Mild intermittent hypoxia exposure induces metabolic and molecular adaptations in men with obesity

The impact of high altitude (hypobaric hypoxia) on insulin resistance in humans

Exposure to hypobaric hypoxia (high altitude) diminishes systemic tissue oxygenation. Tissue hypoxia induces insulin resistance and a metabolic switch that reduces oxidative phosphorylation and glucose storage while enhancing glycolysis. Similarly to hypobaric hypoxia, insulin resistance develops in normal humans undergoing normobaric hypoxia and in patients with obstructive sleep apnea. Following acute exposure to high altitude, insulin resistance returns to baseline values upon returning to sea level or when compensatory mechanisms restore tissue oxygenation. However, insulin resistance persists in subjects unable to achieve sufficient oxygen delivery to tissues. Likewise, long-term residents at high altitude develop persistent insulin resistance when compensatory mechanisms do not attain adequate tissue oxygenation. Among these subjects, insulin resistance may cause clinical complications, such as hypertriglyceridemia, reduced HDL-c, visceral obesity, metabolic dysfunction-associated steatotic liver disease, essential hypertension, type 2 diabetes, subclinical vascular injury, cardiovascular disease, and kidney disease. Impaired tissue oxygenation allows the stabilization of hypoxia-inducible factor-1 (HIF-1), a transcription factor that modulates the transcriptional activity of a number of genes to coordinate the physiological responses to tissue hypoxia. Among them, HIF-1 downregulates PPARG, that codes peroxisome proliferator-activated receptor-gamma (PPAR-γ) and PPARGCA, that codes PPAR-γ coactivator-1α, in order to enable insulin resistance and the metabolic switch from oxidative phosphorylation toward glycolysis.

The role of tissue oxygenation in obesity-related cardiometabolic complications

Obesity is a complex, multifactorial, chronic disease that acts as a gateway to a range of other diseases. Evidence from recent studies suggests that changes in oxygen availability in the microenvironment of metabolic organs may exert an important role in the development of obesity-related cardiometabolic complications. In this review, we will first discuss results from observational and controlled laboratory studies that examined the relationship between reduced oxygen availability and obesity-related metabolic derangements. Next, the effects of alterations in oxygen partial pressure (pO2) in the adipose tissue, skeletal muscle and the liver microenvironment on physiological processes in these key metabolic organs will be addressed, and how this might relate to cardiometabolic complications. Since many obesity-related chronic diseases, including type 2 diabetes mellitus, cardiovascular diseases, chronic kidney disease, chronic obstructive pulmonary disease and obstructive sleep apnea, are characterized by changes in pO2 in the tissue microenvironment, a better understanding of the metabolic impact of altered tissue oxygenation can provide valuable insights into the complex interplay between environmental and biological factors involved in the pathophysiology of metabolic impairments. This may ultimately contribute to the development of novel strategies to prevent and treat obesity-related cardiometabolic diseases.

A randomized controlled crossover trial of acute intermittent and continuous hypoxia exposure in mild-moderate obstructive sleep apnea: A feasibility study

