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The primary organ responsible for regulating metabolism 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 chief functions of the hypothalamus are:

• control and integration of activities of the autonomic nervous system (ANS)

• The ANS regulates contraction of smooth muscle and cardiac muscle, along with secretions of many endocrine organs such as the thyroid gland (associated with many metabolic disorders).

• 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.

• production and regulation of feelings of rage and aggression.

• regulation of body temperature.

• regulation of 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 and leptin is high, then the satiety center is stimulated and sends impulses that inhibit the feeding center. When insufficient food is present in the stomach and ghrelin levels are high, receptors in the hypothalamus initiate the sense of hunger.

• The thirst center operates similarly when certain cells in the hypothalamus are stimulated by the rising osmotic pressure of the extracellular fluid. If thirst is satisfied, osmotic pressure decreases.

All of these functions taken together form a survival mechanism that causes us to sustain the body processes that BMR and RMR measure.

Aerobic vs. anaerobic exercise
Studies published in 1992 and 1997 indicate that the level of aerobic fitness of an individual does not have any correlation with the level of resting metabolism. Both studies find that aerobic fitness levels do not improve the predictive power of fat free mass for resting metabolic rate.

This means it is unclear whether aerobic exercise is effective in raising the RMR (or BMR).

Anaerobic exercise – such as weight lifting – builds additional muscle mass, which is fat-free mass. Additional fat-free mass will lead to a higher resting metabolic rate according to the above studies.

Also, while aerobic exercise is beneficial for cardiovascular reasons as well as direct calorie burning, recent studies seem to suggest that heavy endurance exercise also increases resting metabolism. However, it is unclear if light cardiovascular training has the same effect.

Each person’s metabolism is unique due to his or her unique physical makeup and physical behavior. For some, this makes weight management a very difficult undertaking requiring sophisticated expertise.

There are a number of medical adjustments to natural human processes that can affect one’s metabolism:

Menopause affects metabolism but in different ways for different people, thus hormones are sometimes used to minimize the effects of menopause. Weight training can have a longer impact on metabolism than aerobic training, but there are no formulas currently written which can predict the length and duration of a raised metabolism from trophic changes with anabolic neuromuscular training.

Gastric bypass surgery is used to reduce the content capacity of the stomach, bringing caloric intake down and lowering thermogenesis. Because the surgery significantly reduces caloric consumption, it will decrease BMR and RMR over time in the same fashion as aging, because the volume of the stomach is reduced. The stomach along with the rest of the digestive tract is a major contributor to BMR and RMR.

Heart rate is determined by the medulla oblongata and part of the pons, two organs located inferior to the hypothalamus on the brain stem. Heart rate is important for basal metabolic rate and resting metabolic rate because it drives the blood supply, stimulating the Krebs cycle. During exercise that achieves the anaerobic threshold, it is possible to deliver substrates that are desired for optimal energy utilization.

The anaerobic threshold is defined as the energy utilization level of heart rate exertion that occurs without oxygen during a standardized test with a specific protocol for accuracy of measurement, such as the Bruce Treadmill protocol (see Metabolic equivalent).

With four to six weeks of targeted training the body systems can adapt to a higher perfusion of mitochondrial density for increased oxygen availability for the Krebs cycle, or tricarboxylic cycle, or the glycolitic cycle. This in turn leads to a lower resting heart rate, lower blood pressure, and increased resting or basal metabolic rate.

Knowing what the body burns at rest or through exercise yields (via heart rate monitoring) a targeted program of energy utilization based on metabolic performance. The resting heart rate is correlated to the resting metabolic rate because of the singular contribution made by the heart to survival.

By measuring heart rate we can then derive estimations of what level of substrate utilization is actually causing biochemical metabolism in our bodies at rest or in activity. This in turn can help a person to maintain an appropriate level of consumption and utilization by studying a graphical representation of the anaerobic threshold. This can be confirmed by blood tests and gas analysis using either direct or indirect calorimetry to show the effect of substrate utilization. The measures of basal metabolic rate and resting metabolic rate are becoming essential tools for maintaining a healthy body weight.

Thermic effect of food (also commonly known simply as thermic effect when the context is known), or TEF in shorthand, is the increment in energy expenditure above resting metabolic rate due to the cost of processing food for storage and use.

• 1 It is one of the components of metabolism along with the resting metabolic rate, and the exercise component. Another term commonly used to describe this component of total metabolism is the specific dynamic action (SDA). A common number used to estimate the magnitude of the thermic effect of food is about 10% of the caloric intake of a given time period, though the effect varies substantially for different food components. Dietary fat is very easy to process and has very little thermic effect, while protein is hard to process and has a much larger thermic effect.

• 2 Raw celery and grapefruit are often claimed to have negative caloric balance (requiring they take more energy to digest than usable energy received from the food), presumably because the thermic effect is greater than the caloric content, due to the high fiber matrix that must be unraveled to access their carbohydrates; however, there has been no research carried out to test this theory.

The thermic effect of food is increased by both aerobic training of sufficient duration and intensity and by anaerobic weight training. However, the increase is marginal, amounting to 7-8 cal per hour.1 The primary determinants of daily TEF are the quantity and composition of the food ingested.