Slow Metabolism (Low Metabolic Rate)

The metabolism is all the chemical reactions that occur in the body to sustain life. In the process heat is generated as energy is expended. A measure of the amount of heat generated over time is a direct indication of the energy expenditure and this is known as the metabolic rate. The process of energy production requires nutrients like glucose which are then converted into energy molecule known as ATP (adenosine triphosphate).

These molecules are broken down by the mitochondria in the cell to yield energy that powers the various life-sustaining functions of the cell and the body as a whole. Oxygen and various enzymes are also required for the energy production. The entire process is regulated by various hormones and fluctuates according to the energy demands of any given situation. Simply this means that a person who is sleeping requires less energy than a person that is running.

What is a slow metabolism?

The term slow metabolism or low metabolism is often loosely used to describe a person who is lethargic, easily fatigued and is overweight or obese. In the true sense though, it indicates a lower than normal metabolic rate that is either due to physiological (normal circumstances) or pathological (diseases) causes. For example, a slow metabolism is seen in conditions like untreated hypothyroidism (underactive thyroid gland).

Low Metabolism and Weight Gain

A low metabolism is often blamed as the cause of being overweight or obese. However, in most cases this is due to a lack of exercise and a higher than required calorie intake in food. Energy is measured in calories. Food has energy potential meaning that when it is broken down it will yield a specific amount of energy. Therefore food can be measured in calories. Physical activity requires energy which is also measured in calories. In order to maintain a normal body weight, a person should consume as many calories (food) as expended in physical activity during the course of a day. However, a high calorie intake means that only some of these calories are used for the energy needs and the rest are stored in the body in the form of fat. This is explained further under calorie count and body weight.

Factors that determine metabolic rate

Ultimately the metabolic rate is determined by a person’s level of activity. This is then regulated by various factors, most of which are a result of hormonal influence. A higher metabolic rate is seen in a person who is more active and therefore expends more energy in a day. A lower, or slower, metabolic rate is seen in a person who is less active and utilizes less energy. The metabolic rate can fluctuate during the course of the day – more energy is required and utilized when active like during walking or running and less energy is utilized when inactive like during sleep.

Therefore a person who is less active has a lower metabolic rate than a person who is very active. This does not mean that the person’s metabolism is lower than normal but simply that the energy needs are less due to choice. If a person becomes more active then the metabolic rate increases. There are, however, conditions where a person’s metabolic rate is lower or slower than it should be despite activity but this is either in a diseased state or under certain circumstances.

Hormones and metabolism

The body’s metabolic rate is primarily regulated by the two thyroid hormones – thyroxine (T4) and triiodothyronine (T3). When thyroid hormone binds to specific receptors on the surface of a cell, it triggers the cell to increase energy production. In the process more glucose is taken up from the bloodstream and glucose or even fat is recruited from the stores to meet with the cell’s energy needs.

The metabolism is not solely dependent on the metabolic activity of each cell but also on other factors like the availability of nutrients particularly glucose and oxygen supply. Other hormones may also influence the metabolism either directly or indirectly. This may include the hormones epinephrine, cortisol, insulin and various other hormones which have a smaller effect. Despite this wide array of hormones, it is the thyroid hormones that are the most prominent and important regulator of the metabolic rate.

However, these glands and its hormones do not act in isolation but are regulated by other glands and hormones. The thyroid gland is stimulated by thyroid-stimulating hormone (TSH) that is secreted by the anterior pituitary. TSH secretion is in turn controlled by thyrotropin-releasing hormone (TRH) secreted from the hypothalamus. Therefore if these controlling glands, its hormones or pathways are disrupted, then the metabolism may be slowed down.

Physiological Causes of Slow Metabolism

  • Age. The metabolic rate decreases with age which is associated with a lower muscle mass and possibly even lower than normal levels of certain hormones that regulate metabolism.
  • Gender. Men have a larger muscle mass and muscles are the biggest energy consuming cells because it is rich in mitochondria. Therefore females tend to have a slower metabolism than males.
  • Muscle mass. A more muscular person will have a higher metabolic rate even at rest since muscle cells are the largest energy consumers. Therefore a person with a lower muscle mass will have lower energy needs.

Diseases that Cause Slow Metabolism

These are known as a pathological causes of a slow metabolism.

  • Hypothyroidism is where there is a deficiency of thyroid hormones. This state may be due to a host of other causes that solely affect the thyroid gland, the thyroid hormones or regulating factors.
  • Diabetes mellitus (sugar diabetes) where the uptake and utilization of glucose by cells is diminished due to a deficiency of insulin or a defect in insulin response.
  • Cushing syndrome arises with an excess of cortisol levels (hypercortisolism). Cortisol directly reduces the cellular utilization of glucose and also indirectly does so by reducing the effect of insulin (adrenal diabetes).
  • Malnutrition may be due to a low intake of macronutrients (carbohydrates, protein, fats) or micronutrients (vitamins and minerals). While the macronutrients are directly used for energy production, the micronutrients play important biochemical roles that mediate the metabolic rate and therefore need to be available as well.
  • Galactosemia where a deficiency of the enzymes that break down galactose into glucose. Other carbohydrate sources can be used for glucose supply. Galactosemia is one of the many inborn errors of metabolism.
  • Anemia is deficiency of hemoglobin, the oxygen carrying compound in red blood cells. The low oxygen availability (hypoxia) reduces the metabolic rate since oxygen is required for energy production.

Signs and Symptoms of a Slow Metabolism

  • Weight gain if the calorie intake in food is constantly higher than required.
  • Fatigue and lethargy
  • Depression
  • Poor memory
  • Sleepiness
  • Slow demeanor – speech, movement
  • Diminished libido

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