Basal Metabolism

Basal metabolic rate (BMR), is the rate of metabolism that occurs when an individual is at rest in a warm environment and is in the post absorptive state, and has not eaten for at least 12 hours. The release of energy in this state is only sufficient for the vital organs such as the heart, lungs, nervous system, and kidneys. BMR decreases with age and with the loss of lean body mass. Regular cardiovascular exercise can increase BMR. Other factors can also affect BMR, such as illness, environmental temperature, and stress levels.

Basal metabolic rate is measured under very restrictive circumstances. A more common and closely related term used in many cases is resting metabolic rate (RMR), which is measured under less strict conditions. It is also important that a person is not sympathetically stimulated when the BMR is measured as it could affect the value of the BMR that is being measured.

BMR or RMR are measured by gas analysis through either direct or indirect calorimetry. Studies of energy metabolism using both methods provide convincing evidence for the validity of the respiratory quotient (R.Q.) which measures the inherent composition and utilization of carbohydrates, fats and proteins as they are converted to energy substrate units that can be utilized by the body for energy to survive.

Both Basal Metabolic Rate and Resting Metabolic Rate are usually expressed in terms of daily rates of energy expenditure. The early work of the scientists Harris and Benedict showed that average values could be derived using body surface area computed from height and weight, age and gender along with the oxygen and carbon dioxide measures taken from calorimetry.

Studies also showed that if you eliminate the gender differences that occur with accumulation of fat tissue from genetic determination by expressing metabolic rate per unit of "fat-free" or lean body weight the values between genders for basal metabolism are essentially the same. Exercise Physiology Textbooks have tables to show the conversion of height and surface area as they relate to weight and basal metabolic values.

The primary organ responsible for turning up or turning down the basal metabolic rate or the resting metabolic rate is the hypothalamus. The hypothalamus is located on the brain stem and forms the floor and part of the lateral walls of the third ventricle of the cerebrum. The hypothalamus is divided into a dozen nuclei within four major regions according to most major anatomy and physiology text books. The chief functions of the hypothalamus are:

1) Control and integration of activities of the autonomic nervous system (ANS), which regulates contraction of smooth muscle and cardiac muscle along with secretions of many endocrine organs. Through the ANS, the hypothalamus is the main regulator of visceral activities, such as heart rate, movement of food through the gastrointestinal tract, and contraction of the urinary bladder.

2) It is associated with feelings of rage and aggression.

3) It regulates body temperature.

4) It regulates food intake through two centers. The feeding center or hunger center is responsible for the sensations that cause us to seek food. When sufficient food or substrates have been received, then the satiety center is stimulated and sends impulses that inhibit the feeding center. The thirst center operates similarly when certain cells in the hypothalamus are stimulated by rising osmotic pressure of the extracellular fluid. If thirst is satisfied, osmotic pressure returns to normal.

So all of these functions taken together form a survival mechanism that causes us to sustain the body processes measured by BMR or RMR.

As an example for a 55 year old woman, the estimated BMR is 32 kcal per square meter per hour. If her surface area was 1.40 m², the hourly energy expenditure would be 44.8 kcal/h (32 kcal/(m²·h) x 1.4 m²). On a daily basis, this amounts to an energy expenditure of 1075 kcal per day (44.8 kcal x 24). The value 1075 kcal is the resting metabolic rate or if the more stringent conditions were met, it could also be the basal metabolic rate.

The primary substrates that supply the body with energy for basal metabolic measurement are carbohydrates, fats, and proteins. Each of these substrates have been measured for their caloric values in a bomb calorimeter which determines exact values for energy in units of heat that are expressed as calories.

A calorie is the amount of heat needed to raise the temperature of one kilogram of water one degree Celsius. Chemists often use a small calorie based on the gram rather than the kilogram. The large Calorie (capital "C") is often called a kilocalorie (kcal) which is one thousand small calories. The "calorie" content of food is actually expressed in terms of large calories, whether called Calories or kilocalories.

At restaurants, it is has become popular to provide customers with "Nutrition Facts" that explain the "caloric content" of each menu item. One popular restaurant chain describes its hamburger as having the serving size of 105 grams with 280 calories. In a sub category 90 calories are described as being from fat and 4 of those calories come from saturated fat.

The list is further subdivided into where the grams come from in the total weight content: 30 milligrams of cholesterol, 550 milligrams of sodium, 36 grams of carbohydrates, 2 grams of dietary fiber, and 7 grams of sugar. Therefore the application would be that if a person knew their BMR or RMR they could calculate what amount of caloric content and weight would satisfy their survival needs and what excess or deficit would render a weight loss or weight gain for the day along with thermic effect and effect from activity.

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