Shorter recovery time following high-intensity interval training induced higher body fat loss among overweight women

High-intensity interval training (HIIT) has been widely employed as an exercise protocol to reduce fat, and also in addition, interval recovery periods are the most important factors which can have great impact on weight loss. Therefore, the aim of this study was the evaluation of different recovery time between HIIT program on metabolic responses and weight loss in overweight women. Twenty-four overweight (BMI 29.5 ± 3.5 kg/m2) volunteer women were randomly assigned into three groups (n = 8/group) to study the effects of different recovery times: [group 1; HIIT with 60/60 activity–rest ratio (s), group 2; HIIT with 60/30 activity–rest ratio (s), and group 3 was set as a control]. The participants performed 3 times per week and 4 bouts/session (80% of Heart Rate Reserve). The exercise program gradually increased to 10 bouts/session. The most important findings of this study were a change in the body fat percentage (BF %) in the between group comparison: group1 (40.5 ± 0.9), group 2 (41.2 ± 0.7) and group 3 (41.1 ± 1.1). Compared to the control group the 60/30 s HIIT resulted in a significant decrease in BF % (P = 0.002). However, no other significant differences in the body composition were found. Either there were no significant differences between the groups in T4, T3 and TSH, cortisol, HGH, FBS, blood insulin, insulin resistance insulin sensitivity or fatness-associated hormones. In conclusion, considerable decrease in BF % in the 60/30 s. rest interval group indicates that 30 s recovery period in HIIT may reduce fat % more efficiently than 60 s.


Introduction
Obesity is a physiological condition that may act as a potential risk factor for metabolic syndrome, type-II diabetes, cardiovascular disease, cancer and other diseases [1]. Therefore, obesity demands effective strategies to reduce weight and further the risk for metabolic syndrome. It has been demonstrated that physical activity is an effective neurologic-hormonal and metabolic stimulus to mobilize stored fat to produce energy, and therefore, it is an important way to combat obesity [2]. Recent findings indicate that especially high-intensity interval training (HIIT) is an effective exercise protocol that reduces over-weightiness in obese individuals [3][4][5] and has been shown to cause physiological adaptations similar to endurance training program, such as endocrinologic adaptations [6,7]. Regardless of weight loss in response to exercise programs, the programs result in changes of hormones involved in fat metabolism [8]. For instance, Wahl et al. [9] showed that HIIT results in cortisol secretion after 10 min and during the recovery period. It also seems that compared to continuous exercise, HIIT causes positive changes that are beneficial to blood glucose control and insulin sensitivity [10,11] as well as reduction of insulin resistance [12]. It may be that factors such as the interval recovery periods between the stages of activities are involved in fat metabolism and weight reduction in addition to the duration and intensity of exercise program [13,14]. However, the effects of recovery periods have been scarcely studied, but it has been demonstrated that acute exercise results in an increase of many circulating metabolism-involved hormones [15]. These hormones include catecholamine, corticosteroid, growth hormone, thyroid hormones and gonadotropins (androgen, estrogen), among others, that regulate lipid metabolism [16]. There is a relationship between thyroid hormones and cortisol after exercise; the immediate response of cortisol has a negative and significant correlation with the delayed secretion of TSH and T3 after 24 h. Besides, thyroid hormones decrease up to 24 h after this exhausting activity while cortisol response has an inverse relationship with the reduction of thyroid hormones. In fact, the cortisol secretion can cause thyroid hormone secretion and function [17]. In addition, Bracken et al. [18] reported that performing high-intensity exercise on treadmill with 30/60 s activity-rest intervals increased the level of catecholamine, one of the hormones involved in fat metabolism. Nevill et al. [19] also found that 30 s of high-speed running resulted in ten times higher increase of GH levels compared to the resting state up to 1 h. Related to body composition, Airin and et al. [20] showed that decreasing the rest interval period to 8/12 s caused more body fat % in addition to improvement of anthropometry index in overweight women. Other studies [21] have reported that longer period of recovery (2 min) compared to shorter ones (30 s) resulted in significant fat reduction after 15 weeks of training, while Lousier et al. demonstrated that aerobic metabolism during the recovery period of intensive exercise plays a significant role in rebuilding creatine phosphate [22]. While these studies are promising, the results seem somewhat contradictory. According to the existing studies on the importance of recovery time in HIIT, the recovery time during high-intensity interval training can lead to a change in the metabolic status and a reduction in body fat. Therefore, we seeked to answer the question of whether HIIT training with different recovery period can change some of the metabolic factors that ultimately lead to weight loss and other obesity-related body composition variables in overweight women?

