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Energy in the biological cycle originates from the sun. Plants convert solar energy to chemical energy through a process called photosynthesis. Humans then eat plants and/or animals which also eat plants. From these we obtain the calorie/containing components or macronutrients known as carbohydrates, proteins, fats and alcohol, which contain stored energy. These are then broken down by the body’s cells to provide energy. The thickness of the arrows also illustrates the ease with which each nutrient is converted to energy or stored as fat in fat cells, a thicker arrow indicating easier conversion.
The body’s cells trap the chemical energy released from food in a high-energy compound known as adenosine triphosphate (ATP) which is stored in small quantities in every cell. When energy is needed (say to transport glucose into the cell), the ATP gets broken down to adenosine diphosphate (ADP), thereby releasing the energy needed for the cell’s processes.
Energy derived from fat, carbohydrate, a protein and alcohol is used in the body for catabolic processes which are involved in the break down of cell tissue and for anabolic processes which build up cell tissue. If protein is called upon to fuel these processes, the basic protein units (.amino acids) are first converted to glucose through a process called gluconeogenesis. In addition to supplying the energy for these processes, the four nutrients, but especially protein, can also be the building blocks for growth, such as when muscle size increases with exercise. The sum total of energetic events which occur in the body, i.e. anabolism plus catabolism, is known as metabolism.
Eventually all energy is reduced to heat, therefore the energy produced by living organisms is measured in terms of heat production as kilocalories (kcal). One kilocalorie is defined as the amount of heat energy required to raise 1 kilogram of water 1° Celsius (C) at 15°C. The energy values of food are measured in a similar way through direct calorimetry where a food item is placed in a chamber called a ‘bomb calorimeter’ and combusted in a vat of water. Using the conversion formula above, the rise in water temperature is recorded as kilocalories. The four basic food components have the approximate energy values per gram.
It is important to understand the abbreviations of terms as these are often confused in the popular press. What the lay person usually refers to as T calorie’ is actually 1000 calories’, or 1 kilocalorie’. One real calorie is actually a very small unit, and hence it is multiplied by 1000 to give a kilocalorie (or kcal). This is sometimes also referred to as 1 Calorie (spelt with a capital C). New metric measures also confuse the issue with 1 cal being equivalent to 4.184 joules, or 1 kcal being equal to 4.184 kilojoules (notation ‘kJ’). To round off the figures, 1 kcal is generally regarded as being equal to 4.2 kJ.
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