The appropriate ranges for body weight related to sex, age, and height have been derived from healthy population samples, and there are many sources of this information. The ‘body mass index’ — defined as weight (kg)/height (m)2 — gives a ‘rule of thumb’ guide to obesity: 25–30 = overweight and >30 = obesity. Weight gain above the statistical healthy norm is widely acknowledged to be better avoided for both cosmetic and medical reasons, the latter on good grounds since the links between obesity, liability to ill-health, and earlier death are well established, on a statistical if not an individual basis. Unfortunately we are not — or many or most of us are not — endowed with a built-in physiological system which accurately and automatically matches appetite to requirement. So in affluent society a conscious, deliberate balancing act is required, with the aid of the calorie count and the bathroom scales.
But what does our body weight comprise, to add up to the reading on the scales? In the standard average 70 kg man there are over 40 litres of water, so water accounts for nearly 60% of body weight. Small day-to-day fluctuations in weight are often related to varying retention or loss of water, perhaps by excessive sweat loss, or linked to the hormones which regulate body fluids, including variations in women over the phases of the menstrual cycle. Boxers who are marginally overweight for their class may attempt to achieve their goal by dehydration — although interference with body water balance which is rigorous enough to be effective is perhaps unlikely to be good for their general condition.
The solid components can be separated into fat and lean fractions: adipose tissue and fat-free tissue. Body fat can be estimated from measurement of body density. As for inanimate objects, density can be determined by finding the difference between weight in air and weight in water; the fatter you are, the more you tend to float, because fat is less dense than the lean body mass. A person sits on a seat suspended from a weight gauge in a purpose-built water tank, and is briefly submerged, while holding their breath. The amount of air in the lungs has to be measured and allowed for, because this of course affects buoyancy. A formula is then applied which relates density to percentage fat. More practically, body fat is commonly estimated indirectly from skin-fold thicknesses: at several specified sites a fold of skin and its underlying fatty layer is lifted and its double thickness measured with graduated calipers. The several measurements are added and the sum used to find total body fat from published tables for different age groups: the tables were constructed by comparing skin-fold measurements in a great many people with the ‘gold-standard’ from body density estimations. More sophisticated measurements of all the body fat can now be made from scans using the technique known as DEXA. The norms for percentage fat are between 20% and 30% of body weight — higher in women than in men.
Weight gain is generally equated with an increase in body fat, although there can be a significant increase in muscle weight during recovery from starvation, illness, or injury, or during a rigorous exercise regime. In any state of positive energy balance (more caloric intake than output) most of the surplus is converted into fat and stored in the adipose tissues. In negative energy balance, it is mostly fat which is used up. When weight loss is deemed desirable, intake has to be cut down and/or output increased. Since an individual's basal metabolic rate accounts for a high proportion of energy output, an extra walk or run each week adds little extra: it may do something for general fitness but makes relatively little difference to fatness.
Body weight can be affected in either direction by different illnesses: some hormonal disorders cause weight gain (e.g. thyroid deficiency, or excess of corticosteroids) and others weight loss (e.g. overactive thyroid, adrenal deficiency). Any injury or acute illness can be accompanied by weight loss, if feeding does not match increased metabolic demands, and weight loss can sometimes be a warning sign of serious disease.
Sumner, M. (1981). Thought for food. Oxford University Press.
See also body fluids; development and growth; eating disorders; energy balance; exercise; obesity.
"body weight." The Oxford Companion to the Body. . Encyclopedia.com. (August 19, 2019). https://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/body-weight
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