In this essay food and shelter are considered in respect of a human food-gatherer in an environment which starts off warm, and becomes colder, and with food more scarce. It is not intended as an accurate study of heat loss and thermal comfort, but as an exploration of the broad relation of food and clothing and shelter.

Food

From the point of view of Idle Theory, food is an idleness-increasing 'useful tool'. But while a tool (such as an axe) saves labour by reducing labour time (the axe speeds chopping down trees and cutting up logs), food directly provides the energy to power human life for some period of time. A plate of food converts into a few hours of continued life.

In physical terms, food provides the energy that powers human life. This energy is used up at the rate of about 100W by an inactive human. So that a meal with an energy content of 500kJ will power an inactive human for 5000 seconds, or approaching one and a half hours.

Food energy is stored in the human body in the form of sugars or fats, and this store is slowly 'burned' at a low temperature to power heart and lungs and muscle activity, and the maintenance and repair of every cell in the body. Much of this energy ends up by being dispersed as heat, which serves to maintain the human body's internal temperature, before being lost to the external environment.

As this energy store runs down the sensation of hunger signals the need to top up the store. The pain of the sensation of hunger (or of thirst) is one that becomes more and more insistent the longer it goes unanswered. Pain may be regarded as a form of involuntary work, an interruption drawing attention to the need of some part of the body for relief from some deficit or surfeit. and so someone who is hungry (or otherwise in physical pain) may be considered as less idle than someone who is not.

Hunger is part of a necessary physiological regulatory mechanism for ensuring (as far as possible) a steady food intake, and thus a steady flow of energy into the human body.

Unclothed humans

If food intake and its regulatory sensations of hunger and surfeit are concerned with the influx of energy (in the form of food) into the human body, then clothing and shelter are concerned with the regulation of the outflow of energy (in the form of heat) from the human body to the surrounding environment. The regulatory sensations of warmth and cold are signals requesting the increase or decrease of heat loss from the body, in the same way that sensations of hunger and surfeit are signals requesting the increase or decrease of food energy uptake.

Heat is lost in a variety of ways from the human body. Heat is transported to the skin surface and is lost radiatively or convectively to the external environment. It is also be lost in the form of heated water vapour exhaled into the external environment. The body can increase heat loss by opening up capillary blood flow to increase heat flow to the skin surface , or decrease heat loss by reducing capillary blood flow to the skin surface. Sweating is a further way in which heat may be lost, as sweat vapourising on the skin surface draws up its latent heat of vapourisation of water. Where too much heat is being lost to a cold external environment, the body can generate further heat by shivering or by undertaking some other activity, such as walking or running, which boosts energy consumption, and increases the amount of heat generated within the human body. Or else if too much heat is being lost, humans can reduce the rate of heat loss by putting on thermally insulating clothing, or by moving to a warmer environment inside a shelter.

Food and shelter are both primary human needs. And both are concerned with the flow of energy. Food intake is a flow of energy into the body. And clothing slows or regulates energy loss from the body in the form of heat.

The human body maintains an optimum core body temperature of 37^oC. Considering only conductive heat loss, the thermal conductivity K of human tissue is about 0.5 W/m.C, and with a 10 cm path length from body core to skin surface, heat flow is 5 W/m².^oC. Given a skin surface area of 2m², this gives a net heat loss by conduction of 10 W/^oC.

At rest, the human body generates about 100 W of heat, the basal metabolic rate. If this heat is lost too quickly to the external environment, body core temperature will tend to fall, and a person will begin to 'feel too cold'. If it is lost too slowly, body core temperature will tend to rise, and a person will 'feel too hot'. And so thermal comfort will be achieved when heat is lost from the skin surface at the same rate it is generated. Given conduction heat loss through body tissue

Q = (k.A./l).(T1 - T2)

where Q is 100 W, k.A/l = 10 W/^oC, and body core temperature T1 of 37^o,

T2 = 37 - 100/10 or 27^o. ---(1)

This means that a naked, resting human may be expected to feel 'comfortable' - neither too warm nor too cold - in an ambient air temperature of about 27^o, with no wind.

