13.1: Basal Metabolism

The amount of gas your car uses while idling at a stop light can be likened to how much fuel your body “burns” for basal metabolism. It’s the calories you use when you haven’t eaten for several hours (you are done “processing” food) and are lying down in a comfortable setting, relaxed and awake. (You need less energy when asleep. But when you are awake, it’s easier to make this measurement, e.g., wearing a gas mask to measure how much oxygen—thus how many calories—you are using.)

Unlike an idling car, however, basal metabolism makes up the bulk of our caloric requirement. This is contrary to the popular belief that most of the calories we use each day is for physical activity. If you are sedentary, basal metabolism makes up about two thirds of your energy requirement. It is about half of your energy requirement if you are extremely active—a laborer, football player in training, etc.

Except for pregnant or nursing women,* the calories adults need for basal metabolism depend mostly on body size and body composition. A football lineman has a very high basal metabolism. He is not only big, he has less fat and more lean tissue than most of us. It takes more energy to maintain lean tissue. Figure 13.1 shows how basal metabolism goes up with higher body weight and lower body fat.

Another factor, not taken into account in Figure 13.1, is body surface area (basically, the amount of skin you have—not counting wrinkles). At the same body weight and composition, someone tall and thin has more body surface area and a higher basal metabolism than someone short and wide. A bigger surface area means more body heat is lost, requiring a higher basal metabolism to maintain body temperature.

Figure 13.1: Basal Metabolism in Adults

Think of a cup of hot coffee—it cools faster in a tall, thin cup than a short, squatty one. Or note that when you are cold, you curl into a ball— reduce heat loss by reducing the exposed body surface area. When you are hot, you sprawl out to dissipate body heat. Amputees feel warmer than we do because they don’t lose heat as fast (arms and legs have a lot of surface area relative to their weight).

Factors that affect body size and composition affect basal metabolism. Women generally have a lower basal metabolism than men; they are usually smaller and have a higher percent body fat. Thus, gender affects basic energy needs.

^{*Pregnancy and breastfeeding raise basal metabolism. The RDA (Recommended Dietary Allowance) for pregnancy is an added 300 calories/day during the 2nd and 3rd trimesters. Extra calories aren’t usually allotted during the 1st trimester unless the woman is undernourished. The RDA for nursing mothers is an added 500 calories/day.}

Age

Basal metabolism goes up as we grow in size from infancy to adulthood. But if we look at the calories needed per pound of body weight, it’s highest in infancy. One reason is that growth is greatest during infancy. Another reason is that the brain makes up a large portion of an infant’s body, and uses a lot of energy compared to other organs.† The brain uses about 45% of a 20-pound infant’s basal metabolism, but only about 20% for a 155-pound man.

Starting at about age 30, basal metabolism falls about 2-3% per decade, mostly from loss of lean tissue. As people age at a given body weight, the amount of lean tissue falls and the amount of fat goes up (people incorrectly say muscle is turning into fat). This loss of lean tissue is thought to be partly from an age-related fall in growth hormone.*

In practical terms, this means that even if we keep the same weight and physical activity, we need fewer calories as we age because of the decrease in lean body mass. Thus, if we continue to eat the same number of calories and don’t become more active, we gain weight.

Americans gain about 20 pounds, on average, between age 25 and 45. But part of this weight gain is attributed to a decline in physical activity that often occurs during this time (physical activity uses calories and helps maintain muscle mass).

^{†You might think: If the brain needs so much energy, I must really burn a lot of calories when studying. No such luck. Your brain is very active whether studying or not, so studying doesn’t significantly raise its energy use. But if you are tense, pace the floor, chew pencils, or shake your leg while studying, these accompanying activities do burn calories.
*In one study, growth hormone was given for 6 months to 21 men, ages 61-81, selected for their very low levels. This resulted in more lean tissue, less body fat, and increases in skin thickness by amounts comparable to that lost during 10-20 years of aging. Such studies are made possible by the availability of human growth hormone made by biotech (Chap. 10).}

Hormones

Female hormones fluctuate and affect basal metabolism. Basal metabolism starts to rise around the time of ovulation and peaks just before or at the start of menstruation. Then it falls and stays at that lower level until mid-cycle when ovulation occurs again. This fluctuation is reflected in a fluctuation of body temperature.

