The Human Machine

Thermoregulation and Calorie Intake

“There’s no such thing as bad weather, only bad clothes.” 
My neighbour, a farmer, espoused this anecdotal dinner party gem some years ago and I’ve always wondered if it were true. After years of hillwalking as a larger individual (120kg) and then a much fitter individual (78kg), the science of temperature management and nutrition, and the avoidance of medical conditions arising out of poor temperature management, has always been of interest to me, and must be of supreme interest to ultralightweight backpackers. I am registered First Aid giver but nothing I say here should be taken to be expert advice. This article is designed to make you aware of the issues concerned and you should always be satisfied in your own ability to give care in an emergency situation. I am not a doctor!
There is a wealth of material available, some easy to read, some too scientific for mere mortals like myself. Algebraic equations give me migraines, however, I thought, having read some of it over time, I’d bring it all together in a simple article that might be of use and ensure the reader is aware of the issues. It is all freely available on the internet but some material, for example from the excellent BPL website, is not available and I have sought permission to reproduce some parts of articles by Ryan Jordan and Darin Banner. They are referenced at the end of this article.
Thermoregulation is the ability to keep body temperature within acceptable boundaries, when the surrounding temperature is different.  It is one aspect of homeostasis - the dynamic stability between an organism’s internal environment and its external environment. If the body is unable to maintain a normal temperature and it increases significantly above normal, “hyperthermia” (often through heat exhaustion and, then, heat stroke) ensues. The opposite condition, when body temperature decreases below normal levels, is termed “hypothermia”. Both are serious conditions that could feasibly, in the wrong confluence of circumstances, affect a hillwalker or backpacker.

Calorie Intake

An analysis of the packaging of almost any food product these days demonstrates that an average human male would need to consume 2,500 calories each day for normal activity. As our activity increases, so does our requirement for calorie intake. Calorie requirements will increase as a consequence of, for example, colder temperatures, exertion through ascent or speed, and the composition of the individual concerned (muscle vs fat). Exertion increases our body heat.

On the basis that a 80kg individual, hillwalking with a backpack will burn something in the region of 450-500 calories an hour depending on terrain, weather and fitness, a 5hr walk could end up burning 2,500 calories in addition to the usual necessities for maintaining core body heat and life. That figure may even be higher. Therefore, you must consume enough calories to replace that energy.

Complex carbohydrates are one of the best sources of calories per gram. The body takes longer to break them down into sugars for energy whereas sugary foods give you an instant hit which is not always what a hillwalker requires. Some experts suggest 1g of protein and 1g of fat per 4g of carbohydrates as protein takes even longer to break down for energy. Thus, balancing out carbohydrates, un-saturated fats and protein is essential for energy over the course of the day. Further, nutritionally your food should be predominantly made up of carbohydrates which have a low glycemic index (low GI). This simply means food that releases energy slowly. Generally foods that are unrefined, such as wholegrain bread as opposed to white, are low GI. Complex carbohydrates such as fruit, yoghurt and vegetables take the body longer to break down while simple carbohydrates like sugar release energy quickly. You should identify foods that are high in calories but have a relatively low GI. These could be consumed in the morning, for breakfast, before a hillwalking day starts as they may be impractical to take when lightweight backpacking. Sandwiches on wholemeal bread would be better than white bread. Cheese and ham would provide fat and protein respectively.

Both protein and fat are important to the body's functioning. Protein intake should include as much ‘complete’ protein as possible which is typically found in meat. Good substitutes are nuts, seeds, pulses and fruit eaten in combination. Like protein, fat is an essential part of any diet. Most importantly for hillwalkers, it is needed for certain vitamins to be absorbed into the body.

Don't forget vitamins and minerals which are important for all sorts of bodily functions. Hydration is also crucial to this so you should drink water regularly in small amounts, regardless of whether you’re thirsty - if you're thirsty you are already de-hydrated.

Eating little and often is best when hillwalking. Porridge is excellent as it releases calories slowly over the course of a morning, giving you consistent energy and, in cold weather, it has a conductive heating effect on the interior of your body. Dried fruits provide more potent nutrition than normal fruit and they are lighter. Nuts and seeds are good snacks, are light and small when packed. Kendal Mint cake is excellent for instant energy, depending on whether you can take the taste!

