Every organ, every tissue, and every cell of the body engages in metabolism, the chemical reactions involved in releasing energy, breaking down compounds, and making new compounds. Much like a factory, the body works efficiently to manufacture needed products and dispose of wastes. All these processes are regulated by hormonal signals that coordinate supply and demand.
In disease, metabolic processes can become disturbed, and some diseases are caused by metabolic disturbances. This article introduces metabolism, energy balance, body composition, and the health risks associated with too much or too little body fat.
What is Metabolism?
Metabolism is the way our bodies burn energy. The metabolic work that the body’s cells do, like all work, requires energy, and foods supply that energy. Foods, in turn, get their energy from the sun, either directly (in the case of photosynthesizing plants that make carbohydrates) or indirectly (in the case of animals that eat plants). When chemical reactions in cells release stored energy from energy-yielding nutrients, that energy becomes available to do the cells’ work.
Heat Energy and Body Temperature
The cells of each organ conduct metabolic activities specific to that organ. In addition, all cells must maintain themselves, and many must reproduce. To do this, they must have all the essential nutrients available to them: energy nutrients, vitamins, and minerals, as well as water. As cells do their metabolic work, the chemical reactions that are involved release heat, and this heat keeps the body warm. By regulating the rates at which these metabolic reactions release heat energy, the body maintains its constant normal temperature of about 98.6°F.
During severe stress on the body, metabolism speeds up. Fever sometimes develops. An accelerated metabolism signifies that fuels are being used at a rate more rapid than normal; this may lead to the wasting of body organs and loss of weight, including loss of vital lean tissue.
Building Up and Breaking Down Compounds
When not needed by the cells for energy, the basic units of the energy-yielding nutrients are used to build body compounds. The building up of body compounds is known as anabolism; this book represents anabolic reactions, wherever possible, by “up” arrows in chemical diagrams. Glucose units can be strung together to make glycogen chains. Glycerol and fatty acids can be assembled into triglycerides. Amino acids can be linked together to make proteins. These anabolic reactions, in which simple compounds are put together to form larger, more complex structures, involve doing work and so require the energy provided by the high-energy compound, ATP.
The breaking down of body compounds is known as catabolism. Catabolic reactions usually release energy and are represented, wherever possible, by “down” arrows in chemical diagrams. Glycogen can be broken down into glucose, triglycerides into fatty acids and glycerol, and protein into amino acids.
How Does Your Metabolism Burn Calories?
How does your body get the energy needed to maintain all of its cellular activities from the foods you eat? The answer to this question lies in an understanding of metabolism, the chemical reactions that occur in all living cells.
Although every aspect of our lives depends on energy, the concept of energy can be difficult to grasp because it cannot be seen or touched, and it manifests in various forms, including heat, mechanical, electrical, and chemical energy. In the body, heat energy maintains a constant body temperature, and electrical energy sends nerve impulses, for example. Energy is stored in foods and in the body as chemical energy.
Energy metabolism is the sum total of all the chemical reactions that the body uses to obtain or expend energy from foods. In the process, energy-yielding nutrients—carbohydrate, fat, and protein—are broken down into basic units that are absorbed into the blood:
- From carbohydrates: glucose
- From fat (triglycerides): glycerol and fatty acids
- From proteins: amino acids
The high-energy compound ATP (adenosine triphosphate) is able to transfer small amounts of usable energy to move our muscles and supply our enzymes with the energy they need to catalyze chemical reactions.
When ATP breaks down and releases one of its phosphate groups, a small amount of energy is released and used in the body to build compounds. With the loss of the phosphate group, the ATP becomes ADP (adenosine diphosphate). During energy metabolism, ATP is re-created by attaching a phosphate group to ADP. ATP is produced continuously throughout the day, using the energy from the breakdown of the energy-yielding nutrients.
Breaking Down Nutrients for Energy
To produce ATP, the body breaks down any or all of the four basic units—glucose, fatty acids, glycerol, and amino acids— into even smaller units. Each of these nutrients travels down a different pathway, but all can eventually become acetyl CoA, enter the TCA cycle, and provide hydrogens for the electron transport chain. Details follow.
