Obesity is caused by the accumulation of excess fat, an energy imbalance between excess intake of food and calories burned¹. According to a 2014 report by the World Health Organization (WHO), 39% of the world's adult population (more than 1.9 billion people) are overweight and 13% (6 million) are obese². In Korea, the prevalence of obesity in men decreases with age (32%, 43.9%, 39.6%, 41.5%, 36.9%, and 24.0% in men in their 20s, 30s, 40s, 50s, 60s, and 70s, respectively). However, the prevalence of obesity in women increases rapidly with age (15%, 18.6%, 22.3%, 29.3%, 36.6%, and 37.3% in women in their 20s, 30s, 40s, 50s, 60s, and 70s, respectively)³. Hence, there is a greater chance of women becoming obese with increasing age. These results indicate that long-term and continuous intervention for the treatment of obesity is required for women in their 20s to 40s since their prevalence of obesity is relatively low.

Obesity is a major contributory factor to the increasing prevalence of diabetes, hypertension, cardiovascular disease, fatty liver, gallbladder disease, colorectal cancer, and breast cancer; it also causes serious mental, physical, and social problems^4,5. Currently, there are three ways to prevent and treat obesity: surgery, medication, and a combination of diet and exercise intervention. Among these, surgery and medication may have an immediate effect on weight loss, but due to the costs involved and associated side effects, they are limited to specific patients such as the morbidly obese in clinical settings^6,7. On the other hand, the combination of diet and exercise intervention increases basal metabolic rate by increasing lean body mass and enhancing activation of fat utilization⁸.

In general, the most recommended exercise prescription for the prevention and treatment of obesity is aerobic exercise with moderate intensity (50 to 85% maximal oxygen consumption [VO_2max] or 60 to 90% maximal heart rate [HR_max]), frequency (3 to 5 days per week), time (45 to 60 minutes), and duration (more than 12 weeks)^9,10. However, the most widely used setting of exercise intensity generally shows different levels of aerobic and anaerobic energy metabolism depending on the VO_2max of the individual. Therefore, Menzel & Hilberg¹¹ recommend using the anaerobic threshold (AT) intensity of each individual as the appropriate exercise intensity for weight loss. There are two representative methods of determining AT: lactate threshold using blood lactate levels (LT) and ventilatory threshold (VT) using respiratory gas¹². LT, rather than VT, is more efficient in the determination of AT because it is a point of rapid change in blood lactate concentration, is less subjective in judgment, and more practical¹³.

Recently, the heart rate variability (HRV) test has been widely used in clinical trials as a method for examining the relationship between obesity and the autonomic nervous system^14-16. This method is recognized as a non-invasive and reliable evaluation method that can quantitatively evaluate the activity of the sympathetic and parasympathetic nervous systems^17,18. Autonomic nervous system function is closely related to obesity; the activity of the sympathetic nervous system leads to a decrease in basal metabolic rate, inducing weight gain, and obesity¹⁹. Since autonomic nervous system function has been suggested as an important index for evaluating obesity, it is necessary to examine the relationship between the physiological changes caused by weight loss through diet plus exercise intervention and the autonomic nervous system changes following weight recovery after suspending the intervention.

Regular aerobic exercise, composed of appropriate protocols for obese people, has a positive effect on body composition, aerobic exercise capacity, and autonomic nervous system function. In order to maintain these positive changes, steady exercise and a regular lifestyle should be maintained. However, exercise is easily interrupted by such factors as satisfaction with the achievement of the weight goal, daily economic activity, and low interest in exercise. These lead to the yo-yo phenomenon where there is a return to or increase in weight before the weight is lost again²⁰. However, studies on the improvement of obesity to date are limited to examining the effects of various types of exercise on various dependent variables including weight loss.

Therefore, our study examined the effect of diet plus exercise training at LT and detraining for 12 weeks on body composition, aerobic performance, and stress-related variables in obese women in their 20s to 40s.

Our study examined the effect of diet plus exercise training at LT and detraining for 12 weeks on body composition, aerobic performance, and stress-related variables after classifying obese women in their 20s to 40s into HR_LT and HR_LT + 5% exercise groups.

With regard to dietary intake and daily activity, all participants were encouraged to adopt 70% of the RDA during the 12-week training period. The total calorie intake was significantly decreased during the 12 weeks of exercise training in both groups, and the decrease ratio was found to be 830.8 kcal vs 782.7 kcal (HR_LT vs HR_LT exercise groups), indicating that the treatment for diet was relatively well done. Daily activity showed a similar increase of 221.5 kcal and 284.9 kcal in the HR_LT and HR_LT exercise groups, respectively, during the training period. These results suggest that exercise training is a great motivator for weight loss and health management in the participants, and this motivation changed their lifestyle more actively and positively during the training period²¹. However, 12 weeks of detraining resulted in a significant increase in dietary intake and a significant decrease in daily activity in both groups. Therefore, the detraining in obese women in their 20s to 40s is thought to be the result of both a cessation in physical exercise and an induction of the yo-yo phenomenon of weight gain through increasing dietary intake and decreasing daily activity²².

