Energy balance is the relationship between energy in (energy taken into the blood through food and drink) and energy out (energy being used in the body).
This relationship is underpinned by the first law of thermodynamics, which states that energy cannot be created or destroyed but instead changes from one form to another. In other words, we may convert the energy stored in food into heat or we may store it as fat. The energy balance dictates whether weight is lost, gained or remains the same. Energy is provided by the carbohydrate, protein and fat in the food and drinks we consume. It is also provided by alcohol. Depending on their composition, different food and drinks provide different amounts of energy.
Energy is measured in units of kilocalories(kcal) or kilojoules (kJ), with 1kcal being equal to 4.18kJ.
- Fat contains 9kcal (37kJ) per gram (25g = 1oz)
- Alcohol contains 7kcal (29kJ) per gram
- Protein contains 4kcal (17kJ) per gram
- Carbohydrate contains 3.75kcal (16kJ) per gram
On the flip side, different activities use up different amounts of calories. Strenuous activities such as running and heavy gardening use up more than reading or driving. Cross-country skiing is often billed as one of the most energetic activities, probably from a combination of the effort involved and the fact that it is performed in a cold environment, which means the body has to use up more energy to maintain its core temperature. Because this balance of calorie consumed in food and expended in exerciser is common knowledge, calorie-tracking has become very popular. Sophisticated gadgets track your movement throughout the day and will estimate your total energy expenditure based on personal details such as your age and weight. You can then input details of your diet to see if your energy intake is a near match for your expenditure.
Staying balanced since we can keep tabs on our diets using gadgets, you would think that maintaining energy balance would be quite simple. But in reality it is regulated by a complex network of systems involving the hypothalamus (a part of the brain that controls many functions of the nervous and endocrine systems), nerves and hormones. Information is gathered about energy repletion/depletion, physical activity level, the stage of reproductive cycle or developmental state (such as pregnancy), as well as the types and amounts of foods eaten. This information then influences the processes that affect the body's energy balance.
This means the body is highly adaptable to a variety of energy intakes and outputs. It has to be adaptable in order to survive, and so mechanisms are in place to ensure stable energy transfer regardless of any energy imbalances. For example, a severe negative energy balance can lead to a decline in metabolism. When the body detects an energy deficit, it begins to slow down all functions that aren't essential for survival. Both sides of the energy balance equation are complex and it is the relationship between energy intake and energy expenditure that will determine body composition and health outcomes.
Energy and survival
The next time you hear someone say "I've got no energy", you could in fact point out that as long as they drink water, they probably have enough stored energy to last for three to six weeks (maybe say it silently if you wish t remain friends). The amount of time a person can survive without food depends on factors such as body weight, genes and health status. Most importantly it depends on hydration, as in the absence of water death may occur after 24-72 hours.
Unlike total starvation, a person may survive semi-starvation for many months, or even years. This has been documented by survivors of famine and concentration and prisoner-of-war camps. During this time the body is able to moderate metabolism to conserve energy, although survival rates will differ between individuals and will be influenced by quality of food eaten and intake of the nutrients provided, which can cause or worsen disease through the development of deficiencies. The body's ability to alter its metabolism occurs through changes in thyroid function and over time a shift in the genetic make-up of certain races has evolved to enable them to survive long periods of semi-starvation, such as regular famine, through the better economical use of energy. The persistence of these genes in an obesogenic environment (availability to abundance of high-fat and sugar foods and low levels of physical activity) is a root cause of the increased prevalence of diabetes in these racial groups.
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