INFLUENCES OF SHORT-TERM NORMOBARIC HYPOXIC TRAINING ON METABOLIC SYNDROME-RELATED MARKERS IN OVERWEIGHT AND NORMAL-WEIGHT MEN Normobaric Hypoxic Training on Metabolic Syndrome

Background and Objective This study examined the infl uence of short-term normobaric hypoxic training on metabolic syndromerelated markers in overweight and normal-weight men. Material and Methods Forty-one Japanese men were included and divided into two groups based on their body mass indices (BMIs): BMI≥25 or BMI<25. Participants in the overweight and normal-weight groups were randomly classifi ed into the hypoxic exercise group (hypoxic overweight, HO; hypoxic normal-weight, HN) and the normoxic exercise group (normoxic overweight, NO; normoxic normal-weight, NN). Subjects performed treadmill exercise three days per week for four weeks at an exercise intensity of 60% of maximum heart rate, under either normobaric hypoxic or normobaric normoxic conditions, for 50 min (including 5 minute warm-up and cool-down periods) after a 30-min rest period. The study parameters included weight, body fat percentage, BMI, heart rate, waist circumference, ankle-brachial pulse wave velocity (PWV), blood sugar, triglycerides (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), fasting insulin, Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) scores, and adiponectin levels. Repeated measures two-way analysis of variance was used to examine diff erences in the mean parameter values between the two groups (overweight and normal-weight) before and after training. Original Article DOI: 10.22347/1875-6859.14.1.5 Infl uences of Short-Term Normobaric Hypoxic Training on Metabolic e45 J Mens Health Vol 14(1):e44-e52; February 1, 2018 This article is distributed under the terms of the Creative Commons Attribution-Non Commercial 4.0 International License. Results Hypoxic training improved the weight, body fat percentage, BMI, waist circumference, PWV, TC, LDLC levels, and HOMA-IR scores in the overweight and normal-weight groups (p<0.05). In addition, TG level, HDL-C level, and HOMA-IR scores showed signifi cant interactions with hypoxic training, as these parameters improved in the hypoxic overweight group (p<0.05). Conclusion These results suggest that hypoxic training could be useful for improving arterial stiff ness, circulatory system function, body composition, and energy metabolism in adult males. Obesity and metabolic syndrome are pressing worldwide epidemics that are growing at an alarming rate, with one in three individuals at risk of these conditions. Aerobic and muscle-strength exercises are eff ective at improving lifestyle-related diseases, including obesity. However, middle-aged or elderly people with a motor system disease who perform exercise that strain their back or knees for an extended period can experience increased pain and diffi culty in everyday life. An important question is how training stimuli can be optimized to maximize metabolic and cardiovascular benefi ts and minimize their risk of injury, particularly in overweight individuals. Previous studies have suggested that hypoxic training elicits a similar cardiovascular stimulus to normoxic training at a lower workload. Although the workload is reduced in hypoxic training, its benefi ts are similar to or greater than those of normoxic training in terms of improvements to cardiovascular and metabolic risk factors. Hypoxic training, although not commonly used, may be benefi cial in individuals with clinical conditions, such as coronary artery disease and chronic obstructive pulmonary disease. However, the eff ects of hypoxic training vary greatly according to the oxygen concentration, exercise intensity, time and duration, and among individuals. However, there is disagreement regarding the eff ects of exercise in a hypoxic environment on lipid metabolism, with some studies reporting no change and others reporting improvements in markers. Hence, further studies are needed to develop eff ective and safe hypoxic training programs for the prevention and improvement of metabolic syndrome. Recently, there have been many reports that hypoxic training can eff ectively alleviate obesity and improve metabolic syndrome in overweight and obese people. Shi et al. examined the eff ect of hypoxic training on high-sensitivity C-reactive protein (hs-CRP) levels, an infl ammatory marker, in healthy adult males and reported that hs-CRP levels were signifi cantly lower in the hypoxic group than in the control group after the training. Therefore, hypoxic training may have positive, anti-infl ammatory eff ects on metabolic syndrome-related factors. Most previous hypoxic training studies have focused on specifi c groups, such as obese individuals, athletes, college students, and general adults, and the eff ects of hypoxic training have varied according to the physical fi tness and physical characteristics of the subjects. Since obese people are at greater risk of developing metabolic syndrome than are those of a normal weight, the eff ects of hypoxic training on metabolic syndrome-related factors might be more pronounced in obese individuals. It is also necessary to investigate whether metabolic syndrome markers are improved following hypoxic training in normalweight people. However, no hypoxic training studies have compared obese and normal-weight groups with one another. The purpose of this study was to examine the eff ects of hypoxic training on factors related to metabolic syndrome. In this study, participants were divided into either an overweight or a normal-weight group, and the eff ects of regular normobaric hypoxic training on metabolic syndrome-related factors were examined according to physique.

