Obesity is a complex health issue that results from various causes and contributing factors. Individual factors like genetics and behavior may contribute to overweight and obesity. Genetics played an important role in increasing hunger and food intake of individuals. A few examples of common diseases that are related to obesity are type-2 diabetes, stroke, coronary heart disease and some types of cancer. All these obesity related diseases may not be fully reversible even after weight loss . Body mass index (BMI) is used widely as a screening tool to assess overweight and obesity. Based on the World Health Organization (WHO) international classification of BMI, individuals are considered as obese if their BMI value is equal to 30 kg/m2 or higher and overweight if BMI is 25-29.9 kg/m2. A study was conducted in USA to find the prevalence of obesity among children and adults. Researchers attempted to predict the childhood obesity and analyse the obesity trend among adults in United States. The results of the study showed that the prevalence of obesity among children were 16.9% and adults were 34.9%. There were no obvious changes in prevalence of obesity among children and adults in between 2003-2012 in United States (1). It cannot be denied that there is a link between increasing rate of obesity and increasing medical spending. Medical spending for obese individuals was 42% higher compared to those with normal body weight (2). The researchers reported in their study that obesity was one of the factors that became an economic burden for both public and private payers. South East Asia and Western Pacific countries suffer from diseases associated with obesity such as cardiovascular diseases and diabetes. Malaysia is the country the highest rate of obesity in South East Asia (3). Studies have showed that autonomic nervous system is associated with obesity. In 2001, a study was conducted to identify the effect of weight loss on autonomic nervous system. The findings showed that the activity of parasympathetic nervous system increased with weight loss and cardiac vagal tone improved significantly after weight loss (4). Cardiac vagal tone is controlled mainly by parasympathetic nervous system and this helps to improve the heart rate recovery (HRR). There are two phase of heart rate recovery, fast and slow phase. The fast phase is due to cardiac parasympathetic reactivation while the slow phase is because of cardiac sympathetic withdrawal. If there is any disturbance of the autonomic nervous system then the HRR will slow down (5). Therefore, HRR can be used to assess the cardiac autonomic function and as a marker of cardiac vagal tone. Abnormal heart rate recovery after exercise stress can be considered as dysfunction of autonomic nervous system. It has been suggested in several studies that heart rate recovery after exercise stress is slower among obese individuals compared to BMI individuals. A study was conducted to identify the relationship between heart rate recovery and body mass index. The finding showed that there was a negative association between heart rate recovery and body mass index. 3.5% of 2443 individuals presented with delayed decrease heart rate recovery. The prevalence of decreased heart rate recovery was higher among overweight and obese groups (6). In another study similar findings were observed. Researchers concluded that obese individuals are more likely to have an impaired heart rate recovery during the recovery phase after exercise (7). Aerobic training improves the autonomic cardiovascular control. A study on 4 weeks of aerobic training was done to identify its effects on cardiac vagal tone. Authors reported that 4 weeks of treadmill aerobic training was able to improve the autonomic nervous system among pre-hypertension individuals (8). The aerobic training consisted of 30 minutes treadmill exercise at 65% of maximum oxygen consumption. Another study showed the similar results in which the autonomic nervous system was improved after 6 months of moderate intensity aerobic training (9). The aerobic training was carried out 1 hour each session with 2 sessions in one week for 6 months. The present systematic review shows that aerobic training has been proven to be effective in increasing the heart rate recovery among subjects with established heart disease (10). The effects of 4 weeks of aerobic training in improving the cardiac vagal tone among sedentary healthy adults are still unclear. Therefore, we aim to study the effects of 4 weeks of aerobic training on vagal modulation.
