Energy balance (biology)

Energy balance (biology)

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In biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, energy balance is the biological homeostasis of energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 in living systems. It is measured with the following equation:
Energy intake = internal heat produced + external work + storage. It is also an aspect of bioenergetics
Bioenergetics is the subject of a field of biochemistry that concerns energy flow through living systems. This is an active area of biological research that includes the study of thousands of different cellular processes such as cellular respiration and the many other metabolic processes that can...

, concerning energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 flow through living systems.

It generally uses the energy unit Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...

 (i.e. kilogram calorie), which equals the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.184 kJ
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...


Energy intake

Energy intake is part of the diet
Diet (nutrition)
In nutrition, diet is the sum of food consumed by a person or other organism. Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. With the word diet, it is often implied the use of specific intake of nutrition for health or weight-management...

, which is mainly regulated by hunger
Hunger is the most commonly used term to describe the social condition of people who frequently experience the physical sensation of desiring food.-Malnutrition, famine, starvation:...

 and food energy
Food energy
Food energy is the amount of energy obtained from food that is available through cellular respiration.Food energy is expressed in food calories or kilojoules...

 of what is consumed.

Energy expenditure

Energy expenditure is mainly a sum of internal heat produced and external work.

The internal heat produced is, in turn, mainly a sum of basal metabolic rate
Basal metabolic rate
Basal Metabolic Rate , and the closely related resting metabolic rate , is the amount of daily energy expended by humans and other animals at rest. Rest is defined as existing in a neutrally temperate environment while in the post-absorptive state...

 (BMR) and the thermic effect of food
Thermic effect of food
Thermic effect of food , or TEF in shorthand, is the increment in energy expenditure above resting metabolic rate due to the cost of processing food for storage and use. It is one of the components of metabolism along with the resting metabolic rate, and the exercise component...


External work may be estimated by measuring physical activity level
Physical activity level
The physical activity level is a way to express a person's daily physical activity as a number, and is used to estimate a person's total energy expenditure...


Gaining imbalance

A gaining energy imbalance is a result of energy intake being higher than what is consumed in external work and other bodily means of energy expenditure.

The main preventable causes are:
  • Overeating
    Overeating generally refers to the long-term consumption of excess food in relation to the energy that an organism expends , leading to weight gainingand often obesity. It may be regarded as an eating disorder....

    , resulting in increased energy intake
  • Sedentary lifestyle
    Sedentary lifestyle
    Sedentary lifestyle is a medical term used to denote a type of lifestyle with no or irregular physical activity. A person who lives a sedentary lifestyle may colloquially be known as a couch potato. It is commonly found in both the developed and developing world...

    , resulting in decreased energy expenditure through external work

A gaining imbalance results in energy being stored, primarily as fat
Fats consist of a wide group of compounds that are generally soluble in organic solvents and generally insoluble in water. Chemically, fats are triglycerides, triesters of glycerol and any of several fatty acids. Fats may be either solid or liquid at room temperature, depending on their structure...

, causing weight gain
Weight gain
Weight gain is an increase in body weight. This can be either an increase in muscle mass, fat deposits, or excess fluids such as water.-Description:...

. In time, overweight
Overweight is generally defined as having more body fat than is optimally healthy. Being overweight is a common condition, especially where food supplies are plentiful and lifestyles are sedentary...

 and obesity
Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...

 may develop, with resultant complications.

Losing imbalance

A losing energy imbalance is a result of energy intake being less than what is consumed in external work and other bodily means of energy expenditure.

The main cause is undereating due to a medical condition such as decreased appetite
Anorexia (symptom)
Anorexia is the decreased sensation of appetite...

, anorexia nervosa
Anorexia nervosa
Anorexia nervosa is an eating disorder characterized by refusal to maintain a healthy body weight and an obsessive fear of gaining weight. Although commonly called "anorexia", that term on its own denotes any symptomatic loss of appetite and is not strictly accurate...

, digestive disease
Digestive disease
All diseases that pertain to the gastrointestinal tract are labelled as digestive diseases. This includes diseases of the esophagus, stomach, first, second, and third part of the duodenum, jejunum, ileum, the ileo-cecal complex, large intestine , sigmoid colon, and rectum.-Esophagus:*Esophagitis -...

, or due to some circumstance such as fasting
Fasting is primarily the act of willingly abstaining from some or all food, drink, or both, for a period of time. An absolute fast is normally defined as abstinence from all food and liquid for a defined period, usually a single day , or several days. Other fasts may be only partially restrictive,...

, famine
A famine is a widespread scarcity of food, caused by several factors including crop failure, overpopulation, or government policies. This phenomenon is usually accompanied or followed by regional malnutrition, starvation, epidemic, and increased mortality. Every continent in the world has...

, or overpopulation
Overpopulation is a condition where an organism's numbers exceed the carrying capacity of its habitat. The term often refers to the relationship between the human population and its environment, the Earth...


Energy requirement

Normal energy requirement, and therefore normal energy intake, depends mainly on age, sex and physical activity level
Physical activity level
The physical activity level is a way to express a person's daily physical activity as a number, and is used to estimate a person's total energy expenditure...


The Food and Agriculture Organization
Food and Agriculture Organization
The Food and Agriculture Organization of the United Nations is a specialised agency of the United Nations that leads international efforts to defeat hunger. Serving both developed and developing countries, FAO acts as a neutral forum where all nations meet as equals to negotiate agreements and...

 (FAO) of the United Nations
United Nations
The United Nations is an international organization whose stated aims are facilitating cooperation in international law, international security, economic development, social progress, human rights, and achievement of world peace...

 has compiled a detailed report on human energy requirements: Human energy requirements (Rome, 17–24 October 2001) An older but commonly used and fairly accurate method is the Harris-Benedict equation
Harris-Benedict equation
The Harris-Benedict equation is a method used to estimate the daily calorie requirements of an individual using their basal metabolic rate or BMR. The estimated value is then multiplied by a number that corresponds to the person's activity level. The resulting number is the recommended daily...


Yet, there are currently ongoing studies to show if calorie restriction
Calorie restriction
Caloric restriction , or calorie restriction, is a dietary regimen that restricts calorie intake, where the baseline for the restriction varies, usually being the previous, unrestricted, intake of the subjects...

 to below normal values have beneficial effects, and even though they are showing positive indications in primates it is still not certain if calorie restriction has a positive effect on longevity for primates and humans. Calorie restriction may be viewed as attaining energy balance at a lower intake and expenditure, and is, in this sense, not generally an energy imbalance, except for an initial imbalance where decreased expenditure hasn't yet matched the decreased intake.

See also

  • Dynamic Energy Budget
    Dynamic energy budget
    The Dynamic Energy Budget theory aims to identify simple quantitative rules for the organization of metabolism of individual organisms that can be understood from basic first principles...

    , a theory making explicit use of energy, mass
    Mass balance
    A mass balance is an application of conservation of mass to the analysis of physical systems. By accounting for material entering and leaving a system, mass flows can be identified which might have been unknown, or difficult to measure without this technique...

     and time balances.

External links