The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Gary A. Dudley - One of the best experts on this subject based on the ideXlab platform.
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effect of acute head down Tilt on skeletal muscle cross sectional area and proton transverse relaxation time
Journal of Applied Physiology, 1996Co-Authors: Michael S Conley, Lori L Ploutzsnyder, Ronald A Meyer, J. M. Foley, Gary A. DudleyAbstract:Conley, Michael S., Jeanne M. Foley, Lori L. Ploutz-Snyder, Ronald A. Meyer, and Gary A. Dudley. Effect of acute Head-down Tilt on skeletal muscle cross-sectional area and proton transverse relaxation time. J. Appl. Physiol. 81(4): 1572–1577, 1996.—This study investigated changes in skeletal muscle cross-sectional area (CSA) evoked by fluid shifts that accompany short-term 6° Head-down Tilt (HDT) or horizontal bed rest, the time course of the resolution of these changes after resumption of upright posture, and the effect of altered muscle CSA, in the absence of increased contractile activity, on proton transverse relaxation time (T2). Average muscle CSA and T2 were determined by standard spin-echo magnetic resonance imaging. Analyses were performed on contiguous transaxial images of the neck and calf. After a day of normal activity, 24 h of HDT increased neck muscle CSA 19 ± 4 (SE)% ( P 0.05) of the values assessed after a day of normal activity, with most of the change occurring within the first 30 min. No further change in muscle CSA was observed through 6 h of upright posture. Despite these large alterations in muscle CSA, T2 was not altered by more than 1.1 ± 0.6% ( P > 0.05) and did not relate to muscle size. These results suggest that postural manipulations and subsequent fluid shifts modeling microgravity elicit marked changes in muscle size. Because these responses were not associated with alterations in muscle T2, it does not appear that simple movement of water into muscle can explain the contrast shift observed after exercise.
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effect of acute head down Tilt on skeletal muscle cross sectional area and proton transverse relaxation time
Journal of Applied Physiology, 1996Co-Authors: Michael S Conley, Lori L Ploutzsnyder, Ronald A Meyer, J. M. Foley, Gary A. DudleyAbstract:Conley, Michael S., Jeanne M. Foley, Lori L. Ploutz-Snyder, Ronald A. Meyer, and Gary A. Dudley. Effect of acute Head-down Tilt on skeletal muscle cross-sectional area and proton transverse relaxat...
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effect of acute head down Tilt on skeletal muscle cross sectional area and proton transverse relaxation time
Journal of Applied Physiology, 1996Co-Authors: Michael S Conley, Lori L Ploutzsnyder, Ronald A Meyer, J. M. Foley, Gary A. DudleyAbstract:Conley, Michael S., Jeanne M. Foley, Lori L. Ploutz-Snyder, Ronald A. Meyer, and Gary A. Dudley. Effect of acute Head-down Tilt on skeletal muscle cross-sectional area and proton transverse relaxation time. J. Appl. Physiol. 81(4): 1572–1577, 1996.—This study investigated changes in skeletal muscle cross-sectional area (CSA) evoked by fluid shifts that accompany short-term 6° Head-down Tilt (HDT) or horizontal bed rest, the time course of the resolution of these changes after resumption of upright posture, and the effect of altered muscle CSA, in the absence of increased contractile activity, on proton transverse relaxation time (T2). Average muscle CSA and T2 were determined by standard spin-echo magnetic resonance imaging. Analyses were performed on contiguous transaxial images of the neck and calf. After a day of normal activity, 24 h of HDT increased neck muscle CSA 19 ± 4 (SE)% ( P 0.05) of the values assessed after a day of normal activity, with most of the change occurring within the first 30 min. No further change in muscle CSA was observed through 6 h of upright posture. Despite these large alterations in muscle CSA, T2 was not altered by more than 1.1 ± 0.6% ( P > 0.05) and did not relate to muscle size. These results suggest that postural manipulations and subsequent fluid shifts modeling microgravity elicit marked changes in muscle size. Because these responses were not associated with alterations in muscle T2, it does not appear that simple movement of water into muscle can explain the contrast shift observed after exercise.
