Energy Explained

Everyone understands the meaning of energy in their lives; the word encompasses concepts of vigour and activity in mind, body and spirit.  But what exactly is energy and, more importantly, how can we manage our energy levels and therefore sense of life itself?

Every single living cell, plant and animal, requires energy to grow, reproduce itself and perform crucial metabolic functions. Plants obtain this energy or fuel directly from the sun through a process called photosynethesis. Animals, including humans, need to eat plants or other animals to obtain this energy indirectly.

Energy is typically measured using two different units: calories and joules. One calorie, spelt with a small ‘c’, is scientifically defined as the amount of heat energy that is required to raise the temperature of 1g of water by 1° Celsius.  However, with regard to food, the common use of the term calorie actually refers to a kilocalorie (kcal) and is therefore distinguished by being spelt with a capital ‘C’.  In other words one Calorie is equal to 1 000 calories. The joule (J) is the unit of energy used in the International System of Units (SI) and 1 kcal or Calorie is  equivalent to 2.4 J.

Different foods provide us with different amounts of energy, depending on their molecular structure. When the food molecules are broken down through digestion, energy is released which is then harnessed by the body and used directly by the cells or, if in excess of the body’s requirement, stored as glycogen or fat for later use.

Fat is the most energy-rich type of food yielding 9 kcal per gram. Carbohydrate and protein yield less than half the amount of energy at 4 kcal per gram.  Alcohol yields 7 kcal per gram and can therefore be a strong contributor to energy supply and therefore weight gain.

In the body energy is produced by the mitochondria within the cells. These small structures resemble rechargeable batteries in that they transform a chemical called adenosine diphosphate (ADP) – using oxygen, glucose and fatty acids – into an energy-rich version called adenosine triphosphate (ATP). ATP can then travel anywhere to release its cargo of energy before returning to the mitochondria as ADP to be recharged again into ATP.

Most cells use either fatty acids (from the breakdown of fat) or glucose (from the breakdown of carbohydrate – protein can also be converted to glucose if required).  Brain cells only use glucose as fatty acids cannot pass the blood-brain barrier.

Muscles cells require a lot of energy, especially when we are exercising or doing hard physical work. When muscles are at rest they prefer to use fatty acids for energy but as soon as they become active they switch to glucose (or glycogen, a glucose storage molecule) for fuel. For this reason muscle cells have the highest number of mitochondria. Regular exercise increases both the mitochondrial size and number. This is how a person becomes fitter and has more energy available, and provides a strong inducement to keep up a regular exercise programme.

If our brain or muscle cells do not receive enough oxygen, glucose, fatty acids, or the vitamins and minerals that are required in the energy generation process, we soon start to feel the effects. Fatigue, lack of energy, loss of interest in life, cognitive decline and depression can all follow. Over 20 vitamins and minerals are required directly and indirectly to provide energy for muscles and brain activity. The most important nutrients required directly for energy metabolism include the B complex vitamins, magnesium, phosphorus, sulphur, iron, zinc and copper.

The first step in managing our energy levels so that we are not compromised physically, emotionally or mentally begins, therefore, with a healthy, nutrient-rich diet, which also provides the right amount of Calories.

Our daily energy needs

Note: The values below are approximate as our actual needs depend on our physical makeup as well as our activity levels.

AGE Calories/kcal per day AGE Calories/kcal per day
11-14 1845 11-14 2220
15-18 2110 15-18 2755
19-50 1940 19-50 2550
51-59 1900 51-59 2550
60-64 1900 60-64 2380
65-74 1900 65-74 2330
75+ 1810 75+ 2100