Dairy Consumption, Obesity, and the Insulin Resistance Syndrome in Young Adults: The CARDIA Study
Abstract & Commentary
Synopsis: Dairy consumption was inversely associated with the incidence of all components of the insulin resistance syndrome (IRS) among individuals who were overweight (body mass index > 25 kg/m2). These associations were similar for African Americans and Caucasians and for men and women. Other dietary factors did not explain the association between dairy intake and IRS.
Source: Pereira MA, et al. JAMA. 2002;287: 2081-2089.
This study examined associations between dairy intake and the incidence of insulin resistance syndrome using data generated from the Coronary Artery Risk Development in Young Adults (CARDIA) study, which was a general community sample of 5115 African American and Caucasian adults aged 18-30 years who lived in 1 of 4 major US metropolitan areas. The participants were first seen in 1985-1986 and then again 10 years later. The main outcome variable, insulin resistance syndrome (IRS), is also known as metabolic syndrome or syndrome X. IRS is associated with glucose intolerance, dyslipidemia, hypertension, and impaired fibrinolytic capacity and is a risk factor for cardiovascular disease. The prevalence of IRS among adults has been estimated at 24%. While obesity clearly plays a role in the development of IRS, the role of dietary factors is less well understood. The current recommendation of a low-fat diet has been questioned, in part because its institution has been linked to increased carbohydrate consumption, which may also play a role in the development of IRS. Of the 3157 individuals eligible for participation in this study, 923 had a BMI > 25 kg/m2. A thorough history of diet and other lifestyle variables was obtained. Waist-hip ratio was determined and blood was obtained after an 8-hour fast. Abnormal glucose homeostasis was defined as a fasting plasma insulin > 20 mU/mL, fasting glucose > 110 mg/dL, or use of medications to control blood glucose. Obesity was defined as a BMI > 30 or a waist-hip ratio > 0.85. An elevated blood pressure was defined as > 130/85 or the use of antihypertensive medications. Dyslipidemia was defined as HDL-C < 35 mg/dL or triglycerides > 200 mg/dL. Insulin resistance was defined as the presence of at least 2 of the following 4 components of the IRS: abnormal glucose homeostasis, obesity, elevated blood pressure, and dyslipidemia.
In general, overweight individuals consumed less dairy products. Notably, dairy consumption was positively associated with whole grain, fruit, vegetable, and saturated fat intake and inversely correlated with soft drink intake. No association was observed between dairy intake and incidence of IRS in normal weight participants. Among overweight individuals, regardless of race or sex, incidence of IRS decreased by more than 50% from lowest to highest categories of dairy consumption. Pereira and associates note that the inverse association between calcium intake and IRS was entirely explained by dairy intake. Pereira et al speculate that the complex mixture of lactose, protein, and fat in dairy foods may enhance satiety and reduce the likelihood of eating high-carbohydrate foods and beverages. Also, there may be other biologically active components in dairy foods. The lack of an association in underweight individuals likely reflects the low likelihood of developing IRS in the absence of obesity.
Comment by Sarah L. Berga, MD
This article struck me as having relevance for 2 populations of women often seen in the office, namely, postmenopausal women and women with polycystic ovary syndrome. Women with these conditions struggle to avoid persistent weight gain, commonly display dyslipidemia and hypertension, and are at risk for IRS. Furthermore, they are often seeking dietary recommendations. For many, weight loss is unlikely. However, it is important to recognize that they can change their diet by adding dairy foods, not lose weight, and still accrue health benefits, including a reduction in the risk of diabetes and cardiovascular disease. Another consideration is that dairy products may have health benefits for those who are of normal body weight and it may still be advisable to recommend inclusion of dairy in the diet.
How much dairy consumption is enough? In this study, there was a dose-response curve with no threshold or plateau. The lowest category was 0-10 dairy servings weekly and the highest category was > 35. Interestingly, those with a BMI < 25 average more servings of high-fat dairy products than did overweight individuals. This lends credence to the hypothesis that you need enough fat to turn off appetite.
When queried, most of my patients say they eat the majority of their calories at dinner. They tend to go home from work famished. This places them at risk of overeating at dinner. As glucose levels drop, appetite becomes compelling. As most of us know all too well, appetite does not decline immediately with the first bite. Because of the lag between onset of eating and the sensation of satiety, if one is ravenous at the start of a meal, then one may well overeat. One helpful hint for patients, then, is for them to have a serving of milk in the late afternoon. That way, they go home with their appetite at least somewhat blunted. Of course, dairy products have other benefits as well. They also supply a balanced mineral solution, including calcium, needed for bone accretion. This latter benefit applies also to those of normal weight.
Dr. Berga is Professor and Director, Division of Reproductive Endocrinology and Infertility, University of Pittsburgh.
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