In a prospective, randomized, controlled crossover study, we explored the effects of acute intermittent hypoxia and acute continuous hypoxia on patients with mild-moderate obstructive sleep apnea. Over three single-night sessions, subjects were alternately exposed to normoxia, acute continuous hypoxia and acute intermittent hypoxia before sleep. The apnea-hypopnea index and oxygen desaturation index were used to diagnose obstructive sleep apnea and evaluate efficacy. A responder was defined as a participant with a ≥ 50% reduction in apnea-hypopnea index between normoxia and hypoxia exposure. Sixteen participants with mild-moderate obstructive sleep apnea completed the study. Compared with normoxia, the mean apnea-hypopnea index decreased by 8.9 events per hr (95% confidence interval, 4.2-13.6, p = 0.001) with acute intermittent hypoxia and by 4.1 events per hr (95% confidence interval, 0.5-8.8, p = 0.082) with acute continuous hypoxia, equating to a mean decrease in apnea-hypopnea index of 4.8 events per hr (95% confidence interval, 0.1-9.5, p = 0.046) with acute intermittent hypoxia compared with acute continuous hypoxia. Compared with normoxia, the mean oxygen desaturation index decreased by 9.8 events per hr (95% confidence interval, 4.4-15.1, p = 0.001) with acute intermittent hypoxia but did not significantly decrease with acute continuous hypoxia; the mean oxygen desaturation index decreased by 7.2 events per hr (95% confidence interval, 1.8-12.6, p = 0.010) with acute intermittent hypoxia compared with acute continuous hypoxia. Of the 16 participants, 11 responded to acute intermittent hypoxia and four responded to acute continuous hypoxia (p = 0.032), of whom eight of 11 cases and all four cases had oxygen desaturation indexes <5 events per hr, respectively (p = 0.273). All participants tolerated acute intermittent hypoxia and there were no obvious adverse events during acute intermittent hypoxia exposure. In conclusion, acute intermittent hypoxia exposure improved apnea-hypopnea index and oxygen desaturation index in patients with mild-moderate obstructive sleep apnea, suggesting that further prospective validation of intermittent hypoxia exposure in patients with obstructive sleep apnea is needed to establish its clinical feasibility as a therapeutic modality.

Intermittent normobaric hypoxia alters substrate partitioning and muscle oxygenation in individuals with obesity: implications for fat burning

This single-blind, crossover study aimed to measure and evaluate the short-term metabolic responses to continuous and intermittent hypoxic patterns in individuals with obesity. Indirect calorimetry was used to quantify changes in resting metabolic rate (RMR), carbohydrate (CHOox, %CHO), and fat oxidation (FATox, %FAT) in nine individuals with obesity pre and post: 1) breathing normoxic air [normoxic sham control (NS-control)], 2) breathing continuous hypoxia (CH), or 3) breathing intermittent hypoxia (IH). A mean peripheral oxygen saturation ([Formula: see text]) of 80-85% was achieved over a total of 45 min of hypoxia. Throughout each intervention, pulmonary gas exchanges, oxygen consumption (V̇o2) carbon dioxide production (V̇co2), and deoxyhemoglobin concentration (Δ[HHb]) in the vastus lateralis were measured. Both RMR and CHOox measured pre- and postinterventions were unchanged following each treatment: NS-control, CH, or IH (all P > 0.05). Conversely, a significant increase in FATox was evident between pre- and post-IH (+44%, P = 0.048). Although the mean Δ[HHb] values significantly increased during both IH and CH (P < 0.05), the greatest zenith of Δ[HHb] was achieved in IH compared with CH (P = 0.002). Furthermore, there was a positive correlation between Δ[HHb] and the shift in FATox measured pre- and postintervention. It is suggested that during IH, the increased bouts of muscle hypoxia, revealed by elevated Δ[HHb], coupled with cyclic periods of excess posthypoxia oxygen consumption (EPHOC, inherent to the intermittent pattern) played a significant role in driving the increase in FATox post-IH.

The impact of mild hypoxia exposure on myokine secretion in human obesity

BACKGROUND/OBJECTIVE

Compelling evidence indicates that myokines act in an autocrine, paracrine and endocrine manner to alter metabolic homeostasis. The mechanisms underlying exercise-induced changes in myokine secretion remain to be elucidated. Since exercise acutely decreases oxygen partial pressure (pO₂) in skeletal muscle (SM), the present study was designed to test the hypothesis that (1) hypoxia exposure impacts myokine secretion in primary human myotubes and (2) exposure to mild hypoxia in vivo alters fasting and postprandial plasma myokine concentrations in humans.

