Cold war: time to take on the elements

Andrew Hamilton explains how the cold weather affects the endurance athlete’s body and what this means in terms of the nutritional requirements when training and competing in the colder months ahead

Living and working in centrally heated homes and workplaces, it’s easy to become detached from the reality of the elements. But for those of you who like to train outdoors regularly during the winter, being prepared for the cold is a must. You might think that preparing for colder conditions is only really necessary when living in more severe winter climates, such as Canada, Scandinavia and Russia. However, while it’s true that countries lying in more temperate climate zones such as the UK rarely experience brutal cold, it’s still something you should think about. That’s because thanks to the process of evaporative cooling, the combined effects of wind and rain can chill even the hardiest body to dangerous levels, even if the mercury isn’t sub-zero (see figure 1 for a more detailed explanation).

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FIGURE 1: HOW DOES THE COLD MAKE YOU COLD?

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Understanding heat balance

Being warm-blooded mammals, the human body endeavours to maintain a constant core temperature of around 36.4C. But given that we evolved in an environment where the temperature varied dramatically, we also have the capacity to respond to warm or cold environments.

When the temperature of the environment drops, two mechanisms come into play to help us maintain our core temperature:

*Vasoconstriction – this is when the blood flow to the skin and body peripheries is reduced, and instead diverted into the core. This decrease in peripheral blood flow reduces convective heat transfer between the body’s core and shell (skin, subcutaneous fat, and skeletal muscle) and increases insulation. This in turn means less heat is lost through the skin to the air around us. Put simply, during cold exposure, central core temperature defence occurs at the expense of a decline in skin temperature and the temperature of the peripheral regions of the body. This explains why your fingers and toes tend to feel cold long before you start to shiver – ie when your core temperature has dropped. Figure 2 illustrates this concept, showing how the hand responds when exposed to cold conditions.

*Metabolic heat generation – If vasoconstriction isn’t sufficient to prevent core temperature falling, heat production can increase in order to replace heat lost during cold exposure. Exercise is the best way to increase metabolic heat production but in the absence of an increase in voluntary muscle activity, shivering begins. Shivering is an involuntary pattern of repetitive, rhythmic muscle contractions. Shivering may begin immediately or within several minutes after the onset of cold exposure, usually in torso muscles, followed by a spread to the limbs. When shivering begins and is of relatively low intensity, fat is the main source of fuel. However, if muscle carbohydrate stores (glycogen) are very high, more of this stored carbohydrate can be used for shivering. When shivering is intense (ie under conditions of extreme cold exposure), the preferred source of fuel shifts away from fat and towards glycogen (see figure 3). This partly explains why consuming adequate carbohydrate during cold-weather exposure is particularly important (see later).

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FIGURE 2: VASOCONSTRICTION AND HAND TEMPERATURE

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Individual cold resilience

We all know someone who feels ‘perpetually cold’, even when the heating is on full blast – it may even be you! Likewise, you may envy someone who seems comfortable wearing T-shirts and shorts in the depths of winter. But why is this so? There is an explanation, because research shows that humans vary in their ability to combat cold stress. Part of this can occur through acclimatisation. Just as we can acclimatise to heat, humans can adapt their responses to cold. A good example of this can be seen when we compare cold exposure in habitual cold dwellers (eg the Eskimo Inuit) with those who live in warmer regions (figure 4). However, there are also basic physiological differences between individuals. A major difference is gender related; most women have higher levels of body fat under the skin than men. This thicker subcutaneous fat layer produces greater tissue insulation, which results in a lower critical water temperature – the coldest water tolerated without shivering^{J. Appl. Physiol 1962. 17:961–966}. This explains why women often excel in cold, open-water swimming events such as cross-channel swimming.