Participants
The study was approved by the ethics committee of Sport Sciences Research Institute of Iran. 24 overweight-obese women with the average age of 35.3 ± 3.8 years participated in the study. The height, weight and BMI of the participants were 159 ± 14 cm, 74.6 ± 9.9, and 29.5 ± 3.5 kg, respectively. They were randomly assigned in three groups. The characteristics of the participants are presented in Table 1. For assessing the activity level of the study subject's physical activity questionnaire (RPAQ) Harriss and Atkinson [23] was used. Based on these data, it was determined who had 3-4 days of recreational activities such as gardening, bicycling volleyball, jogging or yoga. No differences between the groups were detected in the physical activity levels at baseline (data not shown).
In addition, health history and nutritional supplements such as the use of creatine, amino acids or vitamins and medical history regarding the medical treatments including surgery, medication or any other information that may interfere with the results of this study within the last 6 months was recorded. They were also asked if they were involved in any other research protocol within the last 2 months [23]. The participants completed the informed consent form before participation in the exercise protocol. In addition, the participants completed a health history questionnaire to reveal whether they had any history of surgery or medical treatment. Finally, after determining inclusion (90% of options) and exclusion criteria (10% of options), any individual who showed two risk factors (Includes in health history questionnaire, nutritional supplement, medical treatments) were excluded from the study. To reduce the influence of previous food consumption on the substrate response during exercise, the subjects were instructed to maintain their normal diet throughout the study.

Body composition and blood pressure
For the purpose of assessing BMI, Asimed analog scale with the discrimination index of 0.1 kg and Asimed height meter with the discrimination index of 1 mm was employed. The waist and hip size was measured in standing position using a flexible tape (cm) to determine the waist-hip ratio (WHR). Body composition was measured by skinfold caliper (Cescorf Brazil) from the right side of the body for seven points [24]. To determine body fat percent, Siri equation was employed [24,25]. These measures were taken once before the start of the protocol and again 1 day after the termination of the exercise program. Blood pressure was measured in sitting position with Mercury manometer 24 h before training and 24 h after 4 weeks training.

Exercise protocol
The exercise protocol was performed under the supervision of 2 researchers and included 12 sessions of HIIT for 4 consecutive weeks (Saturday, Monday, and Wednesday). The exercise protocol included three homogenize groups (n = 8). 4 bouts with 1 min HIIT and 1 min of recovery between each bout (group1 = 60/60 activity-rest ratio); and also 4 bouts with 1 min HIIT and 30 s recovery (group 2 = 60/30 activity-rest ratio) between each bout which increased gradually to 10 bouts × 1 min activity and control group (group 3). Overload was increased by two bouts in each week [26]. Exercise intensity based on the 80% average Heart Rate Reserve (HRR) of each person, which was determined using the Karvonen equation [27]. Speed of treadmill based on 80% HRR, heart rate during rest and activity was monitored by Polar heart rate monitors; Polar Electro Inc (T 31 made in Finland).
To control the treadmill to woman perform the active recovery period, another treadmill was adjusted and used according to the individual HRR. Due to the adaptation caused by exercise, intensity was calculated in each session by Karvonen equation for 80% of HRR, in both experimental groups. Prior to the start of the exercise per session, the participant took part in 3 min of warm-up at 6 k/h and after the termination of the exercise protocol, they took part in 2 min of cool down exercise on treadmill.

Biochemical analyses
Following 12 h of fasting, 10 cc of left brachial venous blood was collected in two different stages 24 h before and 24 h after the end of protocol at 8:00 a.m. Then plasma and serum were separated by centrifugation for 10 min at 4000 rpm and was stored at − 20 °C until analyses. By employing a questionnaire, it was ensured that regular menstruation cycle of 28-32 days was present in all the participants and accordingly blood sampling was conducted at a follicle stage in menstruation cycle.
In addition, cortisol, growth hormone (GH) and insulin were measured by chemiluminescence kit (Zimmense, Germany, Monobind, USA, and Zimmense, Germany, respectively) and CV for those hormones were 6.1, 0.118 and 3%, respectively. Finally, glucose level was measured with a kit (Pars Azmon Co., Iran) with the least measurable sensitivity of 5 mg/dL and CV of 1.28%. The level of glucose and insulin was calculated by AUC method and trapezoid law.

