Metabolism refers to the countless chemical
processes going on continuously inside the body that
allow life and normal functioning. These processes
require energy from food. The amount of kilojoules (kJ)
your body burns at any given time is regulated by your
metabolism. You can't control your metabolism, but you
can make it work for you when you exercise.
Two processes of metabolism
Hormones (chemical 'messages' secreted by the glands of
the endocrine system) and the nervous system control
your body's metabolism. Your metabolism can be upset by
a variety of events, including genetic disorders and
There are two complementary parts to your metabolism,
which are carefully monitored to make sure they remain
in balance. They are:
Metabolic rate (or total energy expenditure)
- Catabolism - the breakdown of food
components (such as carbohydrates, proteins and
fats) into their simpler forms, which can then be
used to create energy. This immediate form of energy
can be converted into heat or burned by cells.
- Anabolism - energy is stored in fat cells
or used to help build and repair structures of the
Your body's metabolic rate (or total energy expenditure)
can be divided into three components:
Basal metabolic rate (BMR)
- Basal metabolic rate (BMR) - is the amount
of kilojoules burned at rest and contributes 50-80
per cent of your energy used.
- Energy used during physical activity - this
is the amount of kilojoules burned during movement
and physical activity; in a normally active person,
this component contributes 20 per cent of daily
- Thermic effect of food - this is the energy
you use to eat, digest and metabolise food. It
contributes about 5-10 per cent of your energy use.
The BMR refers to the amount of energy your body needs
to maintain itself. This accounts for 50-80 per cent of
your total energy use. Total lean mass, especially
muscle mass, is largely responsible for the BMR. So,
anything that reduces lean mass will reduce BMR. That's
why it's important to preserve muscle mass when you try
to lose weight, since the BMR accounts for so much of
the energy we use.
An average male may have a BMR of around 7,100kJ per
day, while an average female may have a BMR of around
5,900kJ per day. Energy expenditure is continuous, but
the rate varies throughout the day. The lowest rate of
energy expenditure is usually in the early morning.
Energy used during physical activity
During heavy physical exertion, the muscles may burn
through as much as 3,000kJ per hour. Energy used during
exercise is the only form of energy expenditure that you
have any control over.
The energy expenditure of the muscles makes up only 20
per cent or so of the total energy expenditure at rest
but, during strenuous exercise, the rate of energy
expenditure of the muscles may go up 50-fold or more.
The following lists the amount of energy used during
Activity Energy (kJ/kg/h)
Sitting quietly 1.7
Standing relaxed 2.1
Driving a car 3.8
Walking rapidly 14.2
Swimming (4km/hour) 33
Rowing in a race 67
Thermic effect of food
Your BMR rises after you eat because you use energy to
eat, digest and metabolise the food you've just eaten.
The rise occurs soon after you start eating and peaks
two to three hours later. This rise in the BMR can range
between 2-3 per cent and up to 25-30 per cent, depending
on the size of the meal and the types of foods eaten.
Factors affecting the BMR
- Fats - raise the BMR 4 per cent
- Carbohydrates - raise BMR 6 per cent
- Proteins - raise BMR 30 per cent
- Hot spicy foods - these can also have a
significant thermic effect: for example foods
containing chilli, horseradish and mustard.
Your BMR is influenced by a number of factors working in
Age-related weight gain
- Body size - larger adult bodies have more
metabolising tissue and a larger BMR.
- Age - metabolism slows with age, due to a
loss in muscle tissue but also due to hormonal and
- Growth - infants and children have higher
energy demand per unit of body weight due to the
energy demands of growth and the energy needed to
maintain their body temperature.
- Gender - generally, men have faster
metabolisms than women because they tend to be
larger and have less body fat.
- Genetic predisposition - your metabolic
rate may be partly decided by your genes.
- Amount of lean muscle tissue - muscle burns
- Amount of body fat - fat cells are sluggish
and burn far fewer kilojoules than most other
tissues and organs of the body.
- Hormonal and nervous controls - BMR is
controlled by the nervous and hormonal systems;
hormonal imbalances can influence how quickly or
slowly the body burns kilojoules.
- Dietary deficiencies - for example, a diet
low in iodine reduces thyroid function, which slows
- Environmental temperature - if temperature
is very low or very high, the body has to work
harder to maintain its normal body temperature; this
increases the BMR.
- Infection or illness - BMR increases
because the body has to work harder to build new
tissues and to create an immune response.
- Crash dieting, starving or fasting - eating
too few kilojoules encourages the body to slow the
metabolism to conserve energy; BMR can drop by up to
15 per cent. There is also loss of lean muscle
tissue, which further contributes to the drop in BMR.
- Amount of physical activity - hard-working
muscles need plenty of energy to burn. Regular
exercise increases muscle mass and 'teaches' the
body to burn kilojoules at a faster rate, even when
- Drugs - some drugs, like caffeine or
nicotine, can increase the BMR.
Muscle tissue has a voracious appetite for kilojoules.
The more muscle mass you have, the more kilojoules you
will burn. People tend to put on fat as they age - this
is because the body slowly loses muscle.
It's not clear whether this muscle loss is a result of
the ageing process or because many people are less
active as they age. However, it probably has more to do
with becoming less active, as research has shown that
strength and resistance training can reduce or prevent
this muscle loss.
If you are over 40 years, have a pre-existing medical
condition or haven't exercised in some time, see your
doctor before embarking on any new fitness program.
Hormones help to regulate the metabolism. Some of the
more common hormonal disorders are concerned with the
thyroid. This gland secretes hormones to regulate many
metabolic processes, including energy expenditure (the
rate at which kilojoules are burned). Disorders include:
Genetic disorders of metabolism
- Hypothyroidism - or underactive thyroid.
The metabolism slows because the thyroid gland
doesn't release enough hormones. A common cause is
the autoimmune condition Hashimoto's disease. Some
of the symptoms of hypothyroidism include unusual
weight gain, lethargy, depression and constipation.
- Hyperthyroidism - or overactive thyroid.
The gland releases greater quantities of hormones
than necessary and speeds the metabolism. The most
common cause of this condition is Graves' disease.
Some of the symptoms of hyperthyroidism include
increased appetite, weight loss, nervousness and
Sometimes a faulty gene affects part of the metabolic
process and stops the body from using food components,
such as carbohydrates, in the normal way. In most cases,
these disorders can be managed under medical
supervision, with strict attention to diet. Some genetic
disorders of the metabolism include:
Where to get help
- Fructose intolerance - the inability to
break down fructose, which is a type of simple sugar
found in fruits, fruit juices, sugar (for example,
cane sugar) and certain vegetables.
- Galactosaemia - the inability to convert
the carbohydrate galactose into glucose. Galactose
is not found in nature; it is produced when lactose
is broken down by the digestive system into glucose
and galactose. Sources of lactose include milk and
milk products, such as yoghurt and cheese.
- Phenylketonuria (PKU) - the inability to
convert the amino acid phenylalanine into tyrosine.
High levels of phenylalanine in the blood can cause
brain damage. High protein foods must be avoided.
Things to remember
- Your doctor
- Your Nutritional Therapist
- Metabolism refers to the countless chemical
processes going on continuously inside the body that
allow life and normal functioning.
- The amount of kilojoules your body burns at any
given time is regulated by your metabolism.
- The metabolic rate is influenced by many factors,
including age, gender, muscle-to-fat ratio, amount
of physical activity and hormone function.
© 2005 HealthSmart Nutrition. All rights reserved.
Revised: May 24, 2006