The breakdown of glucose (a 6-carbon compound) into two molecules of pyruvate (a 3-carbon compound) is called glycolysis, and it produces just two usable ATP. As the carbons in glucose are broken apart to produce pyruvate, the hydrogen atoms attached to the carbons are transferred by coenzymes to the electron transport chain. Thus, the reactions of glycolysis produce a small amount of ATP, pyruvate, and hydrogen-rich coenzymes that are used later in energy metabolism. After glycolysis, pyruvate is converted to acetyl CoA, which consists of a 2-carbon fragment and a coenzyme called CoA.
Acetyl CoA can be produced not only from pyruvate but also from other energy-yielding nutrients. Fatty acids can be broken down into 2-carbon fragments that combine with CoA to form acetyl CoA. As the carbons in fatty acids are broken apart to produce acetyl CoA, hydrogen atoms are released to coenzymes that transfer them to the electron transport chain. Glycerol can easily be converted to pyruvate and therefore can also produce acetyl CoA. The amino acids have various pathways; some can be converted to pyruvate, others can be converted to acetyl CoA, and a few can enter the TCA cycle directly.
The breakdown of energy nutrients continues in the TCA cycle (tricarboxylic acid cycle), as enzymes break down acetyl CoA molecules. With each turn of the TCA cycle, hydrogen atoms are carried by coenzymes to the electron transport chain. The waste product of these reactions is carbon dioxide, which is eventually exhaled.
Electron Transport Chain
The final step in energy metabolism occurs at the electron transport chain. In this process, enzymes attach a phosphate group to ADP, creating ATP. The hydrogen atoms that were collected by coenzymes during glycolysis, fat breakdown, and the TCA cycles provide the chemical energy that drives ATP production. Finally, the same hydrogen atoms are linked with oxygen to produce water.
Aerobic and Anaerobic Metabolism
The production of ATP via the electron transport chain requires oxygen in the final step and is called aerobic metabolism. Glycolysis produces ATP without oxygen and is therefore called anaerobic metabolism. When exercise intensity requires more ATP than can be provided by the electron transport chain (due to limited oxygen or other factors), ATP production from glycolysis is stepped up, making glucose a critical fuel for the exercising muscles.
Metabolism and Weight Loss
Weight gain may seem to be caused by your metabolism. In spite of this, metabolism is a natural process that your body regulates according to your specific needs. Rarely, weight gain can be caused by medical conditions that slow metabolisms, such as Cushing’s syndrome or hypothyroidism.
Weight gain is a complicated process. Put simply, people gain weight when they consume more energy than they expend. Much of the excess is then stored as body fat. Fat can be made from an excess of any energy-yielding nutrient. In addition, excess energy from alcohol is also stored as fat.
Alcohol has also been shown to slow down the body’s use of fat for fuel, causing more fat to be stored, much of it as abdominal fat tissue.1 Alcohol, therefore, is fattening, both through the calories it provides and through its effects on fat metabolism. The fat cells of the adipose tissue enlarge as they fill with fat,
Surplus carbohydrate (glucose) is first stored as glycogen in the liver and muscles, but the glycogen-storing cells have a limited capacity. Once glycogen stores are filled, most of the additional carbohydrate is burned for energy, displacing the body’s use of fat for energy and allowing body fat to accumulate. Thus, excess carbohydrates can contribute to obesity.
Surplus dietary fat contributes more directly to the body’s fat stores. After a meal, fat is routed to the body’s adipose tissue, where it is stored until needed for energy. Thus, excess fat from food easily adds to body fat.
Surplus protein may also contribute to body fat. If not needed to build body protein (as in response to physical activity) or to meet energy needs, amino acids will lose their nitrogens and be converted, through intermediates, to triglycerides. These, too, swell the fat cells and add to body weight. Figure 6-5 shows the metabolic events of feasting.
- Too much food, too little physical activity, or both encourage body fat accumulation.
- A net excess of energy is almost all stored in the body as fat in adipose (fat) tissue.
- Alcohol both delivers calories and encourages the storage of body fat.
- Once glycogen stores are filled, excess carbohydrate is used for energy displacing the body’s use of fat for energy and allowing fat to accumulate.
- Fat from food is particularly easy for the body to store as adipose tissue.
- If not needed to build body protein or to meet energy needs, excess protein can be converted to fat.
- In short, excess energy intake from carbohydrates, fat, protein, and alcohol leads to the storage of body fat.