A loss of body weight has been documented in lowland-living individuals when exposed to hypoxic environments, such as at high altitudes, or under laboratory conditions. This weight loss has been attributed both to a reduced appetite (and subsequent fall in dietary energy intake), and to an increase in resting energy expenditure. Interestingly, a reduction in appetite and energy intake has also been reported during space flight, although the mechanism for this has not been explained. As the gas inside future planetary habitats is likely to have lower partial pressure of oxygen than in Earth's atmospheric air, hypoxia induced appetite reduction could pose a challenge for individuals in these environments. For example, persistent under-eating could compromise long term health due to inadequate intake of essential micronutrients, especially in the presence of altered nutrient metabolism and requirements seen during space flight. Moreover, inadequate macronutrient intake could exacerbate the loss of lean body tissue which occurs in situations (such as microgravity, inactivity and bed rest) where muscles are unloaded. Indeed, a protein intake greater than normal could be required in situations where there is muscle inactivity, to achieve the same postprandial anabolic effect of amino acids seen in ambulatory individuals.

The mechanism for the reduction in appetite observed in hypoxia is not well established. Several incretin hormones and adipokines have been implicated in the control of appetite and may be candidates for inducing this alteration in appetite observed in hypoxia. However, reports in the literature present contradictory findings, perhaps due to the use of different experimental paradigms (hypobaric and normobaric hypoxia, active and resting subjects, variability in the degree and duration of hypoxia).

The protocol of the current study standardises physical activity, ambient temperature, hypoxic stimulus and nutritional composition of the diet, and aims to extend our knowledge of the effects of hypoxia and bedrest on appetite and its hormonal control.

In order to discern the separate and combined effects of microgravity and hypoxia, fourteen normal or over-weight 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 the 3 interventions 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, the targeted energy intakes being 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. 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. Body mass will be assessed 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.

Participants will undergo a mixed meal tolerance test before and on day 17 of each intervention period, in the morning, after a 12 hour fast, with the time of assessment replicated on each study day at every campaign. On arrival, participants will rest supine on a hospital bed and place their hand in a heated hand warming unit (air temperature 50-55oC). An intravenous cannula will then be inserted retrograde into a dorsal hand vein for arterialised-venous blood sampling. After 15mins rest, a baseline, fasting blood sample will be taken for determination of serum insulin, total cholesterol, high density lipoprotein (HDL) cholesterol, low density lipoprotein (LDL) cholesterol, adiponectin and leptin, whole blood glucose and lactate, and plasma catecholamines, ghrelin, PeptideYY (PYY), glucagon-like peptide-1(GLP-1), triglycerides, and non-esterified fatty acid concentration. An expired breath sample will be collected into evacuated tubes for 13 labelled carbon dioxide (13CO2) determination, and a 20min baseline measurement of resting energy expenditure (REE) and respiratory exchange ratio (RER) will be then made using indirect calorimetry, with subjects wearing a mask, and both inspired and expired air being measured on every breath. Appetite assessment will be made by asking subjects to rate their hunger, desire to eat, fullness, and their prospective food intake, by placing a vertical mark on a 0-100mm linear scale. This visual analogue scale will be measured from left to right, with 0 indicating no experience of the variable (e.g. not hungry, unable to eat anything) and 100 indicating the most of each variable that they can imagine experiencing (e.g. intense desire to eat, or completely full). Values for these 4 variables will be combined to calculate a combined appetite score (CAS). Once baseline measurements have been completed, subjects will consume a mixed nutrient milkshake (Ensure Plus, Abbott Nutrition) at 5ml/kg body weight, which will be supplemented (at 1% of carbohydrate content) with 13-Carbon labelled (13C) Glucose. Arterialised venous blood samples will subsequently be taken every 10min for glucose and lactate assessment, and every 20min for assessment of serum insulin and incretin hormones. A measurement of REE and RER will be performed in the last 15min of every 30min period for the following 2 hours to assess fuel oxidation and metabolic rate, and an expired breath sample will be collected into evacuated tubes for 13CO2 determination at a later date. Subjective appetite will be assessed every 15minutes throughout using visual analogue scales, as described above, and at the end of the 2hr postprandial period subjects will be given an ad libitum pasta-based test meal and will be instructed to eat until they feel comfortably full. This meal will be comprised of cooked dried white pasta, commercially available tomato-based pasta sauce, olive oil and grated hard cheese, with a composition of 37% of total energy derived from fat, 48% carbohydrate, and 16% protein. The amount eaten will be recorded and related to subjective appetite ratings.