Cow's Milk and Dairy Consumption: Is There Now Consensus for Cardiometabolic Health?

Abstract

Cow's milk and dairy products derived from this complex food source have long been proposed as beneficial to human health, yet underlying clinical evidence of direct benefit continues to raise controversy. Limited evidence supports positive cardiometabolic effects of a number of dairy macro- and micronutrient components including whey protein and casein, unsaturated fats, milk fat globule membrane (MFGM) and polar phospholipids, vitamin D and calcium, in addition to non-bovine components including bacterial and yeast probiotics. More controversial remain lipid components trans fats, including trans vaccenic acid, trans palmitoleic acid, and conjugated cis trans linoleic acid (CLA), plus medium-chain and odd-chain dairy fats. New evidence is rapidly identifying multiple pathways by which these dairy nutrients may effect health. Processing, including fermentation and homogenization, may also have positive effects. Conversely, the high saturated fat content of dairy has long raised concern, aligned with international guidelines to minimize dietary intake of animal-origin saturated fatty acids (SFA) to achieve better cardiometabolic health. However, led in part by observational studies and meta-analyses showing dairy to have no or even an inverse association with cardiometabolic health, evidence from randomized controlled trials (RCTs) has been scrutinized over the last 5 years, and focus on low-fat dairy has been challenged. Recent evidence supports the hypothesis that adverse effects of SFAs on metabolic health may be ameliorated when these fats are consumed within a complex matrix such as milk, cheese or yogurt, and that dairy food categories may influence outcomes as much as total fat content. For example, yogurt and high-fat, high-SFA cheese have a negative association with risk of type 2 diabetes (T2D) in many, not all, published trials. However, large sample dairy RCTs of long duration with CVD or T2D incidence as primary endpoints are lacking. This is a clear research gap, with these clinical studies required if a causative link between dairy and improved cardiometabolic health is to be confirmed and in turn promoted through dietary guidelines. Current advisories from national guidance groups such as American Heart Association (AHA) and European Society of Cardiology (ESC) continue to promote consumption of low-fat dairy products, whilst liquid milk and yogurt remain part of nutrition guidelines from joint American Diabetes Association (ADA)/European Association for Study of Diabetes (EASD) reports, and as part of a “no-one-size-fits-all” answer to diet and T2D by the ADA in their most recent 2019 Consensus Report.

The Role of Bovine and Non-Bovine Milk in Cardiometabolic Health: Should We Raise the “Baa”?

“…More pertinently, a body of data supports a negative association between milk intake and the risk of developing dysglycaemia, dyslipidaemia, and hypertension [1,4]. However, with gold-standard data from long-term randomised controlled trials (RCTs) featuring type II diabetes (T2D) and cardiovascular disease (CVD) incidence as primary endpoints not currently available, the causality of these findings remains to be confirmed [5]. Nonetheless, putative explanations for a possible metabolic syndrome risk reduction include a direct modulation of the glycaemic response [2,6], and an indirect modulation of body weight through upregulation of postprandial thermogenesis [6][7][8] and/or suppression of appetite [9][10][11].…”

“…However, with the current knowledge coming from trials of limited sample size and inconsistent study design, this verdict cannot yet be determined to be conclusive and remains largely controversial. Indeed, the need for longer-term RCTs evaluating the effect of CLA isomers on incidence of T2D and CVD is emphasised [5].…”

“…Some researchers have also speculated that CLA can stimulate the uptake and subsequent utilisation or storage of glucose, without inducing an excessive secretion of insulin [166]. However, these claims are not supported by the conclusions of recent systematic reviews that have collectively examined clinical RCT data [5,167]. Indeed, one double-blind RCT in men with obesity reported a pro-diabetic effect of the CLA isomer trans-10, cis-12 [168].…”

“…Despite a plethora of different trial designs (e.g., RCTs, observational, in vitro, and animal studies) documenting links between individual milk components (or occasionally whole milk product) and biomarkers of T2D, there remains a lack of high-quality evidence for a causal link between milk consumption and incidence of T2D [5]. A population-based epidemiological study, the CARDIA study, reported a dose-dependent inverse relationship between the inclusion of dairy products in the diet and the risk of overweight individuals developing insulin resistance syndrome over a 10-year period [1].…”

Cardiometabolic health benefits of dairy-milk polar lipids

“… 84 Such guidance arises in part from evidence originating from the Seven Countries Study and similar investigations 85 , 86 that supports recommendations to limit total fat and saturated fat to < 30% and < 10% of total dietary energy, respectively. 87 , 88 These recommendations, especially those related to saturated-fat consumption, are the subject of much debate 89 and are inconsistent with findings of observational studies suggesting that all-cause mortality, CVD, stroke, and T2DM are unrelated to saturated-fat intake. 90 Others also have reported that dietary saturated fat increases circulating low-density lipoprotein cholesterol (LDL-C) but noted that the increase often corresponds with an increase in large LDL particles that have a weaker relationship to CVD risk than do small LDL particles that have greater atherogenicity.…”