Energy and metabolism

Kilocalorie and kilojoules

Food energy is measured in kilojoules (kJ). This unit of measurement describes how much energy a particular food contains and how much energy is burned up during exercise.

A kilojoule or kilocalorie (Calorie) is a unit of measure for food energy.

A calorie is the amount of energy required to heat up water.

The equivalent values are: 4.184 kilojoules = 4 184 joules = 1 Calorie = 1 kilocalorie = 1 000 calories

Foods that we eat provide energy, which is measured in kilojoules. Just how much energy depends on the amount of carbohydrate, protein and fat the food contains.

  • Foods such as fruits, vegetables and legumes are relatively low energy (kilojoule) dense foods.
  • Foods high in fats, added sugars and alcohol are energy-dense foods.

The energy values from macronutrients (per gram) are:

  • Fat                                                    37 kJ (9 Cal)
  • Alcohol                                           29 kJ (7 Cal)
  • Carbohydrates                            16 kJ (4 Cal)
  • Protein                                            17 kJ (4 Cal)
  • Dietary fibre                                 13 kJ (3 Cal) – if fermented by bacteria in the large intestine
  • Water                                               0 kJ (0 Cal)

Consistent over eating will result in the excess energy being stored inside fat cells.

  • One kg of body fat contains the equivalent of 37,000 kJ.
  • To lose one kg of body fat in a month, people will need to reduce energy consumption by about 1200 kJ a day.


Metabolism is the chemical process that occurs in cells and tissues, for example the breakdown of macronutrients to provide energy and nutrients to build and repair of tissues. Metabolism depends on age, gender, the type of physical activity, types of food consumed and lean muscle mass (or muscle to fat ratio).

  • With ageing (over 40 years) the level of physical activity drops and the body slowly loses muscle mass. Conversely physical training (strength and resistance training) may help to reduce or prevent muscle loss.
  • Hormones help regulate metabolism. Hypothyroidism reduces metabolic rate.

The body total energy expenditure consist of 3 components:

  • Basal metabolic rate is energy used by the body at rest, which amounts to 50-80% of our daily energy expenditure
  • Energy used for physical activity, which is up to 20% of daily energy expenditure
  • Energy used to digest and breakdown foods (thermic effect of food), which is about 5-10% of daily energy expenditure

Basal metabolic rate (BMR)

The BMR refers to the amount of energy the body needs to function and maintain equilibrium (homeostasis).
BMR is largely determined by total lean mass, especially skeletal muscle mass. The more muscle mass the more kilojoules is used for muscle metabolism and daily activities. In general men have more muscle mass and hence a higher BMR.
The average BMR is 7100 kJ/day (man) and 5900 kJ/day (woman). Although energy expenditure is continuous throughout the day, the rate varies. The rate of energy expenditure is usually lowest in the early morning and increases depending on the need to burn more energy.

When lean mass is reduced, for example massive weight loss and inadequate exercise or physical training, BMR drops. Since BMR accounts for so much of the total energy consumption, it is important to preserve or even increase lean muscle mass through exercise when trying to lose weight. The exercise physiologist (or personal trainer) assists patients to maintain lean skeletal muscle mass to improve basal metabolism and increase energy expenditure from exercise, both contributing to more excess weight loss.

We often have no voluntary control over the BMR. However BMR can be influenced by many factors:

  • body size – larger adult bodies have more tissue and a result in increased BMR.
  • amount of lean muscle tissue – more muscles result in increased BMR.
  • amount of body fat – fat cells and burn far fewer kilojoules.
  • crash dieting, starving or fasting – eating too few kilojoules encourages the body to slow the metabolism to conserve energy. BMR may drop by up to 15% and the loss of lean muscle tissue further reduces BMR.
  • age – metabolism slows with age, which is associated with loss of muscle tissue, hormonal (e.g. menopause) and neurological changes.
  • growth – infants and children have higher energy requirement per unit of body weight due increased energy expenditure for growth and to maintain body temperature.
  • gender –men usually have faster metabolisms than women.
  • genetic predisposition – basal metabolic rate may be partly decided by genetic makeup.
  • 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 (e.g. hypothyroidism leads to a lower BMR).
  • environmental temperature – if temperature is very low or very high, the body has to work harder to maintain its normal body temperature, thus the BMR increases.
  • infection or illness – BMR increases to build new tissues, repair tisssues and to create an immune response.
  • amount of physical activity – hard-working muscles need plenty of energy to burn. Regular exercise increases muscle mass and facilitate a faster metabolic rate, even when resting metabolic rate.
  • drugs – drugs, like caffeine or nicotine may increase the BMR.
  • dietary deficiencies – a diet low in iodine reduces thyroid function and slows down metabolism.

Energy for physical activity

During intensive physical activities, the muscles may burn about 3000 kJ/hour. Energy used during exercise is the only form of energy expenditure that we can control.

It is estimated that energy expenditure from muscles is up 20 per cent of total energy expenditure at rest but during strenuous exercise, it may increase 50-fold or more.


The thermic effect of food

BMR rises after meals because energy is needed to eat, digest and metabolise the food. The BMR increases soon after you start eating and peaks two to three hours later.

This rise in the BMR can range between 2 to 30%, depending on the size of the meal and the types of foods eaten.

Macronutrients raise BMR by differing amounts.

  • fats increases the BMR 0–5%
  • carbohydrates increases the BMR 5–10%
  • proteins increases the BMR 20–30%
  • Hence protein foods are encouraged after bariatric surgery, to maintain lean muscle mass and more importantly because it induces satiety and increases the thermic effect of food. Hot spicy foods (chilli, horseradish and mustard) may also increases the thermic effect of food and increase total energy expenditure.

Poor metabolism and weight gain 


With age muscle mass is loss and is replaced by fat tissue. With increasing age weight gain is expected. Regular exercises and physical training (both strength and resistance training) after bariatric surgery (or anytime) may help reduce or prevent this muscle loss.

Certain medical/endocrine conditions are associated with reduced metabolism, such as hypothyroidism.

Cushing’s disease or Cushing’s syndrome is a condition associated with high levels of cortisol hormone exposure. Fluid retention, weight gain, increased fat deposition in the trunks (truncal obesity) and elevated blood sugar level are amongst the symptoms. Strictly speaking this is not a problem with metabolism alone but hypothyroidism and hypercortisonism should be excluded and treated before bariatric surgery.