Our findings, based on the most recent nationally representative US data, revealed a significant inverse association between consumption of dairy products and their related nutrients, particularly milk, yogurt, calcium, and magnesium, and health outcomes such as obesity, central obesity, and MetS. However, there was a positive association with cheese consumption and phosphorus intake. For example, the ORs between cheese consumption (servings) and obesity, central obesity, and MetS were 1.14 (1.08, 1.21), 1.11 (1.05, 1.17), and 1.16 (1.04, 1.29), respectively; whereas the corresponding ORs for yogurt consumption (servings) were 0.51 (0.36, 0.73), 0.51 (0.37, 0.70), and 0.40 (0.18, 0.89), respectively. The risks of these conditions were ~20% lower with a 100 mg/d increase in magnesium intake. Compared with magnesium, which appeared to mediate the association between low-fat milk intake and central obesity, calcium had a weak inverse association with obesity but may be mediating the association between yogurt and MetS. The combined variation of magnesium and phosphorus mediated the effect of cheese on MetS. When considering individual metabolic components of MetS, milk was significantly associated with reduced SBP and DBP. All other types of dairy products and related nutrients yielded similar results to those observed with binary outcomes, particularly cheese, yogurt, phosphorus, and magnesium.
Our study confirmed some previous findings regarding the beneficial effects of dairy and calcium intake. Our results also highlight the complex relation between dairy foods and obesity and metabolic outcomes. For example, the beneficial effect was not detected when all types of dairy products were pooled together. In contrast, an adverse effect was detected in some population groups. Among men, dairy increased the risk of central obesity by 5% (per serving) and of MetS by 8%. Consumption of cheese, low-fat milk, and phosphorus were all positively associated with some of these adverse health outcomes as well. Some previous studies reported an inverse association between total dairy intake and adiposity (25
) or obesity co-morbidities (27
), whereas others did not (33
). The differences in findings were likely mainly attributable to variations in study samples (including heterogeneity in their ethnic/genetic background, age groups, and dietary consumption, including the types of dairy products), study design, outcome assessment, and data analysis methods. Our study systematically examined the various types of dairy products and related nutrients and included all adult age groups.
Whereas many studies have focused on the calcium component of dairy (25
), fewer have assessed the effects of dairy fat (26
) and magnesium (53
), and none have examined the effects of phosphorus intake. Our study examined all these conjointly. Unlike our findings, however, one study reported low plasma phosphorus levels in subjects with MetS compared with healthy individuals (60
). In general, most studies observed weakly inverse or null associations between calcium and magnesium and obesity, central obesity, and MetS. Whereas calcium was shown to mainly alter adipocyte metabolism, as discussed earlier (51
), as well as modulate blood pressure (61
), magnesium’s biological pathway involves regulating insulin sensitivity, vascular tone, and blood pressure homeostasis. In addition, intracellular or serum magnesium deficiency is a common feature of both diabetic and hypertensive states, as well as various other cardiovascular and metabolic processes, and aging (62
). This result is in line with our findings regarding magnesium’s inverse association with elevated blood pressure and glucose. Its inverse association with obesity and central obesity found in our present study may be attributable to dietary magnesium’s ability to form soaps with fatty acids in the intestine and thus reduce the digestible energy content of the diet (63
). Finally, the inverse relation between magnesium and plasma TAs, and its direct positive association with HDL cholesterol can be explained by magnesium’s ability to increase lipoprotein lipase activity, which is involved in the conversion of TAs to HDL cholesterol (64
A few previous studies made a distinction between various types of dairy products, including 2 recent large prospective cohort studies that found a general pattern of low-fat dairy being protective against type 2 diabetes and MetS (27
). Similar to our findings, a recent cross-sectional study among older adults in the Netherlands found that whole milk was associated with lower BMI and WC, whereas low-fat milk had the opposite effect(29
). We suspect that in both this study and ours, obese people, when consuming milk, might have modified their eating behaviors and been more likely to choose low-fat milk, whereas thinner subjects were less likely to alter their behavior in response to perceived fat content.
