Human being as a definition of a bio-psycho-social being is a whole, and when one of these mechanisms is affected, the other ones change, also.
The easiest example to observe is, for example, when the baby is sick, the mother experiences a lack of hunger, sleep, etc. This means that a psycho-emotional imbalance has influenced a biochemical balance.
Both weight gain and weight loss below the normal weight limit may have as a trigger an imbalance other than food, but which is reached anyway.