Effects of dietary carbohydrate restriction versus low-fat diet on flow-mediated dilation
Introduction
Endothelial dysfunction is an early event in coronary heart disease that can be detected using a noninvasive technique that evaluates flow-mediated dilation (FMD) of the brachial artery using high-frequency ultrasound [1]. A high-fat meal can transiently impair FMD of the brachial artery [2], [3], [4], and some studies report that the lipemic response is related to vascular dysfunction [5], [6]. Although single meals high in fat induce hypertriacylglycerolemia, adaptation to high-fat diets with a low carbohydrate content dramatically decreases the postprandial lipemic response in normal-weight [7], [8], [9] and overweight [10] individuals. Thus, low-carbohydrate diets, by virtue of their potent triacylglycerol (TAG)-lowering effect, may improve vascular function if subjects are allowed to make metabolic adaptations.
There are other adaptations to carbohydrate-restricted diets (CRD) that could have a benefit on endothelial function, specifically those associated with the metabolic syndrome (insulin resistance syndrome). Metabolic syndrome represents a group of seemingly disparate physiologic signs that indicate a predisposition to obesity, diabetes, and cardiovascular disease. Its origins are generally believed to reside in impairment of insulin action; and its intellectual impact is embodied in a unifying principle for control of the broad range of physiologic effects, which now include vascular dysfunction [11]. Consistent with the idea that an intolerance to carbohydrate is an underlying feature of metabolic syndrome, we have presented results showing that reduction in dietary carbohydrate results in global improvement in traditional and emerging markers associated with metabolic syndrome, particularly the cardiometabolic profile (high-density lipoprotein [HDL] is increased, and TAG and small low-density lipoprotein cholesterol are decreased) [12], [13].
The status of CRD as a method to alter vascular function cannot be adequately addressed by single-meal experiments [14]. Therefore, the primary purpose of this study was to assess postprandial vascular function in the brachial artery after a high-fat meal in subjects with hypertriacylglycerolemia who consumed a CRD for 12 weeks. Their responses were compared with a control group who consumed a low-fat diet (LFD). We also measured an array of cardiometabolic markers implicated in vascular dysfunction including postprandial circulating TAG [5], fatty acids [15], total leukocytes [16], leukocyte subpopulations [17], and proinflammatory markers [18]. Our recent findings in this cohort showed that a CRD was more likely than an LFD to effect global improvement in markers associated with insulin resistance [19], [20]. Here, we extend these findings by showing that a CRD also has a beneficial effect on postprandial lipemia and FMD of the brachial artery compared with an LFD.
Section snippets
Study design and participants
This study was a randomized, controlled, dietary intervention trial that compared a CRD to an LFD over a 12-week period in overweight subjects with atherogenic dyslipidemia. Participants were men and women aged 18 to 55 years with a body mass index (BMI) greater than 25 kg/m2. Exclusion criteria were any metabolic and endocrine disorders; use of glucose-lowering, lipid-lowering, or vasoactive prescriptions or supplements; consumption of a CRD at baseline; or weight loss greater than 5.0 kg in
Baseline characteristics, dietary intake, and weight loss
We previously reported the baseline characteristics, dietary intake, and responses in fatty acid composition [19], [20]. Briefly, the 2 diet groups were equally matched for sex; and there were no significant differences between groups in age, body composition, or metabolic profiles (Table 1). Energy intake was not different between diets, but the nutrient composition varied significantly between the CRD (1504 kcal; percentage of carbohydrate-fat-protein = 12:59:28) and LFD (1478 kcal;
Discussion
The adverse effects of single meals high in fat, especially saturated fat, on postprandial lipemia [2], [3], [4] and vascular and inflammatory function [14] have been used as evidence to discourage low-carbohydrate diets. The prior diet history, however, has a fundamentally important effect on the metabolic response to meals. For example, we have repeatedly shown that adaptation to a very low carbohydrate diet results in a substantial reduction in the postprandial lipemic response to a
Acknowledgment
This work was supported in part by funds from the Graduate School and the Health Disparity EXPORT Center at the University of Connecticut, US Department of Agriculture Hatch, the Dr Robert C Atkins Foundation, the Egg Nutrition Center, and the Research Foundation of the State University of New York.
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JSV, WJK, and MLF had overall responsibility of the study and contributed to experimental design, data analysis and interpretation, and manuscript preparation. KDB, RS, and DAJ performed vascular measurements and analysis as well as biochemical assays. EEQ and CEF were responsible for the dietetic aspects of the study and overseeing the data collection protocols. All authors agreed on the final version of the manuscript, and none had any conflict of interest.