Nutrition is the way our bodies take in and use food to maintain proper functioning, and is the foundation of good health. Eating correctly is critical for proper growth and functioning of our bodies. There is strong evidence that eating correctly can prevent several chronic illnesses or diseases, as well.
| UNDERSTANDING METABOLISM | UNDERSTANDING CALORIES | CALORIES PER GRAM |
| The term metabolism refers to the physical and the chemical process by which a living organism grows, maintains its nutrients and produces energy and its capability to reproduce. | A calorie is the amount of energy that is released as heat when food is metabolized.
It is essential that we understand that the body uses food for fuel. We should not be eating just for taste or satisfaction. |
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ENERGY REQUIREMENTS IN HUMANS
The estimated energy requirement is the average dietary energy intake predicted to maintain an energy balance (that is, when calories consumed are equal to the energy expended) in a healthy adult of a defined age, gender, and height whose weight and level of physical activity are consistent with good health. Differences in the genetics, metabolism, and behaviour of individuals make it difficult to accurately predict a person’s caloric requirements. However, some simple approximations can provide useful estimates: for example, sedentary adults require about 30kcal/kg/day; and very active adults require 40kcal/kg/day.
ENERGY CONTENT OF FOOD
The energy content of food is calculated from the heat released by the total combustion of food in a calorie meter. It is expressed in kilocalories (kcal, or Cal). The standard conversion factors for determining the metabolic calorie value of fat, protein, and carbohydrates are shown below.
Figure: Energy available from the major food components.
Note that the energy content of fat is more than twice that of carbohydrates or protein, whereas the energy content of alcohol is intermediate between fat and carbohydrates.
[Note a joule is a unit of energy that is widely used in countries other than the United States. For uniformity, many scientists are promoting the use of joules (J), rather than calories (1 cal = 4.128 J) However, kcal still predominates.]
HOW ENERGY IS USED IN THE BODY
The energy generated by metabolism of the macronutrients is used for three energy-requiring processes that occur in the body: resting metabolic rate, thermic effect of food (formerly termed specific dynamic action), and physical activity.
- Resting metabolic rate: The energy expended by an individual in a resting, post-absorptive state is called the resting (formerly basal) metabolic rate (RMR). It represents the energy required to carry out the normal body functions, such as respiration, blood flow, ion transport, and maintenance of cellular integrity. In an adult, the RMR is about 1800kcal for men (70kg) and 1300 kcal for women (50kg). Fifty to seventy percent of the daily energy expenditure in sedentary individuals is attributable to the RMR.
- Thermic effect of food: The production of heat by the body increases as much as thirty percent above the resting level during the digestion and absorption of food. This effect is called the thermic effect of food or diet-induced thermogenesis. Over a 24-hour period, the thermic response to food intake may amount to five to ten percent of the total energy expenditure.
- Physical activity: muscular activity provides the greatest variation in the energy expenditure. The amount of energy consumed depends on the duration and the intensity of the exercise. The daily expenditure of energy can be estimated by carefully recording the type and duration of all activities. In general, a sedentary person requires about thirty to fifty percent more that the resting caloric requirement for energy balance. Whereas, a highly active individual may require 100 percent or more calories above the RMR.
