The immediate response to a need for more calories is hunger for food. Feelings of hunger may be mild or intense and can range from slight irritation to extreme agony. Chronic hunger is a miserable experience, but the degree to which it induces mental misery depends greatly on whether the calorie deprivation is voluntary or forced. Although people with plenty of access to food may say “I'm starving” when they haven't eaten for a few hours, the difference between voluntary and forced hunger is so critical that scientific publications and newspaper articles often describe them in different words. Fasting refers to voluntary calorie deprivation, while the forced situations are called starvation or semistarvation. Regardless of what they call it, scientists know exactly what calorie deprivation does to human physiology and psychology. The effects depend entirely on how long the deprivation lasts and how extreme it is. People may feel hungry during the daylong fasts of Yom Kippur and Ramadan or longer “cleansing” fasts, but such short periods of deprivation rarely induce lasting harm. Once people start eating again, they quickly make up for the missing calories. Forced starvation, however, is deadly, and scientists know all too much about it. Over the years, they have repeatedly documented the effects of starvation, and in excruciating detail. During World War II, for example, a group of Jewish doctors meticulously recorded the effects of food deprivation on people imprisoned behind the walls of the Warsaw Ghetto, even going so far as to document the physiological signs of their own decline.1 Case studies of fasting individuals occasionally appear in medical journals. One described the physiological effects of a forty-day total fast undertaken under medical supervision by a cloistered member of a religious community. People who are obese and have large stores of body fat can remain alive for many months without eating. But people who are thin cannot. This difference also has been well researched and is especially known through accounts of hunger strikers who voluntarily refused food in order to achieve political goals. When people who are not obese consume nothing but water, they are able to survive for a remarkably consistent two months, give or take a week or so. In 1981, for example, ten men imprisoned in Northern Ireland starved themselves to death as an act of political protest. Their leader, Bobby Sands, stopped eating on March 1; he died 66 days later, on May 5. For the ten hunger strikers, the average time to death was 62 days, with a range of 46 to 73 days. Make no mistake: these were not easy deaths. As was evident from communications smuggled out of the prison, extreme calorie deprivation—even when voluntary —induces extreme physical suffering.
ANCEL KEYS'S STUDIES OF SEMISTARVATION
Even less extreme calorie deprivation is difficult to endure. This also is well known, not least from the extraordinary experiment in human semistarvation conducted by the Minnesota cardiologist Ancel Keys toward the end of World War II. In 1944 Keys and his colleagues convinced a large group of church, foundation, university, and government sponsors to fund a study of semistarvation. Its purpose would be to help understand how prisoners and ordinary citizens might respond to adverse food conditions such as those expected in Europe at the close of the war and how to rehabilitate those who were starving. The investigators recruited 32 conscientious objectors—all young, lean, and healthy men—who volunteered to lose 25 percent of their body weight in six months while under close observation and monitoring. Keys and his colleagues, in a 1950 monograph reminiscent of the prodigious output of Wilbur Atwater, published the results of these investigations in two volumes of more than 700 pages each.4 To induce weight loss, the investigators allowed the volunteers to eat about 1,600 calories a day in the form of a bland diet largely based on potatoes, designed to mimic what might be expected to be available in concentration camps under wartime conditions. In order to make sure that the men lost 25 percent of their initial body weights, the researchers reduced the already restricted calorie allowance even further over the course of the study. After six months the men had indeed lost a quarter of their initial body weights. A Life magazine photograph of five of them lying shirtless on the grass is a shocking sight. The men were emaciated, with prominent ribs and wasted muscles. Although they were mostly in their twenties, they looked middle-aged. This study examined every conceivable aspect of the physiology and behavior of the semistarved men in astonishing detail. To summarize:
* Basal metabolic rates declined by about 25 percent.
* Body weights fell rapidly during the first two or three weeks of food deprivation, by a pound or two a day. Weight loss then tapered off to one or two pounds a week.
* Body fat levels declined, but so did the size of all muscles and body organs.
* Bones and teeth were not affected. The men showed no signs of vitamin or mineral deficiencies.
* The men's skin became pallid and cold to the touch.