However, women also lose out in the war on cold. Women generally have a greater surface area and smaller total body mass. Assuming an equivalent amount of clothing insulation, a women’s total heat loss is greater due to the larger surface area through which heat loss can occur. Also, because of their smaller body mass, the total body heat content is less in women to begin with, which means that body temperature falls more rapidly for any given thermal gradient and metabolic rate. This probably explains the cliché that many women constantly turn up the heating while their partners are constantly turning it down.

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FIGURE 3: SHIVERING AND CARBOHYDRATE REQUIREMENTS ^{J Exp Biol 2011 214: 286-294}

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Nutrition for the cold

What are the nutritional implications for those training for and participating in cold-weather events? For shorter training sessions or races in less than extreme conditions, simply following the normal guidance (adequate carbohydrate and fluid intake prior to and during exercise) will probably suffice. Of course, this assumes that your clothing is appropriate – for example, the use of thermal base layers worn next to the skin.

However, in more extreme conditions (temperature below 0 degrees C, and maybe combined with strong winds, resulting in a ‘wind chill’), the challenge to stay warm is much greater. For example, 0C combined with a wind of 30mph results in an effective temperature of -8 degrees C (see figure 5). In other words, although the mercury reads 0C (not especially cold), you will lose heat at the same rate as standing around in still air at -8C!

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FIGURE 4: BLOOD FLOW IN THE HANDS OF INUIT AND WHITE CAUCASIAN SUBJECTS DURING COLD-WATER IMMERSION ^{J. Appl. Physiol 1962. 17:326–332}

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FIGURE 5: WIND CHILL AND EFFECTIVE TEMPERATURE

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Nutritional implications of cold weather training and competition

*Carbohydrate - During longer events in very cold weather, participants in prolonged, physically demanding activities are at risk of a condition known as ‘hiker’s hypothermia’ ^{Appl Physiol Nutr Metab. 2007 Aug;32(4):793-8}. This is where prolonged cold exposure and fatigue induced by sustained physical exertion can impair shivering and the vasoconstrictor response to cold, which increases the risk of hypothermia (see figure 6). It’s not clear why this happens but there’s evidence that a drop in blood glucose may increase the risk, which suggests ample carbohydrate intake is particularly important during distance events in very cold conditions.

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FIGURE 6: CORE TEMPERATURE AND HYPOTHERMIA SYMPTOMS

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The importance of carbohydrate while exercising in cold weather has also been explored in depth during studies on military personnel, some of whom are required to undertake operations in very cold environments such as the sub-arctic. These studies indicate that the increase in energy expended on simply staying warm in extremely cold conditions can rise by up to 5-fold compared to temperate conditions. Crucially, carbohydrate oxidation typically rises by 6-fold, whereas fat oxidation rises by only around 2-fold ^{Eur. J. Appl. Physiol 1989. 58:873–878}. And as we have seen, carbohydrate needs can be increased further when shivering occurs, particularly when shivering is intense (refer back to figure 3).

Because fat supplies more than twice the number of calories per gram than carbohydrate, and because body fat is a good insulator, many people have wrongly assumed that high-fat foods are preferable when exercising for long periods in the cold. But for all the reasons outlined above, this is simply incorrect. Maintaining an ample supply of carbohydrate during exercise becomes even more important when the mercury drops.

*Protein - Although carbohydrate is crucial for cold-weather performance, there could be some benefits from consuming a high-protein breakfast before your training session or event. Studies show that the thermic (heat generating) effect of consuming protein is higher than that for both carbohydrates and fat. This can result in increased body warmth for up to six hours following ingestion of a high protein meal^{News in Physiological Sciences 1993. 8:273–277}. This has been confirmed by research specifically looking into breakfast and protein intake, where recent studies confirm this effect^{Nutrients. 2016 Aug 10;8(8). pii: E490 J Nutr. 2015 Oct;145(10):2229-35}. Of course, a high-protein breakfast shouldn’t preclude carbs for all the reasons stated above. Also, it’s important that you start any cold-weather event with your muscles well ‘carbohydrate loaded’ from the previous days!

