Effects of total vitamin A, vitamin C, and fruit intake on risk for metabolic syndrome in Korean women and men

Abstract

Objectives

The question of whether the consumption of antioxidants prevents and alleviates metabolic syndrome (MetS) by reducing insulin resistance remains controversial. The aim of this study was to assess whether the intake of vitamin A (including β-carotene), vitamin C, fruits, or vegetables was negatively associated with MetS in Korean adults aged ≥20 y.

Methods

We conducted a cross-sectional study of 27,656 adults ≥20 y of age who participated in the 2007–2012 Korean National Health and Nutrition Examination Survey. Daily intake of vitamin A and vitamin C was assessed by 24-h recall, and the consumption of fruits and vegetables was determined using a food frequency questionnaire. Odds ratios (ORs) for MetS were calculated for log₂-transformed vitamin A and C intake values and for the interaction of sex with vitamin A and C intake, after covariate adjustment.

Results

Interactions were seen between total vitamin A and C intake and sex for MetS. With a twofold increase in total vitamin A and C intake in women, the ORs (95% confidence intervals) for metabolic syndrome were 0.942 (0.901–0.985) and 0.933 (0.883–0.987), indicating decreases of 5.8% and 6.7% in MetS, respectively. There were no equivalent decreases in men. Women in the second and highest tertiles of fruit intake exhibited 17.5% and 21.8% lower incidences of MetS, respectively, compared with women in the lowest tertile.

Conclusions

The intake of total vitamin A and C, as well as moderate and high fruit intake, may have alleviated MetS in women, but not in men, in a representative sample of the general South Korean population.

Introduction

Oxidative stress results from the balance between production and removal of reactive oxygen species (ROS) by the antioxidant system of the body [1]. Because ROS production increases under certain conditions such as elevated insulin resistance (IR), the antioxidant system should be potentiated to decrease ROS. Antioxidant capacity depends on the activity of antioxidant enzymes, such as superoxide dismutase and glutathione peroxidase, and on the consumption of exogenous antioxidants, such as vitamins A, C, and E, β-carotene, and lycopene [2]. However, the endogenous antioxidant system cannot completely offset the production of ROS alone because antioxidant enzymes have a limited antioxidant capacity. Therefore, dietary antioxidants play an important role in maintaining optimal antioxidant status in the body.

Metabolic syndrome (MetS) is defined as having at least three of the five following conditions: abdominal obesity, IR, dyslipidemia, hyperglycemia, and hypertension, and it is a risk factor for type 2 diabetes and cardiovascular disease (CVD) [3], [4], [5]. The common etiology of MetS begins with IR although the pathophysiological mechanisms involved in the syndrome are complex [6]. Whereas the cause of IR remains unclear, the expansion of fat mass (FM), resulting in energy overload, is the main initiator. Increased FM stimulates adipokines related to inflammation and oxidative stress, leading to cell damage in the liver, pancreatic islets, and brain [3], [6]. Overnutrition also ignites oxidative stress, which contributes to IR development [7]. Thus, oxidative stress can play a substantial role in the etiology of MetS [7], [8], [9].

However, it is still controversial whether antioxidants prevent the development and progression of MetS. Some studies have demonstrated that antioxidants can prevent the development of MetS by attenuating IR [10], [11], [12], [13]. Compared with nonconsumers, consumer of 100% orange juice consumers have a higher intake of total fruit, fruit juice, whole fruit, and whole grains and, as a result, have a higher consumption of vitamins A and C, folate, and magnesium (on the basis of estimated average requirements; the National Health and Nutrition Examination Survey, 2003–2006) [10]. They are also 21% less likely to be obese, regardless of sex, whereas male consumers of orange juice were 36% less likely to have MetS. Additionally, diets rich in antioxidants, such as vitamins A, C, and E, and β-carotene, exhibit beneficial effects on MetS in epidemiologic studies [11], [12], [13]. Consistent with antioxidant intake, several cross-sectional and longitudinal epidemiologic studies have shown an inverse association between MetS and the consumption of fruits and vegetables rich in antioxidants, such as vitamin C and carotenoids [14], [15]. However, some studies showed no significant association between intake of antioxidant-rich diets and risk for MetS [16]. It is thus still controversial whether consumption of antioxidants prevents and alleviates MetS by reducing IR. The disparities in the association between MetS and antioxidants, especially vitamins A and C, involve several factors, such as race and sex, but previous studies did not determine sex and race specificity as an interaction term in the logistic regression models [14], [15], [16].

We hypothesized that the intake of fruits, vegetables, vitamin A (including carotenoids), and vitamin C are negatively associated with MetS in adults aged ≥20 y, independent of other covariates and possible confounders. We conducted a cross-sectional study to test this hypothesis using data from the Korean National Health and Nutrition Examination Surveys (KNHANES) IV (2007–2009) and V (2010–2012), components of a large South Korean population study with a stratified, multistage, probability sampling design.