Thus, a woman can find out when she ovulates by taking her temperature before getting out of bed every morning to look for the rise in body heat (from an increase in basal metabolism) associated with ovulation. Remember that a higher basal metabolism uses more calories. The more calories you use, the more heat you produce. Think of how warm you get when you exercise—it’s from using so many calories.

A woman’s appetite often reflects these changes during the menstrual cycle. Many find they are hungrier and eat more during the last part of the menstrual cycle, and their appetite lessens and they eat less when menstruation begins. Body water also fluctuates during the cycle, generally paralleling the fluctuation in basal metabolism.

Thus, women shouldn’t be unduly concerned with increased appetite and weight late in the cycle. The increased hunger is from an increased use of calories, and the extra water will be lost early in the cycle.

Sex hormones can also affect basal metabolism indirectly. The male sex hormone testosterone promotes an increase in muscle mass, and more energy is then needed to maintain this added tissue.

Thyroid Hormone

The hormone that has the biggest effect on basal metabolism is thyroid hormone (thyroxine), made by the thyroid gland. Either an excess or a deficiency of thyroid hormone has ill effects, and our body has an elaborate system for keeping levels within a narrow range. Excess amounts raise basal metabolism—much like a car with its idle set too high—and inadequate amounts lower it.

Iodine is an essential part of thyroid hormone, and iodine concentrates in the thyroid gland for making the hormone.* If there isn’t enough iodine in the diet, the gland can’t make enough hormone, and it enlarges in its attempt to make more.† This enlarges the neck, because the gland is there.

An enlarged thyroid gland from a lack of iodine is called simple goiter. Until 1924, when iodized salt (iodine added to salt) was introduced, simple goiter was common in the U.S. During World War I, the neck sizes of the men drafted from all parts of the U.S. reflected the geographical distribution of goiter.

The effect of simple goiter depends on the extent of the iodine deficiency and when it occurs. Severe deficiency during fetal life can cause physical and intellectual disability (cretinism), whereas a slightly enlarged thyroid gland may be the only noticeable effect of a mild deficiency that occurs as an adult. Intermediate symptoms include sluggish mental activity and weight gain and puffiness due to abnormal retention of water in tissues.

Iodine deficiency is still a worldwide problem, especially in low-income countries. Worldwide, an estimated 2 billion people don’t get enough iodine. In inland China, many people are intellectually disabled from iodine deficiency.

Seafood is the most consistent source of iodine (sea water is rich in iodine). As long ago as 3000 B.C., the Chinese used burnt sea sponge to successfully treat simple goiter, even though they didn’t know what caused it. Sea sponge has iodine, which, as a mineral, isn’t destroyed when the sponge is burned.

Inland, the iodine in food reflects the amount of iodine in the soil. As might be expected, goiter was least common in areas next to the sea, not only because seafood is available, but because soil next to the sea is rich in iodine. In fact, one method used today to combat iodine deficiency in some countries is to put iodine in the water used to irrigate food crops.

Iodine deficiency is uncommon in the United States. Our main sources of iodine are iodized salt, dairy products, and seafood.

Goiter, by definition, is simply an enlargement of the thyroid gland. The gland not only enlarges in its attempt to make more hormone when there isn’t enough, but it can also enlarge from making excessive amounts. Once the thyroid becomes enlarged, for whatever reason, it doesn’t shrink back to a normal size after the cause is corrected.

There are many causes of goiter. Worldwide, iodine deficiency is the most common cause. There are also other dietary causes of goiter. Ironically, large excesses of iodine can hamper the production of thyroid hormone and cause goiter (another example of too much or too little as a problem). In Japan, some people get goiter from regularly eating large amounts of iodine-rich seaweed.

Another dietary cause of goiter is eating a lot of foods that contain anti-thyroid-hormone substances called goitrogens. This can lower thyroid hormone and cause goiter. Goitrogens act in different ways. Some interfere with production of the hormone; others cause excessive loss.