Keeping Warm
To avoid hypothermia, you need to understand the mechanics of the human body and how it maintains its internal heat. The first law of thermal dynamics states that the increase in the internal energy of a [human being] is equal to the amount of energy added by heating the [human being] minus the amount lost as a result of the work done by the [human being] on its surroundings. Heat cannot be created or destroyed - it is simply transferred from one place to another by a series of mechanisms - conduction, convection, radiation and evaporation.
A human being’s primary source of heat is the metabolism of food and calorie intake but heat is also generated during exercise (which requires food for energy). As we exercise, heat from our bodies is lost to our surroundings and needs to be replaced.
The human body is an ‘open thermodynamic system’ in that matter is constantly being exchanged between it and its environment, specifically food and water, energy, work, and heat. Taking in calories increases the energy available to the human body. The body then loses energy by doing work and transfers that heat to the environment (by the means described above). 

Heat Loss
In order to really understand when and why we become cold, we need to understand how we become cold. The second law of thermal dynamics involves the principle that transfer always occurs from a higher-temperature object to a cooler-temperature one because temperatures will attempt to equalise. We lose body heat as a consequence of processes known as conduction, convection, evaporation (usually through sweating) and radiation.
Conduction, according to the New Oxford American Dictionary, is “...the process by which heat (or electricity, but we’re not concerned with that here) is directly transmitted through a substance when there is a difference of temperature... potential between adjoining regions, without movement of the material”. Simply but, when a cold item touches a warm item, heat is transferred from the warm item to the cold item, resulting in heat loss to the warm item. For us, the cold ground is the most likely place we’ll experience conductive heat loss but it may also occur in contact with cold air.
The rate of heat transfer between two objects of different temperatures depends upon several factors (according to Fourier’s Law) including:
  • The difference in temperature between those objects;
  • The cross-sectional surface area of contact between the two objects; 
  • the time they are in contact; and
  • The effectiveness of the insulation between the objects
The greater the difference in temperature between two objects that are in contact, the more heat that is transferred between them over time. Think of a hot object touching a cold one and the effect induced and how quickly it happens - a cold hand on a radiator housing, for example. The more surface area in contact between two objects, the more quickly heat is transferred between them. putting one finger on a radiator will not warm you up but putting a large amount of your body on it will certainly do some more quickly. The longer we touch a hot object, the warmer we become.
Preventing Conduction Heat Loss
Clearly the easiest way to prevent conductive heat loss is through utilising gear which prevents contact with the cold as much as possible - an insulating ground mat, whilst sleeping, is crucial and, where a torso length mat is used, something else may need to be placed underneath the foot end of your sleeping bag. Often, using a jacket is preferred. Even the resting of an arm, all night, on the cold ground, will cool the body as the time factor is significant.
When taking a break in cold weather, sitting on a cold floor will cause heat loss through conduction - use a sit mat or your sleeping mat. It is also important to minimise heat loss through conduction as a result of contact with other cold surfaces - rocks, trekking poles, rivers, rain and cold air, for example. Proper use of gloves and footwear, as well as the layering of clothing, will reduce heat loss through conduction, but the other potential heat loss avenues must be considered in symbiosis - for example radiation, which is considered below. Layering is important - using only one layer would not prevent heat loss through conduction for example as the outer layer will become cold as a result of conduction and, as the body is in contact with that outer layer, it too will become cold. Layering minimises this effect and insulation, through the auspices of a material that is a poor conductor, is therefore crucial - a down or primaloft insulating layer is a good example. When we are moving, we are losing heat heat which, through conduction, warms the clothing next to our skin. This is why we have fewer layers on when moving - to allow the body to cool.
Convection (or “Hot Air Rises”)
Similar to conduction, convection is different in that the items in contact are also moving relative to each other and the process specifically refers to the way fluid molecules – in the form of liquid or gas – moves when heated. Convection is a direct result of the effect of gravity in relation to the density of the fluid. The colder the fluid, the more densely crammed the molecules and the heavier it is. Conversely, the warmer the fluid, the less densely crammed and lighter.
Wind is a direct result of convection. When air close to Earth’s surface is heated by solar energy it becomes less dense, and rises as a consequence. Immediately cooler (and more dense) air sinks – rushing in to fill the space. The cooler air then heats, rises and the process continues. Stated simply, hot air rises.
Like conduction, the degree of heat transfer between two items is dependent upon their difference in temperature and the amount of surface area in contact. However, the third important component with convection is the speed with which the colder object is moving. Thus, a (naturally) warm face is exposed to cold wind will cool faster than if exposed to simply cold air.
The blood in your body also transfers heat by convection. As our body cools, its response is to move blood away from the extremities in order to keep the body's core at optimal temperature. This may even be before there is a need to do so in order to protect the body’s core temperature but the human body is cautious in this respect.
Preventing Convection Heat Loss
Convective heat loss occurs in response to wind (as the speed of the object is of relevance) and the effect of air movement within clothing. Windproof clothing can assist in preventing or reducing convective heat loss but this is an area where the importance of symbiosis of these principles becomes apparent - overheating is also dangerous as the body will sweat in order to try to cool down which will, in turn, remove minerals and salt as well as de-hydrate the body. The windproof layer should therefore be significantly breathable. Yet a windproof layer alone, particularly when not moving, would not be enough as the layer would become cold through conduction and so would you.
