The biochemistry which controls eating would be a phenotypic variation. This means it would vary from person to person.

Phenotypic variation is something I discovered in the study of the morel mushroom. Knowing what it is, I see it in all of biology.

In the morel mushroom, it shows up as differences in tissue thickness. Some morels have very thick tissue, and some have very thin tissue, while most are in between. They need different tissue thicknesses to cope with weather being either too wet or too dry for shrinking and propelling spores out of the tissue.

Then I noticed it in the color of wildflowers. They vary a lot, when they would be expected to be all the same. Then I noticed it throughout biology. In humans, most of the visible variations would involve phenotypic variation.

Here's what phenotypic variation is: Even when individuals have the same genetic make-up, they can show differences in appearance. Phenotype means appearance. The most obvious example of this is embryonic development. The cells start to become different types, even though all of the cells have exactly the same genes.

The way this works is that there are multiple options for genes. Some genes will be turned on, while others are turned off. The appearance (also called expression or function) will vary depending upon which genes are turned on and which are turned off.

Just as this occurs between cell types within the body, it can occur between individuals. If two sisters differ in height or weight, it's not entirely due to differences in genes. It would be largely due to phenotypic variation. They would have the same genes for height, but different ones would be turned on or off.

Nature needed this mechanism to allow some persons to survive under a wide variety of conditions. If for example, a heavy blizzard were to strike during the winter, a large, heavy person could hunt in the snow easier than a thin, light person. If speed were needed to fight wars or escape predators, a thin, light person would be faster. But genetics cannot change from year to year; so nature produces a variety of types within a population.

The test of phenotypic variation is random distribution through a population rather than the usual distribution of traits being passed from parents to children.

This would apply to eating disorders. Whether a person wants to eat is controlled by chemistry in the body. Hunger is a biochemical reaction. Being full and not wanting to eat is also a biochemical reaction.

It is quite observable that anorexics originally decide not to eat for arbitrary reasons, but after they stop eating, biochemistry takes over, and sometimes it tells them not to eat. For some persons, this tendency is more extreme than for others; and the difference would be a phenotypic variation.

This can be explained in terms of evolution. In the hunter-gatherer culture of early humans, if food is not available, a person should not get overly obsessed with trying to acquire it. Otherwise, they might waste too much time and energy on futility, or they might harm others trying to get it. Therefore, a biochemical reaction tells a person not to eat, if they haven't eaten for some time. But this signal varies from person to person. Usually, it is not so strong that it prevents a person from eating when they decide to. But for some persons, the signal is so strong that it locks them into a mode of not wanting to eat. Such variations would be phenotypic.

The purpose in explaining this is to suggest that an essential element of breaking out of the starvation trap is to change the biochemical signals. This would be done by eating. Of course, an anorexic person doesn't want to eat. But they rationalize to themselves. They need to know that there is a biochemical signal, and it needs to be changed. It would be easier for them to break out if they knew there is a biochemical cause, and it can be controlled.

To break out of the biochemical trap, an anorexic should try to eat sugary fruit. It's the easiest thing to eat and the easiest to digest after fasting; and it would be the most effective in changing the biochemical signal, because sugary fruit is rapidly absorbed and metabolized.