* Their muscles became weaker, their endurance declined precipitously, and they reduced all unnecessary movement. * They became lethargic, depressed, irritable, cold, and uninterested in sex but thoroughly obsessed with thoughts of food.
Some of these findings were expected. Calorie restriction was already well known to reduce the basal metabolism, thermic effect of food, and total energy expenditure of experimental animals. But other findings—the absence of vitamin and mineral deficiencies, for example—seem surprising. They are explained in part by the conditions of this experiment. The men had access to clean beds, hot showers, and soap, and the diet, uninteresting as it might have been, was balanced in nutrients. As body fat and muscles depleted, they released vitamins and minerals that could be used to keep other organs functioning. Keys stopped the experiment before permanent damage occurred, and the men eventually recovered from their ordeal. Decades later they were proud to have participated in a study that produced so much information about how the human body responds to calorie deprivation.
THE PHYSIOLOGY OF CALORIE DEPRIVATION
Bodies do not like being denied food. There is a good reason why hunger is so unpleasant. If you are deprived of calories, you want food, and you want it now. The Minnesota experiment makes sense from the standpoint of the thrifty-gene or the drifty-gene hypotheses. Humans evolved to survive when food was scarce. Times of deprivation may have been difficult, but they were survivable, and for years. Survival of semistarvation is possible because human physiology adjusts — and quickly — to compensate for inadequate food energy. From the standpoint of physiology, hunger is an emergency. The brain requires glucose—the sugar in blood—as fuel. Its need for about 100 grams of glucose a day (nearly a quarter of a pound) is a metabolic priority. Normal mixed diets contain enough carbohydrate to meet this need easily. Hunger is the first sign that the amount of glucose in blood may not be enough to keep the brain functioning. If hunger continues, the balance of regulatory hormones shifts to ensure that blood is continually supplied with adequate amounts of fuel—even when dietary carbohydrates are inadequate or absent. During prolonged calorie deficits, hormone levels shift even further to protect protein stores and preserve vital body enzymes, muscles, and organs as well as to reduce the body's energy requirements. These processes involve the actions and interactions of many hormonal regulatory factors. In the absence of adequate calories, blood glucose levels fall, and the balance of hormones shifts to cause body stores of carbohydrate (liver and muscle glycogen), protein (muscles), and fat to break down into their constituent molecules—glucose, amino acids, and fatty acids—which can be metabolized for energy. It is easiest to explain what happens during food deprivation by separating its effects into stages of acute starvation, when people take in nothing except water. The stages overlap considerably, and the times given are approximate. The duration of each stage depends on the proportion of energy needs met by food intake. In semistarvation, when people get some food but not enough to meet energy needs, the stages are prolonged. The Minnesota experiment, for example, gave the volunteers just barely enough food to maintain them in reasonable health while they were semistarved. If the men were not losing weight rapidly enough, the investigators reduced their rations.
First Few Hours - Glycogen Breaks Down to Glucose
When you are asleep and not eating, your brain still needs glucose. Enzymes break down the glycogen in your liver to glucose. The liver usually stores enough glycogen to take care of glucose needs for relatively short periods of fasting—while you are sleeping, for example.
Overnight or during the First Day - Depletion of Liver and Muscle Glycogen
Once liver glycogen is well on its way to depletion, the regulatory system goes into action, and the balance of hormones shifts to promote disassembly of the glycogen in muscles. Body proteins and fats break down to amino acids and fatty acids to be used for energy. This process also releases the parts of amino acids and the glycerol part of fat that can be used to make glucose. Because glycogen binds up to four times its weight in water, its breakdown releases that water. On average, people store a pound or more of glycogen in muscles and liver. The immediate weight loss that occurs with brief periods of fasting can be accounted for by the loss of glycogen and its associated bound water.
First Few Days - Breakdown of Body - Proteins and Fats
As the total fast continues, hormonal shifts become more pronounced. They signal body proteins and fats to break down more quickly to produce amino acids and glycerol that can be used to make glucose. They also signal the kidneys to excrete salt. The excretion of salt is accompanied by the excretion of water, which is another reason why early weight loss occurs so rapidly. People who are fasting can lose a pound or two a day during this period, but most of the loss is explained by excreted water.
