Basal Metabolic Rate
The metabolic rate is the rate the body uses energy for metabolic reactions. A metabolic rate measured under standard conditions (i.e., when the body is resting, fasting, and quiet) is known as the basalmetabolic rate (BMR).
As mentioned, some energy is used for ATP pro-duction and the rest is converted to heat. The meta-bolic rate of an individual is calculated in calories. A calorie is the amount of heat energy required to raise the temperature of 1 g water from 14°C to 15°C. A kilocalorie (kcal) or calorie (cal) is 1,000 calories.
The adult BMR is about 1,200–1,800 cal/day. The total calories required by an individual depends on his or her activity and physiologic state. For example, teenage boys and active men need about 2,800 calo-ries per day. The requirement is increased during pregnancy and lactation.
To meet the caloric needs of an individual, an ade-quate diet is required. It is recommended that the dis-tribution of calories in the diet should be: carbohy-drates (50% to 60%); fats (about 30% or less), with saturated fat less than 10%; and protein (about 12% to 15%). Metabolism of 1 g of protein or carbohy- drate produces about 4 calories, and 1 g of fat pro-duces 9 calories.
The metabolic rate is regulated by various factors, which depend on the condition of the cells (e.g., avail-ability of ATP and nervous and endocrine stimulus). It also depends on the amount of time after a meal. Soon after a meal and until approximately 4 hours later, when the absorption from the gut is complete, glucose is absorbed from the gut (absorptive state) and is readily available to the tissue. After the absorp-tion is complete (postabsorptive state), the blood glu-cose levels must be maintained from the body re-serves. It is important to maintain normal blood glucose levels because the nervous system and red blood cells depend on glucose for energy production.
Soon after a meal, the rising levels of glucose and certain amino acids stimulate the beta cells of the pan-creas to produce insulin. Insulin facilitates entry of glu-cose into cells, lipogenesis, glycogenesis, and protein synthesis. During the postabsorptive state, blood glu-cose levels begin to fall. Hormones and sympathetic stimulation maintains the level by breakdown of fat and protein, conversion of fat and protein into glucose, and breakdown of glycogen. The primary hormones involved are glucagon (from alpha cells of the pan-creas), epinephrine, norepinephrine, and cortisol.