Milk Fat and Human Health. Oportunities and Challenges

Dairy products are an important source for many vital nutrients including high quality protein, energy, and many essential minerals and vitamins and public health organizations around the world include dairy products in recommendations for a healthy, well-balanced diet (Rice et al., 2013). Dairy products, however, are also a major dietary source of saturated fat, accounting for about 30% of the saturated fat intake (Lock et al., 2014). For over a half-century, saturated fat has been demonized as the major cause of cardiovascular vascular disease (CVD) and public health recommendations are to reduce dietary intake of saturated fat. As a consequence, perception of milk fat is negative and typical dietary advice is to consume only reduced-fat or non-fat dairy products.

Cardiovascular diseases include coronary heart disease, cerebrovascular disease, and other related disorders of the heart and blood vessels. Globally CVD is a leading cause of death, accounting for about one-third of all deaths (Roger et al., 2011). New research and re-evaluation of previous research is challenging long-held dogma on the relationship between saturated fat, milk fat, and CVD. While national organizations and health professionals will be cautious in revising decade-old recommendations, the last few years have provided game-changing scientific evidence that revises the paradigm for the connection between saturated fat and CVD. Furthermore, these new studies have convincingly demonstrated the important role that milk and dairy products play in health maintenance and the prevention of chronic diseases. In the following sections we will deal briefly with some background and historical aspects related to saturated fat and CVD, and then highlight recent findings related to the role of milk and milk fat in human health. 

The idea that dietary fat, specifically saturated fat, was a major public health concern was first proposed by Keys (1953). Using WHO population data, he reported a curvilinear relationship between the intake of fat and deaths from coronary heart disease for six countries. Based on this, Keys (1953; 1980) proposed the “diet-heart hypothesis” suggesting a sequence of etiologic relations between dietary saturated fat, circulating cholesterol, and CVD. In retrospect, other scientists identified several misrepresentations and critical flaws in Keys work; nevertheless, Keys was a charismatic individual and the diet-heart hypothesis was well received by many health practitioners and policy makers. This particularly impacted public perception of dairy products because dairy fat contains 60 to 70% saturated FA (Figure 1). Thus, beginning in the 1970s public health recommendations were to dramatically reduce the intake of saturated fat including a reduction in the consumption of dairy products and/or a shift to low-fat or non-fat dairy products. 

Figure 1. Fatty acid composition of U.S. retail milk supply. Bar coding for fatty acids (FA) is as follows: white + black bars = saturated FA, black bars = saturated FA considered atherogenic, shaded bars = unsaturated FA and crossed hatched bar = trans FA. Adapted from O’DonnellMegrano et al. (2011).

The case against dairy foods was also linked to an imperfect understanding of cholesterol as a surrogate marker of CVD risk (Lock et al., 2014). Lauric acid (12:0), myristic acid (14:0), and palmitic acid (16:0) represent about 40% of milk FA (Figure 1), and are noteworthy because they cause pronounced increases in serum low density lipoprotein-cholesterol (LDL-cholesterol), a biomarker for CVD risk (Hegsted et al., 1965). However, they also cause an increase in serum high-density lipoprotein-cholesterol (HDL-C) and this cholesterol fraction reduces the risk of CVD. Thus, when changes in both LDL-C and HDL-C are considered, these saturated FA do not reflect an increased risk for CVD as convincingly demonstrated by a meta-analysis of data from 60 clinical studies (Mensink et al., 2003). 

For the last 50 years the CVD concern has led to dietary recommendations by nationally recognized bodies to reduce saturated fat intake to as low as possible. The World Health Organization recommends saturated fat intake be reduced to less than 10% of dietary calories and the Guidelines by the American Heart Association recommends saturated fat be reduced to an even lower <7% of total calories (Huth and Park, 2012). However, scientists have started to challenge these recommendations emphasizing flaws in the diet-heart hypothesis of Keys and the lack of support from scientific studies (Ravnskov, 2002; Hoenselaar, 2012; Lawrence, 2013). Several excellent investigations have been reported in the last few years that challenge these recommendations and the following represent key examples.

Elwood et al. (2010) conducted a meta-analysis of prospective cohort studies to examine associations between the intake of milk and dairy products and the incidence of ischemic heart disease and stroke. Results indicated “a reduction in risk in subjects with the highest dairy consumption relative to those with the lowest intake”; relative risk values were 0.92 for ischemic heart disease and 0.79 for stroke (Elwood et al., 2010).

Siri-Tarino et al. (2010) conducted a meta-analysis of 21 prospective epidemiologic studies that covered a 5 to 23 year follow-up of 347,747 subjects and again results indicated “there is no significant evidence that dietary saturated fat is associated with an increased risk of coronary heart disease or CVD” (Siri-Tarino et al., 2010).

Soedamah-Muthu et al. (2011) conducted a similar meta-analysis comparing intake of dairy products and the risk of CVD (including coronary heart disease, stroke, and mortality); their meta-analysis gained greater analytical power by including different dairy food categories and different ranges of intake. Results demonstrated that “milk intake was not associated with total mortality, but may be inversely associated with overall CVD risk”; relative risk for the later was 0.94 (Soedamah-Muthu et al., 2011).

