13.6: Summary

The energy our cells need comes from the food we eat. Food energy is measured in calories. The body releases food energy (burns calories) through the various reactions of metabolism. Unless you’re pregnant, nursing, or growing, the total energy requirement is the sum of what is needed for: (a) basal metabolism, (b) physical activity, and (c) incorporation of food into the body.

Basal metabolism is what’s needed for the body to function at its most basic level (breathing, maintaining tissues, etc.). It’s measured under standard conditions—when a person is lying down, hasn’t eaten for several hours, is relaxed, comfortable, and awake.

Basal metabolism accounts for about two thirds of the total calorie requirement of sedentary people and about half the requirement of “strenuous athletes.” Aside from genetic variations, basal metabolism varies mainly with body size and composition. A large body needs more energy to maintain the greater mass, and lean tissue uses more energy, since it’s more metabolically active.

Women have lower basal metabolism than men because they’re smaller and have proportionately less lean tissue. During pregnancy and nursing, calorie needs go up to provide for the fetus and the making of breast milk. Relative to body weight, infants have the highest resting metabolism, due to their rapid growth. Basal metabolism falls as growth slows and also falls as we age, since aging causes a reduction in lean body mass.

Some hormones, especially thyroid hormone (thyroxine), affect resting metabolism. Basal metabolism falls if there isn’t enough thyroid hormone. This causes symptoms like sluggish mental activity, weight gain, and water retention. Iodine is a vital component of thyroid hormone. Without it, the thyroid can’t make enough hormone.

Goiter—an enlargement of the thyroid gland —has many causes, but the most common cause, worldwide, is a deficiency of iodine in the diet. Excessive thyroid hormone is also a possible cause of goiter, but there are no known dietary causes. Excess thyroid hormone raises basal metabolism (speeds metabolism). This can cause nervousness, sweating, rapid heartbeat, fatigue, and weight loss.

Part of our basal metabolism is for maintaining normal body temperature. We always lose some heat to our surroundings. A tall, thin person loses more than a shorter, wider person of the same weight, and thus has a higher basal metabolism. A thin person has more skin surface and less fat as insulation—an advantage in endurance events where heat generated by the prolonged activity must be dissipated rapidly.

Physical activity can use a lot of energy, and gives us the most control of our energy needs. The amount of energy expended in a given activity is largely dependent on body weight. The more a person weighs, the more energy it takes to move. Calories needed for physical activity can be estimated as a percent of what you need for basal metabolism (since this also reflects body weight).

Digestion, absorption, and assimilation of food into the body uses about 5-10% of the calories we eat. This can make us feel warm after we eat.

The body is well-adapted. For more efficient regulation of body temperature, people from hot climates tend to be thinner, and people from cold climates tend to be fatter. When food is scarce, basal metabolism falls, and we tend to be less physically active. Those who store more calories as fat when food is plentiful survive longer in a famine.

Obesity is epidemic in the U.S. We have an abundance of appealing food combined with less physical effort needed for routine activities. Our genetic heritage includes a compelling urge to eat when body fat falls, an adaptation to food scarcity that’s frustrating to people trying to lose weight.

There are genetic forms of obesity, where the genes dominate, but for most us, obesity is caused by a more even mix of genes and environment. The cause of obesity is an excess of calories from not burning enough and/or eating too many, so the difficult remedy is to do the “unnatural”—exercise more and eat less.

Many discoveries, including that of the hormone leptin made by fat cells, raise the hope that there will soon be more effective drugs to treat obesity.