We are all aware of the importance of proper fluid intake during the hot, sweaty summer months. Dehydration in the cold does not get quite the attention as in the heat, but for endurance athletes, it is a legitimate concern. We can lose as much fluid training in the cold as we do in the heat, but in different ways.
Just like proper hydration regulates body temperature in the heat, it does so in the cold but in different ways. In the heat, we sweat. In the cold and at altitude, increased respiration (breathing) and diuresis (urinating) are methods to regulate body temperature. Cold is defined as temperatures below 30oF. Since cold weather training often accompanies altitude training, this will be addressed as well. Altitude is defined as 5,280 feet or 1,600 meters above sea level.
Fluid loss through breathing: +600-2,000 ml per day
We already lose fluid through respiration when exercising. Cold, dry air and altitude increases these respiratory losses. When breathing in dry air (hot or cold), we must moisten the air to protect the lining of the respiratory track. This requires an additional 1-2 liters of water a day to humidify inhaled air. When you see steam leave your mouth when you exhale, that is water. Body size and duration of activity in the cold impact the amount needed.
Fluid loss through urination: 500 ml per day
The body responds to cold by tightening the blood vessels to conserve heat, which results in increased urination or “Cold-Induced Diuresis” or CID. A similar adaptation occurs at altitude. Estimate an additional 500 ml per day from CID.
Fluid loss though sweating: ½ - 1 liter per hour
Athletes still sweat when training in the cold, particularly if wearing a lot of layers, it just might take a little longer to work up a sweat. Reported sweat rate averages for winter soccer versus summer soccer is 1.13 and 1.46 liters per hour, respectively. Winter half-marathon training versus summer cross country is 1.49 versus 1.77 liters per hour.
Some of those fluid losses in the winter are from respiration. While not as high, there is still significant sweat loss in winter training. This can be quite dangerous when we stop moving and are now wet. Athletes may not realize they are cold until they stop moving. To make things worse, the cold alters thirst sensation and we voluntarily drink less. This is a prime setting for hypothermia, even if it is not that cold out. If the fluid is not replaced, dehydration will decrease blood volume and increase the risk of hypothermia.
Dehydration and Hypothermia
Hypothermia occurs when the body’s core temperature drops below 95oF. Normal is 98.6o F. When dehydrated, blood supply is limited to vital organs and core temperature drops. Hypothermia leads to extreme fatigue, drowsiness, disorientation, and death. Dehydration and being wet increases risk for hypothermia. Once hypothermia sets in, drinking fluids is not enough to raise core temperature so it is best to not get dehydrated. The goal is to hydrate during cold weather training to prevent body temperature from decreasing and have warm, dry clothes available when finished.
Increased carbohydrate needs
Depending on the duration of activity in the cold, energy needs can increase by 25-50 percent in the cold and at altitude. Shivering is an involuntary physiologic response to a drop in body temperature to stay warm. Shivering stops at 92oF (this is bad). Shivering also stops when the skin is warmed even though core temperature is still dropping. Also bad. The best way to warm up cold muscles is with exercise, which requires more calories. A high-carbohydrate diet is preferred in the cold. It is our glycogen stores (carbohydrate) that are depleted to maintain core temperature.
Athletes doing a long run or ride in colder temperatures may need an extra carbohydrate source, ideally from a higher-carbohydrate beverage, to maintain blood sugar levels and prevent performance from dropping. Athletes training to do a race at altitude or who are training at altitude should plan for more carbohydrates than consumed at sea level. Athletes are more susceptible to acute mountain sickness when dehydrated or have a drop in blood sugar.
In the cold and at altitude, athletes have increased fluid and carbohydrate requirements to maintain body functions that preserve heat and acclimate to the elevation changes. When training or racing in the cold or altitude, follow a similar hydration plan to when training and racing in the heat even though you may not feel like you need as much. Your sweat losses will decrease and your urine and respiratory losses will increase.
When we just drink water, we generally only replace about 50% of our fluid needs. The sodium and carbohydrate in a sports drink increases voluntary fluid intake and the extra carbohydrate will support the additional needs from cold and altitude. Salt needs may not be as high as during the summer, but you will still lose some sodium in your urine and sweat. Use a sports drink with 460-800 mg of sodium per liter.
Baker LB, Jeukendrup AE. Optimal composition of fluid-replacement beverages. Comprehensive Physiology. 2014, 4.
Marriott BM, Carlson SJ. Nutritional Needs in Cold and in High-Altitude Environments. Applications for Military Personnel in Field Operations. Committee on Military Nutrition Research. Food and Nutrition Board. Institute of Medicine. National Academy Press. Washington DC. 1996.
Trisha Stavinoha (MAJ, US Army, Retired) just retired from 20 years as an Army dietitian and now consults for Cera Products, Inc. She was a member of the All Army sports program and still competes in triathlon, running, trail running and obstacle course events. For questions, she can be reached at email@example.com or firstname.lastname@example.org.