Goitrogens are found naturally in a wide variety of foods, e.g., cabbage, cassava, turnips, soybeans, and peanuts. There are also some synthetic goitrogens, such as the antibiotic tetracycline, sulfa drugs, and industrial PCBs (polychlorinated biphenyls). Goitrogens only cause goiter when people ingest excessively large amounts over a long time.

Goitrogen-induced goiter is found where goitrogen- containing food is a large part of the diet, e.g., cassava in some tropical countries. But goiter doesn’t always develop with a high goitrogen intake if the diet is ample in iodine. In many countries, the key factor is the relative amounts of goitrogens and iodine in the diet.

There are interesting variations in this balance between dietary iodine and goitrogens. Two neighboring communities in Venezuela were found to have a 10-fold difference in the occurrence of goiter. Both communities got very little iodine, and it appeared that a higher amount of dietary goitrogens was the determining factor.

Excessive thyroid hormone: There are no known dietary causes of excessive thyroid hormone. Iodine deficiency or an excessive amount of iodine or goitrogens causes too little rather than too much hormone. The causes of excessive hormone production (hyperthyroidism) aren’t well understood, but the most common types may be due to a dysfunction in the immune system.

As said earlier, excess thyroid hormone speeds metabolism, like a car with its idle set too high. Symptoms include nervousness, sweating, hypersensitivity to heat, rapid heartbeat, and fatigue (hyper is slang for high-strung or keyed-up, and aptly describes the revved up state of hyperthyroidism).

Basal metabolism can double or triple (a car uses more gas when its idle is set too high), and despite a bigger appetite, weight loss is typical.** To treat hyperthyroidism, part or all of the thyroid gland is removed by surgery or destroyed by giving radioactive iodine. Thereafter, thyroid-hormone pills are taken to achieve normal levels.

^{*Radioactive iodine is released in nuclear accidents like that in Chornobyl in 1986. The World Health Organization recommends that [non-radioactive] iodine pills be available to children living near nuclear power plants, to take in case of an accident. Taking an iodine pill before exposure saturates the thyroid gland, so that very little radioactive iodine will be taken up (moderate excesses of iodine, radioactive or not, are excreted in the urine). Children are especially vulnerable. After the Chornobyl incident, an epidemic of thyroid cancer occurred among children in Belarus (just north of Chornobyl).
†When thyroid hormone is low, a hormone called thyroid-stimulating hormone stimulates the thyroid to make more. When there’s a shortage of iodine, the thyroid simply can’t make enough hormone. Prolonged stimulation by thyroid-stimulating hormone causes the gland to enlarge.
**Former First Lady Barbara Bush lost 18 lbs in 3 months without even trying—she said she should have known something was wrong! She had Graves’ disease, a common type of hyperthyroidism (her husband was found to have it also). It seems that receptors on the thyroid gland for thyroid-stimulating hormone (see earlier footnote) are mistakenly seen as foreign, and the body makes antibodies against them. These antibodies are unusual in that they themselves stimulate the thyroid gland, causing excessive hormone production. The prolonged stimulation causes the gland to enlarge (goiter). Protruding eyes, like hers, is a common symptom.}

Miscellaneous Factors

Scientists measure basal metabolism under standardized conditions (e.g., lying relaxed in a comfortable environment), because many factors can alter the amount of energy used, even when a person is resting. Some examples:

Fever: The higher body heat of a fever causes metabolic reactions to speed up—just as heat speeds up chemical reactions in the laboratory or kitchen. For every 1°F rise in body temperature, basal metabolism goes up about 7%. This means that if you are lying in bed with a fever of 103°F, your calorie need would be about 30% higher.

Prolonged Food Deprivation: Basal metabolism falls when adults don’t get enough calories for a prolonged period. This adaptation is good in a famine—it lets you live longer. But dieters are unhappy with this. They are purposely subjecting themselves to prolonged food deprivation in trying to lose weight. In children, famine doesn’t lower basal metabolism. Instead, growth is stunted, an adaptation to prolonged food deprivation. Smaller bodies need fewer calories and nutrients.

