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The 2010 Dietary Guidelines for Americans indicate the US population is experiencing an epidemic of overweight and obesity while maintaining a nutrient-poor, energy-dense diet associated with an increased risk of osteoarthritis, cardiovascular disease, and type 2 diabetes. To build upon the review of published research in the Report of the Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2010, this article aims to review the scientific literature pertaining to the consumption of dairy foods and the effects of dairy consumption on nutrient intakes and chronic disease risk published between June 2010, when the report was released, and September 2011. PubMed was searched for articles using the following key words: dairy, milk, nutrient intake, bone health, body composition, cardiovascular disease, type 2 diabetes, and blood pressure. Evidence indicates that increasing dairy consumption to the recommended amount, i.e., three servings daily for individuals ≥9 years of age, helps close gaps between current nutrient intakes and recommendations. Consuming more than three servings of dairy per day leads to better nutrient status and improved bone health and is associated with lower blood pressure and reduced risk of cardiovascular disease and type 2 diabetes.
Nearly 70% of adults and one-third of children in the United States are overweight or obese.1,2 Despite being in a state of positive energy balance, Americans, on average, are not meeting recommended intakes for whole grains, vegetables, fruits, low-fat and fat-free dairy foods, seafood, and oils; they are also failing to meet recommended nutrient intakes for potassium, dietary fiber, calcium, and vitamin D, all of which were identified by the 2010 Dietary Guidelines for Americans (DGA) as nutrients for which intake is low enough to be of public health concern.3–5 In the United States, milk is the primary dietary source of three of these nutrients: calcium, vitamin D, and potassium.6
To help Americans meet nutrient recommendations, the 2010 DGA recommends 3 cups per day of fat-free or low-fat milk and milk products for persons 9 years of age and older, 2.5 cups per day for children 4–8 years of age, and 2 cups per day for children 2–3 years of age.3 Data from the National Health and Nutrition Examination Surveys (NHANES) indicate that meeting or exceeding current recommendations for dairy food intake can increase the dietary intake of several nutrients that are underconsumed in the United States, such as calcium, magnesium, and potassium.7
The 2010 DGA recognizes that, in addition to dairy foods being a source of essential nutrients, “moderate evidence shows dairy consumption is associated with improved bone health, especially in children and adolescents, and a reduced risk of cardiovascular disease and type 2 diabetes and with lower blood pressure in adults.”3 To build upon the review of published research in the Report of the Dietary Guidelines Advisory Committee (DGAC) that led to this conclusion, this article reviews the scientific literature pertaining to dairy food consumption and the effects of dairy consumption on nutrient intakes and risk of chronic disease published between June 2010, when the DGAC report was released, and September 2011.
The dairy food group, which includes milk, cheese, and yogurt, contributes to the intake of many nutrients in the American diet, including more than 50% of total vitamin D and calcium and more than 25% of vitamin A, vitamin B12, and phosphorus, while contributing just 10% of total calorie intake (Figure 1).8–10 Dairy intake, however, is significantly below the levels recommended for most Americans,8 with individuals 2 years of age and older consuming 1.8 servings daily on average9 and only 15% of the population meeting recommendations for consumption of dairy foods.11 The DGA encourages increased consumption of fruits, vegetables, whole grains, lean protein sources (e.g., seafood, nuts, legumes), and low-fat and fat-free dairy foods to help individuals meet nutrient recommendations. A review of NHANES data from 2003–2004 indicated that three servings of dairy per day is the minimum amount necessary to ensure adequate intakes of calcium for Americans 9 years of age and older, but four servings or more per day may be necessary to ensure adequate intakes of magnesium and potassium.7 A recent study that utilized NHANES 2003–2006 data to examine the impact of adding one serving of dairy to current intakes supports this conclusion. The addition of a