Should ambient air temperatures fall from 27^o to 17^o, the rate of heat loss will double to 200 W, which is twice what an inactive human generates internally. In the absence of insulating clothing, the only way that a human can maintain a body core temperature of 37^o is to perform physical work of some sort, like shivering, which releases heat as a byproduct of mechanical work. Shivering isn't the only way. Walking, running, and other activities will serve just as well. A human losing 200W of heat has to increase their metabolic rate from 100 W resting, to 200 W shivering or walking.

So while a resting human will feel comfortable with an ambient air temperature of 27^o, should the air temperature fall to 17^o, they will first feel pain, as the body's heat regulatory mechanisms demand action, and they will afterwards respond by becoming more physically active, generating 200 W of heat internally rather than 100 W.

And if temperatures stay low this increased energy consumption will have to be met by increased food consumption. And increased food consumption in turn will demand more work to be done to find food, and this will result in falling idleness, where idleness I is given by

I = 1 - Pm/( Pi - Pe ) --(2)

where Pm is metabolic rate, Pe is additional work performed while finding and gathering food, and Pi is the rate at which food energy is acquired while working. If it is supposed that a human can gain energy, Pi, at the rate of 400 W while working, Pe, at the rate of 100 W, then for a human at 27^o with a metabolic rate, Pm, of 100 W, idleness is given by

I = 1 - 100 / (400 - 100) or 0.66.

If air temperature is reduced to 17^o, and metabolic rate increases to 200 W, then if everything else stays the same,

I = 1 - 200 / (400 - 100) or 0.33

So, in this case, a 10^o fall in ambient air temperature results in human idleness falling from 66% to 33%

And these figures of 'idleness' only indicate the fraction of time that someone is not doing any productive energy-acquiring work. But if someone keeps busy just so as to keep warm, that person might be said to be busy all the time. But, in many cases, it simply means that, in order to maintain thermal comfort, a human will have to restrict himself to the set of activities that are energetic enough to produce the required heat, such as walking, running, dancing, etc.

Clothing

Now let us suppose that, facing air temperatures gradually falling from an optimum of 27^o, instead of simply consuming more food and being more active, humans make insulating garments from wool or animal skins to wear.

The result will be second skin, an extra layer of insulation on top of body fat, which will reduce the rate of body heat loss. If this layer of clothing reduces heat loss from 10 W/^oC to 5 W/^oC, and with basal metabolic rate of 100 W, and body core temperature of 37^o, the ambient air temperature T that can be endured without discomfort is now - from equation (1) -

T = 37 - 100/5, or 17^oC

So as ambient air temperatures fall from 27^o to 17^o the owner of a coat will not need to keep busy to keep warm. When idle, metabolic rate need not be raised above 100 W.

And so where Pm = 100 W, Pi = 400 W, and Pe = 100 W, idleness is once again - from equation (2) -

I = 1 - 100 / ( 400 - 100 ) or 0.66

Except this doesn't take into account the cost of making the coat. If it takes a month to make the clothing, and it lasts a year before a replacement needs to be made, then 1/12th or 0.083 of each year needs to be devoted to making clothes, and this work must be subtracted to give a net idleness

I = 0.66 - 0.083 = 0.58

And if after air temperatures had fallen to 17^o, food also became harder to come by, and could only be acquired at the rate of 300 W for every 100 W expended in work, then in the case of an unclothed man

I = 1 - 200 / (300 - 100), or 0.00

So a 10 degree fall in air temperature, and only slightly scarcer food could reduce a 66% idle human to zero idleness, and to the threshold of death. But in the same conditions, the clothed man would do rather better:

I = 1 - 100 / (300 - 100) - 0.083, or 0.42

Given two strategies - one of eating more and being more active, the other of wearing clothes and being less active - in response to falling temperatures and growing food scarcity, the outcomes may be shown graphically as falling and rising idleness. One man (the red line) remains naked. His idleness falls from 66% to 33% when the temperature falls from 27^o to 17^o, and his idleness falls to zero when food becomes harder to gather, and he dies. The other man's idleness also falls from 66% to 33%, but he then makes a fur coat, and his idleness rises to 58%. And when food becomes harder to find, his idleness falls to 42%, and he survives.

