Metabolic age is just a simple number. However it considered a useful indicator of the overall level of your health and fitness.
The number is obtained by comparing your Basal Metabolic Rate (BMR) with the average BMR of other persons who are the same age in years as you.
If your metabolic age is lower than your actual age, you are fitter and healthier than the average for your age group.
But if your metabolic age is higher than your chronological age, it indicates that you need to improve your level of fitness.
This explanation begs the question… what is your basal metabolic rate?
Firstly, to answer this question, we need to discuss metabolism.
What is metabolism?
The term metabolism refers to all the chemical transformations that take place in your body… changes that are vital for sustaining your life.
These transformations are many and varied, and include:
- your digestive processes… the complex biochemical processes metabolism by which what you eat and drink is combined with oxygen to release the energy your body needs to function
- the transportation of substances into and between cells
- the conversion of glucose inside your cells into the energy that the cells need to function
- the conversion of food into the building blocks for proteins, lipids, nucleic acids etc
- the elimination of nitrogenous wastes
These changes allow your cells to grow and reproduce, maintain their structures and respond to their environment.
Without them your body simple would not work.
Basal metabolic rate (BMR)
Every day your body must breathe, circulate blood, control body temperature, grow and repair cells, adjust your hormone levels, support the activities of your brain and nerves, and so on, even if you do nothing and just lie in bed all day.
Your basal metabolic rate (BMR) is the number of calories your body burns every day to carry out the basic functions needed to support the normal functioning of your vital organs… heart, lungs, nervous system, kidneys, liver, intestines, sex organs, muscles and skin… while you are at rest.
In other words BMR is the amount of energy expressed in calories that you would need to maintain your body function while resting for 24 hours… as if you were lying in bed all day and night.
Surprisingly, your BMR accounts for 60 to 75% of the total energy you burn each day, ie up to three quarters of the calories you consume is spent on just keeping you alive.
The rest of the energy you burn every day is used up in two ways:
Food processing… just digesting, absorbing, transporting and storing the food you consume uses up plenty of calories.
For example, about 10% of the calories you get from the carbs and proteins you eat are expended during their digestion and absorption.
Physical activity… accounts for the rest of the calories your body burns during the day, 15 to 30% of the total energy you use up.
It consists of normal every day activities (such as getting up and moving around) plus deliberate exercise (eg, playing tennis, swimming, taking the dog for a walk and so on).
The energy needs for your body’s basic functions, as well as food processing, stay fairly consistent and do not change easily.
However the calories you burn while undertaking physical activities can vary greatly from day to day.
Indeed physical activity is the most variable of the factors that determine how many calories you burn in a day.
What influences basal metabolic rate (BMR)
Your BMR is influenced by several factors… muscle:fat ratio, gender, size, and age:
Muscle:fat ratio … or composition of your body, ie whether you have more muscle than fat or vice versa.
Just maintaining muscle at rest requires more calories than maintaining fat. So the more muscular you are the higher your BMR will be.
Gender … men usually have more muscle and less body fat than women of the same age and weight, which means that men burn more calories at rest.
Research indicates that the BMR of women is 5 to 10% lower than men.
Size … taller and heavier persons have a higher BMR because the more mass you have, the more fuel you need to sustain larger organs.
When you lose weight your BMR goes down and you need fewer calories per day. And when you gain muscles your BRM will increase.
Age… your BMR decreases as you get older because you muscle mass declines by 5 to 10% each decade after the age of 40.
Heredity… you can inherit your metabolic rate from your ancestors.
Climate … the BMR of persons living in the tropics is can be between 5 and 20% higher than persons living in temperate zones, because keeping the body cool consumes energy.
How to calculate basal metabolic rate (BMR) and metabolic age
Strictly speaking, your digestive system should be inactive when you are calculating your BMR. But in human beings this takes about 12 hours of fasting.
For this reason, the less strict Resting Metabolic Rate (RMR) is calculated in practice instead of BMR.