Goldbohm et al. (2011) reported results from a large cohort study designed to examine the association between the intake of dairy products and mortality; data covered a 10 year period for 120,852 men and women, and results indicated no association between dairy product consumption and stroke mortality for men or women. Likewise, there was no association between total milk intake and ischemic heart disease mortality in men, whereas a small positive association was observed for women (relative risk = 1.07; Goldbohm et al., 2011).

In a remarkable effort, Chowdhury et al. (2014) summarized prospective, observational studies (n = 32 based on dietary intake involving 512,420 participants; n = 17 based on FA biomarkers involving 25,721 participants) and randomized, controlled trials (n = 27 involving 105,085 participants). They concluded, “Current evidence does not clearly support cardiovascular guidelines that encourage high consumption of polyunsaturated fatty acids and low consumption of total saturated fats” (Chowdhury et al., 2014).

There is a growing body of research that supports a paradigm shift in conclusions about the relationship between saturated fat, milk fat, and cardiovascular health. Comprehensive reviews by Parodi (2009), Givens and Minihane (2011), Kratz et al. (2012), Huth and Park (2012), and Lock et al. (2014) all concluded that the majority of studies have failed to support any adverse association between the intake of dairy products and CVD, regardless of milk fat levels. Overall, saturated fat, targeted by nearly all nutrition-related professional organizations and governmental agencies, has little or no relation to CVD. 

Consumers are increasing aware of the connection between diet and health, and scientists are being asked to clarify the role of specific foods in health maintenance and the prevention of chronic diseases. Multidisciplinary studies in developing countries demonstrate that when diets of school children had little or no animal source foods, the intake of essential micronutrients was inadequate resulting in negative health outcomes including severe problems such as poor growth, impaired cognitive performance, neuromuscular deficits, psychiatric disorders and even death (Neumann et al., 2002; Randolph et al., 2007). Milk and dairy products contain 9 essential nutrients, making them a nutrient-rich food that is included in dietary recommendations around the world.

Long-term effects of milk and dairy products on health and the prevention of chronic diseases of the general population are also of interest, and these would best be determined in randomized controlled trials. There have been no such trials and realistically none are likely because of the required number of subjects and the long latency period associated with chronic diseases. The best evidence, therefore, comes from prospective cohort studies with disease events or death as the outcome. A number of prospective cohort studies have evaluated the association between the intake of milk and dairy products and the incident of chronic diseases. Results of meta-analysis of such studies provide convincing evidence that milk and dairy products are associated with beneficial effects for long-term health maintenance and the prevention of chronic diseases. The beneficial effects in reducing the risk of CVD was discussed earlier, and additional examples of chronic diseases for which consumption of dairy products reduces risk include: diabetes, obesity, metabolic syndrome and many types of cancer (Elwood et al., 2008; 2010; Tremblay and Gilbert, 2009; Kliem and Givens, 2011; Grantham et al., 2012; Korhonen, 2012; Kratz et al., 2012; Rice et al., 2013). Overall, the science clearly demonstrates the importance of milk and dairy products in childhood development, health maintenance, and the prevention of chronic diseases. Indeed, linking the benefits of milk consumption with deaths from key chronic diseases led Elwood et al. (2008) to conclude that high milk consumers have an “overall survival advantage”. 

Chowdhury, R., S. Warnakula, S. Kunutsor, F. Crowe et al. 2014. Association of dietary, circulating, and supplement fatty acids with coronary risk. Ann. Int. Med. 160:398-406.

Elwood, P.C., D.I. Givens, A.D. Beswick, A.M. Fehily et al. 2008. The survival advantage of milk and dairy consumption: an overview of evidence from cohort studies of vascular diseases, diabetes and cancer. J. Am. Coll. Nutr. 27:723S-734S.

Elwood, P.C., J.E. Pickering, D.I. Givens, and J.E. Gallagher. 2010. The consumption of milk and dairy foods and the incidence of vascular disease and diabetes: an overview of the evidence. Lipids 45:925-939.

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Goldbohm, R.A., A.M. Chorus, F. Galindo Garre, L.J. Schouten, and P.A. van den Brandt. 2011. Dairy consumption and 10-y total and cardiovascular mortality; a prospective cohort study in the Netherlands. Am. J. Clin. Nutr. 93:615-627.

Grantham, N.M., D.J. Magliano, A. Hodge, J. Jowett, P. Meikle, and J.E. Shaw. 2012. The association between dairy food intake and the incidence of diabetes in Australia: the Australian Diabetes, Obesity and Lifestyle Study. Public Health Nutrition, Available on CJO 2012 doi:10.1017/S1368980012001310.

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Soedamah-Muthu, S.S., E.L. Ding, W.K. Al-Delaimy, F.B. Hu et al. 2011. Milk and dairy consumption and incidence of cardiovascular diseases and all-cause mortality: doseresponse meta-analysis of prospective cohort studies. Am. J. Clin. Nutr. 93:158-171.

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