Part 5

Energy, Calories and Body Weight

Life is enabled by the controlled acquisition and utilization of energy. Green plants obtain energy from sunshine through photosynthesis and nearly all other life including human life obtains energy either by eating plants or by eating other life that has ultimately obtained its energy from plants. In this sense, all food is solar energy.

Living things store energy in chemical bonds: the tiny and invisible "springs" that hold individual atoms together in assemblages that are called molecules. Mammals including humans store most of their energy in the chemical bonds of molecules called fats and those fats are stored in specialized cells called adipocytes (fat cells).

People store fat energy in order to survive famine and since starvation rather than excess has been the normal condition for most of history, they are very efficient at fat storage: nearly all energy that we consume above what we need is saved by making and storing fat. Unfortunately human fat storage is often a process without an "off" switch. This is the ultimate cause of obesity.


A calorie is a specific amount of energy. A dietary calorie is the amount of energy needed to raise the temperature of one liter of water by one degree centigrade.

A pound of pure fat (body fat or dietary fat) contains about 3,200 calories.

Energy Balance

Body fat accumulates when we consume more energy than we burn, and is lost when the reverse is true.

Another way of saying this is that body weight is controlled by the balance of calorie intake and calorie output an axiom so old and basic that nearly everyone has heard it.

Energy Intake

It is easy to understand where dietary calories come from: they come from food, specifically from the chemical bonds in the molecules contained in food. Not all of the molecules in food contain usable energy however. For example, we cannot extract energy from water or from most "fiber". Calories can only come from food, but not all food contains calories.

Energy Output:

Where the energy "goes" is not as easy to describe as where it comes from.

Basal Metabolism:

Most of the body's daily energy use is for basal metabolism which is defined as the background activity of the body that is needed to sustain life and includes cellular metabolism, brain activity and so on.

Thermic Effect of Food:

 Digestion burns calories, more calories than many people realize, and this energy expenditure is called the thermic effect of food (or specific dynamic action of food)


Movement and activity require muscular contraction which in turn is energy driven. Exercise generally consumes less energy than many people realize.


Heat production is often grouped with basal metabolism. In fact heat is an inevitable by-product of metabolism, but under special conditions, the body can be driven to burn calories for no specific metabolic reason but simply to produce heat. The conditions that drive this sort of heat production are generally ones which produce a drop in the body's core temperature; very cold air or moderately cold water.

Malabsorbtion: Digestion is normally very efficient but some medical conditions and certain drugs can cause energy-containing nutrients to pass out of the body undigested. Whether this is energy "output" or a failure of energy "input" is a semantic question.

Control of Input and Output:

Science demonstrates that in most people calorie intake and output is not accidental but in fact very carefully controlled by hormonal and neurochemical systems in the body. Most of these systems are complex and remain poorly understood but they are the focus of nearly all the basic and medical science of human obesity. We will discuss them more later.