Just Accepted by Free Radical Research Lipoprotein Subfraction Oxidation in Acute Exercise and Aging

Exercise and aging can independently increase free radical production that may enhance the susceptibility of LDL to oxidation and create a more atherogenic LDL particle. This investigation was designed to examine exercise and aging on the susceptibility of LDL subfractions to oxidation. Eleven aged (55 + 4 years) and twelve young (21 + 2 years) participants completed a progressive exercise test to exhaustion and within one week performed a 1 hour bout of moderate intensity (65% VO2max) exercise. Blood was assayed for metabolites associated with lipid composition (total cholesterol, free cholesterol, triglycerides) and lipoprotein susceptibility to oxidation. Exercise increased sdLDL oxidation, independently of age (p<0.05). However, sdLDL oxidation further increased 24 hours post exercise in the aged group (p<0.05). With regards to the changes in lipid components within LDL, free and total cholesterol and triglycerides in lbLDL were all elevated 24 hours post exercise in aged compared with young (p<0.05 for all comparisons). There was a decrease in triglycerides in mdLDL 24 hours post exercise in the aged group (p<0.05). The lipid composition of sdLDL, VLDL, HDL2, HDL3 and serum lipid hydroperoxides remained unchanged as a function of exercise and aging (p>0.05). Although regular exercise training is known to be protective against CVD onset, our data demonstrates that acute exercise can increase sdLDL oxidative susceptibility, and this is independent of age and regardless of a change in LDL lipid composition. However, age seems to be a determining factor with regards the susceptibility of sdLDL to oxidation 24 hours following exercise. D ow nl oa de d by [ U ni ve rs ity o f L et hb ri dg e] a t 1 0: 04 2 4 O ct ob er 2 01 5 INTRODUCTION Cardiovascular disease (CVD) and the peripheral complications that it encompasses represent a major health problem in the developed world. Globally, it is estimated that CVD caused the death of 17.3 million people in 2008, with this figure thought to rise to 23.6 million by 2030 [1]. Atherosclerosis, characterised by the deposition of cholesterol and lipids within the vascular wall, represents a major cause of CVD [2]. Known risk factors for atherosclerosis include age, smoking, sex, diabetes, obesity, hypertension and circulating triglycerides and cholesterol [3]. Indeed, even in the absence of specific risk factors, an increase in plasma cholesterol is unique in its ability to develop atherosclerosis [4]. Regular exercise can reduce blood pressure, improve insulin sensitivity, and increase HDL and apolipoprotein B [5], thus help reduce the prevalence of CVD. However, exercise can induce a state of oxidative stress by increasing the production of oxygen-derived free radical species through a number of mechanisms including increased mitochondrial oxygen consumption, activation of oxidase enzymes and autoxidation of metabolites [6]. Under normal physiological conditions, the production of reactive oxygen species (ROS) is counterbalanced by a unique antioxidant defence system. However, when the production of ROS becomes excessive, the antioxidant defence system becomes compromised causing cell and tissue damage [7]. Oxidative modification of LDL and their accumulation within the vascular wall is a recognised key step in the development and progression of atherosclerosis [8]. As such, there may be a paradoxical effect of exercise between the benefits it has on CVD risk factors, and the potential deleterious effects of exercise-induced oxidative stress. Although it has been reported that acute aerobic exercise can increase the susceptibility of LDL to oxidation in vitro [9], there is a distinct lack of research examining the effect of moderate intensity exercise of sustainable duration on LDL oxidation. Furthermore, studies have focused solely on LDL as a homogenous molecule, however, LDL is a heterogeneous particle existing as a set of subspecies and each subspecies is distinct in size, density and composition with the smaller, denser LDL particles D ow nl oa de d by [ U ni ve rs ity o f L et hb ri dg e] a t 1 0: 04 2 4 O ct ob er 2 01 5 showing increased susceptibility to oxidation than the larger buoyant LDL [10]. As such an investigation of these molecules may prove useful in a clinical setting. Age is a non-modifiable risk factor for CVD, and in addition to the age-related changes in lipid and lipoproteins including elevated LDL cholesterol and triglycerides, the aged population represent a cohort of individuals that are potentially at a greater risk of oxidative stress [11]. The ‘oxidative stress theory of aging’ implies an increase in free radical production as we advance in years [12], and an increase in the indices of lipid peroxidation and a decrease in antioxidant capacity have been reported in studies associated with aging [13]. This elevation in oxidative stress in aging may potentiate in vivo oxidation of LDL subfractions following exercise; however evidence in this specific population is sparse. We therefore hypothesize that acute aerobic exercise enhances the susceptibility of LDL particles to oxidation in aged subjects. Thus, the primary aim of this study is to investigate the effect of exercise on LDL subfraction oxidation in young and aged participants. D ow nl oa de d by [ U ni ve rs ity o f L et hb ri dg e] a t 1 0: 04 2 4 O ct ob er 2 01 5 METHODS and STUDY PARTICIPANTS Human participants and experimental design Following ethical approval from a local Research Ethics Committee, twenty-three (n=23) apparently healthy male participants were recruited for the study (Anthropometric data is provided in table 1). A screening questionnaire obtaining medical and exercise history was completed by each participant. Participants were excluded if they used multi-vitamins or antioxidants in the past 2 months, were using lipid lowering/modifying medication, had a diagnosis of heart disease, dyslipidaemia, type 1 or type II diabetes mellitus, orthopaedic limitations or had any other health problem that may interfere with exercise. All participants were non-smokers and only participated in sporadic recreational type activity as assessed by a physical activity questionnaire. Participants were divided into two groups classified as young (18-25yrs, n=12) and aged (50-65yrs, n=11), and asked to refrain from exercise and alcohol consumption for 48 hours prior to all experimental testing. Each participant was required to attend the laboratory on two occasions, and each occasion was separated by at least 3 days. In session 1, all participants performed a graded progressive exercise test to determine baseline fitness. Exercise session 2 required all participants to complete 1 hour of moderate intensity exercise at 65% VO2max on a motorised treadmill (Cosmos, Germany). We specifically chose to use an exercise duration of one hour and an exercise intensity of 65% VO2max due to the following reasons (1) there is a paucity of studies examining LDL oxidation using one hour of exercise at 65% VO2max in the aged, and (2) an exercise intensity of over 65% could not be sustained over a prolonged duration by the aged participants used in this study. Progressive exercise test The progressive maximal exercise test enables the determination of heart rate (HR) and oxygen uptake (VO2) at maximal effort (highest workload achieved). Following a self-determined warm up and familiarisation phase, all participants performed a continuous and progressive incremental treadmill test to exhaustion. A standard protocol was employed to determine maximal oxygen D ow nl oa de d by [ U ni ve rs ity o f L et hb ri dg e] a t 1 0: 04 2 4 O ct ob er 2 01 5 uptake in the young. The test began at a speed determined suitable for each participant, with a 1% increase in treadmill gradient at each minute until volitional exhaustion. The Modified Bruce protocol [14] was employed to determine maximal oxygen uptake in the aged. This protocol employed a slower starting speed (1.7 mph) at 0% gradient with a final exercise stage using a gradient of 10%. Oxygen uptake and heart rate was recorded during the last 5 seconds of every stage, while a heart rate corresponding to 65% VO2max was calculated from the VO2-HR relationship and used as an attainable HR zone during the 1 hour of continuous exercise. Total % body fat was measured using a bioelectrical impedance analyser (Bodystat, UK).