Obesity and metabolic syndrome are pressing worldwide epidemics that are growing at an alarming rate, with one in three individuals at risk of these conditions. 1 Aerobic and muscle-strength exercises are eff ective at improving lifestyle-related diseases, including obesity. 1 However, middle-aged or elderly people with a motor system disease who perform exercise that strain their back or knees for an extended period can experience increased pain and diffi culty in everyday life.An important question is how training stimuli can be optimized to maximize metabolic and cardiovascular benefi ts and minimize their risk of injury, particularly in overweight individuals. 2revious studies have suggested that hypoxic training elicits a similar cardiovascular stimulus to normoxic training at a lower workload. 2,3Although the workload is reduced in hypoxic training, its benefi ts are similar to or greater than those of normoxic training in terms of improvements to cardiovascular and metabolic risk factors.
Hypoxic training, although not commonly used, may be benefi cial in individuals with clinical conditions, such as coronary artery disease and chronic obstructive pulmonary disease. 4However, the eff ects of hypoxic training vary greatly according to the oxygen concentration, exercise intensity, time and duration, and among individuals.However, there is disagreement regarding the eff ects of exercise in a hypoxic environment on lipid metabolism, with some studies reporting no change and others reporting improvements in markers.Hence, further studies are needed to develop eff ective and safe hypoxic training programs for the prevention and improvement of metabolic syndrome.
Recently, there have been many reports that hypoxic training can eff ectively alleviate obesity and improve metabolic syndrome in overweight and obese people.Shi et al. examined the eff ect of hypoxic training on high-sensitivity C-reactive protein (hs-CRP) levels, an infl ammatory marker, in healthy adult males and reported that hs-CRP levels were signifi cantly lower in the hypoxic group than in the control group after the training. 5Therefore, hypoxic training may have positive, anti-infl ammatory eff ects on metabolic syndrome-related factors.
Most previous hypoxic training studies have focused on specifi c groups, such as obese individuals, 6,7 athletes, 8 college students, 5,9,10 and general adults, 11 and the eff ects of hypoxic training have varied according to the physical fi tness and physical characteristics of the subjects.Since obese people are at greater risk of developing metabolic syndrome than are those of a normal weight, the eff ects of hypoxic training on metabolic syndrome-related factors might be more pronounced in obese individuals.It is also necessary to investigate whether metabolic syndrome markers are improved following hypoxic training in normalweight people.However, no hypoxic training studies have compared obese and normal-weight groups with one another.
The purpose of this study was to examine the eff ects of hypoxic training on factors related to metabolic syndrome.In this study, participants were divided into either an overweight or a normal-weight group, and the eff ects of regular normobaric hypoxic training on metabolic syndrome-related factors were examined according to physique.

