Energy from food is one of our prime needs for survival. As well as oxygen and water, without energy from food we would die. Every cell in the body requires a continuous supply. The macro-nutrients carbohydrates, fats, and protein are the sources for energy. When the body uses these for energy, the bonds between the atoms break and energy is released. Energy is expended within the body as electrical energy such as in nerve impulses, kinetic energy such as muscle movement, chemical reactions such as synthesis of new molecules; or the energy can be liberated as heat. (1,2)
Units of measuring the food energy supplied are the joule (amount of work performed when a mass of one kilogram is moved one metre by a force of one newton) or the calorie (heat required to raise temperature of one gram of water one degree Celsius). Being small units kilojoules (kJ) or kilocalories (kcal) are more commonly used. The standard measurement in Australia is kilojoules. One kcal is approximately 4.18 kJ. The energy supplied by each macro-nutrient differs. Fat averages 37 kJ (9kcal) per gram, protein 17 kJ (4 kcal); carbohydrates 17 kJ (4 kcal) and alcohol (in adults) 29 kJ (7kcal). (1)
The macro-nutrients are broken down into smaller units during digestion after a meal; carbohydrates into sugars, fats into fatty acids and protein into amino acids; which allows their absorption from the gut into the blood stream. They are then transported by the blood around the body for immediate use or converted into storage forms for later use. It is these storage forms of macro-nutrients that allows a continual supply of energy to each cell, between meals. (1,2)
In summary, in simple terms nutrients digested from food are absorbed into the blood stream and from there fluctuate within three main states.
The Fed State
After a meal (called post-prandial), the absorbed macro-nutrients flood into the blood stream. The influx of these nutrients are considered to be obtained exogenously which means from ‘outside the body’, or in other words – from food. The level of these nutrients rise in the blood in the fed state.
Carbohydrates are stored as glycogen in the liver and muscles. Fat is stored in adipose tissue. Protein does not have a storage form as such and there is a continual cycle of protein synthesis, breakdown, and replenishment. However, breakdown substrates can be used for fuel; and muscle and other tissue are considered protein reservoirs that can be broken down and used as fuel in an emergency. (1,2)
The Fasting State
Between meals, over-night or in periods of food deprivation; nutrients are considered to be obtained endogenously or from ‘inside the body’. This is when nutrients are drawn from storage or synthesised from other substrates to ensure adequate supply to every cell.
The body has an amazing capacity to keep essential nutrients, including macro-nutrients, within a narrow range in the blood stream to ensure a constant supply. One of the main nutrients and a prime need of the body is glucose …
To Be Continued
Disclaimer: Nothing in this article or website should be taken as medical or dietary advice. Anyone reading any information provided within should seek advice from their own medical practitioner for any issue, disease, illness or health-related issue they may have. Always seek advice from a medical practitioner or dietitian before changing your own diet.
(1) E Whitney, S R Rolfes, T Crowe, D Cameron-Smith, A Walsh. Understanding Nutrition. Australia and New Zealand Edition. 2nd Edition. Cengate Learning. 2014.
(2) M L Wahlqvist. Ed. Food and Nutrition in Australia. Methuen Australia. 1982.
(3) Image courtesy: [Copyright] Grant Cochrane