During the time of eating, carbohydrates, fats and proteins are broken down into glucose, fatty acids and amino acids respectively. In this broken down form, they are used for muscle glycogen and liver stores, body fat stores, urea where there is loss of nitrogen and body proteins.
When the body fails to receive any food whether partially or completely (fasting), the body gets fats and glucose from the respective stores. The storage in the muscle glycogen and the liver are broken down to glucose. This glucose is used to provide energy to the red blood cells, the nervous system and the brain cells. Metabolic activities also enable the body to break down into fatty acids from the body fat stores. The fatty acids are used to energy to the other cells relevant to the body.
When the body is deprived of food for more than 24 hours, the glycogen stores begin to dwindle (Whitney & Rolfes, 2010, 222). Since the body still needs energy, it reaches out to the protein stores. The lean tissues and muscles are broken down to amino acids in order to have enough glucose to serve the brain cells and other body components. The fats are converted into ketone bodies that serve as another source of energy thus sparing the protein breakdown. This function is carried out by the liver.
Diets that facilitate weight loss are meant to make the body receive less energy than the body actually needs. This enables the body to use up the excess body fats and glucose stored up in the cells in order to supplement the deficiency. By the continuous intake of these diets, the body continues consuming the excess fats and glucose. The person then consumes the needed diet only in little excess of the needed energy and fats in order to prevent obesity and to keep the body fit when there is a deficiency.
Physical activity consumes more energy more in terms of kcalories as compared to sedentary persons or people just dieting. Activities such as running, walking, cycling, swimming or any other kind of sorting activity that requires vigorous body activity requires high expenditures of the body’s energy. Some of the major components that contribute energy expenditure include metabolism, discretionary kCalorie allowance and body composition. All these are related to body activity (Sizer & Whitney, 2010, 42). Since each component is related to body activity, it is therefore significance to note thus the higher the body activity, the higher the chances are of increasing the energy usage of the body. However, it is also significant to note that the body has its own limit. Extremes end up harming the body.
When one compares a marathon runner and a person reading a book in a library but dieting, the marathon runner loses more weight than the person reading a book does. If it is assumed that the runner is running for 3 miles in 30 minutes, he/she will loose around 150 kcalories more than the one reading a book (Whitney & Rolfes, 2010, 291). The marathon runner also consumes more discretionary kcalorie allowance. This allowance is the difference between kcalories needed to keep the energy balance constant and those needed the supply of nutrients to the body cells. Exercise uses up energy so the energy required in maintaining body weight is increased in a runner while the energy to supply nutrients remains the same. In such a case, the marathon runner looses more weight. If the sedentary person is consuming 1500kCalories to maintain body weight, the runner is using up 2000 kCalories thus a discretionary allowance of 500kCalories.