1. Introduction
The metabolic syndrome, which encompasses excess abdominal adiposity, insulin resistance, dyslipidaemia, and hypertension, represents the largest public health challenge in developed countries [1]. The rise in metabolic syndrome prevalence in recent decades has been mirrored by changes in dietary patterns, reflecting increased nutrient availability [2]. Diets rich in fat and rapidly-digestible carbohydrates have increased total energy intake [2]. At the same time, fortification of staple foods and widespread supplement use has increased folic acid intake in many Western countries, placing importance on investigations into possible adverse effects [3].Folates are a family of structurally-similar compounds involved in the transfer of one-carbon units for the production of nucleotides used in DNA synthesis; for the methylation of a variety of biological substrates; and for cell division [4]. These functions make folates especially important during the anabolic stages of foetal and childhood development [4]. Sources of natural folates (pteroylpolyglutamates) include green leafy vegetables, orange juice and legumes [4]. Folic acid (monoglutamate) is a synthetic member of the folate family commonly used in fortified foods and supplements due to its stability and low cost [4]. The current Recommended Daily Allowance (RDA) for folate is 400 μg Dietary Folate Equivalents (DFEs)/day for the general adult population [5]. While mandatory folic acid fortification of grains since 1998 has reduced the incidence of neural tube defects (NTDs) and other developmental disorders in Canada and the USA, population-wide intake of folic acid has increased to unprecedented levels, leading to concern that there may be adverse consequences [3,6,7]. Children and elderly populations are likely to have high folic acid intake because large proportions of their diet typically consist of cereals and bread [3]; while pregnant women are likely to have high intakes due to high supplement use [8]. High folic acid intake by women planning pregnancy is prevalent in many countries, including countries without mandatory folic acid fortification, due to worldwide recommendations for this population to consume 400 µg/day folic acid for the prevention of neural tube defects [9,10]. Human observational evidence has linked high folic acid intake to increased risk of colorectal and prostate cancer [11,12]; impaired immune function [13,14], and impaired cognition [15]. Further observational evidence has linked folate status to obesity, sparking investigations into the relationship between folic acid intake and lipid and energy metabolism [16,17].Maternal excess folate [17,18] or methyl donor [19] intake during pregnancy in animal models causes weight gain or components of the metabolic syndrome in offspring. These effects may be more pronounced when offspring are fed a high fat diet [20,21]. In humans, high erythrocyte folate status during pregnancy was associated with increased fat mass of children at six years of age [22]. Folic acid appears to influence energy and lipid metabolism by modulating DNA methylation and gene expression patterns [17,18,23]. Diet-gene interactions remain important determinants of health throughout the lifespan [24], and so excess folic acid may continue to promote changes to energy and lipid metabolism in adulthood. However, the effects of excess folic acid intake on metabolic syndrome risk and adiposity in adulthood remains poorly understood.The effects of excess dietary folic acid intake on the liver, an important site of both folate and lipid metabolism, have been investigated in rodent models [23,25]. Excess folic acid intake may promote changes to one carbon metabolic pathways and gene expression patterns, leading to liver injury [25]. There is evidence that the influence of methyl donors, including folic acid, on gene expression may be tissue-, site-, and gene-specific, and so investigations into the influence of excess folic acid on other tissues (e.g., adipose) are warranted [3].The aim of our study was to investigate the effects of excess folic acid (EFA) intake compared to adequate folic acid (AFA) intake on metabolic health of rats. We hypothesized that consumption of a diet containing EFA would induce changes to lipid and glucose metabolism. High fat diets are commonly used to study weight gain and components of the metabolic syndrome in animal models. Therefore, we conducted our investigations in the setting of a 15% of energy low fat (LF) and a 60% of energy high fat (HF) diet. Our data suggest that EFA, in combination with a HF diet increases weight gain, adipose tissue mass and markers of inflammation compared to AFA, and that these effects are not seen in the setting of a LF diet. We conducted supporting experiments in vitro, the results of which suggest that folic acid can increase triglyceride accumulation in 3T3-L1 cells by inducing peroxisome proliferator-activated receptor γ (PPARγ).
