starvation

starvation The sight of the ravages of starvation in far too many parts of the modern world is all too familiar on the television screens of Western society. In our ‘developed’ countries starvation is encountered only occasionally: self-inflicted as an ultimate political statement by the hunger striker, or in the effort to lose weight, whether reasonably in the obese, or unreasonably by sufferers from eating disorders; or even more rarely in prolonged survival at sea or other circumstances of accidental isolation.

In normal circumstances, with an adequate and balanced diet, all tissues take up the required amounts of nutrients from the passing blood, in the form of simple sugars (mainly glucose), fatty acids, and amino acids. Tissues differ in the ratio of different fuels used: the brain and spinal cord, and also the red blood cells, can normally utilize only glucose. Excesses from the diet are converted to storage forms, mainly as lipid in fatty tissue and as glycogen in the liver.

Adults can survive for many weeks without food, provided they have water. For just how long depends partly on the extent of their body stores of nutrients, mainly fat. But unfortunately it is not only the fat which is broken down to simpler substances to be used for metabolic energy production and for essential repair and maintenance of the body's tissues. As soon as carbohydrate stores have run out, proteins are mobilized from muscles for the manufacture of sugars by the liver, causing progressive physical weakness.

The physiological priorities in the face of zero food intake no doubt evolved in early millenia when hunting and gathering was an unpredictable and variable source of food. The first priority is to provide the brain with glucose, which is its staple diet, and this requires a certain level of glucose in the circulating blood. The carbohydrate store in the form of liver glycogen is used first to provide this glucose but is used up in the first day or two. Then glucose has to be made from lactic acid and from amino acids derived from muscle protein, released into the blood and taken up in the liver.

But how do the various parts of the body involved ‘know’ that there is a state of starvation and ‘act’ accordingly? A fall in blood glucose directly affects the endocrine part of the pancreas to change the balance of its hormonal secretions, suppressing insulin and enhancing glucagon synthesis and release. A fall in blood glucose is sensed also in the hypothalamus in the brain, which is the co-ordinating centre for homeostatic processes — those which tend to maintain the body's status quo. This orchestrates a complex hormonal response and also switches on autonomic nervous mechanisms, which stimulate the release or synthesis of glucose in the liver; adrenaline secretion is increased from the adrenal medulla; and the anterior pituitary gland is brought into action, releasing growth hormone, and ACTH which in turn stimulates the release of cortisol from the adrenal cortex.

Glucagon promotes all the processes which tend to raise blood glucose — first its release from liver glycogen, then its synthesis from amino acids. Adrenaline, growth hormone and cortisol all promote mobilization of lipid from adipose tissue (fat stores) and the use of the resulting circulating fatty acids as metabolic fuel in preference to glucose — everywhere except the brain and the red blood cells, which do not have the necessary chemical apparatus to use them. In the liver, the predominant metabolic use of fatty acids produces the ‘ketone bodies’, acetoacetate and b-hydroxybutyrate, which circulate in the blood and can be used by other tissues for energy production. If starvation is prolonged even the brain is able to utilize these when glucose is seriously depleted. Some of the acetoacetate is converted to acetone — another ‘ketone body’ — mainly in the lungs, and this becomes noticeable on the breath. The blood becomes progressively more acidic.

In the end, of course, there can be no hope of maintaining life in the absence of food intake. Some of the defence mechanisms become counter-productive. Thus the necessary production of extra acid as a by-product of altered metabolism puts an additional load on the kidneys to deal with H⁺ excretion; the acidity also stimulates breathing, helpfully counteracting acidity by the excretion of more carbon dioxide, but imposing extra breathing work on a weakening body. Muscles waste away; oedema and accumulation of fluid in body cavities follows depletion of plasma proteins. Ultimately there is multi-organ failure from lack of fuel and loss of vital enzyme activity. Shortages of minerals and vitamins will also take their toll if starvation is not total, and hence attenuated enough for these deficiencies to take effect, but in terms of aid to starving communities, the urgent requirements are for the basic nutrients.

Sheila Jennett

See also acid base homeostasis; blood sugar; fasting; hormones; hunger.