ENERGY: an Introduction

In order to live and carry out all daily activities we need energy. Most of the energy supplied by our food and drink is used to maintain basic body functions, i.e. it keeps theheart beating, the blood circulating, and the lungs and other major organs, including the brain, constantly working. A variable amount of energy is needed in addition to the basal metabolism to account for activity for mostpeople this adds up to half as much as the basal requirement to the day’s needs. Energy is measured in kilocalories(kcal) or kilojoules (kJ) (1 kcal is 4.18 kJ). Although scientists use the precise term kcal, most people simply call this unit a ‘calorie’, and adults generally require between 1500 and 3000 kcal or calories per day to maintain body weight, depending on gender, body size, and activity level.
Taking in more energy than required (or not compensating for a high energy intake by increasing activity) resultsin weight gain; insufficient energy intake for the level of activity you do results in weight loss...

Energy is supplied by the major components of our food, i.e. carbohydrate, fats, and protein (sometimes called macronutrients), as well as by alcohol. One gram of pure
carbohydrate or protein provides approximately 4 kcal,while fat supplies 9 kcal per gram and alcohol 7 kcal pergram; however, as few foods are composed of just one
nutrient, the energy content of the food depends largely on the amount of water in the food and the proportions of protein, fat, and carbohydrate (including dietary fibre)making up the dry weight. For example, foods such as fruits and vegetables generally contain large amounts of water, no fat, little protein, and varying amounts of carbohydrate, but because of the high water content may have energy values as low as 1 kcal per gram, whereas foods with a high fat and low water content, such as nuts, will have energy values in excess of 6 kcal per gram.Dietary fibre, or non-starch polysaccharides (see below),are considered to be part of the carbohydrate component of foods but supply about 2 kcal per gram. A high dietary fibre
content therefore acts to dilute the energy content of a food.