The most commonly used method of treating obesity is a combination of diet and exercise, and it has been reported that dietary treatment should not be limited to less than 1,200-1,600 kcal/day for men and 1,000-1,200 kcal/day for women to avoid limiting the energy balance and micronutrient intake required by individuals²³. Additionally, it is widely known that aerobic exercise of 60 to 90 minutes or more daily is the most commonly used method of treating obesity as it reduces body fat²⁴. The combination of diet and exercise has been reported to not only reduce body weight and body fat, but also to improve various clinical factors associated with diabetes and cardiovascular disease^8,25. It also increases endorphin secretion to make the subject feel good and has a motivating effect on the treatment of obesity, which can be maintained for a relatively long time after treatment²⁶.

Based on this previous study, our study provided proper diet and exercise treatment to all participants of the HR_LT and HR_LT + 5% exercise groups during the 12-week training period. As a result, body weight showed a decrease of 4.9 kg and 4.8 kg in the HR_LT and HR_LT + 5% exercise groups, respectively, which persisted after 12 weeks of detraining. The percentage of body fat decreased by 3.1% and 3% in the HR_LT and HR_LT + 5% exercise groups, respectively, but showed slightly increased tendencies of 0.3% and 0.8% in the HR_LT and HR_LT + 5% exercise groups, respectively, after 12 weeks of detraining. In other words, our study showed a positive effect contrary to the results of previous studies, in which obese people stopped the long-term diet plus exercise intervention for weight loss and showed the yo-yo phenomenon. The differences compared to previous studies cannot be clearly interpreted, but these results are believed to be due to the fact that although both groups had higher dietary intake and lower daily activity after 12 weeks of detraining than after 12 weeks of training, there was a relatively lower dietary intake and higher daily activity after 12 weeks of detraining than before training.

Generally, the improvement of aerobic performance by long-term exercise training has been reported to improve body composition and various clinically-related variables, thereby reducing the risk of obesity and metabolic syndrome^27,28. The most commonly used parameters in evaluating aerobic performance are VO_2max and HR_max. VO_2LT and HR_LT have also been reported to be appropriate as evaluation parameters^29,30. Our study showed a significant increase in VO_2LT and increased tendency in VO_2max by 12 weeks of exercise training in both groups. However, despite a higher daily activity being maintained during the detraining period than before training, the aerobic performance was reduced to the initial level. These results suggest that aerobic performance is not maintained even if high daily activity is shown through various activities in daily life, unless physical activity of high intensity is performed.

The HRV test is recognized as a very effective method for determining stress levels by quantitatively evaluating the activity and balance of the autonomic nervous system, and the result is a relatively simple non-invasive measurement that can be quickly obtained through computer analysis^30,31. Furthermore, HRV has the advantage of being able to objectively and easily measure the activity of the autonomic nervous system and changes in its activity and balance due to stress have been reported to have characteristics that make it possible to diagnose the degree of stress and stress-related diseases in the field of psychiatry^14,32. Therefore, the HRV test is regarded as a useful tool for objectively evaluating the psychological-emotional status of an individual³³.

In this study, the HRV test was performed to examine the effects of 12 weeks of training and detraining on the stress in obese women in their 20s to 40s. No significant change was observed in the HR_LT exercise group, but the HR_LT + 5% exercise group showed a relatively marked increase tendency. In particular, mean RR and RMSSD showed significant increases of 38.77 ms and 5.87 ms, respectively. Mean RR and RMSSD, which are prominent in exercise training, are very simple methods that use the RR intervals obtained from electrocardiograms³⁴. The RR interval is the interval between R and the next consecutive R between adjacent QRS complexes on the ECG, and mean RR generally corresponds to the mean value of the time interval between R and R for 5 minutes. The RMSSD is expressed as the square root of the average of the sums of squares of differences between adjacent RR intervals. These variables are reported to indicate short-term cardiac variability and activity of parasympathetic nerves. Therefore, it is reported that the autonomic nervous system activity and balance are stable, less stressed, and healthy people have higher mean RR and RMSSD levels^14,17. Also, these HRV variables have been reported to improve with various indicators for obesity and lifestyle diseases when applying various types of exercise training^14,17,30,33. Therefore, dietary treatment equivalent to 70% of RDA and exercise treatment above moderate intensity corresponding to HR_LT and HR_LT + 5% for 12 weeks enhance the balance of the autonomic nervous system and resistance to stress by improving HRV, and these effects are believed to persist even after 12 weeks of detraining.

Our results suggest that 70% RDA of dietary intervention and exercise training corresponding to HR_LT and HR_LT + 5% for 12 weeks were effective in improving body composition, aerobic performance, and stress. In particular, the improvement of HRV, an indicator of stress, persisted for up to 12 weeks after the end of exercise training in obese women.