Sugar, fructose, uric acid and hypertension in children and adolescents

Abstract

Fructose consumption has been increasing over recent decades and is believed to play a role in the rising epidemic of metabolic disorders and hypertension (HT) in children [1,2]. This theory is supported by epidemiological and experimental studies in animals and humans. High-fructose diets upregulate sodium and chloride transporters, resulting in salt overload that increases blood pressure (BP) [3]. Moreover, excess fructose has also been found to deregulate vasoconstrictors and vasodilators, and over-stimulate the sympathetic nervous system. Metabolism of fructose is mediated by fructokinase, which uses ATP as a phosphate donor. Unlike glucose, there is no feedback mechanism regulating fructokinase. As a result, AMP is continuously involved in the production of uric acid (UA) [4]. In adolescents in the US, serum UA was showed to increase from the lowest to the highest category of fructose-sweetened beverage intake and this increment was paralleled by an increase in BP, even independently of body mass index [5]. These data suggest pathways other than obesity relating soft drinks to the development of HT. Epidemiological studies demonstrate an association between serum UA and both prevalence and new onset of essential HT in adolescents [6]. Recently, UA showed a strong independent relationship with BP values across different BP categories, from normal BP up to pre-and finally to established HT in children at relatively high cardiovascular risk [7]. Animal models support a two-phase mechanism for the development of hyperuricemic HT. Initially, UA induces vasoconstriction by activation of the renin-angiotensin system and reduction of nitric oxide. Over time, UA uptake into vascular smooth muscle cells (VSMC) causes cellular proliferation and arteriolosclerosis that impair pressure natriuresis, causing sodium-sensitive HT [8]. Increased UA causes endothelial dysfunction by inflicting oxidative stress once inside cells. UA internalization is mediated by URAT-1, and stimulates production of growth factors and chemokines in human VSMC [9] and increases ROS activating NADPH-oxidase, leading to apoptosis in human tubular cells [10]. These actions may, at least in part, explain the association between increased serum UA levels and initial cardiovascular and renal damage described in adolescents with obesity or HT [11,12]. Interestingly, in both animal and human studies, allopurinol attenuates the development of fructose-induced HT by lowering UA. Indeed, lowering UA with either allopurinol or probenecid reduces BP in adolescents with HT or pre-HT [13,14]. While larger studies are needed, fructose assumption and serum UA are emerging as potentially modifiable risk factors for the prevention and treatment of HT in children.