Planetary Habitat Simulation: Bone Metabolism Studies
Planetary Habitat Simulation: Bone Metabolism Studies
Bone losses are well known to occur in response to unloading (in microgravity or during
 immobilisation) and in patients with chronic obstructive airway disease (COPD). However, it
 is unknown whether there is an interactive effect between hypoxia and musculoskeletal
 unloading upon bone and mineral metabolism. Fourteen non-obese men, who are otherwise
 healthy, will undergo 3x 21-day interventions; normobaric normoxic bed rest (NBR; FiO2=21%),
 normobaric hypoxic ambulatory confinement (HAMB; FiO2=14%; ~4000 m simulated altitude), and
 normobaric hypoxic bed rest (HBR; FiO2=14%). The effects of hypoxia and bedrest on bone
 metabolism and phosphor-calcic homeostasis will be assessed (before and during each
 intervention, and 14 days after each intervention period) using venous blood sampling, 24hr
 urine collections, and peripheral quantitative computerized tomography (pQCT).
The risk of bone loss in response to immobilization and space flight is widely recognized,
 with such bone losses occurring predominantly in the legs. Loading forces in the lower
 extremities are typically low in space and during bed rest, but musculoskeletal loading
 countermeasures can prevent or reduce bone losses induced in these conditions. Although the
 primary origin may be of a mechanical nature, bone losses in astronauts and immobilized
 patients are likely to be modulated by the endocrine and the internal environment.

 Bone loss and osteoporosis is also prevalent in those with chronic obstructive pulmonary
 disease (COPD). The most discussed etiological factors for this condition include lack of
 physical activity, vitamin-D deficiency, hypogonadism, use of corticosteroids and smoking.
 However, COPD is associated with tissue hypoxemia and it is unclear whether hypoxia per se
 will affect bone turnover or metabolism.

 The protocol of the current study standardises potential confounding factors, such as
 physical activity, ambient temperature, hypoxic stimulus and nutritional composition of the
 diet across all 3 interventions, and aims to extend our knowledge of the effects of hypoxia
 and bedrest on bone metabolism and phospho-calcic homeostasis.

 Fourteen non-obese men, who are otherwise healthy, will be recruited following medical and
 psychological screening. They will be invited to attend the Olympic Sport Centre, Planica,
 Slovenia on 3 occasions, with each visit being 31 days in duration and separated by 5 months.
 Each 31-day visit ('campaign') includes a baseline recording period (5 days), 21 days of
 intervention, and a recovery period (5 days), with a follow-up visit being carried out 14
 days after each intervention. The 3 interventions will be allocated in a randomized, cross
 over design: i) Normobaric normoxic bed rest (NBR; FiO2=21%), ii) Normobaric hypoxic
 ambulatory confinement (HAMB; FiO2=14%; ~4000 m simulated altitude), and iii) Normobaric
 hypoxic bed rest (HBR; FiO2=14%). A standardized, repeating, 14-day dietary menu, comprised
 of foods commonly consumed in the Slovenian diet, will be applied during all campaigns.
 Targeted energy intakes will be calculated individually using a modified Benedict-Harris
 formula, with physical activity factor multipliers of 1.2 for the HBR and NBR campaigns and
 1.4 for the HAMB campaign, used to promote energy balance. Body mass will be monitored daily
 during the campaigns using a gurney incorporating load cells, and whole body composition will
 be determined before and immediately after each intervention using fan beam dual-emission
 X-ray absorptiometry. Macronutrient composition of the diet will be approximately 17%
 protein, 53% carbohydrate and 30% fat, with >1.1g of protein per kg body weight provided per
 day, and daily salt (sodium chloride) intake being <10g. Food will be provided in weighed
 portions and subjects will be encouraged to eat all food supplied. However, any food not
 eaten will be weighed and actual amount consumed recorded in a diet analysis programme.

 Participants will have bone mineral content assessed at 5 time points (before, 3 during and
 one after each intervention) using pQCT scans of the calf and thigh. Horizontal scans will be
 taken at 4%, 14%, 38%, and 66% of the tibia (assessed from its distal end), and at 4% and 33%
 of the femur. Twenty-four hour urine collections will be obtained before (2 time points),
 during (11 time points) and after (3 time points) each intervention, and will be assessed for
 urinary calcium and phosphate content, and for a marker of bone reabsorption (N-terminal
 telopeptide). In addition, early morning, fasting venous blood samples will also be taken
 before (2 time points), and during (5 time points) each intervention. These will be analysed
 for calcium, phosphate, bone specific alkaline phosphatase, parathyroid hormone,
 25-Hydroxyvitamin D, Procollagen-I-N-terminal propeptide and regulators of bone metabolism,
 (Dickkopf-related protein 1 and Sclerostin).
Inclusion Criteria:

 -  Physically and mentally healthy subjects

 -  Body mass index < 25 kg/m2

 -  Height 158 - 190 cm

 -  Waist circumference < 94 cm

 -  Volunteers that are able to declare their willingness to participate in the entire
 study

 -  Willing to be assigned randomly to the three groups

 -  Successfully passing the psychological and medical screening

 -  competent to sign informed consent

 -  Slovenian social insurance

 -  English language fluency

 Exclusion Criteria:

 -  Medication required that may interfere with the interpretation of the results

 -  Bone mineral density (as measured by DEXA) more than 1.5 standard deviations < t score

 -  Recent sub-standard nutritional status

 -  Family history of thrombosis or positive response in thrombosis screening procedure.

 (Biochemical analysis of the following parameters: ATIII, High sensitive C-reactive
 protein, S-Akt., Factor V-Leiden, Prothrombin, Lupus-partial thromboplastin time, Factor
 II)

 -  History of: thyroid dysfunction, renal stones, diabetes, allergies, hypertension,
 hypocalcemia, uric acidaemia, lipidaemia, or hyperhomocysteinaemia

 -  Gastro-esophageal reflux disease or renal function disorder, Hiatus hernia

 -  History of medical illness

 -  Smoker within six months prior to the start of the study

 -  Abuse of drugs, medicine or alcohol

 -  Participation in another study up to two months before study onset

 -  No signed consent form before the onset of the experiment

 -  Blood donors in the past three months before the onset of the experiment

 -  Vegetarian and Vegans

 -  Migraines

 -  History of orthostatic intolerance

 -  History of vestibular disorders

 -  Claustrophobia

 -  metallic implants, osteosynthesis material

 -  Chronic back pain