consumption-weighted composite of dairy foods (milk, cheese, and yogurt) increased average daily dairy intake to 2.8 servings and average calcium, magnesium, and potassium intakes to 1,245 mg, 301 mg, and 2,916 mg, respectively.12 Changes in additional dietary intakes are presented in Table 1. According to another analysis of NHANES data, from 2003 to 2006, more than one-third of Americans 2 years of age and older have usual intakes below the estimated average requirement (EAR) for vitamin A and calcium, nearly half have intakes below the EAR for magnesium, more than two-thirds have intakes below the EAR for vitamin D, and just 3% of the population have usual intakes of potassium greater than the adequate intake.5 Despite the naturally occurring amounts of these nutrients in food sources and the additional contribution from enriched foods and dietary supplements, shortfalls still exist.5
Several randomized clinical trials (RCTs) have demonstrated that consuming three or more servings of dairy foods per day has beneficial effects on nutrient intakes in adults.13–16 In middle-aged obese men and women fed a weight-maintenance diet for 6 months, the consumption of three or more servings of dairy foods per day resulted in significantly higher intakes of energy and macronutrients as well as of calcium and vitamin D compared with intakes of individuals who consumed one or fewer serving of dairy foods per day. For those who consumed higher amounts of dairy foods, the average calcium intake at the end of the study (women, 1,330 ± 39 mg/day; men, 1,448 ± 41 mg/day) was above the EAR (800 mg/day) for their age group, and vitamin D intakes doubled compared with baseline intake levels (women, 3.6 ± 0.5 to 7.1 ± 0.4 μg/day; men, 4.9 ± 0.8 to 9.7 ± 0.6 μg/day), which approached the EAR (10 μg/day) for their age group. For those who consumed lower amounts of dairy foods, the average calcium intake after 6 months was below the EAR (women, 587 ± 45; men, 624 ± 36 mg/day) for their age group, and vitamin D intakes did not improve from baseline and were well below the EAR (women, 4.2 ± 0.6 to 2.9 ± 0.4; men, 4.1 ± 0.7 to 3.8 ± 0.5 μg/day) for their age group.13 In another study in which total energy intake did not differ between groups, the consumption of four servings of dairy foods daily for 21 weeks by middle-aged obese adults resulted in significantly higher calcium intakes (1,200 ± 370 mg/day) compared with intakes in the group who consumed two servings of dairy foods per day (668 ± 273 mg/day).15 The group who consumed four servings of dairy foods per day had calcium intakes above the EAR for their age group, whereas the group who consumed two servings of dairy foods per day did not reach the EAR for calcium intake.15 Another RCT showed that, in groups of postmenopausal women between the ages of 55 and 65 years, matched for total energy and macronutrient consumption, consumption of three servings of low-fat calcium- and vitamin-D-fortified dairy products (2 servings of milk and 1 serving of yogurt) per day for 30 months resulted in no significant difference in intakes of total calories or macronutrients and significantly greater intakes of calcium, magnesium, vitamin D, and phosphorus (1,337 ± 500 mg/day, 329 ± 124 mg/day, 18.5 ± 1.4 μg/day, and 1,561 ± 675 mg/day, respectively) compared with intakes in a control group who did not consume three servings of fortified dairy products daily (564 ± 310 mg/day, 247 ± 104 mg/day, 0.8 ± 1.4 μg/day, and 1,133 mg/day, respectively).16 Similarly, in another study in which three servings of low-fat calcium- and vitamin-D-fortified dairy products were provided to postmenopausal women daily for 30 months, calcium, phosphorus, and vitamin D intakes were higher (1,183 ± 283 mg/day, 1,409 ± 627 mg/day, and 18.8 ± 1.3 μg/day, respectively) compared with intakes in the control group (671 ± 334 mg/day, 1,091 ± 484 mg/day, and 1.2 ± 0.6 μg/day, respectively).14 There were no significant differences in calorie, carbohydrate, or fat intakes, and protein intake was significantly higher in the dairy intervention group. Both of these studies reported intakes that increased from sub-EAR levels to levels that exceeded the EARs for calcium, vitamin D, and magnesium (1,000 mg/day, 10 μg/day, and 265 mg/day, respectively).14,16 These higher nutrient intakes were a significant improvement for this age group, which may be at risk for underconsuming calcium and vitamin D.17 The results of these studies indicate that exceeding daily dairy recommendations has beneficial effects on nutrient intakes.