Tomas Jogestrand - One of the best experts on this subject based on the ideXlab platform.
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is chronic tension type headache a vascular headache the relation between chronic tension type headache and cranial hemodynamics
Headache, 1998Co-Authors: Jan Hannerz, Tomas JogestrandAbstract:Twenty-seven patients with chronic tension-type headache were studied as to end-tidal Pco2, heart rate, mean blood pressure, diameter and blood flow of the common carotid arteries, cranial vascular resistance, and headache intensity at supine rest, after administration of nitroglycerin, and at head down Tilt. The results were compared to the results of nitroglycerin and head down Tilt provocations in age- and sex-matched controls. During supine rest, no change in chronic tension-type headache occurred. Nitroglycerin and Tilting induced significant increase of the headache intensity compared to baseline in patients with chronic tension-type headache (P=0.01 and P<0.05, respectively) in contradistinction to controls who did not develop significant headache. Common carotid artery blood flow changes were similar during nitroglycerin provocations in the two groups, but greater (P<0.05) during head down Tilt in patients than in controls. Lumbar cerebrospinal fluid pressure was found to be greater than 20 but less than 26 cm H2O in 45% of the 22 patients studied with chronic tension-type headache. The results indicate that the pain in chronic tension-type headache is related to cranial hemodynamics, presumably to distention of intracranial veins.
Michael S Conley - One of the best experts on this subject based on the ideXlab platform.
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effect of acute head down Tilt on skeletal muscle cross sectional area and proton transverse relaxation time
Journal of Applied Physiology, 1996Co-Authors: Michael S Conley, Lori L Ploutzsnyder, Ronald A Meyer, J. M. Foley, Gary A. DudleyAbstract:Conley, Michael S., Jeanne M. Foley, Lori L. Ploutz-Snyder, Ronald A. Meyer, and Gary A. Dudley. Effect of acute Head-down Tilt on skeletal muscle cross-sectional area and proton transverse relaxation time. J. Appl. Physiol. 81(4): 1572–1577, 1996.—This study investigated changes in skeletal muscle cross-sectional area (CSA) evoked by fluid shifts that accompany short-term 6° Head-down Tilt (HDT) or horizontal bed rest, the time course of the resolution of these changes after resumption of upright posture, and the effect of altered muscle CSA, in the absence of increased contractile activity, on proton transverse relaxation time (T2). Average muscle CSA and T2 were determined by standard spin-echo magnetic resonance imaging. Analyses were performed on contiguous transaxial images of the neck and calf. After a day of normal activity, 24 h of HDT increased neck muscle CSA 19 ± 4 (SE)% ( P 0.05) of the values assessed after a day of normal activity, with most of the change occurring within the first 30 min. No further change in muscle CSA was observed through 6 h of upright posture. Despite these large alterations in muscle CSA, T2 was not altered by more than 1.1 ± 0.6% ( P > 0.05) and did not relate to muscle size. These results suggest that postural manipulations and subsequent fluid shifts modeling microgravity elicit marked changes in muscle size. Because these responses were not associated with alterations in muscle T2, it does not appear that simple movement of water into muscle can explain the contrast shift observed after exercise.
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effect of acute head down Tilt on skeletal muscle cross sectional area and proton transverse relaxation time
Journal of Applied Physiology, 1996Co-Authors: Michael S Conley, Lori L Ploutzsnyder, Ronald A Meyer, J. M. Foley, Gary A. DudleyAbstract:Conley, Michael S., Jeanne M. Foley, Lori L. Ploutz-Snyder, Ronald A. Meyer, and Gary A. Dudley. Effect of acute Head-down Tilt on skeletal muscle cross-sectional area and proton transverse relaxat...