METHODS

Differentiated primary human myotubes were exposed to different physiological pO₂ levels for 24 h, and cell culture medium was harvested to determine myokine secretion. Furthermore, we performed a randomized single-blind crossover trial to investigate the impact of mild intermittent hypoxia exposure (MIH: 7-day exposure to 15% O₂, 3x2h/day vs. normoxia: 21% O₂) on in vivo SM pO₂ and plasma myokine concentrations in 12 individuals with overweight and obesity (body-mass index ≥ 28 kg/m²).

RESULTS

Hypoxia exposure (1% O₂) increased secreted protein acidic and rich in cysteine (SPARC, p = 0.043) and follistatin like 1 (FSTL1, p = 0.021), and reduced leukemia inhibitory factor (LIF) secretion (p = 0.009) compared to 3% O₂ in primary human myotubes. In addition, 1% O₂ exposure increased interleukin-6 (IL-6, p = 0.004) and SPARC secretion (p = 0.021), whilst reducing fatty acid binding protein 3 (FABP3) secretion (p = 0.021) compared to 21% O₂. MIH exposure in vivo markedly decreased SM pO₂ (≈40%, p = 0.002) but did not alter plasma myokine concentrations.

CONCLUSIONS

Hypoxia exposure altered the secretion of several myokines in primary human myotubes, revealing hypoxia as a novel modulator of myokine secretion. However, both acute and 7-day MIH exposure did not induce alterations in plasma myokine concentrations in individuals with overweight and obesity.

CLINICAL TRIALS IDENTIFIER

This study is registered at the Netherlands Trial Register (NL7120/NTR7325).

Effects of hypoxic exercise on 24-hour glucose profile and substrate metabolism in overweight and obese men with impaired glucose metabolism

Hypoxic exercise (HE) may have more pronounced effects on glucose homeostasis than exercise under normoxic conditions (NE), but effects on 24-h glucose profile and substrate utilization remain unclear. We investigated the effects of moderate-intensity HE compared with NE on 24-h glucose profile and substrate metabolism in overweight/obese individuals. Ten overweight/obese men with impaired glucose homeostasis participated in a randomized, single-blind, crossover trial. Participants performed moderate-intensity cycling exercise for 4 consecutive days under mild normobaric hypoxic ([Formula: see text]: 15%) or normoxic ([Formula: see text]: 21%) conditions at similar relative exercise intensity (2 × 30 min/day at 50% of maximal heart rate, with a ∼4-wk washout period. Twenty-four-hour glucose levels and systemic oxygen saturation ([Formula: see text]) were monitored throughout the study. At day 5, plasma metabolites and substrate oxidation were determined during a mixed-meal test under normoxic conditions. [Formula: see text] and absolute workload were lower (both P < 0.001), whereas heart rate was comparable during HE compared with NE. HE did not alter mean 24-h, daytime, and nighttime glucose concentrations, and measures of glycemic variability. However, the HE-induced decrease in [Formula: see text] was positively correlated with HE-induced improvements in mean 24-h (r_s = 0.683, P = 0.042) and daytime (r_s = 0.783, P = 0.013) glucose concentrations. HE at similar relative exercise intensity reduces [Formula: see text] and has comparable effects on mean 24-h glucose concentration and glycemic variability than NE in overweight/obese men with impaired glucose metabolism. Nevertheless, a more pronounced reduction in [Formula: see text] during HE was associated with lower 24-h glucose concentrations, suggesting that a marked hypoxic stimulus is needed to improve glucose homeostasis.NEW & NOTEWORTHY We demonstrate that hypoxic exercise at similar relative exercise intensity (i.e. lower absolute workload) reduces systemic oxygen saturation ([Formula: see text]) and has comparable effects on mean 24-h glucose concentrations and glycemic variability than normoxic exercise in men with overweight/obesity and impaired glucose metabolism. A more pronounced reduction in [Formula: see text] during hypoxic exercise, however, was associated with lower 24-h and daytime glucose concentrations. Our findings suggest that a marked hypoxic stimulus may improve glucose homeostasis.

