Vitamin D and Hypophosphatemia in Patients with Anorexia Nervosa and Avoidant/Restrictive Food Intake Disorder: A Case Control Study

Background Refeeding hypophosphatemia (RH) is a common complication of nutritional restoration in malnourished individuals, yet clear risk stratification remains elusive. Individuals with anorexia nervosa (AN) and avoidant/restrictive food intake disorder (ARFID) may be deficient in vitamin D, an important component of dietary phosphorus absorption in the gut. The relationship between vitamin D and RH in AN and ARFID is unknown. Therefore, the aims of this of this study were to 1) describe the prevalence of low serum 25-hydroxy vitamin D levels and RH in AN and ARFID 2) report associations between nadir phosphorus level and variables associated with RH in extant literature and 3) examine the relationship between 25-hydroxy vitamin D levels and serum phosphorus nadir in AN and ARFID. Method Analyses included retrospective chart review of 307 individuals admitted to the ACUTE Center for Eating Disorders and Severe Malnutrition with a diagnosis of AN or ARFID. Variables of interest included admission laboratory values (vitamin D level, comprehensive metabolic panel, hemoglobin, point-of-care blood glucose), anthropometric measures (weight, body mass index [BMI], % ideal body weight [IBW]), age, duration of illness, length of stay, feeding method, and serum phosphorus nadir. Pearson and Spearman rank correlation, one-way ANOVA, and regression analyses were used to determine the relationship between variables and serum phosphorus. Results Over 1/3 of the sample (35.3%) had serum phosphorus levels ≤ 2.9 mg/dL. There were no significant differences between groups in phosphorus nadir (p = .17, η2 = 0.12) or hypophosphatemia (p = .16, ϕc = 0.11); 44% of individuals with ARFID and 33% of individuals with AN had hypophosphatemia. Nadir phosphorus showed a positive association with weight, BMI, %IBW, potassium, and calcium on admission, and a negative association with length of stay, hemoglobin, and total number of tube-fed days. Higher levels of 25-hydroxy vitamin D moderated the relationship between serum phosphorus nadir and weight on admission (p = .0004). Conclusion Individuals diagnosed with ARFID are as nutritionally fragile as those with AN regarding vitamin D and RH. The negative feedback loop involving vitamin D that maintains phosphorus homeostasis may play a role in the development of RH in AN and ARFID.


Introduction
2][3][4][5][6] During hospitalization, one critical area of clinical concern is refeeding syndrome, a potentially life-threatening cluster of metabolic and electrolyte abnormalities that can occur when a person in a starvation state shifts from catabolism to anabolism during nutrition restoration. 7Refeeding syndrome is associated with impaired respiratory and cardiac function, hemolysis, rhabdomyolysis, encephalopathy, uid overload, and death. 7[13] One key etiological factor for the development of refeeding syndrome, and a main target of clinical monitoring, is low serum phosphorus level, known as refeeding hypophosphatemia (RH). 14RH has been observed in up to 38% of individuals with AN. 15 Although many factors have suggested associations with RH (e.g.low body mass index [BMI], rate and magnitude of weight loss, low serum magnesium and potassium, higher levels of hemoglobin), [15][16][17][18][19][20][21][22][23] they are not su cient to understand and identify who is at greatest risk for RH in populations of AN.Additionally, there has been little investigation into the development of RH in individuals with ARFID.One area yet to be fully investigated is the relationship between vitamin D and RH in this population.
5][26] The Endocrine Society recommends serum 25-hydroxy vitamin D as a reliable measure of vitamin D, with levels below 20 ng/mL classi ed as de cient, levels between 21 and 29 classi ed as insu cient, and levels 30 or greater classi ed as su cient. 279][30][31][32] Low 25-hydroxy vitamin D levels have also been described in those with ARFID. 26,33e role of vitamin D in osteoporosis and osteopenia has been a topic of interest in literature pertaining to eating disorders, but has yet to be described in this population with regards to RH. 26,34,35 As a component of a negative feedback loop involving phosphorus, calcium, parathyroid hormone, and broblast growth factor 23, vitamin D activates sodium-phosphate co-transporters on intestinal epithelial cells that are responsible for phosphorus absorption in the gut. 36,37Therefore, low levels of 25-hydroxy vitamin D may play a role in the development of RH and risk of refeeding syndrome.

Current Study
Due to vitamin D's potential role in the development of RH, the aims of this study were to 1) describe the prevalence of low serum 25-hydroxy vitamin D levels and refeeding hypophosphatemia in AN and ARFID; 2) report associations between nadir phosphorus level and variables associated with hypophosphatemia in the literature (admission labs, weight on admission, % IBW on admission, length of stay, nasogastric tube feeding, age, caloric prescription, and duration of illness); and 3) examine the relationship between serum 25-hydroxy vitamin D levels and the serum phosphorus nadir during nutritional rehabilitation in these groups.We hypothesized that there would be an association between the levels of 25 Patients at ACUTE were started on oral nutrition restoration unless oral feeding was not possible.Nasogastric tube feeding was used for oral food refusal on a short-term basis and discontinued as soon as an individual could tolerate oral intake.Electrolytes were monitored daily and replenished via oral supplementation at the discretion and clinical judgement of the provider.Intravenous electrolyte repletion was necessary in cases where rapid or dangerous drop in levels were detected.

