What Is Metabolism? How Does It Affect Weight Loss

The term “metabolism” has become part of the fitness vernacular, thrown around at weight management classes and fitness centers as well as water coolers and cocktail parties. Generally, people use the term to refer to the speed in which calories are burned. This, of course, leads to metabolism envy of thin people who appear to have the magical capacity to indulge in pizza and cake without gaining an ounce, unlike others who seem to gain weight merely from thinking about those indulgences. In reality, metabolism is a term of medical science, more specifically nutrition science, and the reason for its colloquial nature is the intriguing connection between this normal bodily function and weight loss.

What is the meaning of metabolism?

To understand what metabolism really is, it helps to understand the basics of digestion and absorption, and how the body uses the food we eat. There is an overarching ongoing series of events that takes place in the human body, and it is metabolism that enables this process.

When we eat, human cells cannot immediately process the food. To be of any use to the body, the food needs to be broken down (digested) and then absorbed into the lining of the gastrointestinal tract. The completion of a successful digestion process essentially liberates the vitamins and minerals to circulate around the body. However, the GI tract acts as a gatekeeper, making critical decisions about how much of these nutrients will actually be absorbed.

This is exciting because while all this is going on, massive chemical changes take place in the cells, which ultimately produces energy and builds tissue. When carbohydrates, protein, and fat are digested and move through the bloodstream, these macronutrients break down into the basic building blocks of life. Conversion into energy or storage follows, and the party begins. It is all these chemical cell changes combined that define metabolism. Essentially, metabolism is how our cells maintain life. However, there is a connection to weight loss worth exploring.

What is the connection between metabolism and weight loss?

So the life-sustaining activities of metabolism produce energy. Energy intake is derived from food plus energy stored. To lose weight, energy has to be expended beyond the intake required to sustain life. But physical activity is only one of the three demands for the energy required to achieve the intake/output balance.

The body demands energy in three forms: Resting metabolic rate (RMR), the thermic effect of food and physical movement. It may come as a surprise to learn that the body expends most of its energy on its operations at rest, performing internal activities such as sleeping, digestion and other involuntary functions. The thermic effect of food is the stimulating impact that the efforts of digesting, absorbing and transport have on metabolism. In other words, you burn energy by the physical act of eating. However, the impact is low and cannot be controlled. Weight management is about controlling what can be controlled; hence the focus on physical activity for weight loss.

How do you determine your metabolic rate?

When people say that metabolism varies by individual, what is really being said is that REE varies from one person to the next, which is impacted by such factors as gender and age.

Knowing your REE informs you of the level of calorie intake that should result in weight maintenance, gain or loss. If you know, for example, that your REE is 1500 calories, that would tell you that your daily calorie consumption needs to be 1500 calories to maintain the same weight. In other words, if you did absolutely nothing all day but simply exist, your body is naturally burning 1500 calories. This interesting fact is why some people want to know their personal REE. It should be noted that resting energy expenditure is often used interchangeably with resting metabolic rate (RMR), basal energy expenditure (BEE) and basic metabolic rate (BMR).

There are devices to measure the metabolic rate, which involve measuring the exchange of oxygen and carbon dioxide. Measuring the body’s utilization of oxygen is considered to be highly accurate. Additionally, there are formulas designed to estimate daily REE needs. These formulas are gender- and age-specific, and use kilocalories (kcal). One kilocalorie equals 1000 calories. Following is a general formula that takes into account gender:

BMR = 1 kcal x kg body weight x 24 hours

BMR = 0.9 kcal x kg body weight x 24 hours

To convert pounds to kilograms, divide the number of pounds by 2.2

For example, following is the calculation for a woman who is 121 pounds:
121/2.2 = 55 kg
0.9 x 55 x 24 = 1188

In this example, the woman needs to consume 1188 calories per day to maintain her weight with no physical activity. To lose weight, she needs to either reduce daily calories below 1188 or add exercise to her daily routine to expend more calories.

There are more complex formulas that allow you to factor in age, height and even physical lifestyle, ranging from sedentary to extra active. Online calculators are available for Harris-Benedict and Mifflin-St. Jeor equations.

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