The value of the coat is the idle time that it affords its owner. Before the coat was made, its owner's idleness was 33%. Wearing the coat, his idleness rose to 58%. And so the amount of idle time that the coat afforded its owner over its lifetime was (0.58 - 0.42) years, or 0.16 years.

But when food became harder to find, the coat became even more valuable. For without it, its owner would have an idleness of 0. Instead, he has an idleness of 42%. So the value of a coat becomes (0.42 - 0) or 0.42 years.

This assumes that it's cheaper to make clothes than to eat more food. But if food is easy to come by, another strategy to keep warm might be to add an extra layer of body fat in place of clothing, by eating more food but allowing it to be stored as a 'second skin' of fat.

Shelter

Yet another strategy that might be adopted in the face of falling ambient air temperatures is to construct a small enclosed shelter - a hut -, warmed by body heat, to spend idle time inside when not busy gathering food.

A hut of this sort would provide shelter not only against cold, but against wind and rain. The effect of wind is to increase the rate of heat loss from exposed skin (or clothing). And the effect of rain is also to increase heat loss, as rain evaporates from skin or clothing in the same way as sweat. In this sense, wind and rain act to increase body heat loss in exactly the same way as a cold environment. Keeping dry means keeping warm. A wet and windy environment may easily draw off as much heat from the human body as a cold, dry, windless environment.

If the thermal conductance of the walls was 2.5 W/m², and the hut was a cube of side 1 m, and the internal air temperature of the hut was Ta, then at equilibrium, the naked man would lose heat through his 2 m² of skin to the internal air at a rate

Q1 = 5 . 2 .(37 - Ta)

And the hut would lose heat to the 17^o outside air at the rate

Q2 = 2.5 . 6 .(Ta - 17)

And when Q1 equalled Q2, Ta = 25^o. Which is almost a comfortable temperature.

The hut might be small and cramped and airless, but life would be more comfortable inside it than outside.

It would perhaps be the work of only a few days, if there were enough loose stones and branches lying about, to construct such a small hut with stone walls, and a roof of branches and leaves. And the hut would probably have a longer lifetime than a garment like a coat, because it would not be subject to wear and tear in use. It might take only a week or so to build a shelter that might last for several years.

The unclothed man would spend all his idle time inside the hut, and only go out to energetically gather food, keeping warm by keeping busy. And so his idleness in respect of food gathering would again be

I = 1 - 100 / ( 400 - 100 ) or 0.66

But the additional cost of spending a week making a new hut every 5 years would entail extra work at the rate of 7/( 365 . 5 ) or 0.0027.

So the hut owner would enjoy an idleness of 0.66 - 0.0027, which is still just about 0.66, or 66%. And so, at an air temperature of 17^o, the owner of a 1 cubic metre hut will enjoy an idler existence than his rabbitskin-clad companion with a 58% idleness.

In general, the smaller the hut, and the nearer it becomes a 'skin-tight' fit like a garment, the nearer its required thermal resistance approaches that of a coat in the same conditions. The smallest hut possible would be a 'shallow grave' with an entrance at one end through which to crawl into it (which prompts the thought that when archaeologists find such graves, they may not have been graves at all). The bigger the hut, the thicker and more thermally resistant its walls need to be to achieve the same equilibrium temperatures. Or conversely the bigger the hut, the greater the need to heat it with something more than body heat.

Such additional heating might not entail lighting fires inside huts. A 1 Kg piece of limestone with a specific heat of 800 J/Kg^oC heated to 220^o in an external fire could hold about 160 kJ of heat to release into the hut. A 1 kg log of pine wood with a specific heat of 2.5kJ/Kg^oC would release about 500 kJ of heat as it cooled. Compare this to the 360 kJ of heat released by a resting man in one hour.

A larger hut would be more comfortable and airy, but would take longer to heat up with body heat alone. But this could be supplemented with a candle used as heat source rather than a light source. Beeswax contains about 50 kJ/g of energy, and if a candle burns 0.001 gms of wax per second, it will release 50 J/s or 50 watts, which is half the resting metabolic rate of a man. If the candle weighs 10g, its total energy content would be 50 kJ, released over about 100 minutes.