Statistical Analyses
Variables were summarized using descriptive statistics.Biological sex was not included in the analyses due to an insu cient number of males in this cohort.To test relationships between nadir phosphorus and variables of interest, Pearson's correlations and Spearman rank correlations were used.Analysis of variance (ANOVA) was used to address the hypothesis that lower 25-hydroxy vitamin D level on admission would be associated with lower serum nadir phosphorus level.Tukey post-hoc analyses were performed for signi cant relationships, as appropriate, and reported in Table 2.As serum phosphorus nadir may in uence clinical decision making even without meeting the threshold for hypophosphatemia (e.g. increase or start supplemental phosphorus if serum phosphorus is down trending), we examined both serum phosphorus nadir as well as whether or not the individual experienced hypophosphatemia.
Binary logistic regression was used to test the hypothesis that individuals with lower 25-hydroxy vitamin D level on admission would be more likely to demonstrate refeeding hypophosphatemia (de ned as serum phosphorus<2.9mg/dL) during hospitalization.A logistic regression model was created with variables signi cantly associated with the outcome of hypophosphatemia, using a p-value of <.10; all variables were entered into the model simultaneously.Moderation analyses were then run to examine interaction between variables of interest and vitamin D on the outcome of nadir phosphorus using PROCESS macro package. 38Effect sizes were reported as partial eta squared (η 2 , small=0.01,medium=0.06,large=0.14),and Cramer's V (ϕc; small=0.10,medium=0.30,large=0.50).Statistical analyses were conducted using SPSS version 27 statistical software, 39 and p-values of <0.05 were considered statistically signi cant.This study was approved by the Institutional Review Board.

Results
The mean age of the study cohort was 29 years (SD = 11.42) and the sample was overwhelmingly female (Table 1).There were no signi cant differences between individuals with AN and individuals with ARFID in age, length of stay, admit weight, admit BMI, admit %IBW, duration of illness, or caloric prescription.

Nadir Serum Phosphorus and Refeeding Hypophosphatemia by Diagnosis
Over one-third of the patients (n=109, 35.3%) recorded a serum phosphorus of 2.9 mg/dL or less over the course of admission.Hypophosphatemia during admission was found in 44% of individuals with ARFID compared to 33% individuals with AN (Table 2).There were no signi cant differences between groups with respect to mean phosphorus nadir (F=1.80,p=.17, η 2 =0.12) or prevalence of hypophosphatemia

Relationship between Nadir Phosphorus and Other Variables of Interest
Nadir phosphorus level showed a signi cant positive association with weight, BMI, %IBW, serum potassium, calcium, and prealbumin on admission (Table 3).A negative association was found with length of stay (r 2 =-.170, p=.003), total number of tube-fed days (r 2 =-.170, p=.003), and hemoglobin on admission (r 2 =-.140, p=.014).A non-signi cant trend was found between use of tube feeding (dichotomized as present versus absent) and nadir phosphorus level (F=3.85,p=.051, η 2 =0.012).Nadir phosphorus level was not associated with age, weight gain per week during hospitalization, kcal prescribed on admission, duration of illness, or remaining laboratory studies (admission magnesium, AST, ALT, point-of-care blood glucose).

Relationship between Nadir Phosphorus and Vitamin D
For the whole sample, there was no correlation between 25-hydroxy vitamin D level on admission and nadir serum phosphorus level (p=.447).At a trend-level approaching signi cance, individuals who were vitamin D de cient (<20 ng/mL) were more likely to become hypophosphatemic during admission than those whose 25-hydroxy vitamin D levels were greater than 20 ng/mL (either insu cient or su cient; x 2 =3.796, p=.051, ϕc=0.112).This trend was not observed when comparing across three groups (de cient, insu cient, and su cient; p=.066, ϕc=0.133) or when comparing individuals whose vitamin D was su cient to those with either de cient or insu cient vitamin D levels (i.e. when vitamin D level was dichotomized as less than or greater than 30 ng/mL; p=.75, ϕc=0.018).

Likelihood of Refeeding Hypophosphatemia
When analyzing phosphorus as a dichotomous variable (whether or not a threshold level of < 2.9 mg/dL was met), associations with variables of interest were the same as with nadir phosphorus level.A logistic regression model evaluating likelihood of hypophosphatemia included admission weight, length of stay, potassium, calcium, hemoglobin, prealbumin, total number of tube-fed days, 25-hydroxy vitamin D level and was controlled for total dose of vitamin D supplement during hospitalization.The nal model was signi cant (p<.001), and admission weight, potassium, hemoglobin, prealbumin, and calcium contributed signi cantly to the model.Odds ratios are found in Table 4.

