The US Pro Cycling Challenge (USAPC) is back and Colorado is ready to witness the best cyclists in the world compete on some of the toughest and most gorgeous roads in the world. What makes this race particularly challenging is the altitude.   If you're a Colorado local you've seen big-name cyclists flocking to the area over the past month to gain precious acclimatization time in Colorado's high altitude. We spoke with world-reknowned sports scientist Dr. Inigo San Milan, who works with many of the teams and top riders to get his take on how to take on high altitude during the US Pro Challenge, and how you can extend this to your own riding and racing: The first five stages of the US Pro Challenge are raced in "the high country" where most of the time riders will be racing at altitudes between 8,000 ft and nearly 12,000 ft. These high altitudes represent a very important challenge for the human physiology. During a cycling race, how the body reacts to altitude will be decisive to decide the winner of the race. Here we will explore some of the challenges that cyclists will face due to the altitude at the US Pro Cycling Challenge: Oxygen (O2) is obviously the main challenge of high altitude. There is a misconception that oxygen concentration at altitude is lower than at sea level. When exercising at altitude how many times do we hear “There is no oxygen up here!”. Well, that is not really true; the oxygen on the top of Mt. Everest (the highest peak in the world at almost 30,000ft) and at sea level are exactly the same: about 21% of oxygen.  What actually differs is the atmospheric pressure.   As we go up to higher altitudes, atmospheric pressure decreases. The reason we feel like there is less oxygen is because a lower atmospheric pressure results in a decreased oxygen supply to the tissues (aka: hypoxia). The oxygen saturation in the blood decreases and therefore the oxygenation of the tissues is decreased. At sea level (0 feet altitude) the atmospheric pressure is 760 mmHg and the oxygen saturation (Also measured as Saturation of Peripheral Oxygen-SpO2) is 98%. At about 8,000 ft which is the altitude of the majority of Stage 1 of the USAPC, the barometric pressure is about 575mmHg which is about 75% of the O2 available at sea level (resulting in a O2 saturation (SpO2) of 91%). But that's nothing, because during Stage 2 the riders crest Independence Pass, which is at a whopping 12,000 feet with a barometric pressure of 495mmHg and a SpO2 of 79%. These are some numbers that won't make it very "fun" to be up there watching the bike race, let alone racing it. Altitude and it's lower oxygen saturation presents major challenges to exercise and we can see them many different manifested in metabolic responses: 1. The first noticeable response we can perceive is an increased number of breaths per minute accompanied by an increase in heart rate. The major purpose of this response is to increase oxygen entry into the lungs. Since there is lower oxygen saturation in the blood, the heart rate increases to try to pump more oxygen to the tissues. If there is less oxygen content in the blood the speed of the blood flow to the tissues must be is higher. However, regardless of how hard the body works to increase oxygenation, the maximal oxygen consumption (VO2max) decreases the higher you go at a rate of about 6% per every 1000m (3,280 ft). Therefore during the opening stages of the USAPC, VO2max for all rider will be decreased by about 15% during the Aspen and Vail stages and reaching about 20% at the top of Independence Pass. As a result of this decrease, power output is reduced by about 3-5% at about 5,300 ft, and could be reduced to about 10-15% at 9,500 ft. 2. Other important responses of altitude pertain to fuel utilization. At altitude the muscles have a much higher need for glucose. Altitude increases adrenergic activity. Adrenergic activity not only increases heart rate but also increases the mobilization of glycogen from the muscles and liver to produce glucose to use as fuel. While it is very important to increase carbohydrate utilization at sea level, it is even more important to increase carbohydrate utilization at altitude where by nature we have a higher chance of bonking. At altitude the recommendation for carbohydrate intake per hour during a race is about 40-60grams/hour. At altitude it should be more. I have tried as much as 80-100g/h and it seems to work quite well. Altitude also increases metabolism so an increase in calories should be contemplated. Note: dieting or restricting calories at altitude is not the best idea. 3. Hydration is very important at altitude. When you're taking more breaths, you have a higher rate of respiration and therefore there is a higher loss of water from the lungs. High-altitude areas also tend to be very dry, so evaporation of water from the body is  accelerated. High altitudes may also elicit increased urinary output (diuresis) especially at night, so it is important to replace liquids constantly. The higher the altitude the more liquids we need to drink. By using electrolyte enhanced sports drinks we you can replace liquids lost at altitude as well as adjust for the increased carbohydrates that need to be consumed during exercise at altitude. 4. Finally, it is important to adapt correctly to compete at altitude. It is very well known that altitude exposure increases red blood cells and therefore tissue oxygenation. This is the whole theory behind "sleep high, train low." The higher the altitude exposure the higher the red blood cell stimulation. However it is very important to perform a proper adaptation. At low altitudes ~5,000-6,000 ft a period of 8-12 days could be needed for a good adaptation. A full adaptation at higher altitudes of ~ 8,000-9,000 ft could take about 3 weeks. Training should be also controlled and monitored properly. At higher altitudes it is not possible to train and recover the same way you do at sea level. A reduction in volume and intensity should be taken into consideration while you're acclimating. For a 3 week training camp at altitude it would be ideal to perform a phased approach to acclimatization. For the first phase plan about 5-7 days at a moderate altitude, about 5,000-6,000 ft. During this time do short and easy rides for the first 5 days and increase the duration and intensity between the 5th -7th day. In  phase two go to a higher altitude,  about 8,000-9,000 ft, for another 2 weeks. Allow  4-5 days to adapt to the new altitude and again decrease the intensity and duration of workouts.  It is always important to be conservative when training at altitude. Way too often do I see athletes go to altitude and continue training like they were at sea level. Time and time again this results in overtraining and fatigue. The riders leave altitude training in worse shape than they arrived and have a major decrease in performance. If you have any questions for Inigo, email us at