Determining "ideal" body weight in children who suffer from anorexia nervosa is complex. Pediatric patients cannot be treated like "little adults". An example of this principle is the way medication is dosed in childhood. The right dose of an antibiotic for a newborn is different than the right dose for a two year old or for a 14 year old. And so it is for setting "weight goals" in pediatric eating disorder patients.
Bear with me then, because this is not simple. And not only is it not simple, but it is even more complicated and fraught with "special cases" than the following summary would indicate. Yet, I believe, parents can use it as a general guide.
A true discussion of goal weights cannot be separated from knowledge of a child's developmental stage. Have they gone through puberty? If so, is puberty complete? Has breast development begun (if a girl)? Has she ever had a period?
Children, like adults, will fall along some kind of a bell curve of normal weights: the vast majority will be in the average range with some being in the "obese" range and some being in the "growth-stunted" range where the eating disorder struck at a very young age causing stunting of both height and weight (and probably brain growth). I will address these groups below:
Group One : Children who were a normal weight before the onset of their anorexia nervosa
These children need to be divided further into four sub-categories:
A. Those who have not yet begun puberty
B. Those who are in early to mid puberty with some breast and pubic hair development (if a girl) or pubic and penile development (if a boy)
C. Those who seem to have full breast and pubic hair development but who may have had only one or a very few periods (if a girl) or who seem to have adult pattern pubic hair, but are not shaving and/or do not have adult pattern underarm hair (if a boy).
D. Those who had completed puberty and had two years of menstruation (if a girl) or who had completed puberty and had been shaving (if a boy) before the onset of their eating disorder.
Group A will have the most uncompleted growth potential. Their "goal weight" is a moving target. First return them to the highest weight they have ever experienced. Then go up. You will be looking for normalization of heart rate, blood pressure, temperature, and soon, resumption of height growth. Do not consider your child "done with gaining weight" until they are done with height growth or until they have had normal periods for two years (if a girl) or are shaving (if a boy).
Group B will also still be growing, though some of their growth will be behind them. Look at their growth chart and see what percentile they were growing along two years before you think the eating disorder started. You want this cushion of two years because research shows that, in retrospect, the eating disorder often started much earlier than anyone knew. Aim now for that weight which will return them to this previous ?centile and be aware that they will need to continue to gain more weight as they gain height.
Group C will have some, if reduced, height potential, but still have growing brains! Return them to their previous growth ?centile and expect them to need to put on a little more weight if they get taller.
Group D will be kids who were fully grown in height before their eating disorder started (but don't forget that brain growth continues). Return them to their highest pre-eating disorder weight. Unless they were objectively obese, resist the temptation to give in to their pleas to be returned to a weight lower than they weighed before their anorexia nervosa , no matter who in the family or circle of friends "weighs that little and they are ok". Remember also that you cannot alleviate anxiety by allowing your child to keep a "lower-end weight" since there is no weight low enough to appease the eating disorder. It's about health. Period.
Group Two: children who were obese before the onset of their eating disorder
These children represent a special case. No one wants to return a child to an obese state. If this was the case for your child please understand that your weight goals will be educated guesses and fraught with more anxiety than normal. A formerly obese child is subjectively afraid of "becoming fat" as all eating disordered children are and objectively afraid of it, too. In these cases we look at the family height and weight pattern, the child's growth pattern as a younger child and begin our climb up towards a "state" rather than a "weight". That "state" is normalization of heart rate, blood pressure, temperature and return to normal social behavior. In girls it also includes return of menstruation or the start of it, and in children category A-C (above) the resumption of growth in height. Rarely can a child who is genetically programmed to be larger than average be safely held at a "thin" body weight. Size acceptance may be a part of the family's treatment challenge.
Group Three: children who are growth stunted
These children also represent a special case. Sometimes girls who have been growth stunted for years prior to receiving treatment for their anorexia nervosa have been treated by family and friends as "petite" "dainty" and "elfin". They may like this. Parents may like it or at least accept it. As such children begin to grow both they and their parents may have to re-adjust their expectations of what this child will look like once they are healthy. Look at the family growth patterns, look at the growth ?centile along which the child grew before they showed signs of growth slow-down. Do not accept partial treatment. Not only is the body being stunted, but the brain as well.
In summary, what is a young person's ideal body weight, down the line, when they are fully grown? It is: that weight (in females) which allows them to have normal ovulatory periods and which they can maintain when not engaged in eating disorder behaviors.
In other words, if the only way a person can maintain a certain weight is by constant restrained eating and exercising for the sake of weight control, then that weight is not their body's ideal weight.
Children need to grow, they need to play, hang out with friends and family, learn and did I mention grow? Don't settle for anything less.