Basal metabolic rate
Basal metabolic rate is the rate of energy expenditure per unit time by endothermic animals at rest. It is reported in energy units per unit time ranging from watt to ml O2/min or joule per hour per kg body mass J/. Proper measurement requires a strict set of criteria to be met. These criteria include being in a physically and psychologically undisturbed state and being in a thermally neutral environment while in the post-absorptive state. In bradymetabolic animals, such as fish and reptiles, the equivalent term standard metabolic rate applies. It follows the same criteria as BMR, but requires the documentation of the temperature at which the metabolic rate was measured. This makes BMR a variant of standard metabolic rate measurement that excludes the temperature data, a practice that has led to problems in defining "standard" rates of metabolism for many mammals.
Metabolism comprises the processes that the body needs to function. Basal metabolic rate is the amount of energy per unit of time that a person needs to keep the body functioning at rest. Some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. Basal metabolic rate affects the rate that a person burns calories and ultimately whether that individual maintains, gains, or loses weight. The basal metabolic rate accounts for about 70% of the daily calorie expenditure by individuals. It is influenced by several factors. In humans, BMR typically declines by 1–2% per decade after age 20, mostly due to loss of fat-free mass, although the variability between individuals is high.
Description
The body's generation of heat is known as thermogenesis and it can be measured to determine the amount of energy expended. BMR generally decreases with age, and with the decrease in lean body mass. Increasing muscle mass has the effect of increasing BMR. Aerobic fitness level, a product of cardiovascular exercise, while previously thought to have effect on BMR, has been shown in the 1990s not to correlate with BMR when adjusted for fat-free body mass. But anaerobic exercise does increase resting energy consumption. Illness, previously consumed food and beverages, environmental temperature, and stress levels can affect one's overall energy expenditure as well as one's BMR.BMR is measured under very restrictive circumstances when a person is awake. An accurate BMR measurement requires that the person's sympathetic nervous system not be stimulated, a condition which requires complete rest. A more common measurement, which uses less strict criteria, is resting metabolic rate.
BMR may be measured by gas analysis through either direct or indirect calorimetry, though a rough estimation can be acquired through an equation using age, sex, height, and weight. Studies of energy metabolism using both methods provide convincing evidence for the validity of the respiratory quotient, which measures the inherent composition and utilization of carbohydrates, fats and proteins as they are converted to energy substrate units that can be used by the body as energy.
Phenotypic flexibility
BMR is a flexible trait, with, for example, lower temperatures generally resulting in higher basal metabolic rates for both birds and rodents. There are two models to explain how BMR changes in response to temperature: the variable maximum model and variable fraction model. The VMM states that the summit metabolism increases during the winter, and that the sustained metabolism remains a constant fraction of the former. The VFM says that the summit metabolism does not change, but that the sustained metabolism is a larger fraction of it. The VMM is supported in mammals, and, when using whole-body rates, passerine birds. The VFM is supported in studies of passerine birds using mass-specific metabolic rates. This latter measurement has been criticized by Eric Liknes, Sarah Scott, and David Swanson, who say that mass-specific metabolic rates are inconsistent seasonally.In addition to adjusting to temperature, BMR also may adjust before annual migration cycles. The red knot increases its BMR by about 40% before migrating northward. This is because of the energetic demand of long-distance flights. The increase is likely primarily due to increased mass in organs related to flight. The end destination of migrants affects their BMR: yellow-rumped warblers migrating northward were found to have a 31% higher BMR than those migrating southward.
In humans, BMR is directly proportional to a person's lean body mass. In other words, the more lean body mass a person has, the higher their BMR; but BMR is also affected by acute illnesses and increases with conditions like burns, fractures, infections, fevers, etc. In menstruating females, BMR varies to some extent with the phases of their menstrual cycle. Due to the increase in progesterone, BMR rises at the start of the luteal phase and stays at its highest until this phase ends. There are different findings in research how much of an increase usually occurs. Small sample, early studies, found various figures, such as; a 6% higher postovulatory sleep metabolism, a 7% to 15% higher 24 hour expenditure following ovulation, and an increase and a luteal phase BMR increase by up to 12%. A study by the American Society of Clinical Nutrition found that an experimental group of female volunteers had an 11.5% average increase in 24 hour energy expenditure in the two weeks following ovulation, with a range of 8% to 16%. This group was measured via simultaneously direct and indirect calorimetry and had standardized daily meals and sedentary schedule in order to prevent the increase from being manipulated by change in food intake or activity level. A 2011 study conducted by the Mandya Institute of Medical Sciences found that during a woman's follicular phase and menstrual cycle is no significant difference in BMR, however the calories burned per hour is significantly higher, up to 18%, during the luteal phase. Increased state anxiety also temporarily increased BMR.
Physiology
The primary organ responsible for regulating metabolism is the hypothalamus. The hypothalamus is located on the diencephalon 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
- * The ANS regulates contraction of smooth muscle and cardiac muscle, along with secretions of many endocrine organs such as the thyroid gland.
- * 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.
Variation and causes
The basic metabolic rate varies between individuals. One 2005 study of 150 adults representative of the population in Scotland reported basal metabolic rates from as low as per day to as high as, with a mean BMR of per day.The early work of the scientists J. Arthur Harris and Francis G. Benedict showed that approximate values for BMR could be derived using body surface area, age, and sex, along with the oxygen and carbon dioxide measures taken from calorimetry.
Studies also showed that by eliminating the sex differences that occur with the accumulation of adipose tissue by expressing metabolic rate per unit of "fat-free" or lean body mass, the values between sexes for basal metabolism are essentially the same.
The aforementioned 2005 study in particular found that 62% of the variation in BMR among participants was explained by differences in fat free mass. Other factors explaining the variation included fat mass, age, and experimental error including within-subject difference. The rest of the variation was unexplained. This remaining difference was not explained by sex, leptin levels, triiodothyronine levels, or by differing tissue size of highly energetic organs such as the brain.
In a 2012 study of 8780 white obese subjects, it was found that body weight and fat-free mass had similar predictive power when used to estimate BMR, their respective combination with gender and age being able to explain 59% or 60% of the variation. Gender plays a role in determining BMR in children and adolescents, but not in adults. In addition, the age-related reduction in BMR is mostly explained by the decrease in FFM.
The BMR of the average person has also changed over time. A cross-sectional study of more than 1400 subjects in Europe and the US showed that once adjusted for differences in body composition and age, BMR has fallen over the past 35 years. The decline was also observed in a meta-analysis of more than 150 studies dating back to the early 1920s, translating into a decline in total energy expenditure of about 6%.