Participants
Forty-one Japanese men were included in this study.Information on aerobic and hypoxic exercise programs were posted on notice boards in the Gifu University and Gifu University hospital in order to recruit participants, and those who wanted to participate in this exercise program were included in this study.Participants were divided into two groups based on their body mass indices (BMIs): BMI≥25 or BMI<25.The overweight and normal-weight groups were randomly subdivided into the hypoxic exercise group (hypoxic overweight, HO; hypoxic normal-weight, HN) and the normoxic exercise group (normoxic overweight, NO; normoxic normal-weight, NN).The sample sizes were 8, 12, 9, and 12 in the HO, HN, NO, and NN groups, respectively.Participant characteristics are shown in Table 1.
The exclusion criteria included a history of coronary heart disease, cardiac insuffi ciency, pulmonary disease, or uncontrolled hypertension.Only men were included in this study since certain blood parameters diff er across sexes.The purpose and procedures of the study were explained in detail to all participants, and written informed consent was obtained prior to their participation.This study was approved by the institutional review board of the Gifu University School of Medicine (reference number 24-392) and was performed in accordance with the ethical standards established in the 1964 Declaration of Helsinki.

Measurements
Baseline and follow-up investigations were performed in the laboratory.Body weight and body fat were measured to the nearest 0.1 kg with a BC-118D body composition analyzer (TANITA Co., Tokyo, Japan).Heart rate was measured with an automated blood pressure cuff (HEM-7500F, OMRON Healthcare Co., Ltd., Kyoto, Japan).Waist circumference was measured in the standing position to the nearest 0.1 cm with a vinyl tape, at the narrowest circumference between the lowest rib and the iliac crest.
Ankle-brachial pulse wave velocity (PWV), representing arterial stiff ness, was measured noninvasively with a form-I automated PWV/ABI analyzer (Colin Co. Ltd., Komaki, Japan) attached to the four limbs with subjects in the supine position. 13PWV is generally assessed as the time required for a pulse wave to travel a given distance along the blood vessel, and serves as an objective index of atherosclerosis. 14 fasting blood sample was taken at 12:30 P.M. on the day of the experiment from the antecubital vein.The study parameters included blood sugar, triglycerides (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), fasting insulin, Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) scores, and adiponectin levels.HOMA-IR scores were calculated based on fasting insulin and glucose levels using the following formula: insulin (μU/mL) × glucose (mmol/L) / 22.5. 12ased on the protocol of Shi et al. 5 They trained on a treadmill 3 days per week for 4 weeks, under either normobaric hypoxic (15.4% O 2 , equivalent to an altitude of 2500 m) or normobaric normoxic (20.9% O 2 , equivalent to sea level) conditions for 50 min (including 5-minute warm-up and cool-down periods).In both environmental conditions, a 30-minute rest period preceded and followed each exercise  session.Exercise was performed at an intensity corresponding to 60% of the maximum heart rate (HR max), which was calculated from the age and heart rate at rest (HRrest) using the following formula: [(220−age−HRrest)×0.6+HRrest)].All subjects were required to rest for 30 minutes after exercise.During the exercise session, HR was monitored to ensure that the exercise intensity did not exceed 60% of HR max.Subjects were asked to report any symptoms of acute altitude sickness (e.g., headache, nausea, or a typical weakness in the legs).Percutaneous oxygen saturation (SpO 2 ) was also monitored for safety with a PULSOX-M24 pulse oximeter (Teijin Pharma Limited, Tokyo, Japan).

Statistical Analyses
Values are presented as the mean ± standard deviation.A two-factor analysis of variance (ANOVA) with repeated measures of one factor (before and after training) was used to examine diff erences in the mean parameter values between the two groups (overweight and normal-weight) and before and after training.A two-way ANOVA was conducted for hypoxic and normoxic training.Multiple post-hoc comparisons were performed with Tukey's honestly signifi cant diff erence test if a signifi cant main eff ect or interaction was identifi ed.A probability level of p<0.05 was considered statistically signifi cant.STATISTICA 10.0 software (StatSoft Inc., Tulsa, OK, USA) was used for all statistical analyses.

