The Components of the Food We Eat: Energy Why Energy?

Energy is needed for the body to survive and function properly. Food provides the energy, which is used to power bodily functions (even at rest and while asleep) and physical activity - e.g. walking, running, lifting, etc.


The body derives energy from the main constituents in the diet i.e. carbohydrates, fats, protein and even alcohol. The first three are referred to as energy nutrients. Energy is measured in Calories (kcal) or kilo-Joules (On food labels, energy is shown in kilo-Calories kcal and kilo-Joules kJ). in the same way that weight is measured in stones and pounds or kilograms. For simplicity, energy is generally referred to as calories. In this article, all units of energy are expressed as kcal.


In the body, energy is released gradually from food by a series of steps (each carefully controlled by an enzyme). This energy is used to perform muscular work (mechanical energy), maintain body temperature (thermal energy) and digest, absorb and metabolise food (specific dynamic action of food) in addition to powering processes such as breathing. But it is worth bearing in mind that a considerable amount of energy is lost as heat. The energy provided by carbohydrate, fat, protein and other constituents can be measured and used to calculate the energy value (calorific value) of any food, diet or recipe or even a meal.

Energy Value of Food

1g (gram) dietary carbohydrate provides 3.75 (4) kcal
1g dietary protein provides 4 kcal
1g dietary fat provides 9 kcal
1g dietary alcohol provides 7 kcal

In addition to the above there are Nutritive sweeteners such as polyols (e.g. sorbitol, maltitol, xylitol, isomalt and mannitol) which also provide energy. Furthermore, a small number of calories can be derived from organic acids such as citric acid in fruits and acetic acid in vinegar, with each gram providing approximately 3 kcal. Vitamins, minerals, fibre and water do not provide energy. For some people, sweets, deserts and fizzy and alcoholic drinks represent significant sources of energy. 

Knowledge of the above values is quite essential as they help in identifying the source/s of calories in the diet. For instance, one may be eating a healthy, well-balanced diet, but still experience some weight problems if alcohol intake isn't taken into account.

Sources of Energy in the Diet

Almost all foods are composed of protein, fat, carbohydrate and water in varying degrees. Foods which contain a large amount of water such as salad, vegetables, fruits and clear soups usually contain little protein, fat, or carbohydrate and hence little energy (kcal).

Conversely, dry foods such as cereal grains, breakfast cereals and foods rich in fat are good sources of energy (N.B. one gram of fat provides more than twice the energy of one gram or protein or carbohydrate). Sugars are also concentrated sources of energy. This clearly indicates that reducing calories means reducing the consumption of both energy dense foods (i.e. those containing high proportions of fat and sugar) as well as portion sizes - a practical approach that works and continues to work very well anywhere. The following nutrients are usually present in the food we eat, albeit to a varying degree:

CARBOHYDRATE

Carbohydrates supply the body with energy and some vitamins, and may also be converted into body fat if taken in excess of the body's needs. Three major categories exist under this group; (a) starch, (b) sugar and (c) non-starch polysaccharides (NSP). Sugars and starches are major sources of energy for humans all over the world. Non-starch polysaccharides are known collectively as fibre.



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FAT

The term fat includes both fats and oils and provides energy in a more concentrated form than carbohydrate as indicated above. Fats derived from food can easily be converted into body fat and stored in adipose tissue if consumed in excess. Two types of fat exist; (a) saturated fat and (b) unsaturated fat (monounsaturated and polyunsaturated).

PROTEIN

Protein can originate from both animals and plants. It provides the body with amino acids that are essential for growth and repair. It can also be converted into carbohydrate (glucose) and used to provide energy, especially when energy intake is severely restricted and/or during starvation.

MINERALS

These include macro-minerals like Calcium, Magnesium, Manganese, etc and trace metals such as Zinc, Copper, Selenium, etc. They are required for tissue growth and repair and to regulate various body processes.

VITAMINS

Some are 'water soluble' such as the B-complexes and Vitamin C and some are 'fat soluble' and include vitamins D, A, K and E. Their prime role is to regulate body processes.

WATER

Like oxygen, water is also essential for life. However, it is not usually considered as a food or nutrient.
OTHER CONSTITUENTS OF FOOD:


The group of carbohydrate compounds, collectively known as 'fibre' or non-starch polysaccharides, are not strictly nutrients but have important, positive effects on health.

On the other hand, alcohol is considered as a food because it provides energy, even though it has no other nutritive value.

Iron (Fe) from cooking utensils is regarded as a nutrient as it may be incorporated in foods and consequently used to renew red cells.

Clearly, there is hardly any food that contains only one nutrient. The majority of foods are very complex mixtures of a variety of different nutrients, which consist mainly of carbohydrates, fats and proteins, together with water. 

Minerals and vitamins are usually present in smaller amounts despite their overriding roles in metabolic processes. On the basis of this illustration, there is more in food than just calories.