Statistical analysis
All statistical analyses were carried out using SPSS version 16.0. The normality of data was confirmed by Shapiro-Wilk test. The difference between the pretest-posttest of all the measured variables was used to test the hypothesis. One-way analysis of variance (ANOVA) was employed to compare the results and Tukey post hoc test was used to compare the means if a significant difference was found. Paired and independent t tests were used to investigate the changes in the variables as a result of the effect of recovery time separately in each group and to compare two types of recovery in the training groups. The correlation of cortisol hormone with thyroid hormones was tested by Pearson correlation coefficient. All the hypotheses were examined at the alpha level set to 0.05.

Physical characteristics
The results indicated that none of the exercise modes caused significant changes in weight [F (2,18) Table 1).

Blood lipids
There were no significant differences in the serum level of cholesterol [F (2,18) (Table 2).

Thyroid, cortisol and growth hormones
No significant differences were found in the levels of thyroid hormones T3 [F (2,18)

Fasting blood glucose and blood insulin
None of the exercise modes caused significant changes in fasting blood sugar [F (2,18) Table 3].

Discussion
The purpose of this research was to examine the effect of 4 weeks of high-intensity interval training program on anthropometric indices that are employed in assessing fatness in overweight women. According to the heart disability of obese people and exercise intolerance related to the age range [30], the probability of skeletal joints injuries due to overweight in women, especially in HIT, increases. Therefore, this study was done in 4 weeks. The results of the present research indicated that besides a decrease in fat % in the 60/30 group, HIIT program had no significant effects on the other body composition variables in overweight women. In agreement with our findings, Keating et al. [31] have also shown that HIIT program in comparison with the long-term endurance program had no significant effects on anthropometrics used to assess fatness in adult men and women. However, conversely, Heydari et al. [32] demonstrated that HIIT programs with shorter activity to rest interval of 8-12 s in young men led to significant changes in body composition. Thus, shorter period of training could contribute to the different findings. The results of the present study showed a significant change in the fat % of the three groups that may be attributed to the 60/30 activity-rest interval compared to the control group. In agreement, Tjonna et al. [33] found that 12 weeks of aerobic interval training (with rest interval shorter than the training time of 3-4 min decreased significantly subcutaneous fat mass), visceral fat mass and WHR of obese participants. However, in a contrary, Gillen et al. [34] demonstrated that 6 weeks of HIIT program with 60/60 rest-activity interval caused improvement in body composition variables and muscle oxidative capacity of obese women. The importance of the recovery times between HIIIT has been pointed out before [35]. For instance, it has been shown that if the recovery period in high-intensity exercises is decreased, the proportion of glycolysis for providing energy is reduced and as a result the aerobic metabolism increases [22]. This may be one of the contributing mechanisms in HIIT programs that use rest intervals shorter than the activity periods. It is likely that the shorter recovery period compared with the 60/60 s rest-activity group is one of the causes of changes that have not been indicated in previous studies. Trapp et al. [36] reported that participation in 15 weeks of HIIT program with activity-rest period of 8-12 s compared to the steady-state exercise programs led to significant decrease in plasma leptin level and measures of fatness in young women. In addition, Moher et al. [21] also compared prolonged exercise program with 15 weeks of HIIT program of 30 s activity with 2-min rest intervals and concluded that fat mass in sedentary women decreased significantly in longer training time. Despite these findings, there are other results that show greater fat mass reduction in prolonged exercise Romain et al. [37] and most of the indices of fatness versus the HIIT program [38]. These results in regard to different activity-rest intervals suggest that the recovery period is an important variable affecting the changes in physiological indices in an exercise program.
It should be noted that many variables including the type of exercise, intensity, duration, body condition during the blood collection, lab methods [39], individual difference [6] and gender [40] may be confounding factors that could lead to the contradictory findings presented in the current literature.
