The subject was a 30-year old male with a body height of 1.80 m, a body mass of 93.8 kg, and a body mass index of 29.0 kg/m2. He was an experienced ultraendurance athlete (world champion in the Deca Iron ultratriathlon 2008, a world record holder in the 10-day triathlon, a finisher of the Trans Europe Footrace in 2009 and of the Race across Germany in 2010), and intending to run the first marathon in history under these conditions.
One day before the experiment, a maximal treadmill test was performed to determine maximum oxygen uptake (VO2max). A running speed of 8.0 km/hour was set as the start speed. Every 3 minutes, the treadmill speed was increased by 2 km/hour until volitional fatigue. VO2 was determined using an expired air analysis by Meta Max 3B  (Cortex Biophysik GmbH, Leipzig, Germany) and heart rate was measured using a portable heart rate monitor (RCX5, Polar Electro Oy, Kempele, Finland). Lactate threshold was determined using Ergonizer version 3.2.1 (K. Rocker, www.ergonizer.de) to enable data analysis.
For the experiment, the athlete ran clockwise laps of 17.6 m in the climate chamber at the CEBra (Centrum fur Energietechnologie Brandenburg) of Brandenburg Technische Universitat in Cottbus, Germany (www.tu-cottbus.de/cebra). The investigating team was located outside the climate chamber. Each lap was counted using a laser-assisted photoelectric barrier. A computer counted the laps continuously and showed the distance covered at each moment.
During the marathon, Tcore was measured in the rectum along with Tskin at three sites. Before the start, after 10.5 km, 21.1 km, 31.6 km, and at the end of the study, venous blood samples were drawn, urine and sweat were collected, and body mass was measured. Each measurement took on average 10 (8-12) minutes. The athlete changed clothes for comfort during the breaks. He started running with winter boots, winter gloves, a winter jacket, and a winter hat over his running clothes. After 10.5 km (at about -20 C), no changes in clothing were made. After 21.1 km (approximately +5 C), he took off his winter clothes and ran in his usual running clothes. After 31.6 km (around +30 C), he took off his shirt. Ratings of perceived exertion were not recorded. The hydration status of the athlete was not assessed before the marathon and he consumed food and drinks as he was used to doing during his races.
Tambient was set at -45 C at the start and continuously increased to +55 C (Figure 1) at a rate of 1 C per 4.5-minute interval. At the start, humidity was set at 60%. After 210 minutes of running at an Tambient of 0 C, humidity was steadily reduced until it reached 20% by the end of the marathon. Temperature and humidity in the climate chamber were recorded every minute. Percent change was rounded to two decimal digits.
The day before the climate chamber marathon, the athlete went with his crew to a supermarket to buy conventional food, such as bread, meat, cheese, fruit, and fluids, including mineral water and soft drinks. The athlete consumed food and drinks ad libitum during the marathon. All food and drinks were provided at ambient room temperature. At each point of measurement, his coached prepared the food and drinks as requested by the athlete. Intake of food and drinks was recorded to estimate the fluid and energy intake in a cumulative histogram (Figure 2). The composition of the fluids and solid food was estimated using the information on the products consumed by the athlete and a nutrition chart.
While running, sweat was collected using a plastic bag attached to a 17 x 25 cm area on the back of the runner. To estimate total sweat production, the sweat produced on this area of the back was extrapolated to the whole body surface area. Body surface area (BSA) was estimated using the formula BSA = weight (kg)0.425 x height (m)0.725 x 0.007184 following Dubois and Dubois.
Heart rate was recorded continuously during the marathon using a portable heart rate monitor (RCX5, Polar Electro Oy) to estimate energy expenditure, using the heart rate method.
Tcore in the rectum and Tskin on the forehead, right wrist, and right ankle were measured using an EndoTherm Med  device (EndoTherm GmbH, Arlesheim, Switzerland) every 10 seconds while running. The EndoTherm Med thermoelectric probes sensor has a guaranteed accuracy of 0.0625 C. The sensor for measurement of Tcore in the rectum was encased in a silicone capsule and packed into a fingerstall. The sensors used to measure Tskin were fixed using textile ribbons on defined spots. The sensors were evaluated by EndoTherm Control software (www.endotherm.ch) and showed a temperature variation of +-0.1 C (Figure 3).