Another way to add extra heat is to consume hot food and drink. Given core body temperature of 37^o, a litre of hot water at 77 ^o. Water has a high heat capacity of 4.2 kJ/kgC and a density of 1000 kg/m3. A litre of water weighs 1 kg, and heated by 50^o above body core temperature will release about 200 kJ of heat as it cools to body temperature.

The above calculations are very far from exact. There is much more to body heat loss than is suggested. And also to the estimation of comfort. But they give an outline idea of the approach ttaken

Substituting Food, Clothing, and Shelter

Clothing and shelter may be regarded, up to a point, as substitutes for food, and substitutes for each other. In a cold environment someone may opt either to stay warm by keeping physically busy while naked, and consuming more food in the process, or they may opt to stay warm by wearing clothes or staying in warm shelters. The actual choice that anyone makes will depend upon the environment in which they find themselves, and on the relative availability of food, and the materials for making clothes or building shelters.

Shelters will be larger if they are made for families or for entire communities. And they will be larger if they are required to shelter tools and luxuries which may rust or rot outside. And they will be larger if there are varieties of work performed inside them as well as outside.

In cold climates, people may be expected to wear heavier clothes, and build more substantial and well-insulated houses. In warm climates, they may need little clothing, and only simple shelters which act more as shelters against rain and wind than cold. In desert regions, where there is little material for building shelters, humans may largely live outdoors as aborigines. And in warm tropical forests, the same may often apply.

This will also affect body shape and body fat and activity levels. In cold climates, indigenous people may tend to become more 'rotund' than in cold climates, to reduce skin surface area nearer to a sphere. And in cold climates, people may be expected to carry more insulating body fat than in warm climates. In cold climates people may be expected to be physically busier than in warm climates where unnecessary physical activity may result in body overheating.

What is 'right' is what results in maximum idleness in any particular set of environmental circumstances. There is no one size that fits all. All the variables are dependent on each other, so that as food intake decreases, clothing or shelter must increase.

It may even be that the supposed modern 'epidemic' of obesity is a perfectly natural response to living a relatively inactive life in a cold climate with abundant cheap food but expensive clothing and housing, with the result that the natural equilibrium is found where people don't wear much, live in underheated houses, and so carry a lot of thermally insulating body fat to make up for their underclothed inactivity. When food is cheap, getting fat may be the easiest way to stay warm. And there may also be positive feedback loops involved, whereby once someone has become obese, it becomes difficult for them to be active, and so they become more inactive, and more obese, and so on. And as they become more obese, it may become harder to dress adequately, because large size clothes may be hard to obtain, with the result that they become yet more obese.

And there may be nothing wrong with such a strategy. In a cold climate, it is better to be fat to be thin. And body fat is a store of future days of life, and is an indicator of prosperity.

The idea that there is an optimum 'athletic' body type and body weight, and an optimum level of physical activity, and an optimum food consumption, and an optimum clothing level, and an optimum house size, and so on, is a cultural prejudice no different from one which favours an 'optimum' skin colour.

Subjective Valuations of Food, Clothing, and Shelter

Food and clothing and shelter are also subjectively valued in ways that extend beyond their utilitarian value. They are very often desired in themselves, as luxuries. People often enjoy food for its taste and texture and appearance. And they like clothes and houses for their appearance, as fashion items and architectural statements.

And in many cases subjective values may take priority over any 'objective' values. In idle societies, in which food and clothing and shelter are easy to come by, the luxury value of foods as culinary experiences, and of clothing and footwear as cultural identifiers and fashion statements may take precedence over more utilitarian concerns of warmth and nutrition. They may even become entirely lost, as people starve themselves to become fashionably thin, or wear too little clothing for the same reason.

Summary

Food and clothing and shelter serve to maintain core body temperature at an optimum level, and to maintain continued life. And they are to a great extent interchangeable with each other. In a cold climate people may go naked, if they keep physically active and eat more food. Or they may wear clothes and live inside houses, being less physically active and eating less. In Idle Theory, the optimum balance is achieved when net idleness is maximized for any given environment. There is no single fixed 'best' combination applicable to all environments and all circumstances.

And the same sorts of considerations apply with heating, cooking, and a variety of other strategies for maintaining body temperature.