Many people associate dehydration only with exercise in hot environments. While this can certainly be true, dehydration is also a common concern during exercise in the cold. In fact, one must stay on top of hydration in the cold just as much as in the heat – perhaps even more so.
One obvious factor that could promote dehydration during exercise in the winter has do to with overdressing for conditions. Exercising in the cold with too much or the wrong type of clothing can lead to increased thermal stress. This hot microclimate around the body can result in excessive sweating and overheating to levels similar to exercise in the heat. Therefore, it is important to dress appropriately.
Exercise in the cold is also different than exercise in the heat in a few subtle and less appreciated ways that can impact hydration status. Below is a discussion of a few differences between exercise in cold vs. temperate environments that every athlete should know.
Increased water loss in winter
Many times when individuals exercise in the cold for winter sports, it is at a higher altitude than normal. Relative to exercise at sea level, the same exercise at altitude results in increased ventilation to maintain blood oxygenation. Increased ventilation promotes dehydration because our breath is saturated with water, and so increased breathing results in increased water loss. Physiologists call this “insensible” water loss, as it is out of our control. This route of dehydration is usually chronic (since ventilation is also increased at rest at altitude) and can really add up without an individual perceiving that any water has been lost!
Altered thirst sensation in winter
One important factor to consider when exercising in the cold is that our bodies have a decreased perception of thirst when in cold environments. This has been shown in animal studies where exposure to cold air at 5°C during rest for a period of time resulted in decreased water intake and increased evaporative water loss relative to animals at room temperature. When the animals were moved to room temperature of 26°C, they began drinking water within 15 min which lasted for about 1 hour (1). Similar studies showing less water intake during rest and exercise in the cold have been performed in humans as well (2). When individuals exercised in the cold in either a hydrated or dehydrated state, the perception of thirst was always blunted by close to 40% in the cold compared to a thermal neutral environment (3, 4). Therefore, individuals exercising in the cold are going to be predisposed to dehydration due to decreased perception of thirst! Therefore, athletes exercising in the cold need to be extra vigilant to maintain good hydration habits.
Alterations in perception of dehydration in the cold
Other than the decrease in thirst in the cold, there is also an alteration in the brain’s perception of dehydration in the cold. This effect can be explained in part by the redistribution of blood volume. To maintain core temperature in the cold, our bodies constrict blood from the periphery (arms, legs, and skin) to maintain the warm blood around the core of our body. This is called vasoconstriction, and this tricks our brain (the hypothalamus actually) to thinking our bodies have plenty of blood volume – even though dehydration is a common finding in the cold. Normally when we are dehydrated, plasma arginine vasopressin (AVP – also known as anti-diuretic hormone) increases to minimize kidney urine production to reduce urinary water loss and help maintain blood volume. In the cold, despite dehydration, central blood volume shifts prevent the increase in plasma AVP relative to the same amount of dehydration in a warm environment (3). This results in increased urine production, promoting dehydration. Interestingly, there is a strong relationship between plasma AVP and thirst, which means that part of decreased thirst in the cold may be attributed to lower concentrations of plasma AVP. Therefore, athletes exercising in the cold often have increased urine production compared to a more temperate environment. These changes result in an increased risk of dehydration in the cold that are not always obvious.
Are there other metabolic changes during exercise in the cold?
There are no consistent differences in exercise fuel utilization during exercise in the cold compared to exercise at the same relative intensity in a warm or hot environment. Some reports suggest mobilization of fatty acids from adipose tissue is decreased in the cold (perhaps due to constricting blood flow to adipose tissue), but this does not change working muscle substrate use. Similarly, there are no consistent changes in blood glucose or glycogen degradation during exercise in the cold (5). Interestingly, there are some data suggesting that exercise in the cold with cold muscles can decrease maximum voluntary force and promote fatigue (6). Some of this may be due to an increased use of fast twitch muscle fibers in the cold (7), which would be less fatigue resistant, and may also explain the increase blood lactate concentration often found during exercise in the cold (8). So other than a decrease in muscle strength and increased fatigue when exercising with cold muscles, there are few metabolic alterations that need to be considered during exercise in the cold.
Other than dressing properly to not over- or under-heat, athletes need to be aware they will have increased water loss and decreased thirst while exercising in the cold. All the same principles for maximal exercise performance exist in both cold and warm environments. These principles include making sure to start exercise in a hydrated state, and consuming enough fluids during exercise to prevent dehydration and the decrease in performance it brings.
Post by By Bryan Bergman, PhD and Brent Mann
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