Weight restoration is a crucial component of anorexia nervosa treatment. It is a challenging process for a multitude of reasons. Adding to the complexity and the challenge is the fact that during weight restoration, individuals with anorexia nervosa tend to require increasingly more calories to maintain the same rate of weight gain.
That is, individuals need to continually increase their caloric intake, in steps, sometimes upwards of 100 calories (technically, kilocalories) per kilogram per day, to continue gaining weight. For instance, an individual weighing 45 kg may need to eat 4,500+ calories to continue gaining 1-1.5kg (2.2-3.3lbs) a week. Indeed, studies have found that standard resting energy expenditure (REE) equations tend to overestimate caloric needs at the beginning of refeeding but underestimate them in the later stages (Forman-Hoffmann et al. 2006; Krahn et al., 1993).
After achieving a healthy weight, individuals recovering from anorexia nervosa still typically need to eat more calories to maintain their new healthy weight — more than healthy individuals of the same weight who do not have eating disorder histories — usually at least 50 to 60 calories per kilogram per day (e.g., about 2500-3000 calories for an individual weighing 50 kg (110 lb). This hypermetabolic periods tends to last between 3 – 6 months after weight restoration.
The graphic below shows the typical course of weight gain and calorie intakes for someone entering treatment at ~ 70% of average body weight for that height:
I do want to be clear that studies on metabolic rates, calorie requirements, and weight gain vary widely in their results. According to Salisbury et al. (1995), some studies have shown that individuals with AN need to consume an average of 5,350 extra calories to gain a kilo whereas others have suggested that the number is closer to 9,750 — that’s a big range. This may be due, in part, to methodological differences between studies, lack of differentiation between AN subtypes, and small sample sizes. And, as I blogged about in my last post: exercise. Exercise can lead to a threefold range in the amount of calories that individuals with AN need to gain (and maintain) weight (Kaye et al., 1988). As Zipfel and colleagues (2013) show, this is, at least in part, due to the higher calories needs of lean body mass among AN patients who engage in (excessive) exercise.
Still, what might explain the increased caloric requirements during refeeding? The most obvious answer is of course weight gain itself: As individuals gain weight, they require more calories to maintain that weight. Moreover, gaining 1 kg of fat requires more calories than gaining 1 kg of fat-free mass (9300 vs. 5300 calories). If individuals are gaining more fat mass during the latter stages of the refeeding process, then they would require more calories to gain 1 kilo:
Overall, the body composition data seem to suggest that at least 50%, and perhaps more, of weight regained is fat tissue. This conclusion is supported by the fact that the average excess number of kilocalories required to gain 1 kg of body weight in these studies was 7,462, which is approximately the average of 9,300 and 5,300 kcal (required for the gain of 1 kg of fat and protein, respectively). Furthermore, it may be that the initial very low body weights and percent body fat predispose [AN] patients to gain more lean tissue than fat early in refeeding. As patients approach a more normal weight later in refeeding, more fat than lean tissue is gained.
In addition to weight gain, there’s also the fact that patients with AN appear to be metabolically inefficient. Evidence suggests that individuals with AN convert more energy to heat (as opposed to building tissue) than do healthy controls:
Our clinical experience is that AN patients often complain of becoming hot and sweaty during nutritional restoration, particularly during the night. It is not uncommon that they will wake up sweating and their sheets are soaked. . . . This notion is supported by studies showing that the thermic effect of food in AN patients during renutrition is high, [63,65,66] representing up to the 30% of energy expenditure instead of the 14-16% in healthy controls  and being particularly high at the beginning of refeeding .
Some previous studies found that AN patients who binge and purge require fewer calories than AN patients who don’t. According to Sunday and Halmi (2003), however, those findings may have been due to differences in lean body mass vs. fat mass on admission. Individuals who have proportionately more lean body mass will require more calories to gain and maintain weight compared to those who start out with proportionately more fat mass (Sunday and Halmi, 2003).
Conversely, previous studies (e.g., Stordy et al. 1997, Walker et al., 1979) have found that patients with a history of obesity tent to need fewer calories to gain and maintain weight compared to patients without a history of obesity.
Despite decades of research we still don’t know all that much about metabolic changes during and immediately after refeeding. There’s a lot more research to be done. But for now, I’ll leave you with some practical suggestions regarding nutritional rehabilitation and weight gain.
From Mehler et al. (2010):
Caloric requirements for weight restoration in patients with anorexia nervosa are best determined by monitoring an individual’s rate of weight gain. Given this dynamic process, caloric requirements may have to be recalculated if weight gain is not being achieved as expected during the refeeding process. . . .
Ultimately, as the process of refeeding progresses and [ideal body weight] is achieved, very high levels of caloric intake may be temporarily necessary (70 to 80 kcal/kg) to promote ongoing weight gain. It has recently been elucidated that refeeding a patient with anorexia nervosa may be associated with an actual increase in REE during the weight gain process. Although the mechanism of this phenomenon is presently unknown, its clinical implications are quite clear: unusually high-calorie diets may be necessary to provide continued weight gain towards the end of the weight restoration process. A plateau in the desired rate of achievement of a patient’s target weight may be observed because of underestimation of caloric needs during the late stages of weight restoration due to the aforementioned change in the REE value.
Finally, to conclude, I’ll quote Marzola et al. (2013):
To obtain the best chance of long-term weight maintenance recovery, AN patients should persist with an increased [and varied!] caloric intake treatment plan.
Please keep in mind that this post was meant to be a very short and relatively simple overview of a complex topic, so, as always, feel free to ask questions if something is confusing or if you feel I overlooked something important. I really appreciate it :-).
Marzola, E., Nasser, J.A., Hashim, S.A., Shih, P.A., & Kaye, W.H. (2013). Nutritional rehabilitation in anorexia nervosa: review of the literature and implications for treatment. BMC Psychiatry, 13 PMID: 24200367