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effect of acute head down Tilt on skeletal muscle cross sectional area and proton transverse relaxation time
Journal of Applied Physiology, 1996Co-Authors: Michael S Conley, Lori L Ploutzsnyder, Ronald A Meyer, J. M. Foley, Gary A. DudleyAbstract:Conley, Michael S., Jeanne M. Foley, Lori L. Ploutz-Snyder, Ronald A. Meyer, and Gary A. Dudley. Effect of acute Head-down Tilt on skeletal muscle cross-sectional area and proton transverse relaxation time. J. Appl. Physiol. 81(4): 1572–1577, 1996.—This study investigated changes in skeletal muscle cross-sectional area (CSA) evoked by fluid shifts that accompany short-term 6° Head-down Tilt (HDT) or horizontal bed rest, the time course of the resolution of these changes after resumption of upright posture, and the effect of altered muscle CSA, in the absence of increased contractile activity, on proton transverse relaxation time (T2). Average muscle CSA and T2 were determined by standard spin-echo magnetic resonance imaging. Analyses were performed on contiguous transaxial images of the neck and calf. After a day of normal activity, 24 h of HDT increased neck muscle CSA 19 ± 4 (SE)% ( P 0.05) of the values assessed after a day of normal activity, with most of the change occurring within the first 30 min. No further change in muscle CSA was observed through 6 h of upright posture. Despite these large alterations in muscle CSA, T2 was not altered by more than 1.1 ± 0.6% ( P > 0.05) and did not relate to muscle size. These results suggest that postural manipulations and subsequent fluid shifts modeling microgravity elicit marked changes in muscle size. Because these responses were not associated with alterations in muscle T2, it does not appear that simple movement of water into muscle can explain the contrast shift observed after exercise.
Benjamin D. Levine - One of the best experts on this subject based on the ideXlab platform.
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effect of gravity and microgravity on intracranial pressure
The Journal of Physiology, 2017Co-Authors: Justin S Lawley, Lonnie G Petersen, Satyam Sarma, Michael A Williams, Louis A Whitworth, Rong Zhang, Erin J Howden, William K Cornwell, Benjamin D. LevineAbstract:Key Points Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure on Earth. Gravity has a profound effect on fluid distribution and pressure within the human circulation. In contrast to prevailing theory, we observed that microgravity reduces central venous and intracranial pressure. This being said, intracranial pressure is not reduced to the levels observed in the 90 deg seated upright posture on Earth. Thus, over 24 h in zero gravity, pressure in the brain is slightly above that observed on Earth, which may explain remodelling of the eye in astronauts. Abstract Astronauts have recently been discovered to have impaired vision, with a presentation that resembles syndromes of elevated intracranial pressure (ICP). This syndrome is considered the most mission-critical medical problem identified in the past decade of manned spaceflight. We recruited five men and three women who had an Ommaya reservoir inserted for the delivery of prophylactic CNS chemotherapy, but were free of their malignant disease for at least 1 year. ICP was assessed by placing a fluid-filled 25 gauge butterfly needle into the Ommaya reservoir. Subjects were studied in the upright and supine position, during acute zero gravity (parabolic flight) and prolonged simulated microgravity (6 deg Head-down Tilt bedrest). ICP was lower when seated in the 90 deg upright posture compared to lying supine (seated, 4 ± 1 vs. supine, 15 ± 2 mmHg). Whilst lying in the supine posture, central venous pressure (supine, 7 ± 3 vs. microgravity, 4 ± 2 mmHg) and ICP (supine, 17 ± 2 vs. microgravity, 13 ± 2 mmHg) were reduced in acute zero gravity, although not to the levels observed in the 90 deg seated upright posture on Earth. Prolonged periods of simulated microgravity did not cause progressive elevations in ICP (supine, 15 ± 2 vs. 24 h Head-down Tilt, 15 ± 4 mmHg). Complete removal of gravity does not pathologically elevate ICP but does prevent the normal lowering of ICP when upright. These findings suggest the human brain is protected by the daily circadian cycles in regional ICPs, without which pathology may occur.