Hypoxia as a Double-Edged Sword to Combat Obesity and Comorbidities

The global epidemic of obesity is tightly associated with numerous comorbidities, such as type II diabetes, cardiovascular diseases and the metabolic syndrome. Among the key features of obesity, some studies have suggested the abnormal expansion of adipose-tissue-induced local endogenous hypoxic, while other studies indicated endogenous hyperoxia as the opposite trend. Endogenous hypoxic aggravates dysfunction in adipose tissue and stimulates secretion of inflammatory molecules, which contribute to obesity. In contrast, hypoxic exposure combined with training effectively generate exogenous hypoxic to reduce body weight and downregulate metabolic risks. The (patho)physiological effects in adipose tissue are distinct from those of endogenous hypoxic. We critically assess the latest advances on the molecular mediators of endogenous hypoxic that regulate the dysfunction in adipose tissue. Subsequently we propose potential therapeutic targets in adipose tissues and the small molecules that may reverse the detrimental effect of local endogenous hypoxic. More importantly, we discuss alterations of metabolic pathways in adipose tissue and the metabolic benefits brought by hypoxic exercise. In terms of therapeutic intervention, numerous approaches have been developed to treat obesity, nevertheless durability and safety remain the major concern. Thus, a combination of the therapies that suppress endogenous hypoxic with exercise plans that augment exogenous hypoxic may accelerate the development of more effective and durable medications to treat obesity and comorbidities.

Mild intermittent hypoxia exposure alters gut microbiota composition in men with overweight and obesity

Results from high altitude studies in humans and controlled animal experiments suggest that hypoxia exposure induces alterations in gut microbiota composition, which may in turn affect host metabolism. However, well-controlled studies investigating the effects of normobaric hypoxia exposure on gut microbiota composition in humans are lacking. The aim of this study was to explore the impact of mild intermittent hypoxia (MIH) exposure on gut microbiota composition in men with overweight and/or obesity. We performed a randomised, single-blind crossover study, in which participants were exposed to MIH (FiO2: 15%, 3×2 h per day) and normoxia (FiO2: 21%) for seven consecutive days. Following the MIH and normoxia exposure regimens, faecal samples were collected for determination of faecal microbiota composition using 16S rRNA gene-amplicon sequencing in the morning of day 8. Paired faecal samples were available for five individuals. Furthermore, tissue-specific insulin sensitivity was determined using the gold-standard two-step hyperinsulinemic-euglycemic clamp. MIH did not affect microbial alpha and beta-diversity but reduced the relative abundance of Christensenellaceae and Clostridiaceae bacterial families. MIH significantly increased the abundances of obligate anaerobic bacterial genera including Fusicatenibacter, Butyricicoccus and Holdemania, whilst reducing Christensenellaceae R-7 group and Clostridium sensu stricto 1, although these findings were not statistically significant after correction for multiple testing. Furthermore, MIH-induced alterations in abundances of several genera were associated with changes in metabolic parameters such as adipose and peripheral insulin sensitivity, plasma levels of insulin, fatty acids, triacylglycerol and lactate, and substrate oxidation. In conclusion, we demonstrate for the first time that MIH exposure induces modest effects on faecal microbiota composition in humans, shifting several bacterial families and genera towards higher abundances of anaerobic butyrate-producing bacteria. Moreover, MIH-induced effects on faecal microbial composition were associated with parameters related to glucose and lipid homeostasis, supporting a link between MIH-induced alterations in faecal microbiota composition and host metabolism. The study was registered at the Netherlands Trial Register: NL7120/NTR7325.

Effects of Intermittent Hypoxia-Hyperoxia on Performance- and Health-Related Outcomes in Humans: A Systematic Review

BACKGROUND

Intermittent hypoxia applied at rest or in combination with exercise promotes multiple beneficial adaptations with regard to performance and health in humans. It was hypothesized that replacing normoxia by moderate hyperoxia can increase the adaptive response to the intermittent hypoxic stimulus.