The protective effect of yogurt (23
) and adverse effect of cheese (29
) have been shown in other selective study samples. The deleterious effect of cheese may be the result of its higher energy density compared with other dairy products (eg, 336 kcal/per 100 g compared with 110 kcal for yogurt and 35–61 kcal for milk of various fat content, skim to whole milk), its elevated content in saturated fat, and possibly its higher phosphorus content compared with other dairy products. Another possible explanation is its high level of sodium compared with other dairy products, which may also trigger excessive intake of high-calorie beverages. Yogurt, a fermented dairy product, was shown to reduce absorption of cholesterol and therefore prevent dyslipidemia (24
) and is thought to increase calcium bioavailability through its high acidity (24
Another main finding of our analysis is that large ethnic disparities exist for intakes of dairy and calcium, and for all metabolic outcomes. In particular, NH blacks had lower dairy intake levels, and the highest prevalence of obesity and central obesity, whereas the “others” ethnic group had the highest mean number of metabolic disturbances. Interestingly, NH blacks had the fewest metabolic disturbances compared with other ethnic groups. The differences appear to be mainly the result of their high HDL cholesterol and low TAs.
Our analyses suggest that ethnic differences in metabolic outcomes, particularly higher BMI among NH blacks and SBP among NH blacks and Mexican Americans compared with NH whites, may be at least in part explained by variations in dairy-related nutrients. Inadequate dairy consumption (< 3 servings) was found in over 80% of the study population, and mean dairy servings were particularly low among minority groups. Only 6.5% of NH blacks and 11.4% of Mexican Americans reported consuming 3 servings or more per day, as recommended. Greater efforts are needed to help all Americans consume more dairy products, in particular the low-fat items, to have adequate calcium and vitamin D intake. The lower intake of dairy products by minority groups, particularly NH blacks, may be the result of their higher prevalence of lactose intolerance compared with NH whites (66
). However, this concept has been recently challenged by studies suggesting that NH blacks can consume at least one cup (8 oz) of milk without experiencing symptoms, and that tolerance can be improved by consuming the milk with a meal, choosing yogurt or hard cheeses, or using products that aid in the digestion of lactose (such as lactase supplements or lactose-reduced milks) (67
). Our lack of ability to detect significant associations between dairy consumption and binary metabolic outcomes (obesity, central obesity, and MetS) within the NH black group may be partly because of their smaller sample size.
The present study has a number of strengths. First, it is based on recent, nationally representative data of adults in the United States. Our analysis accounted for design complexity and yielded national estimates. Second, this study is one of very few that examined both dairy consumption of varying types and dairy-related nutrients in relation to adiposity and various metabolic outcomes of interest. Third, to our knowledge, this attempt is the first to examine the role of dairy consumption in explaining ethnic disparities in adiposity and metabolic outcomes, through use of structural equations modeling techniques.
A main limitation of this study is its cross-sectional design, which precludes inferences on causation. Residual confounding caused by measurement error in covariates included, or noninclusion of important confounders, as well as possible eating behavior changes as a result of health conditions are possible explanations for some of the associations, including the positive association of low-fat milk with metabolic outcomes. In addition, some recent studies suggest that MetS may not be a clinically useful endpoint to study and that the clustering of disturbances may not be more predictive of cardiovascular disease and mortality than its separate components (69
). Many studies, however, still consider MetS an important endpoint that can be targeted and prevented by lifestyle and environmental changes.
In summary, our results suggest that the health effects of dairy products and related nutrients are complex and may not be uniform across the population, at least for obesity and related metabolic disorders. The data also indicate that variations in consumption of dairy products and dairy-related nutrients appear to be factors that account for some of the disparities in risk of obesity and its co-morbidities between major ethnic groups in the US.