Clothing should be evaluated for its ability to control convection by preventing warm air (warmed by the radiation effect of the body’s own heat) from escaping via convection. Adjustable cuffs, collars and hems are crucial. Opening them prevents over-heating and closing them prevents cooling down - both examples of controlling convection. The same can be said of how you pitch your shelter - it can provide a windbreak itself, and should be pitched in a sheltered area away from wind. In hot weather, ventilation is important too.
Thermal radiation is the transfer of infra-red or ultra-violet electromagnetic energy between two objects of different temperatures. We receive radiative heat from the sun, a campfire, and light reflected off from snow, rocks and sand, or water and, as our core temperature is often higher than that of our surroundings, we in turn radiate heat to our surroundings (imagine being on the Underground in London when empty as opposed to when full, or indeed, inside a tent). Consequently, as we radiate heat, we are losing heat (the first law of Thermodynamics). When clouds cover the sun, its thermal radiation decreases.
Preventing Radiation Heat Loss
Radiative heat loss tends to be more significant at night, especially if other weather conditions are not causing heat loss in other ways (wind and rain, for example). You might ask why the air overhead doesn’t radiate heat toward you to compensate for the heat you radiate toward the sky - air is reasonably transparent to infrared light, absorbing and emitting relatively little of it. Only water vapor, carbon dioxide, and a few other gases in air interact with infrared light. Thus most of this exchange of energy by radiation is between you and empty space. 
Reflective materials prevent radiation (think of the blankets given to marathon runners who are cooling down as a consequence of suddenly stopping exercise and sweating heavily). Thick clothing also reduces radiative heat loss because infrared radiation cannot move through the right insulation material (particularly if it is thick), and thus, most of the infrared radiation lost by the body remains trapped in the clothing system rather than exiting out to the environment.
Evaporation occurs when a liquid becomes gaseous as a result of heat. When a liquid evaporates its change in state from liquid to a gas takes up a great deal of energy and lowers the temperature of the surface on which it occurs. This is the process of evaporation. This is why we sweat when we are too warm. This is also why rain is dangerous - the wetness on the skin, when it evaporates, takes heat with it. When considering wet clothing, it is conduction that is important - the wetness evaporates, making the clothing colder and, as you are in contact with the clothing, so do you become colder. That is one of the reasons we use a layering system.
Evaporation also occurs when breathing. This loss of heat is increased when the air you are breathing in is very cold or very dry. The colder and drier the air and more you breathe, the more significant the loss of heat. Sweat also, in warmer conditions and as a result of exercise, contains levels of sodium (salt). It contains less in cooler conditions or where there is not so much exercise undertaken.
The Consequences of Heat Loss: Hypothermia
Hypothermia is a lowering of the body's core temperature. It can become life-threatening if ignored. There are different types of hypothermia, which depend on how quickly the body loses heat.
  • Acute or immersion hypothermia occurs when a person loses heat very rapidly, for example by falling into cold water.
  • Exhaustion hypothermia occurs when a persons body is so tired it can no longer generate heat - this can occur as a result of insufficient calorie intake, inadequate hydration and fatigue.
  • Chronic hypothermia is when heat loss occurs slowly over time. This is common in elderly people living in a poorly heated house, or in people sleeping rough.
When your body gets cold, the normal response is to warm up by becoming more active, putting on more layers or moving indoors. But if exposure to the cold continues, your body's automatic defence system will try to prevent any further heat loss by:
  • shivering (which keeps the major organs at normal temperature in the same way as exercise but burns calories and consequently will result in further heat loss if that energy is not replaced quickly),
  • restricting blood flow to the skin, and
  • releasing hormones to generate heat.
After prolonged exposure to the cold, these responses are not enough to maintain body temperature, as they also drain energy.
When the body’s energy is exhausted, it slowly starts to shut down. Shivering stops and your heartbeat begins to slow. This life-threatening stage can develop very quickly, so it is vital that hypothermia is treated as a medical emergency.
The drop in body temperature of results in shivering becoming more violent as hypothermia gets worse. Other symptoms will include being unable to think or pay attention, confusion (some people don't realise they are affected), loss of judgement and reasoning, difficulty moving around or stumbling (and weakness as the body uses energy to keep warm), feeling afraid, memory loss, fumbling hands and loss of coordination, drowsiness, slurred speech, listlessness and indifference, or slow, shallow breathing and a weak pulse.
To treat mild to moderate hypothermia, you must remove the sources of heat loss and increase your calorie intake. Shivering, and consuming sweet foods and drink to replace the consequential energy loss, can be enough to raise your body temperature to normal. Warmed drinks (not hot) will add energy to your system as the body will not need to expend energy heating itself; however, the temperature of the drink is less important than its calorie content. Therefore, a fruit juice would be preferable to tea.
You can speed the process of rewarming by employing a “hypo-wrap”. Described by Darin Banner in his excellent article on BPL: 
“The traditional method is to lay down a wind and water proof sheet like a tarp or a tent fly. This is used to eliminate convective and evaporative heat loss and keep the wrap dry. Next put at least one insulated sleeping pad on top of the tarp. This reduces conductive heat loss. Ideally you would have enough sleeping bags to put an empty one on top of the sleeping pad, then one with the patient inside on top of the first, and finally an empty bag on top of the patient. All this is then wrapped up with the wind and waterproof sheet.” 

