This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
When a man is deprived of food the supply of things which can bo oxidized in his body is cut off. The tissues and organs are used up and not renewed ; his temperature falls, his muscles become weaker and weaker, and at last he dies. The body does not live, and work, and keep warm, by means of a peculiar vital force or energy which inhabits it, but by utilizing the energy set free in it by the oxidation of foods, or of things made in it from foods. If the food supply be cut off, the body first uses up any reserve of nutritious matter which may have been stored up in it when the starvation commenced, and as this is expended it becomes weaker and weaker until death supervenes. *
What must we conclude from the fact that our bodies keep at nearly the same temperature all the time? How do we know that they generate heat? Give a reason for taking food, in addition to its use as a source of energy to be spent by the muscles.
What happens when a man is starving ? How does the body live, and keep warm; and work? What first happens when the food supply is stopped?
How long a man, totally deprived of food, can keep alive, will depend, partly, on how much reserve material, capable of oxidation, he has stored up in him when the starvation period commences; but largely, also, on the extent to which he can spare himself muscular work and loss of heat. The breathing movements and beat of the heart must go on, but if the individual lies quiet in bed he need do little or no other muscular work ; and if he is well covered up with blankets, the loss of heat from the body is slight and calls for but little oxidation of the tissues to compensate for it. † Also, a fat person will survive starvation longer than a lean one ; during the process his fat is slowly burnt; but so long as it lasts he can supply his muscles with something which can be oxidized to yield working power, and he also, by its burning, can maintain his temperature. Fat is, in fact, a sort of reserve fuel, laid up in the body, and a man, in the strict sense of the word, can hardly be said to begin to starve until his fat has nearly all been used up. ‡
Upon what does the length of life of a man getting no food depend? What expenditures of energy must go on all the time? How does lying in bed diminish the expenditure of energy? Why will a fat man deprived of food live longer than a lean one?
When does a fasting man really begin to starve?
* When warm-blooded animals are starved their temperature slowly falls; and when it comes down to about 77° F. (25° c.) death occurs: the various tissues at that temperature can no longer work so as to maintain life.
† Hence Dr. Tanner, and " fasting girls " keep in bed, warmly covered up, most of the time: the losses of the body in mechanical work and heat are thus reduced to a minimum, and consequently the oxidation of the food reserves stored in the body at the beginning of the fast.
‡ Some warm-blooded animals, as bears, hibernate; that is, sleep all through the winter and take no food. They feed well in the warm weather, and are quite fat at the close of autumn, when they seek some sheltered place to winter in. This shelter and their warm, furry coats make the loss of heat very little; the animal, except for its breathing and the beat of its heart, hardly ever moves during the winter, and even those necessary movements are reduced to the least possible, the breathing and heart-beat being much slower than daring the summer. With return of warm weather the creature wakes up again, but is then lean, having burnt up its fat during its winter sleep.