This is an article about how the body reacts to restricted nutrition. Aside from weight loss, something else goes when you radically limit caloric intake: your hair.

More than half the population in the developed world struggles with excess weight, but another, smaller segment is afflicted with eating disorders of a different kind, anorexia nervosa and bulimia. These diseases are characterized by severe and often dangerous weight loss: anorexia by simple undernourishment, bulimia by alternating bouts of compulsive overeating and purging (through self-induced vomiting or the use of diuretics, laxatives and enemas).

In some cases with either disease, the individuals will engage in excessive exercise. Indeed, proprietors of health clubs are told to look out for members who use cardiovascular equipment for extended periods of time in correlation with related behaviors and appearance. Both are considered medical-psychological disorders, and in extreme cases the individual can die of her or his disease. Anorexia and bulimia occur most frequently in young females, as young as 9 years old and up through the mid-40s, although one out of eight victims are males of similar ages. An estimated 8 million Americans suffer from one form or the other.

Both disorders illustrate how the body throws off hair when stressed. The mechanisms are fairly straightforward:

1. Lack of proper nutrients, organ function insufficiency and gastric abnormalities starve the hair in the anagen phase (when hair grows, at the roots).
2. Hair shafts skip the catagen phase — the part where hair shafts are seen.
3. Hair shafts go directly into the exogen (loss) phase. This premature loss of hair is known as telogen effluvium.

Hair, of course, is not the only part of the patient’s physiology affected by this seemingly willful act of starvation (in fact, the patient may wish to end the behavior but is psychologically incapable of doing so without professional help). Seriously affected are the cardiovascular, nervous, and endocrine systems as well as the kidneys and the blood. According to the National Eating Disorders Association, a nonprofit organization, both anorexia and bulimia arise when people “use food and the control of food in an attempt to compensate for feelings and emotions that may otherwise seem overwhelming.” In broad terms, the association says that professional help is recommended for treatment and that addressing the concerns of any one individual is a complex task.

The phenomenon of how eating disorders affect hair health is instructive. Consider the basics of what makes up hair in a healthy individual:

• Hair is largely made of the protein keratin.
• The root of the hair is below the skin surface and is produced by a hair bulb, which also determines hair color.
• Normal hair loss at any age is between 50 and 100 hairs a day. (Until permanent hair loss begins, new hair grows back to replace those that are lost.)
• The body places hair low in its priorities. In times of scarcity, nutrients are first used by vital organs, leading to a starvation of the hair shaft.

It bears noting that this same type of hair loss, telogen effluvium, often occurs as a result of gastric bypass surgery and other treatments for obesity that involve forced dietary restriction and rapid weight loss. Other shocks to the body, including major surgery of any kind, can have a similar effect.

In otherwise healthy individuals — including surgical patients who adjust their eating with quality nutrition — hair growth resumes 6 to 12 months following surgery. Telogen effluvium as experienced by persons with anorexia or bulimia is usually temporary as well, according to research reported in Dermatological Nursing (13, no. 4 [August 2002]: 269-272, 277-278; F. Mulinari-Grenner et al., Departments of Dermatology and Pathology, Cleveland Clinic Foundation, Cleveland, Ohio).

Certainly, the lesson learned is that a healthy diet — not too little, and not too much — is as central to the condition of your hair as it is to general wellbeing. But it also illustrates the interdependencies and relationships of various parts of the body. Psychological conditions may affect how and what we eat. What we eat (or do not eat) sets off a chain reaction among organs, muscles, bones and nerves. The body determines what it needs to keep functioning under limited nutrition (heart keeps ticking, hair can fall out). At the very least, we can see that something isn’t working or is deficient when a very visible part of our physiology breaks down.

And on that level it’s clear that hair is sometimes more than just about vanity. If it is related to inadequate nutrition, its loss is similar to a canary in a coal mine: It is a warning sign.