Passive Heat Therapy in Sedentary Humans Improves Aerobic Capacity and Insulin Sensitivity via Increases in Skeletal Muscle Capillarisation and eNOS
Authors: Wagenmakers Anton JM , Hesketh Katie , Shepherd Sam O , Strauss Juliette A , Low David A , Cooper Robert J , Cocks Matthew 
Published: June 2021

OBJECTIVESNegative associations have been reported in a sedentary Finnish population between sauna bathing and all‐cause mortality1, while a joint association of sauna bathing and cardiorespiratory fitness was seen on cardiovascular and all‐cause mortality in a more recent study2. These associations give scientific support to the ‘proverbial wisdom’ in Scandinavia that frequent sauna bathing has multiple health and fitness benefits. The first objective of the current study was to investigate the hypotheses that 1) passive heat therapy (PHT, as occurs in sauna bathing) similar to moderate intensity continuous exercise training (MICT) increases whole body insulin sensitivity and VO2max and that 2) this is the result of increases both in a) skeletal muscle capillarisation and endothelial specific eNOS content and b) mitochondrial, GLUT4 and IMTG content.METHODSTwenty young sedentary males (21 ± 1 years; Body Mass Index (BMI) 25 ± 1 kg/m2) were allocated to either 6 weeks of PHT (n=10; 40–50 min at 40°C in a heat chamber, 3 x per week) or MICT (n=10; time matched cycling at ~65% VO2peak). VO2peak and whole‐body insulin sensitivity were measured and muscle biopsies were taken from the vastus lateralis muscle pre and post the interventions. Immunofluorescence microscopy was used to assess changes in capillarisation measures and endothelial specific eNOS content, as described before 3 and skeletal muscle mitochondrial density, GLUT4 content and IMTG content as described elsewhere4.RESULTSPHT and MICT increased VO2peak (PHT 5%, MICT 7%; P<0.05) and insulin sensitivity index (ISI, Matsuda) (PHT 15%, MICT 36%; P<0.05). PHT and MICT both increased capillary density, capillary–fibre perimeter exchange index, and endothelial specific eNOS content (Table 1). However, unlike MICT (P<0.05) PHT did not increase mitochondrial density (P=0.443), total GLUT4 content (P=0.217) and IMTG content (P=0.957).CONCLUSIONThis study is the first to show that 6 weeks PHT induces similar increases in aerobic capacity and whole‐body insulin sensitivity as a time matched traditional MICT intervention. The study also shows and that these improvements coincide with increases in skeletal muscle capillarisation and endothelial specific eNOS content similar in magnitude to MICT. As skeletal muscle mitochondrial density, GLUT4 content and IMTG concentration were not changed by PHT the data suggest that the microvascular adaptations alone are responsible for the observed effects. This leads to the conclusion that PHT (and sauna bathing) may be effective alternative interventions to achieve significant health benefits in the large and growing population of sedentary and elderly people unable to exercise or to chronically adhere to high physical activity levels.Support or Funding InformationNAThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.