If you have ultralight equipment, you may need to extend these principles by using other items to prevent conductive and evaporative heat loss - the idea is to make sure the victim is wrapped and well off the ground. Place insulating layers where the head and torso are as these are the major organs to protect.

I am grateful for permission from BPL to re-produce parts of this article. I have referenced the full article below which you should read.
If you are with a partner or a group, yours or the group's equipment could to be used to augment the hypo-wrap. Remember, however, that the health and safety of the caregivers is paramount. Do not take measures that will put you or members of your group at risk of hypothermia. Build a fire if possible, to increase the safety of both the caregivers and the patient.
Darin also says this, which dispels a long-held myth:
“The question of whether to put a cold person in a sleeping bag with a warm person often comes up. Testing has shown that a mildly hypothermic person in a bag with a warm person will rewarm at the same rate as a hypothermic person shivering vigorously in a bag by themselves.4 Caring for a hypothermic person in the backcountry is a lot of work; a warm person's energy could be more effectively used outside of the bag caring for the patient.”
The treatment of moderate to severe hypothermia is much more difficult than mild hypothermia. The colder a person gets, the more stress their heart is under. To prevent cardiac arrhythmia as a consequence of rough handling, the victim should be moved gently and left in a horizontal position. As with mild hypothermia, you must remove the sources of heat loss. Wet clothing may need to be cut off. If the person is conscious and can take liquids, hydrate them with sweet liquids. Use the hypo-wrap to minimize further heat loss. Medical attention will be required in what will become a life-threatening situation. This is not something that can be dealt with without professional medical help.


Hyperthermia is an elevated body temperature due to failed thermoregulation. Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate. When the elevated body temperatures are sufficiently high, hyperthermia is a medical emergency and requires immediate treatment to prevent disability and death.