Based on the review of the scientific literature by the DGAC, the 2010 DGA stated there was moderate evidence linking dairy consumption with improved bone health, especially in children and adolescents.3 Since the release of the DGAC report, several RCTs reinforced that dairy consumption also has benefits on bone health in adults, particularly in women (Table 2).14,16,18–20 In a 4-month trial in overweight premenopausal women who consumed a calorie-restricted diet, three or more daily servings of low-fat dairy foods (average daily calcium intakes above 1,300 mg) combined with resistance exercise increased lumbar spine bone mineral density compared with one or fewer servings of low-fat dairy foods per day combined with resistance exercise.20 In another study in healthy, young premenopausal women who followed a 12-week diet and resistance-exercise plan, four daily servings of fat-free milk added to participants' normal diets resulted in improved markers of bone turnover compared with an isoenergetic carbohydrate control.19 In the group that consumed four additional servings of fat-free milk per day, serum vitamin D (25-[OH]-D) increased and parathyroid hormone decreased more than in the group that did not consume any additional fat-free milk per day.19
Serum 25-(OH)-D levels above 80 nM/L have been related to improved bone health.17 Participants who consumed the four additional servings of fat-free milk each day had serum 25-(OH)-D levels that increased to between 48.2 nM/L and 79.2 nM/L; thus some were still vitamin D insufficient (50–80 nM/L) or deficient (<50 nM/L).17,19 The beneficial effects on markers of bone health that resulted from consuming four additional servings of fat-free milk per day may have been due to the approximately 1,200 mg of added calcium contributed by milk to the diet, which allowed serum calcium to be maintained within the recommended circulating range.19 Greater gains in lean muscle mass and a decrease in fat mass compared with the control group were also observed, showing a beneficial effect of four daily servings of milk on total body composition during weight loss.19 In a trial that examined the effect of milk consumption on body composition in lean and obese premenopausal females, an average of just two added servings of reduced-fat milk three days a week to the normal diet of both lean and obese participants for 8 weeks resulted in increased bone mineral density of the hip and spine compared with that in controls who did not consume any additional dairy foods.18 Additionally, lean body mass increased and percent body fat decreased in the lean milk-consuming group.18 The results of these studies indicate that exceeding daily dairy recommendations may have beneficial effects on markers of bone health and body composition in female adults.
In a study of obese men and premenopausal women who participated in a 12-week weight-loss intervention followed by a 24-week weight-maintenance period, total body bone mineral content was increased from baseline in women who consumed ≥3 servings of dairy per day (recommended) as well as in those who consumed ≤1 serving of dairy per day (low consumption).13 Despite increases in total body bone mineral content, no changes in total body bone mineral density were detected.13 In the men, both low dairy consumption and recommended dairy consumption resulted in increased total body bone mineral density over the course of the intervention. Total body bone mineral content, however, decreased in the men over time.13 Estrogen is known to inhibit bone resorption,21 perhaps explaining why differences in treatment effects on bone health were observed between the women and the men. Why differences in total body bone mineral content were not reflected in total body bone mineral density cannot be explained. In another trial, Palacios et al.15 reported that Puerto Rican obese men and women who consumed four servings of low-fat milk, cheese, and yogurt daily for 21 weeks had no differences in total body bone mineral content or density from baseline or compared with those who consumed 1,200–1,300 mg of calcium per day from a dietary supplement or those who consumed less-than-recommended amounts of dairy foods and no calcium supplement (<700 mg calcium/day). Mixed results from these two trials that studied dairy intake and bone health in obese subjects indicate that obesity, in addition to gender, may have a considerable effect on bone health regardless of dairy and micronutrient intakes. This may be because, in obese individuals, compounds vital to bone health such as vitamin D are incorporated into adipose tissue and do not contribute to serum 25-(OH)-D concentrations.22 Whereas trials in obese individuals have produced inconsistent results, RCTs in normal-weight individuals show a beneficial effect of dairy consumption on bone health.