When people undergo repeated cycles of feast and famine, their basal metabolism falls faster during famine. This adaptation is helpful to the many people worldwide who involuntarily cycle between feast and famine. But for the many regular dieters in this country, this is disheartening.

Mental State: Being excited, nervous, upset, stressed, etc., uses calories. Hormones like adrenalin—the “fight or flight” hormone—speed the rate of metabolism. Also, things like increased muscle tension uses calories, and we have a lot of muscle that we can “make tense.” On the other hand, meditation or self-hypnosis can lower energy needs by about 15% (blood pressure falls, breathing is slower, etc.). Meditation is, in essence, the opposite of being tense.

Environmental Temperature: Greater or lesser loss of body heat affects our energy needs. When we are in comfortable surroundings, it doesn’t take as much energy to maintain normal body temperature. But even at a comfortable temperature, we differ in how much body heat we lose. As noted earlier, body shape—body surface area—makes a difference. Body fat also matters, because the fat stored in a layer just under the skin is a good insulator. A lean person loses more body heat than a fat person of the same weight—just as hot coffee cools faster in a paper cup than in a styrofoam cup.

In fact, getting rid of excess body heat is often a problem for obese people in hot weather or during even moderate exercise. Their body shape and insulation minimize heat loss. In contrast, the lean and thin physique of marathon runners maximizes heat loss, which can be crucial to winning in warm weather. Recall that heat is a byproduct of ATP production. In running a marathon, a lot of ATP and heat is generated over an extended time. Runners need to get rid of excess heat as fast as possible to avoid heat prostration.

If we are too cold or too hot, it takes more calories to maintain normal body temperature. Like a house with a thermostat, the heater or air conditioner goes on if it is too cold or too hot, and both appliances use energy. One of the functions of the circulatory system is to distribute body heat (Chap. 7). By blood going in or out of capillaries in the skin, more or less blood contacts the cooler body surface and the body can adjust the amount of heat lost.

We sweat when hot. Evaporation of sweat cools the skin and the blood in the skin’s capillaries. (When hot—or embarrassed—capillaries in our facial skin fill up and our face “turns red.”) Dry weather or a breeze evaporates sweat faster, and we feel cooler. (A cool breeze is actually a cooling breeze.) We feel hotter when the weather is hot and humid, because the humidity slows the rate of evaporation.

We shiver when cold. Blood moves out of the capillaries in our skin to minimize heat loss. We get goose bumps, meant to stand our once-abundant body hair on end—a thicker and warmer fur coat. We take action ourselves—put on more clothes (insulation) and wrap our arms across our chest or curling up in a ball to lessen the body surface exposed to the cold.

Interestingly, there’s a special type of fat called brown fat that, unlike regular fat, is geared to produce heat.* We have it in only small amounts and in specific parts of the body, like around the neck and kidneys. Brown fat cells have lots of mitochondria and have a rich blood supply, both of which give them their distinctive brown color.

They work like miniature heaters that burn fat in response to cold-exposure. Blood picks up the heat as it circulates through brown fat and then circulates that heat to the rest of the body.

It appears that we are born with a limited amount of brown fat, which lessens thereafter, unless we are chronically exposed to a severely cold climate—“use it or lose it?” Complete loss of brown fat has been reported in adults and children who have died of cold-exposure.

Smoking: Smoking raises basal metabolism, one reason why people often gain weight when they quit (smoking is far worse for health than being moderately overweight). Concern with this weight gain can make it even harder to quit smoking; it’s already very hard because of nicotine addiction. In this country, women have a harder time quitting. This may be due to the greater emphasis they put on thinness (also, depression is more common in women, and nicotine may help alleviate depression).

^{*The other more abundant and familiar type of fat (white fat) is mostly in a layer under the skin (and as “belly fat”). The main function of this fat is storage of “excess calories” as triglycerides (which are white). When ATP energy is needed, fat cells release fat into the blood, so that other cells can use it to make ATP. In essence, white fat is for ATP production at other sites, and brown fat is for “on site” heat production}