Our further analyses indicated that no significant changes in blood lipid levels occurred in the three groups. Such finding is in agreement with what was reported by Mohr et al. [21] who examined the effects of 15 weeks of HIIT swimming program with activity-rest intervals ratio of 30 s/2 min on lipid profiles and found no effects. In addition, Hydari et al. [32] reported that 12 weeks of HIIT program with activity-rest intervals ratio of 8/12 s did not affect blood lipids. Mannaing et al. [41] claimed that the initial level of blood lipids, being non-obese or the level of fat storage. Blaize et al. [42] may lead to the effectiveness of an exercise program that results in significant changes in blood lipid profiles. Therefore, in our study, the activity level of the participants in 3-4 sessions of leisure activity per month, low sample size and duration of exercise program are limitations of the present study, and may be the reason that the measures of fatness other than fat % did not change significantly. The results of the present study did not show any significant effect for HIIT program on insulin sensitivity and insulin resistance, of course, decreasing trend has observed in insulin resistance in the experimental group 60/30 activity-rest interval group, which however, was not statistically significant. Boutcher [43] in a review research concluded that HIIT programs considerably decrease insulin resistance and glucose tolerance. On the other hand, Baldwin and Haddad [44] stated that exercise program may decrease the levels of thyroid hormones as a result of a decrease in blood glucose and fatty acid increase, but after the termination of the exercise activity and during the recovery period, the secretion is resumed. Similarly, the level of the hormones changed in our study, but the change was not statistically significant.
Krotkiewski et al. [45] showed that the level of T4 in obese women increased considerably in response to 3 months bicycle ergometer training, but no significant change was seen in T3 and TSH. Huanga et al. [46] did not report any significant change in TSH level as a result of participation in maximum intensity exercise on treadmill but showed a considerable increase in T3 level, Ciloglu et al. [47] showed a significant effect of participation in 3 min of 90% of maximum heart rate on the level of T4 and TSH but reported a decrease in the level of T3. Researchers believe that the hormonal changes may depend on many interfering factors including insufficient sleep [48], emotional pressure and caloric demands [49,50], and therefore these factors all may disrupt the correct thyroid function [51]. As a result, controlling these nuisance factors increases the validity of findings. In addition, it seems that previous experience of the participants in the physical activity, training variable and resting time for recovery all may have effect on thyroid secretion and cause conflicting findings in different research protocols [51].
Other researchers including Anthony et al. [52] showed a negative correlation between the decrease in free T3 and increase of reverse T3 12 h of following the last training program, whereas the negative correlation between the increase in cortisol and decrease of free T3 in 12 h after the last session of training was present. In this regard, the authors examined the effects of psychological pressure on the subjects and showed that despite the absence of significant change in cortisol levels, negative correlation between cortisol and T3 was found after the training program. Therefore, it is likely that the change in thyroid hormone levels as a result of stress hormones such as cortisol occurs.
On the other hand, acute exercise activity results in an increase of plasma GH probably due to the increase in parasympathetic nervous system [53]. However, the results of our study did not show any significant changes. Pritzlaff et al. [54] claimed that during the recovery period there is a direct association between the GH level and the level of fat oxidation. That is, fat metabolism is increasingly associated with GH release during the recovery [53]. In addition, the level of physical fitness, age and sex are also factors that influence this hormone's release [55]. Increasing lactate is an effective stimulus for GH secretion [56]. In this regard, Gosselin et al. [56] revealed that HIIT with activity-rest periods of 90/30 and 60/90 led to higher lactate levels in comparison with activity-rest periods of 60/60 and 30/30. It could be concluded that increasing lactate levels due to the reduction of recovery time during HIIT increase fat burning-induced GH secretion. It needs to be pointed out that examining the effect of exercise to the recovery time ratio on GH is also important. The effects of recovery time between the exercise bouts on the obesity indices have not been examined and thus it remains unclear which recovery to exercise time ratio in high-intensity workout results in increase of metabolism that triggers fat mobilization.
In conclusion, compared to the control group, the 60/30 s HIIT resulted in a significant decrease in body fat % while no other significant differences in the body composition were found. However, according to the present study and the results of studies mentioned above, 30 s recovery period in HIIT may reduce fat % more efficiently than 60 s.