Metabolism: An Overview

Metabolism is a term covering all the chemical reactions that occur in the body that are necessary to maintain life. These metabolic processes include growth and repair, cell renewal and movement. 

At any one time, there are millions of metabolic processes occurring simultaneously in our bodies. These processes can be divided into two groups. Anabolic processes use simple chemicals and molecules to manufacture a vast array of finished products - for example, the building of proteins from amino acids. On the other hand, catabolic processes break down complex molecules to release energy, which our bodies need - for example, the breakdown of starch into glucose. 

The speed of these metabolic processes is commonly known as metabolic rate or energy expenditure and this determines the number of calories your body will use up in a given amount of time. The faster your metabolism, the more calories your body needs. 

Metabolic rate can be divided into the following categories: 

• Basal metabolic rate (BMR)is your metabolic rate, i.e. your digestive system is inactive and you have rested for at least eight hours. It is the amount of energy needed to control your temperature and breathing and to keep your heart, brain and nerves functioning. 
• Resting metabolic rate (RMR) is often used interchangeably with BMR, but is measured using different technique. It is the minimum metabolic rate required to keep you alive and functioning while not physically active. On average, it accounts for up to 50-75% of total calorie expenditure. 
• Thermic effect of food (TEF) is the energy consumed when your body is digesting and processing food. When you eat, your metabolic rate rises. This rise represents approximately 10% of total energy expenditure. 
• Thermic effect of exercise (TEE) is the energy consumed when you exercise. 
• Non-exercise activity thermogenesis (NEAT) is the energy consumed during other activities. This includes everything from typing and hanging laundry to fidgeting. 

How to calculate your metabolic rate

For research purposes, metabolic rate can be measured by two methods:

Direct calorimetry, which measures the subject's heat output through direct observation inside a calorimeter. Indirect calorimetry, which measures the heat output by using a calculation based on the difference between the amount of oxygen inhaled and the amount of carbon dioxide exhaled. While man-sized calorimeters are not widely accessible, there are several handheld devices available to buy, but these can be very expensive and not completely accurate. 

If you don't have a machine to hand, equations can be used to work out your BMR. The formula known as the Harris-Benedict equation was first developed in 1918 and over the years has been tweaked and updated to be more suited to our improved health status a century later. 

BMR Calculation for Women:

(10 x weight in kg) + (6.25 x height in cm) - 5 x age in years) - 161

BMR Calculation for Men:

(10 x weight in kg) + (6.25 x heaight in cm) - (5 x age in years) + 5

To then find out your body's energy needs, you should factor in your daily activity and exercise level. 

Level of Exercise	Daily Calories Needed
Little to no exercise	BMR x 1.2
Light exercise (1 to 3 days per week)	BMR x 1.4
Moderate exercise (3 to 5 days per week)	BMR x 1.6
Heavy exercise (6 to 7 days per week)	BMR x 1.8
Very heavy exercise (intense workouts twice a day)	BMR x 1.9

An individuals BMR will differ according to their body size (how tall they are and how much they weigh), their body composition (how much muscle they have in relation to fat) and their age. The bigger you are the more calories you need for basic functions. And the more muscle you have the more calories you use, even if those muscles are resting. Gender is also a factor and men have a higher basal metabolic rate than women because their muscle-to-fat ratio is generally higher than women's. This means that an average man will burn more calories than an average woman of the same age and weight. 

What effects your metabolism?

As we get older, changes in body composition, hormones and lifestyle result in lower calories requirement. With ageing, muscle mass drops, resulting in a higher fat-to-muscle ratio, and the natural decline in BMR means we need fewer calories. Hormones levels drop as we age, with men producing less testosterone and women producing less oestrogen, both of which are involved in anabolic processes that use up energy. It s particularly a problem for women approaching the menopause, as this brings with it a significant drop in hormones that normally promote energy use. Many women find it harder to lose weight during this time. 

Other factors that can influence metabolic rate include environmental temperature and physical activity. When your body is exposed to cold, it needs to burn more calories to prevent its temperature from falling. Most people think that it is the amount of exercise you do that will have the biggest impact on energy expenditure, whereas NEAT (energy consumed through non-exercise activity) could account for as much as 30%. 

Genetics influence metabolism too, which means that metabolic rates do vary between individuals from birth. However, experts disagree on just how much these genetic differences can actually account for variations in weight, as there are so many more factors that can have a much larger impact. It is a common belief that slim people have a faster metabolism and overweight people have a slower metabolism, but this is very rarely the case. Despite the common belief that metabolism determines whether someone is fat or thin, weight gain is primarily due to energy imbalance - taking on more calories than you expends. 

What affect your BMR?

Your BMR may be higher if...

• You have a fever
• You smoke
• Your thyroid is hyperactive
• You are stressed or drink caffeine
• You are in a hot or cold environment
• You are young and growing
• You have more muscle
• You are male
• You are female and experiencing PMS
• You are female and pregnant
• You are tall and thin

Your BMR may be lower if...

• You are older
• You are female
• Your thyroid is sluggish
• You are sleeping
• You have less muscle or are small in stature
• You are fasting or suffering malnutrition
• You inherited a slow metabolism