Dairy foods provide bioavailable calcium and have been associated with beneficial effects on risk of cardiovascular disease.8 Two meta-analyses that aimed to investigate the effects of calcium supplementation on cardiovascular events indicated that nondietary calcium supplementation may increase the risk of cardiovascular events in women over 40 years of age.23,24 The DGAC report indicated calcium naturally occurring in foods is the recommended source.8
Based on the DGAC's review of the scientific literature, which presented evidence from two systematic reviews and meta-analyses and one case-controlled study,8 the 2010 DGA stated, “Moderate evidence also indicates that intake of milk and milk products is associated with a reduced risk of cardiovascular disease and type 2 diabetes and with lower blood pressure in adults.”3 The 2010 DGA recommends the consumption of low-fat and fat-free milk and milk products (milk, yogurt, and cheese) as a means to obtaining the same nutrients that are available in full-fat varieties but with a reduced amount of calories and saturated fat.3 While dairy foods can contribute to saturated fat intake, the research summarized in the 2010 DGAC report indicates consumption of milk products may not have a predictable effect on serum lipids; it may not raise total cholesterol and may favorably impact high-density lipoprotein cholesterol.
Since the release of the 2010 DGAC report, several epidemiological studies have examined the associations between dairy consumption and cardiovascular disease endpoints, such as coronary heart disease and stroke (Table 3).25–31 A meta-analysis of prospective cohort studies examining the associations of milk and total dairy intakes with the risk of cardiovascular diseases in over 600,000 participants in the United States, Europe, and Japan found that milk intake was associated with a reduced risk of overall cardiovascular diseases (dose response of 6% reduced risk for each 200 mL/day).30 Mean intake among participants was one serving of dairy foods per day.30 Notably, when analyzed according to fat content, no association between high-fat, low-fat, or combined total fat content of dairy foods and coronary heart disease was detected, indicating a neutral effect of milk fat on risk of coronary heart disease.30
In a prospective study among adult women in the United States, substituting dairy foods for fish was associated with increased risk of coronary heart disease, and consumption of high-fat dairy foods was associated with an increased risk of coronary heart disease.23 Consumption of low-fat and high-fat dairy foods, however, when compared with consumption of red meat, was associated with a substantially lower risk of coronary heart disease.26 The highest median dairy food intake was two servings daily.26 In another prospective investigation among Dutch adults who consumed on average three servings of dairy foods per day, fermented full-fat milk was inversely associated with all-cause mortality, but dairy fat was associated with an increased risk of all-cause and ischemic heart disease mortality among women only.28 In a case-control study of over 3,000 Costa Rican adults, however, no association between dairy fat and risk of myocardial infarction was detected.25 In another investigation of over 1,500 Australian adults followed for 16 years, higher intake of higher-fat dairy foods was associated with decreased risk of cardiovascular mortality; 69% less risk was associated with consuming about 1.5 servings of higher-fat dairy foods per day versus 27% less risk associated with consuming less than 0.5 serving of higher-fat dairy foods per day.27 In over 400 Swedish adults who consumed one to two servings of dairy foods daily, higher intake of dairy foods, as measured by biomarkers of milk fat in adipose tissue, was associated with 26% less likelihood of first myocardial infarction.31 The men and women studied consumed both low- and high-fat dairy products such as milk, fermented milk, cheese, and cream.31 In postmenopausal women in Australia, consumption of milk, cheese, and total dairy product (mean intake of 2.3 servings per day) was not associated with coronary heart disease as measured by common carotid artery intima-media thickness.29 Yogurt consumption was inversely associated with coronary heart disease, and in those women who consumed more than one-half serving of yogurt daily, arterial thickness was less than in those who consumed less than one-half serving daily.29 The results of these epidemiological studies indicate that low-fat dairy foods are associated with decreased risk of coronary heart disease. Whereas some studies pointed to detrimental effects, the majority indicated that full-fat dairy foods may have beneficial effects on risk of coronary heart disease. Research on the effects of milk fat on risk of coronary heart disease is ongoing.
