hearts, lungs, brains and the like . . . the less energy will remain in our energy stores.
for example, or thumb leisurely through the pages of the local newspaper. The more energy we use to support the work of our hearts, lungs, brains and the like (in other words, the higher our
BMR), the less energy will re- main in our energy stores. As always, the energy we use for the needs of our resting bodies is derived from food. If food
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is not consumed, either intentionally (for example, during an overnight fast or during an attempt to lose weight) or unintentionally (if one is starving, as a result of either food deprivation or another disease), the energy to provide for the basic functions of the body is mobilized from the energy stores. This energy is initially derived from glycogen ( carbohydrates stored in the liver and in muscle), which runs out fairly quickly, and then from fat (stored as fat tissue), which lasts longer.
Now that we know that the energy to support the life of an organism is derived from foodstuffs, let us briefly review how that actually happens. How does my slice of pecan pie convert into the energy I spend moving from the sofa to the refrigerator?
Dr. Hans Krebs, a scientist who received a Nobel Prize in 1953 for his discoveries of the major steps in the biochemistry
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of energy production, elegantly divided this process into three stages. Thanks to food labels, everyone now knows that the items we consume are composed of three major nutrients (as we call them, ‘‘macronutrients’’): proteins, carbohydrates, and fats. In the first stage of energy production from food, large molecules are broken down into smaller units. Proteins are reduced to amino acids; large carbohydrates are converted into simple sugars, such as glucose; and fats are broken down into glycerol and fatty acids. Even though no energy is generated at this point, this is the critical preparatory step as only these simple molecules can be used to generate energy at stages two and three. Certain medications that we use to prevent weight gain or to ameliorate diabetes work specifically at this stage.
For example, acarbose interferes with the breakdown of complex carbohydrates into simple sugars, thereby retarding and diminishing the absorption of carbohydrates into the bloodstream.
In the second stage of energy production from food, these smaller molecules enter various cells, and most of them are
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further reduced or degraded into a very few simple units that enter the mitochondria—the energy-making factories of the cells of our bodies. Although some tissues, such as heart tissue, prefer using fatty acids to generate energy, in most cells of the human organism a healthy competition exists between fatty acids and glucose for the privilege of being burnt for the sake of producing new cellular energy.
Stage three is the real factory for production of energy.
Remnants of sugars and fats are burnt in the energy-producing furnace to generate energy that is stored in high-energy compounds known as ‘‘adenosine triphosphate,’’
abbreviated ATP. The ATP molecules function like a battery, supporting the life of each cell in the body.
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Carbohydrates, the most abundant source of calories in the human diet, are present in both plant and animal products, and they are easily broken down into simple sugars for speedy absorption. Carbohydrates are an excellent source of quick energy, with 1 g of carbohydrate providing four calories. Excess carbohydrate is readily stored in the liver and muscles in a form of glycogen that is also easily broken down into single molecules of glucose for quick utilization.
Proteins are built from twenty-two amino acids, eight of which can only be obtained from food. Because these eight amino acids cannot be produced in our bodies, they are termed ‘‘essential.’’ The richest source of protein is meat, which, in combination with milk, cheese, and eggs, provides all eight essential amino acids. Many plant foods also contain substantial amounts of protein. Although, like carbohydrates, a gram of protein provides approximately four calories, proteins are rarely used to cover energy needs.
They are much more suitable as building materials, to create new proteins in muscle, and everywhere else.
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Fat is the most significant metabolic fuel we have. One gram of fat provides nine calories, twice as many as carbohydrates or proteins. Therefore, in terms of acquisition of energy, fat is the most efficient source. It is particularly important for tissues that use great quantities of energy in their work. These tissues are skeletal muscle and heart muscle. For absorption, dietary fats are broken down into single fatty acids and glycerol. In the bloodstream, they travel throughout the body and are either used as a source of energy or deposited in storage, in fat tissue. Approximately 85% of the body’s energy is stored as fat.
How is this relevant to what, and how much, we eat? How is it related to our ultimate weight? Or to our ability to lose weight? The answers are complex, but they are directly and
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critically relevant to the regulation of body weight.
According to the law of conservation of energy, if we are to maintain our weight, the energy we generate during these three stages must be equal to the energy we ex-
pend. If we generate more energy than we expend, we gain weight. If we expend more energy than we generate, we