After Several Days - Ketones Replace Glucose as Fuel for the Brain
The increasingly rapid breakdown of body proteins to provide the makings of blood glucose cannot go on for long without destroying muscle and enzyme function and causing serious harm. To preserve life, vital body proteins must be conserved. But so must fuel for the brain. Here we have a physiological dilemma. Most of the stored energy in the body is in the form of fat. Fat triglycerides break down to fatty acids just as rapidly as protein breaks down to amino acids, but except for their glycerol portion, triglycerides cannot be used to make glucose. Neither can the fatty acids. Glycerol converts rapidly to a compound that can be used to make glucose (two glycerols form one molecule of glucose). But the amount of glucose that can be made from the glycerol part of fat is not nearly enough to meet the brain's daily energy requirements, even when combined with the glucose made from parts of disassembled amino acids.
This unfortunate problem—you can make fat from glucose, but you can't make enough glucose from fat—has a solution. When blood glucose is falling and fat is breaking down rapidly, the metabolic reactions that produce energy are too slow to keep up with the influx of fatty acids released from the breakdown of fat. The reactions are swamped with surplus fatty acids. To relieve these metabolic pressures, enzymes in the liver and kidney convert the surplus fatty acids into compounds called ketones. As ketone levels rise in blood, the brain gradually adapts to using them for fuel.
After a Week or So - Conservation of Body Proteins
Once the brain substitutes ketones for glucose, the breakdown of body fat becomes the primary source of energy. Body proteins also continue to break down, but more gradually, thereby conserving vital muscle functions such as those of the heart and diaphragm (the muscle that controls breathing). The intestinal tract, having nothing to do, begins to shed its lining of villi, the cellular structures where digestion and absorption take place. This leads to further weight loss. At this stage, weight loss increasingly reflects losses of body fat and protein.
After One to Three Weeks - Reduction in Basal Metabolic Rate
The weight loss—and the continued shifts in the balance of hormones—slows down the body's energy requirements. The result is that it takes less energy to support body functions. The basal metabolic rate drops in direct proportion to the loss in body weight. Weight loss slows. Most of the weight loss derives from body fat, but small amounts of protein continue to be lost from all organs. In prolonged semistarvation, blood pressure falls, and people feel nauseated, become dizzy when they stand up, and do everything they can to avoid unnecessary movement. The Minnesota volunteers, for example, were deeply lethargic and could barely manage the tasks they were assigned. Few voluntary fasts last beyond this stage, as the risk of death increases rapidly from then on.
After Four Weeks or So - Depletion of Body Fat
Fasting or starvation can go on for only so long. At some point—sooner in leaner people but later in people with more body fat—fat stores run out and body proteins are all that remain to draw on for body energy and brain fuel. Once that stage is reached, all body systems collapse. The Irish hunger strikers became dangerously ill. Their wounds would not heal. They became blind and delirious. Eventually, they lapsed into coma. Once the heart and diaphragm muscles become weak, starving people are no longer able to clear fluids from the lungs. This makes them increasingly susceptible to lung infection, explaining why pneumonia is so often the cause of death of undernourished people.
RECOVERY FROM STARVATION
One relatively early effect of calorie deprivation is the reduction in size and complexity of the lining of the digestive tract. Ordinarily it is lined with villi, tiny threads of cells that produce digestive enzymes. These form a thick carpet with an enormous surface area for digesting and absorbing nutrients. Villi are especially sensitive to food deprivation. Without food, they atrophy and stop producing digestive enzymes. This reduces the surface area of the entire length of the digestive tract, making it increasingly difficult to absorb food molecules. Starving people cannot digest or absorb food very well. If they eat beyond the absorptive capacity of the intestine, unabsorbed molecules pass into the large intestine, where bacteria ferment them, producing gas and diarrhea. Refeeding must take place slowly and gradually until the intestinal tract is fully functional. Even when permitted to consume thousands of extra calories a day, the men in the Minnesota experiment took many weeks to gain back the weight they had lost. They had much repairing to do.
By Marion Nestle & Malden Nesheim in "Why Calories Count" - From Science to Politics, University of California Press, USA, 2012, p. 197-204. Adapted and illustrated to be posted by Leopoldo Costa.
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