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deterioration of left ventricular chamber performance after bed rest cardiovascular deconditioning or hypovolemia
Circulation, 2001Co-Authors: Merja A Perhonen, Julie H Zuckerman, Benjamin D. LevineAbstract:Background—Orthostatic intolerance after bed rest is characterized by hypovolemia and an excessive reduction in stroke volume (SV) in the upright position. We studied whether the reduction in SV is due to a specific adaptation of the heart to Head-down Tilt bed rest (HDTBR) or acute hypovolemia alone. Methods and Results—We constructed left ventricular (LV) pressure-volume curves from pulmonary capillary wedge pressure and LV end-diastolic volume and Starling curves from pulmonary capillary wedge pressure and SV during lower body negative pressure and saline loading in 7 men (25±2 years) before and after 2 weeks of −6° HDTBR and after the acute administration of intravenous furosemide. Both HDTBR and hypovolemia led to a similar reduction in plasma volume. However, baseline LV end-diastolic volume decreased by 20±4% after HDTBR and by 7±2% after hypovolemia (interaction P<0.001). Moreover, SV was reduced more and the Starling curve was steeper during orthostatic stress after HDTBR than after hypovolemia. ...
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effect of head down Tilt bed rest and hypovolemia on dynamic regulation of heart rate and blood pressure
American Journal of Physiology-regulatory Integrative and Comparative Physiology, 2000Co-Authors: Kenichi Iwasaki, Rong Zhang, Julie H Zuckerman, James A Pawelczyk, Benjamin D. LevineAbstract:Adaptation to Head-down-Tilt bed rest leads to an apparent abnormality of baroreflex regulation of cardiac period. We hypothesized that this “deconditioning response” could primarily be a result of...
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effects of head down Tilt bed rest on cerebral hemodynamics during orthostatic stress
Journal of Applied Physiology, 1997Co-Authors: Rong Zhang, Julie H Zuckerman, James A Pawelczyk, Benjamin D. LevineAbstract:Zhang, Rong, Julie H. Zuckerman, James A. Pawelczyk, and Benjamin D. Levine. Effects of Head-down-Tilt bed rest on cerebral hemodynamics during orthostatic stress. J. Appl. Physiol. 83(6): 2139–214...
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cardiac atrophy after bed rest deconditioning a nonneural mechanism for orthostatic intolerance
Circulation, 1997Co-Authors: Benjamin D. Levine, Julie H Zuckerman, James A PawelczykAbstract:BACKGROUND: The cardiovascular adaptation to bed rest leads to orthostatic intolerance, characterized by an excessive fall in stroke volume (SV) in the upright position. We hypothesized that this large fall in SV is due to a change in cardiac mechanics. METHODS AND RESULTS: We measured pulmonary capillary wedge pressure (PCWP), SV, left ventricular end-diastolic volume (LVEDV), and left ventricular mass (by echocardiography) at rest, during lower-body negative pressure, and after saline infusion before and after 2 weeks of bed rest with -6 degrees Head-down Tilt (n=12 subjects aged 24+/-5 years). Pressure (P)-volume (V) curves were modeled exponentially by P=ae(kV)+b and logarithmically by P=-Sln[(Vm-V)/(Vm-V0)], where V0 indicates volume at P=0, and the constants k and S were used as indices of normalized chamber stiffness. Dynamic stiffness (dP/dV) was calculated at baseline LVEDV. The slope of the line relating SV to PCWP during lower-body negative pressure characterized the steepness of the Starling curve. We also measured plasma volume (with Evans blue dye) and maximal orthostatic tolerance. Bed rest led to a reduction in plasma volume (17%), baseline PCWP (18%), SV (12%), LVEDV (16%), V0 (33%), and orthostatic tolerance (24%) (all P<.05). The slope of the SV/PCWP curve increased from 4.6+/-0.4 to 8.8+/-0.9 mL/mm Hg (P<.01) owing to a parallel leftward shift in the P-V curve. Normalized chamber stiffness was unchanged, but dP/dV was reduced by 50% at baseline LVEDV, and cardiac mass tended to be reduced by 5% (P<.10). CONCLUSIONS: Two weeks of Head-down-Tilt bed rest leads to a smaller, less distensible left ventricle but a shift to a more compliant portion of the P-V curve. This results in a steeper Starling relationship, which contributes to orthostatic intolerance by causing an excessive reduction in SV during orthostasis.
Edwin Mulder - One of the best experts on this subject based on the ideXlab platform.
