Introduction
The living body requires energy for its maintenance, metabolism, vital activities and to perform various tasks. Food comprises various substrates which releases energy, when subjected to oxidation.
Food contains macronutrients as well as micronutrients. These are carbohydrates, fats and proteins, lipids etc. Each substrate, upon oxidation, releases energy at a different rate and extent. The order of oxidation is as follows: carbohydrate, fats and proteins.
Carbohydrates are the preferred substrates of oxidation. Energy also gets stored in the body in the form of high energy phosphate bonds in the molecules of ATP, which when required by the body gets released by breaking the bonds of ATP.
Fats and Calories
Calories are defined as the amount of released energy when the body oxidises down the food. The energy requirements of different individuals are different depending on gender, age, weight and other factors. The calorific value is measured in the kilojoule (kJ) and the kilocalorie (kcal). The higher the calories, the higher will be the energy that the food provides. When the food consumed contains extra calories than are required, the body accumulates it as “body fat” in the form of adipose tissues.
Fatty foods (such as sausage, bacon or potato chips) are rich in calories and have saturated fats with few essential nutrients than other low-calorie foods. Saturated fat increases low-density lipoprotein (LDL) or bad cholesterol in the blood which is not required by the body. Bad cholesterol is one of the major contributing risk factors of many cardiovascular diseases such as CHF and atherosclerosis.
Good cholesterol or high density lipoprotein (HDL) is required by the body for healthy living. Sources of unsaturated fats are majorly protein sources like lean meat, fish, seafood, poultry without skin, beans, lentils, tofu, dairy products, cottage cheese and flax seeds. They contain unsaturated fat and also Omega-3-fatty acids.High levels of HDL cholesterol can lower your risk for heart disease and stroke.
Fat contains more than twice the calories than carbohydrates and proteins present per gram of substrate. One gram of fat contains about 9 calories, while one gram of carbohydrate or protein contains nearly 4 calories.
How much Fat is Recommended?
Proper dietary intake including moderate fat content is important for maintaining body weight and health and for the prevention of diseases by gaining essential and adequate nutrients. According to the Institute of Medicine and the American Heart Association, the recommended fat intake level should be 25-35% of the total calories. For the majority of the population, approximately 1500-2000 calories per day in a diet are sufficient to maintain the healthy and normal functions of the body.
Measuring the Calorific Value of Fats
There are two methods used to measure the calorific value of food containing carbohydrates, fats and proteins:
- Direct method
- Indirect method
-
Direct Method
Through this method, the caloric value of food can be calculated by measuring the heat produced when a given amount of substrate is completely burnt in oxygen. It is performed with the help of a “bomb calorimeter” in which the oxygen is under constant pressure. It involves a heavy steel bomb with platinum or gold plated copper lining. It also has a cover that is tightly held by a strong screw collar. The sample is weighed and placed and the bomb is charged with an oxygen valve. The valve is then closed and the bomb is immersed in the amount of water being weighed. The burning of the substrate is terminated by an electric spark and the liberated heat is measured by the rise in temperature of the surrounding water. The temperature variation can be measured by a differential thermometer which can read up to one-thousandth of a degree.
-
Indirect Method
Through this method, the caloric value can be determined indirectly by burning the food in oxygen in an oxy-calorimeter. The volume of oxygen that is utilised to burn the food sample is measured and the caloric value can be determined. The amount of energy produced is directly proportional to the oxygen utilisation. The principle used to calculate the calorific value is that when 1 litre of oxygen is consumed in the oxidation of organic nutrients, approximately 4.8 kcal of heat is released. The oxygen consumption can be determined by quite a simple technique and is now applied universally to estimate the metabolic rate.
Daily Calorie Requirement
The daily requirement of calories varies from person to person depending on their activity level, age, gender, health status, lifestyle, pregnancy status, etc. The normal calorie requirement is as follows:
- For people with a sedentary lifestyle, it is usually between 12 and 15 calories per pound
- For people who are moderately active, it is usually between 14 and 17 calories
- For people who are very active, it is usually between 16 and 18 calories
- Women usually require a lesser range of calories than men
- Pregnant females require a high amount of calories
Conclusion
Fats are one of the main sources of energy fuel for the metabolism in the body. They are a rich source of energy as they provide 9 cal per gram of the food. While excessive fats contribute to various health disorders, the required amount is essential for maintaining the normal health and functioning of the body.
Fats generally have higher calorific value than carbohydrates and proteins whether at the gross level or at the physiologic level.
- Gross calorific value is the heat energy that is released when one gram of food is combusted in a bomb calorimeter (a device used to measure calorific value in laboratories).
- Gross calorific values are carbohydrate = 4.1 kcal/g, proteins = 5.65 kcal/g and fats = 9.45 kcal/g.
- Physiologic calorific value is the heat energy that is released when one gram of food is oxidised inside the body. Physiologic calorific values are usually less than gross calorific values as simultaneous consumption of energy takes place, for the normal functioning and maintenance of the living body.
- Physiologic calorific values are carbohydrate = 4.0 kcal/g, proteins = 4.0 kcal/g and fats = 9.0 kcal/g.
Fats get oxidised very slowly after heavy exercises or during fasting when carbohydrates are not present in sufficient amounts. In men, about 15% of the body weight is composed of fats, whereas in women, the average weight of body fat is approximately 25%.