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Metabolism and Its Role in Weight Regulation

In simple terms, metabolism is the body’s own chemical process of using food for energy and growth. It includes two major processes: catabolism and anabolism. Catabolism is the process in which foods and stored fat are broken down into simpler compounds like glucose, glycerol or fatty acids, which may readily release or store energy. Anabolism is the process which uses simple substances such as glucose, glycerol or fatty acids and builds them up into more complex substances. This process consumes energy.

Metabolism plays a key role in weight gain and loss. Calories are the body’s source of energy and the body’s metabolism regulates how these calories are used. Excess energy is generally stored in the body in the form of fat. When the body takes in more calories (energy) than it uses, the result is weight gain. And vice-versa, if more energy is expended than is taken in, the body uses its fat stores for energy and loses weight.

There are three principal methods by which the body uses calories, regardless of whether they are taken from food or from stored fat:

  • Basal metabolic rate (BMR) is the amount of energy used when the body is at rest. It is also called resting metabolic rate. It involves processes such as breathing, organ function, blood circulation, adjusting hormone levels, growing and repairing cells. The BMR accounts for about two-thirds to three-quarters of an adult’s energy expenditure.
  • Thermic effect of food (TEF) is the energy used to process (e.g., absorb, digest, transport and store) consumed food. The TEF makes up about 10% of the body’s energy expenditure.
  • Physical activity. Energy is also used during physical activities like walking, exercising or any other movement.

The sum of these three methods of burning calories is known as the total energy expenditure (TEE) of the body.

Changes in Metabolism

An individual’s metabolic rate is not the same at all times. Certain events, such as aging, pregnancy or breastfeeding, and activities, such as changes in eating habits or exercise, can cause metabolism to decrease or increase. For example, metabolism slows down as people age which is due to changes in hormones and a reduced muscle-to-fat ratio. It is also very obvious that feeding the fetus, breastfeeding as well as physical exercise increase a person’s rate of metabolism. Another factor affecting the metabolic rate is eating. When calories are scarce, for example when fasting, the body attempts to conserve energy by slowing down metabolism.

Chemical Reactions Occurring within a Cell

Metabolism involves all the ways in which the body obtains and uses calories (energy) from food. Calories represent a measure of the energy produced by food or burned by activity. They are provided by the three energy-producing nutrients – carbohydrates, fats and proteins. The approximate number of calories provided by a gram of carbohydrates and protein is 4 compared with 8 calories provided by a gram of fat.

Some energy from catabolism is stored as high-energy storage compounds such as adenosine triphosphate (ATP). These substances readily release energy and can be easily stored as fatty acids in the adipose tissues. The breakdown of food into these substances occurs as a series of chemical reactions called metabolic pathways.

Carbohydrates, fats and proteins are broken down in different ways to produce energy for the body. First, they create a metabolite called acetyl coenzyme A. Acetyl coenzyme A can be used to make fats or adenosine triphosphate, two forms of stored energy. If the body has plenty of adenosine triphosphate, acetyl coenzyme A is built up to make fats. If the body’s adenosine triphosphate levels are low, acetyl coenzyme A enters a metabolic pathway to produce more adenosine triphosphate.