5).

The 2010 DGAC concluded that the evidence for a relationship between increased intakes of milk and milk products and positive effects on health is moderate (bone health in children, cardiovascular disease, blood pressure and type 2 diabetes in adults) to limited (bone health and metabolic syndrome in adults) (2). The long latency period for chronic disease outcomes make randomized controlled trials (RCTs) with food imprac- tical, except in some cases in vulnerable populations. Consequently, meta-analyses and systematic reviews on the relation between milk and milk product intake and health tend to use RCTs of biomarkers or on prospective or observational studies. The consequences of excluding dairy in the diet are most associated with compromised bone health. Effects apparently can begin in utero, as evidenced by a study that showed increased consumption of milk and milk products by pregnant women at 28 week gestation significantly predicted total body and bone mineral content (BMC) of children at age 6 years (6). The pubertal growth spurt is a critical time for building peak bone mass to protect against fracture risk as a child and later in life. Almost half of adult peak bone mass is acquired during adolescence (7). Approximately 95% of adult peak bone mass is acquired by the age of 16.2 years (8), emphasizing that nutrition can only influence peak bone mass appreciably before the end of adolescence. Thereafter, any benefits are to minimizing loss of peak bone mass, a much lower return on investment approach. A meta-analysis of trials of dairy products and dietary calcium on BMC in children showed significantly higher total body and lumbar spine BMC with higher intakes when the comparison group had low calcium intakes (9). A meta-analysis of prospective cohort studies concluded that there is no overall association between milk intake and hip fracture risk in women, whereas, in men, evidence was suggestive of a benefit of higher milk intake (10). There are also cohort studies in milk avoiders and milk consumers. In women aged 38–57 years, women who were lactose into- lerant consumed 570 mg calcium daily compared with 850 mg daily in the lactose-tolerant group, and had double the risk of lower body bone fracture (11).

As is the case with bones, there is a lack of adequately powered RCTs on cardiovascular disease and blood pressure; the evidence used by the 2010 DGAC for a benefit of dairy is based on systematic reviews and meta-analyses of prospective and cohort studies. A systematic review and meta-analysis (11) showed a reduction in risk of myocardial infarction, ischemic heart disease, hypertension, and stroke in those consu- ming the highest amount of milk compared with those consuming the lowest amount. This was consistent with another systematic review (12) and with large prospective cohort studies. A recent meta-analysis reported a 13% reduction in risk of all-cause mortality, an 8% reduction in risk of ischemic heart disease, and a 21% reduction in risk of stroke in those with the highest compared with the lowest intake of dairy (13). Little evi- dence exists for individual dairy foods, although yogurt was associated with better systolic blood pressure, and fluid milk was associated with reduced systolic and diastolic blood pressure (14).

One meta-analysis of four prospective studies constituted the evidence used by the 2010 DGAC to determine the benefit of dairy in reducing risk of diabetes. Those with the highest milk consumption compared with those with the lowest milk consumption had a 15% reduction in risk of type 2 diabetes (15). In a more recent meta-analysis of seven cohort studies, there was a reduction of 18% in the risk of type 2 diabetes associated with low-fat dairy and a reduction of 17% with yogurt (16). The benefit of dairy on reduced risk of metabolic syndrome was based on one systematic review with meta-analysis, one prospective cohort study, and three cross-sectional cohort studies (2). The systematic review and meta-analysis reported a 26% reduction in risk of metabolic syndrome in those consuming the highest amounts of milk compared with those consuming the lowest amounts (17).

Meeting nutrient recommendations has little to do with fat content or flavorings in the dairy products. The recommendation for low-fat dairy is more of a philosophical argument to reduce energy intake from fat and added sugar than from evidence of health concerns. Milk and cheese contribute 9.2% of intake of energy, 10.9% of fat, and 8.3% of carbohydrate in the diet of Americans, but these products also contribute 46.3% of calcium, 11.6% of potassium, and 7.9% of magnesium in the American diet, which may provide overriding benefits to health (18).”

Based on: Weaver C. M. How sound is the science behind the dietary recommendations for dairy? AJCN. Published online March 2014.

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18. Huth P. J. et al. Major food sources of calories, added sugars, and saturated fat and their contribution to essential nutrient intake in the U.S. diet: data from the national health and nutrition examination survey (2003-2006). Nutr J. 2013; 12:116.