Since the release of the DGAC report, four RCTs have examined the effects of dairy consumption on biomarkers associated with risk of cardiovascular disease (Table 4).32–35 In addition to traditional lipid screenings, biomarkers of inflammation have emerged as equally if not more valuable indicators of coronary heart disease risk.36 In overweight and obese subjects, a weight-maintenance diet including three daily servings of low-fat dairy smoothies that provided a total of 1,050 mg of calcium per day was shown to reduce biomarkers of oxidative stress and chronic inflammation within 28 days compared with a soy-based placebo smoothie that provided 500–600 mg of calcium per day.35 The dairy intervention also lowered serum low-density lipoprotein cholesterol and had no adverse effects on blood pressure or serum total cholesterol, high-density lipoprotein cholesterol, or triglycerides.35 Notably, in another study in postmenopausal women, when three servings of reduced-fat (2% fat) cow's milk was compared with a vanilla soy beverage containing comparable amounts of macronutrients and calcium for 1 month, no differences in serum lipids between groups, or from baseline, were detected.32 In middle-aged overweight and obese adults, it was demonstrated that daily consumption of three servings of low-fat milk and yogurt had beneficial effects on several biomarkers of inflammation.34 Another intervention designed to test the differences between effects of high-fat and low-fat dairy foods on biomarkers of inflammation following a single dairy meal showed that high-fat dairy foods such as butter, cheese, cream, and yogurt (45 g of fat per single-meal intervention) reduced several biomarkers of inflammation as well as levels of both low-density and high-density lipoprotein cholesterol.33
Collectively, these studies indicate an inverse association between consumption of dairy foods and coronary heart disease, adding to the evidence that led the DGAC to conclude that “bioactive components that alter serum lipid levels may be contained in milk fat.”8 Additionally, controlled trials that examined differences in coronary heart disease risk based on the fat level of dairy foods showed that the consumption of milk fat does not detrimentally – and may beneficially – affect biomarkers of coronary heart disease.
The review of the scientific literature by the DGAC presented findings from a meta-analysis of four prospective studies and found the relative risk of type 2 diabetes was estimated to be 10% lower in people with higher milk intake than in those with lower consumption.8 Based on this finding, the 2010 DGA stated that moderate evidence indicates the intake of dairy foods is associated with a reduced risk of type 2 diabetes.3 Several epidemiological studies designed to assess the relationship between dairy consumption and type 2 diabetes have been published since the release of the 2010 DGAC report and all have reported an inverse association between the intake of milk and milk products and the risk of type 2 diabetes (Table 5).37–40 A prospective study that assessed how dairy consumption by adolescents was associated with the incidence of type 2 diabetes in adulthood found that adults who reported consistently consuming two servings of dairy per day during adolescence had a 43% less risk of developing type 2 diabetes in adulthood when compared with adults who reported consuming one-half serving or less of dairy per day during adolescence.38 Consumption of both low-fat and full-fat dairy foods in adolescence was inversely associated with type 2 diabetes in adulthood.38 A prospective study of older adults found that circulating trans-9 C16:1, a biomarker indicative of dairy fat consumption, was associated with a 62% lower incidence of type 2 diabetes in adults who consumed at least one serving compared to those who consumed less than one serving of reduced-fat (2% milk fat) and/or full-fat dairy foods daily.39 Another study conducted in French adults found that those who consumed more than three servings of milk or yogurt daily had a 15% lower risk of incident metabolic syndrome and/or type 2 diabetes compared with those who consumed less than one serving of milk or yogurt daily.37 Cheese consumption had no effect on the incidence of metabolic syndrome and/or type 2 diabetes.37 In a meta-analysis of over 300,000 American, Japanese, and Chinese middle-aged participants, higher dairy consumption (>3 servings/day versus <1 serving/day) was associated with a 15% reduced risk of type 2 diabetes.40 A dose-response analysis showed that the risk of type 2 diabetes could be reduced by 5% for each additional serving of total dairy products and by 10% for each additional serving of low-fat dairy products consumed.40 Findings from these epidemiological investigations indicate milk and milk products, regardless of fat level, are associated with a reduced risk of type 2 diabetes. Furthermore, exceeding current recommendations of three servings daily may further reduce risk.
