What is metabolism?

Metabolism is the term to describe the sum of all chemical reactions in the body. Reactions can be catabolic or anabolic.

Catabolic reactions are those involving the breakdown of molecules (e.g., carbohydrates are catabolized into glucose, and proteins are catabolized into amino acids). These reactions release energy and heat.

Anabolic reactions involve the building of new molecules from smaller ones (e.g., using amino acids to build proteins during muscle hypertrophy or creating adipose (fat) tissue that deposits as fat). These reactions require energy, which is usually obtained from the breakdown of ATP.

What is metabolic rate?

All metabolic processes produce heat, and so metabolic rate relates to body temperature.

Basal metabolic rate (BMR, measured in units per time) describes the minimum amount of energy required by the body at rest.

Generally, BMR accounts for 60-70% of a person’s daily calorie expenditure and varies between individuals depending on several factors, such as age, body mass, and body composition (muscle/fat). There are also studies that suggest BMR has a genetic component.

The thermal effect of food describes the total energy required to fuel all digestive and absorptive processes and accounts for 10% of a person’s daily calorie expenditure. Eating less food throughout the day can lead to a decreased metabolic rate as less energy is required for the breakdown of food in the body.

Calculating BMR

Approximately 25 kcals are used for every kilogram of weight per day just to sustain life, and this is known as the basal metabolic rate (BMR). To calculate BMR, use the following equation:

BMR = 25 (kcal) x body weight (kg)

For example, an individual that weighed 60kg  - BMR = 25 x 60 = 1500 kcals/day.

So that person would require 1500 kcals a day just to meet their basal energy requirements and stay alive. Obviously, this equation does not take into account any type of movement. When we exercise or go about our daily lives, we require additional calories.

It is recommended that those who have a fairly sedentary lifestyle require around 120% of their BMR, those living a moderately active lifestyle require 150% of their BMR, and very active individuals need around 200% of their BMR.

For example, if the individual from above that weighed 60kg had a moderately active lifestyle, their daily requirements would be  - BMR = 1500 kcals/day, 1500 x 1.5 = 2250 kcals/day.