RESULTS
Table 2 displays the diff erences in parameters between the overweight and normal-weight hypoxic groups before and after training.Signifi cant main eff ects were observed before and after training for weight, body fat percentage, BMI, waist circumference, PWV, TC, LDL-C levels, and HOMA-IR scores, as indicated by the elevated levels for these values in the HO group compared to the HN group (p<0.05).Signifi cant main eff ects were observed between the groups for weight, body fat percentage, BMI, heart rate, waist circumference, PWV, TG, HDL-C, and HOMA-IR scores, as indicated by the lower levels for these values after training in both groups (p<0.05).Signifi cant interactions were observed with TG, HDL-C, and HOMA-IR scores (p<0.05).In the HO group, TG and HOMA-IR levels were lower after the training than before the training, and HDL-C levels were higher after the training than before the training (p<0.05).
Table 3 displays the diff erences in parameters between the overweight and normal-weight normoxic groups before and after training.Signifi cant main eff ects were observed before and after training for weight, body fat percentage, BMI, waist circumference, PWV, blood sugar, TG, LDL-C, HOMA-IR, and adiponectin levels, as indicated by the elevated levels for these values in the NO group compared to the NN group (p<0.05).A signifi cant main eff ect was observed between the groups for heart rate since it was lower after training compared to before training (p<0.05).A signifi cant interaction was observed with waist circumference, which was smaller after training than before training in the NO group (p<0.05).

DISCUSSION
This study evaluated the eff ects of regular normobaric hypoxic training on metabolic syndrome relatedfactors according to physique.First, the weight, body fat percentage, BMI, waist circumference, PWV, and TC and LDL levels were reduced by hypoxic training in both groups, regardless of body weight.The improvements in body composition and reduction in body weight associated with hypoxic training may correlate with the increased basal metabolic rate observed under hypoxic conditions. 4An increased metabolic rate may result from improved substrate utilization and mitochondrial oxidation, 15 via signalling pathways that stimulate GLUT-4 transport. 4,16ypoxic conditions are also associated with increased leptin production, 17,18 which can suppress appetite and hence improve body composition. 16A previous study involving hypoxic training (O 2 , 15%) for middle-aged obese subjects over 4 weeks indicated that the reduction in body fat content with training was greater in the hypoxic group than in the normoxic group. 18,19n this study, the weight, BMI, body fat percentage and waist circumference were reduced after hypoxic training, regardless of physique, supporting the results of previous studies.PWV is the rate at which blood leaving the heart circulates back to the heart, and is an indicator of the degree of arterial sclerosis (i.e., a faster rate indicates a greater risk of cardiovascular disease).Hirai et al. reported that arterial stiff ness correlated highly with cardiovascular disease, and that PWV independently predicted cardiovascular risk. 20In a crossover study by Shi et al. involving 4 weeks of aerobic exercise in a 2500-m hypoxic environment for male college students, PWV was signifi cantly lower in the hypoxic group after training. 5In addition, Nishiwaki et al. reported that hypoxic training increased fl ow-mediated vasodilation (FMD) (an indicator of increased blood fl ow) and reduced PWV, similar to the results of this study. 21Katayama et al. reported that aerobic exercise improved vascular endothelial function to a greater extent in a hypoxic environment than in a sea-level environment. 3Exercise-induced augmentation of blood fl ow and the subsequent increase in laminar shear stress have been reported to cause vasodilation and to upregulate endothelial nitric oxide (NO) synthase. 10,22,23Systemic acute hypoxia induces vasodilation in conduit arteries.Furthermore, a combination of submaximal exercise and hypoxia also causes vasodilation.Casey et al. reported that vasodilation during exercise in hypoxia was due to NO from the endothelium. 24These results are similar to the physiological adaptation induced by aerobic exercise, but regular aerobic exercise is expected to lead to more eff ective cardiovascular adaptation in hypoxic environments than under sea-level conditions.
Meanwhile, signifi cant interactions were observed in TG, HDL, and HOMA-IR levels.This suggests that the eff ects of hypoxic training are greater for people who are overweight than for those who are of a normal weight.The eff ects of hypoxic exercise on lipid metabolism vary with the study subjects and environmental conditions. 3Our fi nding that hypoxic training did not increase LDL levels is consistent with the fi ndings of a number of previous studies; however, our HDL results are inconsistent. 4 total cholesterol (TC) levels in obese subjects with dyslipidemia and metabolic syndrome. 4However, the extent of the reduction beyond the eff ects of exercise has varied, and there have been few controlled studies.HDL and LDL levels have been strongly associated with metabolic syndrome, though no clear conclusions can be drawn on the eff ects of hypoxic training on these parameters. 25Katayama reported that glucose metabolism increased with exercise in an acute hypoxic environment, while lipid metabolism improved marginally. 25 The improvement in HDL levels may be related to weight loss and improvements in body composition through an increase in the metabolic rate.Shin et al. reported that HDL signifi cantly increased after training in the hypoxic group, but did not differ from that in the normoxic group. 11HDL and TG levels are closely related to weight gain, and regular aerobic exercise induces weight loss and improves lipid metabolism.In the overweight group, the extent of weight loss and improvements in body composition by hypoxic training was relatively large, and the positive eff ects on both variables were also signifi cant.Although insulin sensitivity was not determined directly in this study, we obtained HOMA index values, which highly correlate with insulin sensitivity or resistance.The HOMA-IR index also signifi cantly decreased in the HO group.Morishima et al. found that 4 weeks of hypoxic training in obese subjects suppressed elevated blood glucose levels more eff ectively than the same training under sea-level conditions. 26ackenzie et al. analyzed insulin sensitivity in sealevel and hypoxic environments in patients with type 2 diabetes, and reported that insulin sensitivity increased signifi cantly after exercise under hypoxic conditions. 27aufe et al. reported that HOMA index values as well as glucose and insulin responses to oral glucose tolerance testing improved with training, and even more so when training was combined with hypoxia. 2 The relative increase in the glucose oxidation rate during physical activity after hypoxic training was attributed to transactivation of hypoxia-inducible factor 1 (HIF-1). 28Activation of the regulatory subunit HIF-1α leads to cellular adaptations that counteract the eff ects of the reduced oxygen supply to cells under hypoxic conditions. 2 These adaptations include the induction of several genes, such as those encoding phosphofructokinase, GLUT-1, and other proteins involved in glucose metabolism. 2,29In this study, HOMA-IR decreased signifi cantly with hypoxic training, thereby suggesting that insulin resistance had improved.