The most common causes are heat stroke and adverse reactions to drugs. We are concerned with the former. Heat stroke is an acute condition of hyperthermia that is caused by prolonged exposure to excessive heat and/or humidity. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to effectively deal with the heat, causing the body temperature to climb uncontrollably. Heat Stroke begins with Heat Exhaustion.

Heat Exhaustion is when the temperature inside the body, known as the core temperature, rises to 37-40°C (98.6-104°F). At that temperature, the levels of water and salt in the body begin to drop. Hydration, therefore, is pivotal.

Heat Stroke occurs when a person’s core temperature rises above 40°C (104°F). Cells inside the body begin to break down and important parts of the body stop working.

Heat Exhaustion and Heat Stroke can be caused as a consequence of exertion, but need not be, and will often depend on whether the person has been exercising in the heat. Significant physical exertion on a very hot day can generate heat beyond a healthy body's ability to cool itself, because the heat and humidity of the environment reduces the efficiency of the body's normal cooling mechanisms. Other factors, such as drinking too little water, can exacerbate the condition.

Preventing Heat Exhaustion

Clearly, adapting clothing according to the conditions is crucial - breathable clothing which allows sweat to escape and the body to cool down is important. Exposing as much of the skin to the air as possible is the most effective way of permitting sweat to evaporate but exposes you to the risk of cooling down too quickly and/or skin cancer. Keeping properly hydrated is important. Wearing a hat can reduce the risk of heat exhaustion. A damp bandana, which hangs down the back of your neck will cool you down. Symptoms

The symptoms of heat exhaustion include:
  • Excess sweating
  • Nausea and vomiting
  • Feeling light-headed/faint
  • Tiredness
  • Headache
  • Muscle cramps
Hot, dry skin is a typical sign of hyperthermia. The skin may become red and hot as blood vessels dilate in an attempt to increase heat dissipation, sometimes leading to swollen lips. The inability to cool the body through perspiration causes the skin to feel dry.
The dehydration associated with heat stroke can produce nausea, vomiting, headaches, and low blood pressure. This can lead to fainting or dizziness, especially if the person stands suddenly. The person may become confused or hostile, and may seem intoxicated. Heart rate and respiration rate will increase as blood pressure drops and the heart attempts to supply enough oxygen to the body. The decrease in blood pressure can then cause blood vessels to contract, resulting in a pale or bluish skin color in advanced cases of heat stroke. Some victims, especially young children, may have seizures. Eventually, as body organs begin to fail, unconsciousness and coma will result.
Someone affected by heat exhaustion needs to:
  • Rest in a cool place - an air-conditioned room is ideal
  • Sip cool drinks - drinks especially designed for rehydration (e.g., electrolyte replacement sports drinks) are useful because they contain a good balance of different salts
  • Take a cool shower or bath, soak feet in a bucket of water or place a wet towel on the shoulders
They should keep doing this until they feel better.
Heatstroke is a medical emergency. If left untreated, an affected person may slip into a coma, often within minutes. Tragically, some people die from heatstroke - the excess body heat can lead to heart failure, kidney failure or brain damage.
Anyone believed to be suffering from heatstroke should be admitted to hospital as an emergency. They will usually be cared for in an intensive care unit where their body temperature will be lowered to a safe level and the abnormal salt and water content in the body corrected.
Darin Banner has been certified as a wilderness first responder by the Wilderness Medicine Institute. They have posted his patient assessment and incident report form as a resource on their website. Darin is also a First Aid and CPR instructor for the American Red Cross. His article was critical in my decision to write my own article and I thank him.
The Physics Hypertextbook -
The NHS Website -
Wikipedia - various articles on Thermodynamics
Giesbrecht, Gordon et al., "Treatment of mild immersion hypothermia by direct body-to-body contact," Journal of Applied Physiology 76, Issue 6 (1994): 2373-2379.
Banner, “Hypothermia: Avoiding and treating hypothermia in the backcountry with ultralight equipment”, BPL, 6th May 2008
Jordan, “Thermoregulation: An Overview of Heat Loss Mechanisms and Practical Guidelines for Staying Warm with Lightweight Gear”, BPL, 24th October 2003
Weiss, “Secrets of Warmth: For Comfort or Survival”, The Mountaineers, 1998