Metabolism: what is it and how it's important?


We hear it often: metabolism is the key for weight management. But what does metabolism mean exactly? Is it simply the way our bodies "burn" energy?

First we need to understand how our bodies work in terms of energy expenditure. Foods are energy "IN" and how our bodies process that energy is the energy "OUT". The total amount of energy the body needs during a period of 24 hours is named Daily Total Energy Expenditure. For a sedentary individual, this expenditure of energy is divided into the following:

Resting metabolism makes up 70% of the total energy spent;

Thermic effect of food makes up about 10% of the total energy spent; and

Physical activity (even light ones such as getting up from a chair) accounts for approximately 20% of the Total Energy Expenditure.

Basal and Resting Metabolism (or Metabolic Rate)

The major component of TEE is basal metabolism, which refers to the energy necessary to keep the body alive at complete rest. Many physiological processes require energy such as breathing, blood circulation, digestion, temperature regulation, nerve signal conduction and so on. This measurement is the minimal amount of energy needed for existence and it's referred as Basal Metabolic Rate (BMR). This can be measured on a semi-controlled or laboratory setting with the person being measured soon after waking after a overnight fast.

A more common situation is a person in state of wakefulness at different times of the day and this is referred to Resting Metabolic Rate (RMR), which is 10% higher than the BMR, but is more accurate in describing metabolism outside research setting.

So the resting metabolism actually comprises about 70% of a person's total daily energy expenditure but it can be affected by a wide variety of factors including:

age

gender

genetics

hormonal changes

body size

body composition (especially the amount of skeletal muscle)

exercise

environmental temperature

altitude

food and caffeine intake

cigarette smoking

Decreasing or Increasing Metabolism

Research has determined that among the above factors, only 2 can be voluntarily controlled to exert substantial change in metabolism - since the other factors are either not under our control (ex: aging) or if we control, they have only temporary influence (ex: caffeine intake).

Factor that decrease RMR: starvation (or severe dieting)

Studies have shown that starvation state can reduce resting metabolic rate by 20% or more. The body quickly adapts to this new condition by protecting both fat and lean tissue from being further reduced as a physiological defense - which may be the reason why people on a very restricted, low calorie diet reach a "plateau" with no further weight loss and, if they go back to old patterns of eating, gain even more weight than when they started dieting. This also explains the effect of "yo-yo" diets on lowering metabolism with time.

Factor that increase RMR: body composition

On the other hand, studies have shown that one of the greatest influences on metabolism is the amount of fat-free (or lean body) mass of an individual. This can be achieved by increasing the size of skeletal muscle through strength training. This explains why only aerobic activities and diet usually reach a plateau of no further weight reduction: the body adapts to its conditions.

So a combination of aerobic activities and weight training for muscle hypertrophy is recommended to achieve an increase in metabolism and a change of body composition towards higher lean body mass.

The old formula: exercise + diet should be modified to weight training + healthy eating!

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© Updated 2020 by Personal Nutrition.