LIMITATIONS
In this study, the eff ects of participant factors, such as social status, lifestyle, and exercise experience, on the training eff ect were not considered.The mean BMI of the normal and overweight groups in this study were 21.3 and 26.9, respectively.Generally, BMI averages of normal weight and overweight are 21.7 and 27.5, respectively, based on the BMI range of normal (18.5-24.9BMI) and overweight (25.0-29.9)individuals.Therefore, the BMI of the participants were slightly lower than the averages of participants included in this study.

CONCLUSION
This study investigated the eff ects of regular normobaric hypoxic training on metabolic syndromerelated factors according to physique.Hypoxic training improved the weight, body fat percentage, BMI, waist circumference, PWV, and TC and LDL levels in overweight and normal-weight men.In addition, there were signifi cant interactions between improvements in TG, HDL, and HOMA-IR levels, and hypoxia exposure in the overweight group.These results indicate that hypoxic training can improve arterial stiff ness, circulatory system function, body composition, and energy metabolism in adult males.
Wee and Climstein found that hypoxic training signifi cantly reduced J Mens Health Vol 14(1):e44-e52; February 1, 2018 This article is distributed under the terms of the Creative Commons Attribution-Non Commercial 4.0 International License.

TABLE 1
Characteristics of Participants Infl uences of Short-Term Normobaric Hypoxic Training on Metabolic e47 J Mens Health Vol 14(1):e44-e52; February 1, 2018 This article is distributed under the terms of the Creative Commons Attribution-Non Commercial 4.0 International License.
February 1, 2018This article is distributed under the terms of the Creative Commons Attribution-Non Commercial 4.0 International License.

TABLE 2
Diff erences between the Overweight and Normal-Weight Hypoxic Groups Before and After Training February 1, 2018 This article is distributed under the terms of the Creative Commons Attribution-Non Commercial 4.0 International License.

TABLE 3
Differences between the Overweight and Normal-Weight Normoxic Groups Before and After Training February 1, 2018 This article is distributed under the terms of the Creative Commons Attribution-Non Commercial 4.0 International License.