OBJECTIVE

Our objective was to systematically review the current state of the literature on the effects of chronic intermittent hypoxia-hyperoxia (IHH) on performance- and health-related outcomes in humans.

METHODS

PubMed, Web of Science™, Scopus, and Cochrane Library databases were searched in accordance with PRISMA guidelines (January 2000 to September 2021) using the following inclusion criteria: (1) original research articles involving humans, (2) investigation of the chronic effect of IHH, (3) inclusion of a control group being not exposed to IHH, and (4) articles published in peer-reviewed journals written in English.

RESULTS

Of 1085 articles initially found, eight studies were included. IHH was solely performed at rest in different populations including geriatric patients (n = 1), older patients with cardiovascular (n = 3) and metabolic disease (n = 2) or cognitive impairment (n = 1), and young athletes with overtraining syndrome (n = 1). The included studies confirmed the beneficial effects of chronic exposure to IHH, showing improvements in exercise tolerance, peak oxygen uptake, and global cognitive functions, as well as lowered blood glucose levels. A trend was discernible that chronic exposure to IHH can trigger a reduction in systolic and diastolic blood pressure. The evidence of whether IHH exerts beneficial effects on blood lipid levels and haematological parameters is currently inconclusive. A meta-analysis was not possible because the reviewed studies had a considerable heterogeneity concerning the investigated populations and outcome parameters.

CONCLUSION

Based on the published literature, it can be suggested that chronic exposure to IHH might be a promising non-pharmacological intervention strategy for improving peak oxygen consumption, exercise tolerance, and cognitive performance as well as reducing blood glucose levels, and systolic and diastolic blood pressure in older patients with cardiovascular and metabolic diseases or cognitive impairment. However, further randomized controlled trials with adequate sample sizes are needed to confirm and extend the evidence. This systematic review was registered on the international prospective register of systematic reviews (PROSPERO-ID: CRD42021281248) ( https://www.crd.york.ac.uk/prospero/ ).

The Effects of Mild Intermittent Hypoxia Exposure on the Abdominal Subcutaneous Adipose Tissue Proteome in Overweight and Obese Men: A First-in-Human Randomized, Single-Blind, and Cross-Over Study

Adipose tissue (AT) oxygen tension (pO₂) has been implicated in AT dysfunction and metabolic perturbations in both rodents and humans. Compelling evidence suggests that hypoxia exposure alters metabolism, at least partly through effects on AT. However, it remains to be elucidated whether mild intermittent hypoxia (MIH) exposure impacts the AT proteome. We performed a randomized, single-blind, and cross-over study to investigate the effects of seven consecutive days of MIH (FiO₂ 15%, 3x2h/d) compared to normoxia (FiO₂ 21%) exposure on the AT proteome in overweight/obese men. In vivo AT insulin sensitivity was determined by the gold standard hyperinsulinemic-euglycemic clamp, and abdominal subcutaneous AT biopsies were collected under normoxic fasting conditions following both exposure regimens (day 8). AT proteins were isolated and quantified using liquid chromatography-mass spectrometry. After correction for blood contamination, 1,022 AT protein IDs were identified, of which 123 were differentially expressed following MIH (p < 0.05). We demonstrate for the first time that MIH exposure, which markedly reduces in vivo AT oxygen tension, impacts the human AT proteome. Although we cannot exclude that a single differentially expressed protein might be a false positive finding, several functional pathways were altered by MIH exposure, also after adjustment for multiple testing. Specifically, differentially expressed proteins were involved in redox systems, cell-adhesion, actin cytoskeleton organization, extracellular matrix composition, and energy metabolism. The MIH-induced change in AT TMOD3 expression was strongly related to altered in vivo AT insulin sensitivity, thus linking MIH-induced effects on the AT proteome to metabolic changes in overweight/obese humans.