How does a lack of emotion result from anorexia?

Anorexia and emotion: Your question perhaps needs to be reversed, since emotions can play a significant role in anorexia. Talk to your family doctor, and consider also talking with a psychiatrist or clinical psychologist.

The brain: Lack of emotion, or dysregulated emotion, occurs frequently when the brain has been starved. If the starvation is of long enough duration, apathy and indifference to one's surroundings sets in. This is also seen in famines and with intentional starvation in times of war. Children are especially vulnerable to this, but adults are affected too.

See below: In addition to the effect of starvation on brain function, brain imaging data suggests that the brain of someone with anorexia is unable to distinguish a reward from a loss. As food reward is processed by this system, deriving pleasure from eating is not "natural" in them.