BMR ( Basal Metabolic Rate ) is the amount of energy expended while at resting time. Everyday requires a minimum number of calories to live. This minimum number is called The Basal Metabolic Rate ( BMR). Your BMR is the number of calories your organ needs to function while you perform no activity whatsoever like if you stayed in bed all day.
It is highly variable and the causes variation include factors such as Fat Free Mass (FFM), Fat Mass (FM), Age , Sex ,Hormonal Status ,Disease etc. This article will explain to you details about these factors.
Body Size And Composition
Basal and resting energy expenditures are related to body size, being most closely correlated with the size Fat Free Mass (FFM), which is the weight of the body less then the weight of its mass.
The size of the Fat Free Mass (FFM) generally explains about 70-80 percent of the variance in Resting Metabolic Rate (RMR). FFM is the metabolically active tissue in the body which is also called the Lean Body Mass (LBM) and so most of the variation in BMR between people can be accounted for by the variation in their FFM.
For example athletes with greater muscular development have higher BMR than non-athletes individuals. Thus exercise can help maintain a higher Lean Body Mass and hence a higher metabolic rate. Similarly, the lower basal metabolic energy requirements of women is primarily related to their generally lower amount of lean muscle mass ( more fat mass ) as compared to men.
The decline in BMR with increasing age is also to some extent the consequence of changes in the relative size of organs and tissues. Further larger people have higher BMR than people of smaller size. In fact individuals with greater surface area have a higher metabolic rate.
To illustrate , if two people of different heights and same weights , the taller individual with larger body surface area will have a higher metabolic rate. In adults with a higher percentage of body fat composition , mechanical hindrances can also increase the energy expenditure associated with certain types of activity.
The BMR per unit weight also varies with age being higher in children and lower in elderly. The loss of FFM with ageing is associated with a decline in the metabolic rate, amounting to about 1-2% decline per decade after early adulthood. The Resting Energy Expenditure (REE) is highest during the periods of rapid growth , chiefly during the first and second year of life, and reaches a lesser peak through the periods of puberty and adolescence in both sexes.
Women who have generally more fat in proportion to muscle than men , have metabolic rates which are 5-10% lower than men of the same weight and height. Thus differences in BMR between gender are due to the greater level of body fatness in women.
Thyroid status may be the most important factor and can make differences of up to plus or minus 50% for hyperthyroidism or hypothyroidism respectively. Hyperthyroidism increases the Resting Metabolic Rate (RMR). Stimulation of the sympathetic nervous system ( eg – during the period of stress or emotional excitement or fear , anxiety) causes the release of epinephrine, which directly promotes glycogenolysis and increases cellular activity.
In adult premenopausal women ,the metabolic rate fluctuates with the menstrual cycle. An average of 359 KCAL per day difference in the BMR has been measured between the low point about 1 week before ovulation on day 14, and the higher point ,just before the onset of menstruation.
Extremes in environmental temperatures also affect the metabolic rate. Energy expenditure will be increased if extra heat production is needed to maintain body temperature in cold time. The extent to which the energy metabolism increases in an extremely cold environment depends on the insulation available from body fat and protective clothing . Conversely, there is some evidence that the Basal Metabolic Rate is reduced in hot climates.
For example BMR is on an average 10% lower in Indians than North Europeans. Further exercise undertaken in temperatures greater than 86 Degree Fahrenheit also imposes an additional metabolic load of about 5% from increased sweat gland activity.
Beside climate conditions, altitude too has been shown to affect metabolic rates ,hypoxia ( Lack of oxygen in tissues ) of high altitudes increases BMR. Hypoxia increases glucose utilization , which might affect metabolism rate. However, these are temporary effects ,which disappear with acclimatization .
Pregnancy And Lactation
These periods of physiological stress also have an impact on the metabolic rates . REE ( Resting Energy Expenditure) is highest during the periods of rapid growth, ie – Chiefly during the first and second year of ,In pregnancy too. During the foetal growth period ,the metabolic rate increases particularly later in pregnancy because of uterine placental and foetal growth and the mothers increase cardiac workload.
Illness And Injury
Any illness or fever caused by an illness influences the metabolic rate. Fevers increases the metabolic rate by about 7% for each degree increase in body temperature above 98.6 degree fahrenheit or in other words 13% for each degree above 37 degree celsius.
During injury or infections there is an increased BMR, and this increase is dependent on the severity of the injury.
For example BMR even doubles with burns more than 40% of the body surface, in severe sepsis, multiple traumas, whereas it may only increase by about 25% in patients with long bone fractures and even less after surgery. But we need to understand that in sick patients who are likely to be in bed, the increase in the BMR due to the stress imposed by the disease may be offset by the decrease in physical activity, such that the total daily energy expenditure may not change drastically.
Undernutrition and starvation are the factors which require consideration. Prolonged undernutrition or starvation causes a reduction of about 10-20% in BMR. In semi-starvation studies, data suggest Lhnt the subjects BMR decrease by about 25% when expressed per kilogram of their fat free mass (FFM) (or metabolic active tissue).
Reductions in BMR are partly mediated through weight loss itself, in which metabolically demanding tissue of the body (the IjFM i.e. the lean body tissue) are reduced in size, and partly through reduction in the metabolic activity of these tissues.
Smoking is one variable thought to influence the metabolic rate. Smoking increases BMR, cessation of smoking lowers BMR. The BMR in sleep is about 5% less than in the basal condition.