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from human terrestrial models to new preventive measures for ocular changes in astronauts results of the german aerospace center studies
Ophthalmologe, 2020Co-Authors: Edwin Mulder, Jens Jordan, Christine E Hellweg, C SternAbstract:Background Ocular changes in astronauts, particularly the spaceflight associated neuro-ocular syndrome (SANS), pose a medical challenge for which no suitable preventive measures exist. During long-duration spaceflight missions, e.g. to the Moon and Mars, SANS and radiation-induced cataract could affect the health and performance of crews and jeopardize the success of missions. Mechanistic studies and development of preventive measures require suitable terrestrial models. Objective Overview on the most recent research and future plans in space medicine. Material and methods Search for relevant publications using PubMed. Results Bed rest studies at the German Aerospace Center (DLR) demonstrated that strict bed rest in a -6° head down Tilt position reproduces changes just like SANS on Earth. This model including creation of optic disc edema is applied in human studies testing influences of artificial gravity through short arm centrifugation as a preventive method. The unique research facility :envihab provides the opportunity to also simulate the ambient conditions of the International Space Station during bed rest studies. Conclusion Future head down Tilt bed rest studies will serve to systematically test preventive measures for SANS. Similar investigations would be difficult to realize under real space conditions. Through close collaboration between space medicine and terrestrial ophthalmology, this research can benefit patients on Earth.
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unchanged cerebrovascular co2 reactivity and hypercapnic ventilatory response during strict head down Tilt bed rest in a mild hypercapnic environment
The Journal of Physiology, 2020Co-Authors: Steven S. Laurie, Brandon R Macias, Edwin Mulder, Kate H Christian, Jacob Kysar, Stuart M C Lee, Andrew T Lovering, Stefan Moestl, Wolfram Sies, Millennia YoungAbstract:KEY POINTS Carbon dioxide levels are mildly elevated on the International Space Station and it is unknown whether this chronic exposure causes physiological changes to astronauts. We combined ∼4 mmHg ambient PCO2 with the strict Head-down Tilt bed rest model of spaceflight and this led to the development of optic disc oedema in one-half of the subjects. We demonstrate no change in arterialized PCO2 , cerebrovascular reactivity to CO2 or the hypercapnic ventilatory response. Our data suggest that the mild hypercapnic environment does not contribute to the development of spaceflight associated neuro-ocular syndrome. ABSTRACT Chronically elevated carbon dioxide (CO2 ) levels can occur in confined spaces such as the International Space Station. Using the spaceflight analogue 30 days of strict 6° Head-down Tilt bed rest (HDTBR) in a mild hypercapnic environment ( PCO2 = ∼4 mmHg), we investigated arterialized PCO2 , cerebrovascular reactivity and the hypercapnic ventilatory response in 11 healthy subjects (five females) before, on days 1, 9, 15 and 30 of bed rest (BR), and 6 and 13 days after HDTBR. During all HDTBR time points, arterialized PCO2 was not significantly different from the pre-HDTBR measured in the 6° HDT posture, with a mean (95% confidence interval) increase of 1.2 mmHg (-0.2 to 2.5 mmHg, P = 0.122) on day 30 of HDTBR. Respiratory acidosis was never detected, although a mild metabolic alkalosis developed on day 30 of HDTBR by a mean (95% confidence interval) pH change of 0.032 (0.022-0.043; P < 0.001), which remained elevated by 0.021 (0.011-0.031; P < 0.001) 6 days after HDTBR. Arterialized pH returned to pre-HDTBR levels 13 days after BR with a change of -0.001 (-0.009 to 0.007; P = 0.991). Compared to pre-HDTBR, cerebrovascular reactivity during and after HDTBR did not change. Baseline ventilation, ventilatory recruitment threshold and the slope of the ventilatory response were similar between pre-HDTBR and all other time points. Taken together, these data suggest that the mildly increased ambient PCO2 combined with 30 days of strict 6° HDTBR did not change arterialized PCO2 levels. Therefore, the experimental conditions were not sufficient to elicit a detectable physiological response.
