Posted at 8:36 • 31 Jan• Trisha Stavinoha, MS, RD, CSSD, CSCS, TSAC-F

You get just as dehydrated in the cold as you do training in the heat. Today we dive into the what to look out for and how to stay hydrated!

You already lose fluid through respiration when exercising.

Cold, dry air and altitude increases your respiratory losses. When breathing in dry air (hot or cold), you 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.

The body also 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.

Athletes still sweat when training in the cold, particularly if wearing a lot of layers.

It might take a little longer to to notice you are sweating. 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. 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. To make things worse, the cold alters thirst sensation and we voluntarily drink less in the cold. 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, increasing your risk of hypothermia.

Hypothermia occurs when the body’s core temperature drops below 95F. Your normal body temperature is 98.6F. When dehydrated, your blood supply is limited to vital organs and core temperature drops.  

Hypothermia leads to extreme fatigue, drowsiness, disorientation, and death. Once hypothermia sets in, drinking fluids is not enough to raise core temperature. Staying hydrated in cold weather is critical to prevent your body temperature from decreasing. Having warm, dry clothes available when you finish exercising is also critical to avoiding hypothermia.

Depending on the duration of activity in the cold, energy needs can increase by 25-50% in the cold and at altitude. Shivering is an involuntary physiological response to a drop in body temperature to stay warm.

Shivering stops at 92F. That is really bad. Shivering also stops when the skin is warmed even though core temperature is still dropping. Also really 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 your glycogen stores (carbohydrate) that are depleted to maintain core temperature.

When you are doing a long run or ride in colder temperatures you 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 if you are training at altitude you should plan to consume more carbohydrates than at sea level. You are also more susceptible to acute mountain sickness when dehydrated or if you have a drop in blood sugar.

In the cold and at altitude, you 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. You may not feel like you need as much. Yet your sweat losses will decrease but your urine and respiratory losses will increase.

Water is not enough. When we just drink water, we 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 your 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 sodium per liter.

References:

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.