Since the release of the 2010 DGAC report, two RCTs have investigated the effects of dairy consumption on risk factors associated with type 2 diabetes.41,42 It was demonstrated in middle-aged overweight and obese individuals that consumption of three and one-half servings of dairy per day, with two of the three being milk and/or yogurt (fat level not reported), resulted in increased insulin sensitivity and decreased plasma insulin within 1 week of consuming dairy foods, changes that were sustained throughout the 12-week duration of the study.42 Additionally, beneficial effects on several lipid and inflammatory markers of metabolic health were observed when comparing these individuals with those who consumed less than one-half serving of dairy daily.42 In middle-aged subjects with type 2 diabetes, vitamin-D- and calcium-fortified yogurt drinks consumed twice daily for 3 months led to decreases in fasting serum glucose, glycated hemoglobin, and insulin resistance measured by the Homeostasis Model of Assessment–Insulin Resistance, which are indicators of glycemic status.41
The 2010 DGA stated there was moderate evidence that dairy consumption is associated with lower blood pressure in adults.3 Dietary patterns that incorporate at least three servings of dairy daily, such as the Dietary Approaches to Stop Hypertension eating plan, which includes low-fat milk, cheese, and yogurt and one serving of full-fat cheese each day, have been successful in reducing blood pressure.8 The DGAC, however, noted that an independent relationship between dairy intake and blood pressure is complicated to evaluate due to varying types of dairy products consumed in research trials, confounding effects of calcium intake from other food sources, and the beneficial effect of weight loss on blood pressure.8 A systematic review and meta-analysis of studies evaluating the association between dairy intake and blood pressure in approximately 45,000 adults found that consumption of dairy foods was associated with a 13% reduced risk of elevated blood pressure.43 Separation of high-fat and low-fat dairy foods revealed an inverse association between only low-fat dairy foods and elevated blood pressure, with no effect from high-fat dairy foods or cheese.43
Since the release of the 2010 DGAC report, four RCTs have investigated the effects of dairy foods on blood pressure and components of blood pressure control (Table 6).42,44–46 While other RCTs have tested the effects of specific dairy ingredients on blood pressure control,47–50 those trials were beyond the scope of this review. In middle-aged overweight and obese adults, daily consumption of greater than two and one-half servings of low-fat milk and yogurt for 8 weeks was demonstrated to reduce systolic blood pressure by 2.9 mmHg.45 In another study of overweight and obese middle-aged adults, consumption of greater than three and one-half compared with less than three servings of dairy foods, two of which were milk and/or yogurt (fat content not reported), daily for 12 weeks decreased systolic blood pressure by 7.1 ± 3.1 mmHg and diastolic blood pressure by 4.1 ± 1.9 mmHg.42 In two studies of borderline hypertensive adults fed just over one serving daily of fermented milk for 8 weeks, no changes in systolic or diastolic blood pressure or components of blood pressure control were observed.44,46 These studies indicate that at least two and one-half servings of dairy per day is the level at which the effects of dairy on blood pressure are seen. Further studies in normal-weight and obese individuals with and without underlying hypertension will aid in better understanding the relationship between dairy consumption and blood pressure control.
Since the release of the 2010 DGAC report on the 2010 DGA, many studies have been published that make important contributions to the literature on the relationship between dairy consumption, nutrient intakes, and reduced risk of chronic diseases. As the US population continues to underconsume nutrients such as vitamin D, calcium, magnesium, and potassium,3,5 they also underconsume the recommended daily servings of dairy foods. Since milk and milk products are major contributors of these nutrients to the typical diet in the United States,8–10 consuming, at minimum, three servings of dairy foods each day would help close the gap on several nutrients that are typically underconsumed.7 The totality of the evidence indicates that consumption of three or more servings of dairy per day has beneficial effects on bone in adults, but four servings per day may be necessary to achieve the nutrient status associated with optimal bone health in adulthood. Recent studies continue to build on the evidence that dairy consumption is inversely associated with the development of cardiovascular disease, even at intakes below recommended levels. Milk fat may contribute to the beneficial effects of dairy foods on cardiovascular disease risk, but more research is needed. Recent findings further support the evidence reviewed by the DGAC indicating that meeting recommendations for dairy consumption reduces the risk of type 2 diabetes and has beneficial effects on blood pressure, and that exceeding recommendations may provide further benefits.
In conclusion, meeting and exceeding recommendations for consumption of dairy products each day, in combination with a healthy dietary pattern, leads to better nutrient status, can lead to improved bone health, and is associated with lower blood pressure and a reduced risk of cardiovascular disease and type 2 diabetes.
Declaration of interest, The authors are employees of the Dairy Research Institute/Dairy Management Inc., Rosemont, Illinois, USA.