Page 39 - 86 human physiology part-2

Chapter 13

sedentary (physically inactive) individuals with an average body composition.[1] For these individuals,
the current value settings are as follows: a BMI of 18.5 to 25 may indicate optimal weight; a BMI
lower than 18.5 suggests the person is underweight while a number above 25 may indicate the person is
overweight; a BMI below 15 may indicate the person has an eating disorder; a number above 30
suggests the person is obese (over 40, morbidly obese).

In physiology the term “weight” is used interchangeably with “mass”. For a given body shape and
given density, the BMI will be proportional to weight e.g. if all body weight increase by 50%, the BMI
increases by 50%.

BMI is defined as the individual's body weight divided by the square of their height. The formulas
universally used in medicine produce a unit of measure that is not dimensionless; it has units of kg/m2.
Body mass index may be accurately calculated using any of the formulas below.

SI units US units UK mixed units

BMI Weight Status
Below 18.5 Underweight
18.5 - 24.9 Normal
25.0 - 29.9 Overweight
30.0 and Above Obese
The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of
American men and 49% of women have BMIs over 25. Extreme obesity — a BMI of 40 or more —
was found in 2% of the men and 4% of the women. There are differing opinions on the threshold for
being underweight in females, doctors quote anything from 18.5 to 20 as being the lowest weight, the
most frequently stated being 19. A BMI nearing 15 is usually used as an indicator for starvation and the
health risks involved, with a BMI <17.5 being one of the criteria for the diagnosis of anorexia nervosa.

Anorexia nervosa: is a psychiatric diagnosis that describes an eating disorder characterized by
low body weight and body image distortion with an obsessive fear of gaining weight. Individuals with
anorexia often control body weight by voluntary starvation, purging, vomiting, excessive exercise, or
other weight control measures, such as diet pills or diuretic drugs. It primarily affects young adolescent
girls in the Western world and has one of the highest mortality rates of any psychiatric condition, with
approximately 10% of people diagnosed with the condition eventually dying due to related factors.[1]
Anorexia nervosa is a complex condition, involving psychological, neurobiological, and sociological
components.[2]

Bulimia nervosa: commonly known as bulimia, is generally considered a psychological condition
in which the subject engages in recurrent binge eating followed by an intentional purging. This purging
is done in order to compensate for the excessive intake of the food and to prevent weight gain. Purging
typically takes the form of vomiting; inappropriate use of laxatives, enemas, diuretics or other
medication; and excessive physical exercise.

254 | Human Physiology