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lower body negative pressure reduces optic nerve sheath diameter during head down Tilt
Journal of Applied Physiology, 2017Co-Authors: Edwin Mulder, Karina Marshallgoebel, Robert Terlevic, Darius A Gerlach, Simone Kuehn, Jorn RittwegerAbstract:The microgravity ocular syndrome (MOS) results in significant structural and functional ophthalmic changes during 6-mo spaceflight missions consistent with an increase in cerebrospinal fluid (CSF) pressure compared with the preflight upright position. A ground-based study was performed to assess two of the major hypothesized contributors to MOS, headward fluid shifting and increased ambient CO2, on intracranial and periorbital CSF. In addition, lower body negative pressure (LBNP) was assessed as a countermeasure to headward fluid shifting. Nine healthy male subjects participated in a crossover design study with five Head-down Tilt (HDT) conditions: -6, -12, and -18° HDT, -12° HDT with -20 mmHg LBNP, and -12° HDT with a 1% CO2 environment, each for 5 h total. A three-dimensional volumetric scan of the cranium and transverse slices of the orbita were collected with MRI, and intracranial CSF volume and optic nerve sheath diameter (ONSD) were measured after 4.5 h HDT. ONSD increased during -6° (P < 0.001), -12° (P < 0.001), and -18° HDT (P < 0.001) and intracranial CSF increased during -12° HDT (P = 0.01) compared with supine baseline. Notably, LBNP was able to reduce the increases in ONSD and intracranial CSF during HDT. The addition of 1% CO2 during HDT, however, had no further effect on ONSD, but rather ONSD increased from baseline in a similar magnitude to -12° HDT with ambient air (P = 0.001). These findings demonstrate the ability of LBNP, a technique that targets fluid distribution in the lower limbs, to directly influence CSF and may be a promising countermeasure to help reduce increases in CSF.NEW & NOTEWORTHY This is the first study to demonstrate the ability of lower body negative pressure to directly influence cerebrospinal fluid surrounding the optic nerve, indicating potential use as a countermeasure for increased cerebrospinal fluid on Earth or in space.
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an international collaboration studying the physiological and anatomical cerebral effects of carbon dioxide during head down Tilt bed rest the spacecot study
Journal of Applied Physiology, 2017Co-Authors: Edwin Mulder, Karina Marshallgoebel, Dorit Donoviel, Gary E Strangman, Jose I Suarez, Chethan Venkatasubba P Rao, Petra FringsmeuthenAbstract:A new approach for investigating the combined effects of cephalad fluid shifting and increased ambient carbon dioxide (CO2) is presented. This may be useful for studying the neuroophthalmic and cer...
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microgravity simulated by the 6 head down Tilt bed rest test increases intestinal motility but fails to induce gastrointestinal symptoms of space motion sickness
Digestive Diseases and Sciences, 2015Co-Authors: Meher Prakash, Edwin Mulder, Petra Fringsmeuthen, Ron Fried, Oliver Gotze, Francisca May, Judith Valentini, Mark Fox, Michael Fried, Werner SchwizerAbstract:Background Space motion sickness (SMS) is the most relevant medical problem during the first days in microgravity. Studies addressing pathophysiology in space face severe technical challenges and microgravity is frequently simulated using the 6 Head-down Tilt bed rest test (HDT). Aim We were aiming to test whether SMS could be simulated by HDT, identify related changes in gastrointestinal physiology and test for beneficial effects of exercise interventions. Methods HDT was performed in ten healthy individuals. Each individual was tested in three study campaigns varying by a 30-min daily exercise intervention of either standing, an upright exercise regimen, or no intervention. Gastrointestinal symptoms, stool characteristics, gastric emptying time, and small intestinal transit were assessed using standardized questionnaires, 13 C octanoate breath test, and H2 lactulose breath test, respectively, before and at day 2 and 5 of HDT. Results Individuals described no or minimal gastrointestinal symptoms during HDT. Gastric emptying remained unchangedrelativetobaselinedatacollection(BDC).Atday 2 of HDT the H2 peak of the lactulose test appeared earlier (mean ± standarderrorforBDC-1,HDT2,HDT5:198 ± 7, 139 ± 18, 183 ± 10 min; p: 0.040), indicating accelerated small intestinal transit. Furthermore, during HDT, stool was softer and stool mass increased (BDC: 47 ± 6, HDT: 91 ± 12, recovery: 53 ± 8 g/day; p: 0.014), indicating accelerated colonic transit. Exercise interventions had no effect.