*Hydration – While you might think that fluid loss and dehydration isn’t a problem in cold weather, the reverse is actually true – you are more likely to become dehydrated while exercising in very cold conditions than in mild or cool conditions. There are three main reasons for this:

*Cold-induced dieresis (CID) – this is an effect where urine (and therefore fluid) losses are considerably increased during exposure to cold. Although this effect has long been known about, researchers are still unclear about the exact mechanism. One explanation is that the movement of blood and fluid towards the core as a result of vasoconstriction increases the kidneys’ output of urine. Regardless, CID can dramatically increase fluid loss, which will increase your fluid intake needs.

*Respiratory loss – Very cold air is very dry because it can’t hold much water vapour. When inhaled into the lungs, this dry, cold air encourages the transport of water vapour from the bloodstream into the air in the lungs. This water vapour is then exhaled and lost. Over time, the water losses via this route can be considerable, especially during hard exercise.

*Sweating – while you might not associate sweating with cold weather exercise, sweating can still occur. This is most likely when your event involves periods of different exercise intensity. If you dress to keep warm during the lower-intensity periods, you’ll almost certainly sweat during the higher-intensity periods. You could wear only enough garments to maintain warmth during the intense periods but that would risk becoming very cold during the less-intense periods! Overall then - contrary to what you might expect - in cold conditions you might actually need more fluid than in mild conditions. Athletes who therefore neglect to ensure ample fluid intake in the cold can expect to pay a performance penalty.

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HOT DRINKS

Given the need for ample fluid and that of staying warm, the use of hot drinks can be particularly useful for endurance athletes training or competing in cold conditions. If you can remember back to your physics lessons at school, you may recall that water has a very high ‘specific heat capacity’. This means it takes a lot of energy to warm even a small amount of water – and also that water gives up a lot of energy to its surroundings as it cools down. In fact, it takes one calorie of energy to warm a litre (1000mls) of water by just 1 degree C. A hot drink or soup at 60C is 24C warmer than body core temperature; therefore, each litre of hot drink you consume will release 24 calories (over 100,000 joules) of heat energy into the body. This is over and above the energy released when any carbohydrate and/or protein in the drink is broken down to release energy. Hot drinks are therefore highly recommended if you’re struggling to maintain heat balance – for example in extreme conditions or in multi-stage events where you end up hanging around in between your physical efforts!

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PRACTICAL RECOMMENDATIONS

How can you use the findings above to prepare yourself for a cold-weather training session or event and maximise your performance? Below are some practical recommendations:

• *Take advantage of your ability to cold acclimatise. Try and perform some longer training sessions in cold conditions during the run-up to the event.
• *You don’t need to wait for the weather to turn severely cold in order to acclimatise. You can simply ‘under dress’ – ie wear less warm garments than you normally would. Exposing your body to cold is also easily achieved when the weather is cool and wet, and evaporative cooling takes place.
• *It is recommended that cold acclimatisation training is NOT carried out in a remote area; if you really begin to suffer, you need to be able to throw in the towel and get back home/to the car to a warm environment quickly!
• *For an actual event, dress appropriately; try to wear enough garments to keep you warm but not overly warm.
• *Factor in the effects of wind chill on race day. Don’t just consider temperature, but how hard the wind is blowing. Make a note of effective wind chill temperatures during previous training sessions and the level of clothing required to maintain adequate warmth.
• *Consider a high-protein breakfast on a training or race day morning, but be sure to include some goodquality carbohydrate too.
• *During your event, carbohydrate is king. Consume plenty, but little and often. Consider solid carbs rather than drinks, which may well be ice cold by the time you get to drink them. Remember, the heat capacity of water can also work against you when the fluid you ingest is cold!
• *Drink plenty of fluid during the event. Hot drinks (if available) such as tea, coffee and soup provide a useful additional source of heat if you are struggling to stay warm.