Before the start of the marathon and every 10.5 km, body mass was measured and samples of blood, urine, and sweat were collected. Body mass was measured using a commercial scale (Beurer BF 15, Beurer GmbH, Ulm, Germany) to the nearest 0.1 kg, after voiding the urinary bladder. Venous blood samples were drawn in a sitting position. Two serum tubes (Serum Sep Clot Activator, 5 mL, Greiner/Bio-One, Frickenhausen, Germany) and three ethylenediamine tetra-acetic acid (EDTA) tubes (BD Vacutainer , 3 mL, Becton Dickinson, Heidelberg, Germany) were drawn. One of the EDTA tubes, for the later performed blood count and measurement of lactate dehydrogenase, was stored at room temperature. The two tubes for serum and the remaining two tubes for EDTA plasma were centrifuged immediately at 3000 g for 10 minutes and the plasma was frozen on dry ice.
Urine samples were collected after measuring total urinary volume at the defined times into a Vacuette  system (10 mL, Greiner/Bio-One) and were also stored on dry ice. Sweat samples were collected from the defined skin area on the back. Foil was fixed using Tegaderm film (3M Medica, Neuss, Germany) to the 17 x 25 cm area on the back leading to a small reservoir at the caudal end. After documenting the quantity in mL, the sweat was transferred in empty sterile tubes (7 mL, Greiner/Bio-One) and also stored on dry ice.
After collection, the EDTA blood samples were immediately transferred to the laboratory, and the blood counts were performed within 4 hours of blood sampling. The other serum, plasma, urine, and sweat samples remained stored on dry ice and were transported to the laboratory immediately after the experiment and stored there at -20 C until analysis.
Blood counts were performed using the EDTA blood samples, and included a white blood count with differentiation, red blood count, hemoglobin, hematocrit, and platelets. Hematologic parameters were determined using the Cell-Dyn Sapphire  (Abbott Diagnostics, Wiesbaden, Germany). Osmolality in serum, urine, and sweat was measured using an Osmomat 030 device (Gonotec, Berlin, Germany). Glucose in serum and lactate in plasma were measured using a Biosen 5140 system (Barleben, Magdeburg, Germany). Sodium, potassium, calcium, chloride, creatinine, urea, creatine kinase in serum and urine, C-reactive protein in serum, lactate dehydrogenase in plasma, and glucose in urine were measured using a DxC 800 system (Beckman Coulter, Krefeld Germany). Myoglobin, cardiac troponin I, free triiodothyronine, free thyroxine, thyroid-stimulating hormone (TSH), prolactin, cortisol in serum, and brain natriuretic peptide (BNP) in plasma were measured using an Advia Centaur XP (Siemens, Eschborn, Germany). Insulin in serum was measured using an AxSYM device (Abbott Diagnostics). Insulin-like growth factor 1 and interleukin-6 in serum were measured using the Immulite 2000 (Siemens). Somatotropic hormone in serum, and adrenocorticotropic hormone (ACTH) and renin in plasma were measured using a Liaison  device (diaSorin, Dietzenbach, Germany). Procalcitonin was measured using a Mini-Vidas system (Bio-Merieux, Marcy l'Etoile, France). Aldosterone in plasma was measured using a radioimmunoassay kit (Beckman Coulter). Midregional proatrial natriuretic peptide, midregional proadrenomedullin, c-terminal proendothelin-1, and copeptin in plasma were measured using a Kryptor device (Brahms GmbH, Hennigsdorf, Germany). Sodium, potassium, and calcium in sweat were measured using the AAnalyst 100 (Perkin Elmer, Rodgau, Germany). Albumin in urine was measured using an Immage  800 device (Beckman Coulter). Metanephrines and normetanephrines in urine were measured using high-performance liquid chromatography (Recipe, Munich, Germany).
We expressed urine concentration as fractional excretions. This applied to sodium, potassium, urea, and osmolality in urine. We used the general formula: fractional excretion of parameter = [(parameterurine x creatinineserum)/(parameterserumx creatinineurine)] x 100, following Espinel. Transtubular potassium gradient was calculated using the equation transtubular potassium gradient = (potassiumurine x osmolalityserum)/(potassiumserum x osmolalityurine) according to West et al. Glomerular filtration rate (GFR) was estimated using the four-variable MDRD4 formula (GFR [mL/min/1.73 m2] = 32841 x (creatinine [mumol/L][-1.154]) x age [years][-020]) following Levey et al. Creatinine clearance rate was estimated using the Cockcroft-Gault formula.