Basal Metabolism Measurement
Basal Metabolism is the rate of metabolism when the body is at complete’ rest and when, after a 12-hour fast, digestion is at its normal daily minimum. Normally there is a constant relation between the basal metabolism and the surface area, being represented by the total hourly heat production to the square meter (approximately a square yard) of body surface. It may be determined directly by measuring the actual heat given off by the individual in a stated time, or indirectly by measuring at the same time the amount of oxygen used and the carbon dioxide given off, the surface area being determined by the subject’s height and weight.
The figure obtained, in proportion to the body surface, is fairly constant for normal people of the same age and sex. but varies in some measure in disease. For instance, hyper-thyroidism, fever diseases and leukemia exhibit high rates, while hypothyroidism, inanition, asthenia, and deficiencies of glandular activity have a low rate, which is present also during rest in bed.
Thus the basal metabolism rate differentiates diseases into three distinct groups: those with normal basal metabolic rates (normal heat production); those with increased rates; and those with decreased rates. There is a small variation nor¬mally, but not sufficient to disturb these three groups, for the increased and the decreased rates are respectively much above and below the normal range of variance.
Calories are used in expressing the metabolic rate; either the total in 24 hours or the number each hour to the square meter of surface area. However, in practice only the oxygen intake need be measured.
The metabolic rate is modified by various factors: It is greater in males than in females and greater in early life than later life; various foods modify the rate; muscular activity makes the test valueless; certain stimulating psychic reactions alter the rate; increase in temperature above normal increases the metabolic rate 7.2 per cent, for each degree Fahrenheit of temperature above normal.
Various diseases increase, while others decrease the rate. Children, as shown by basal metab¬olism determinations, require a larger proportion of food than do adults, the basal requirement in boys being 25 per cent, above that of adults of equal size. Before puberty the require-ment is especially high, but on the approach of full develop¬ment it becomes progressively lower. The same conditions pertain to girls, also, except that before and during the age of puberty there is not so pronounced an increase as there is in boys at the same period.
The figures in basal metabolism vary widely according to the rate of internal combustion and this latter is influenced by many internal and external causes.