Dairy Products and MetS
Several studies suggest that the consumption of dairy food may influence the development of metabolic syndrome (MetS). The metabolic syndrome, described for the first time by Kilin (Kilin 1923), is a disorder of energy utilization and storage. It is also known as cardiometabolic syndrome or insulin resistance syndrome. It is described as a cluster of cardiometabolic risk factors, such as abdominal obesity, insulin resistance, high fasting blood glucose, hypertension, and dyslipidemia that create an increased risk of cardiovascular disease (Alberti et al., 2006). In fact, MetS can favor the onset of more serious diseases such as cardiovascular disease (CVD) (Isomaa et al., 2001) and type II diabetes (Ford et al., 2010).
Nutrition, a highly modifiable element in the habits of people, may sensibly reduce risk factors. A large, comparative study has been carried out on literature produced over a period of about 50 years concerning the association between the consumption of milk products and MetS. Thirteen reviews were analyzed: ten cross-sectional studies and three prospective cohort studies (Crichton et al., 2011). Among the cross-sectional studies, five indicated that dairy products were inversely associated with MetS considering an intake of milk (Elwood et al., 2007), milk and cheese (Mennen et al., 2000), milk, cheese, and yogurt (Azadbakht et al., 2005), low-fat dairy, high-fat dairy, and total dairy (Liu et al., 2005). Two studies found there was no association between milk products intake and MetS (Shin et al., 2009; Snijder et al., 2007). One study reported mixed findings: a direct association with cheese intake and an inverse association with yogurt intake (Beydoun et al., 2008). Two out of three prospective studies found inverse associations between incidence of MetS and dairy consumption not only in overweight adults at baseline, but also in normal weight adults (Lutsey et al., 2008; Pereira et al., 2002). One of these prospective studies found no association (Snijder et al., 2008). The hypothesis is that metabolic risk factors that are part of the MetS could be reduced thanks to the action of nutrients found in milk and its derivatives such as: calcium, whey protein, monounsaturated fatty acid, and polyunsaturated fatty acid (Gibson et al., 2009; Skeaff and Miller 2009). In order to better understand the effect of reduced-fat milk products on cardiometabolic health, a 12-month randomized crossover intervention trial was carried out in a total of 61 overweight or obese adults, randomly assigned to two groups (Crichton et al., 2012)—a high dairy diet, 4 servings/day of reduced-fat dairy; or a low-dairy diet, less than 1 serving/day of reduced-fat dairy. After 6 months, the participants were crossed over to the alternate diet for another 6 months. The results of the trial showed no significant difference in body weight, body fat, or abdominal adiposity between the two different groups. It is plausible that the increased consumption of calcium in the phase ofthe high dairy diet reduced fat absorption preventing gains in body weight or fat mass. These results are comparable to those obtained by other similar studies (Zemel et al., 2008; Zemel et al., 2005).
Thus, it has been shown a possible role of the intracellular calcium levels in modulating the processes of synthesis and degradation of lipids within the adipocytes. Calcium rich-diet promotes lipolysis, a diet low in calcium stimulates lipogenesis (Zemel et al., 2000): A possible explanation for the inverse relationship between dairy calcium intake and body weight/body fat is that dietary calcium interferes with the absorption of fat in the intestine forming insoluble calcium soaps with fatty acids and/or bonding bile acids, causing a decrease of the digestible energy in the diet (Christensen et al., 2009). Dairy products could also modulate body weight by calcium-independent mechanisms. Milk proteins suppress food intake in the short term, increase the feeling of satiety, and stimulate the regulatory mechanisms of food intake known to signal satiation (Major et al., 2008). Crichton et al. concluded their clinical trial attributing an anti-obesity effect to dairy foods and recommending an intake of reduced fat dairy products, with a control of the total energy intake, to prevent and manage overweight and obesity (Crichton et al., 2012).
However, a large study conducted on a population-based cohort of 1,807 Australian adults aged 49 years and over evaluated the association between dairy intake and the MetS finding a ~59% reduction of the risk of developing MetS in people who consumed approximately two servings of regular fat dairy compared to those who consumed less than one serving of regular fat dairy. No relationship was observed with total or low/ reduced fat dairy consumption (Louie et al., 2013).
Moreover, although the dietary guidelines recommend a reduction in saturated fat so that cardiovascular risks can be prevented, a recent meta-analysis found no association between these fats and CVD (Siri-Tarino et al., 2010). Milk derivatives do not have the negative effects on blood lipids due to their content in saturated fat. Bioactive components such as calcium, a mineral prevalent in dairy products, can alter the effects on LDL cholesterol and triglycerides (Astrup 2014). These results are supported by a recent study that considered more than 7,000 middle-aged Koreans examined to evaluate the effects of dairy consumption (Shin et al., 2013). The study concluded that daily intake of milk derivatives can protect against MetS mainly by an association with decreased central obesity.
Hypertension is one of the risk factors of MetS; in 1997, an interesting research in Dietary Approaches to Stop Hypertension (DASH) showed that the level of blood pressure can be reduced by increasing the consumption of dairy products with low fat content (Sacks et al., 2001).
The results of the DASH study showed that the addition of three servings/day of low-fat dairy produces a significant reduction in blood pressure. The study compared a group of subjects who ate a typical American diet (rich in calories and low in fiber and micronutrients) with a second group that followed a diet rich in fruits and vegetables (five to six servings/day) and a third group who took three servings/day of low-fat milk derivatives in addition to diet rich in fruits and vegetables. The addition of dairy to the diet doubled the effect of fruit and vegetables produced on pressure reduction. Reductions in blood pressure appeared within two weeks and were maintained over time.
Furthermore a link has been demonstrated between the consumption of milk products in childhood and the type I diabetes (Saukkonen et al., 1998). Even if the specific mechanism(s) responsible for the beneficial effects of dairy consumption on the development of MetS have not yet been clarified, current studies suggest that dairy prod- ucts—in particular, milk—could be associated with a reduced risk of MetS. Randomized clinical trials are necessary to obtain definitive answers, and more studies are also needed to better understand which mechanisms govern the prevention of the MetS. The results of these studies should not be underestimated for choosing a diet; however every single person should learn to choose from the vast family of dairy products those products that are best suited for calcium intake, the amount of fat and energy assets, the individual metabolic needs, or a particular lifestyle.