Two main features must be present to make a diagnosis of anorexia nervosa:
- Active restriction of energy intake relative to requirements, leading to a significant low body weight in the context of age, sex, developmental trajectory, and physical health. The threshold for considering someone significantly underweight in adults is debated and varies – this can be a body mass index (BMI) below 17.0, 18.0 or 18.5.
- Over-evaluation of shape and weight and their control; that is, judging self-worth largely, or even exclusively in terms of one’s shape and weight and one’s ability to control them.
Rather than worrying about being underweight, people with anorexia nervosa are typically terrified of gaining weight and becoming fat. Indeed, many judge themselves as already being “fat” despite their low weight. For this reason they are sometimes said to have a “morbid fear of fatness” or a “weight phobia,” and their dieting has been described as being driven by a “relentless pursuit of thinness”.
Anorexia nervosa mainly affects teenage girls and young women, but about 10% of people with anorexia nervosa are male. In typical cases, the disorder starts with the adoption of a restrictive diet and, in some cases, with excessive exercising. People with anorexia nervosa tend to avoid eating foods they view as fattening and they may fast at times. About one third have recurrent episodes of binge eating, most of which are small in size (i.e. they are “subjective” episodes of binge eating). Some cases of anorexia nervosa, especially among teens, are short-lived and the person makes a complete recovery with or without treatment. Alternatively, it may evolve into bulimia nervosa or other forms of eating disorder. A small proportion of people with anorexia nervosa have a severe and enduring form of the disorder from which it can be hard to recover.
The Body Mass Index (BMI)
The body mass index (BMI) is used to determine if a person is underweight, normal weight or overweight. It is calculated by dividing the weight (in kg) by height squared (in m) (i.e. Wt/Ht2). BMI can be applied to all adults, of both sexes, between the ages of 18 and 60.
Below are the BMI thresholds (note that they are based on health risks, not appearance):
People of Asian origin have greater health risks at lower BMIs than Caucasians. It is also important to bear in mind that BMI does not apply to those below 18 years of age, adults above 60, and people with large muscle mass.
The BMI-for-age percentile growth charts are the most commonly used indicator to measure the size and growth patterns of children and teens (up to 19 years of age). The weight status categories defined by the Centers for Disease Control and Prevention (CDC) are as follows: underweight (less than the 5th percentile); normal or healthy weight (5th percentile to less than the 85th percentile); overweight (85th to less than the 95th percentile); obese (Equal to or greater than the 95th percentile). However, with patients with eating disorders it is advisable to consider a BMI-for-age percentile corresponding to a BMI of 19.0 in adults as the minimal threshold of healthy weight. Indeed, below of this BMI most people experience some adverse physical and psychosocial effects of being underweight.