Moderation by Vitamin D
Vitamin D did not moderate the relationship between low body weight and nadir phosphorus level (B=.0006,SE=.0003, p=.056).However, the conditional effects show that this interaction was signi cant at higher levels of 25-hydroxy vitamin D, such that higher levels of vitamin D were associated with signi cantly higher phosphorus nadir level as weight increased (Figure 1).This interaction was signi cant for 25-hydroxy vitamin D levels at the mean (40.6 ng/mL and higher but not for levels one standard deviation below the mean (23.0 ng/mL).For those whose vitamin D level was at least 40.6, nadir serum phosphorus was signi cantly higher than those whose vitamin D level was lower as admission weight increased (p=.0001).For individuals with low levels of vitamin D, there were no signi cant differences between nadir phosphorus level as admission weight increased (p=.17).No interaction effects were found between vitamin D and age, BMI, %IBW, duration of illness, potassium, magnesium, hemoglobin, prealbumin, calcium, or caloric prescription on the outcome of nadir phosphorus level.

Discussion
Refeeding hypophosphatemia, a common complication during nutrition restoration for restrictive eating disorders, 40 is a target of clinical monitoring and need for supplementation. 41Despite awareness, close clinical monitoring, prophylaxis, and intervention, RH risk remains a poorly understood phenomenon.
Consequently, there exists wide variability in the interventions utilized during nutrition rehabilitation. 9,42his study is the rst, to the authors' knowledge, to examine the risk of RH in relation to 25-hydroxy vitamin D levels upon admission for a medical stabilization program among a cohort of severely ill individuals with AN and ARFID.Additionally, information about prevalence rates of vitamin D de ciency in AN and ARFID is limited.The results presented here describe both the prevalence of vitamin D de ciency, as well as risk of RH in the context of that nding.
Overall, there has been a paucity of research focusing on patients with ARFID.In this cohort, individuals with ARFID had signi cantly lower 25-hydroxy vitamin D levels than individuals with AN-R, and similar rates of de ciency and insu ciency in Vitamin D compared to individuals with AN-BP.These ndings occurred despite no signi cant differences in patient characteristics including weight and BMI on admission.This result highlights the need for careful consideration of nutritional state of individuals with ARFID, especially those who are demonstrating signs of severe malnutrition.Although ARFID notably includes a heterogeneous cluster of symptom presentation and nutritional intake pro les, previous studies have found that macro-and micronutrient intake in ARFID is signi cantly reduced compared to controls. 43,44However, the authors of that report found no difference in vitamin D intake and did not speci cally detail phosphorus intake.It remains unclear why individuals with ARFID had signi cantly lower 25-hydroxy vitamin D level than individuals with AN-R in this cohort.Speci c dietary sources of vitamin D such as eggs, yogurt, cheese, and sh may be consumed less by individuals with sensory challenges and contribute to this phenomenon.However, generally accepted foods such as cereal forti ed with vitamin D may offset the lack of intake from other sources.Additionally, it has been proposed that there is a high prevalence of ARFID in children with Autism Spectrum Disorders, 45 which commonly present with severe food selectivity and associated vitamin D de ciency. 46Future studies examining causes for low vitamin D in individuals with ARFID and AN should be explored.Research to date suggests that clinicians in both ambulatory and inpatient settings should consider individuals with ARFID as nutritionally fragile regardless of weight, and ARFID may warrant an approach similar to the thoughtful nutrition restoration that occurs in AN.
Low weight on admission has been identi ed as a risk factor for RH, yet low percent of variance in nadir serum phosphorus has been explained by low weight on admission. 15Although recent position statements note that clinicians should have a high index of suspicion for RH in individuals who present at a low weight, have lost a large percent of body weight, or lost weight quickly, 47 it remains unclear what additional factors intersect with low weight to create greatest risk.In this work, 25-hydroxy vitamin D at higher levels (40.6 ng/mL or greater) signi cantly moderated the relationship between admission weight and nadir serum phosphorus level.This suggests a possibility that vitamin D may be protective against the development of RH, although signi cant effects at higher weights may be of less clinical utility.Further research is necessary to elucidate the relationship between weight and vitamin D as it pertains to RH.
The ndings of this study point toward a need to further investigate a possible link between vitamin D and serum phosphorus levels as components of a negative feedback loop that maintains phosphorus and calcium homeostasis.In this system, serum phosphorus levels are maintained through gut absorption mediated by vitamin D, parathyroid hormone stimulation of renal phosphorus and calcitriol excretion, as well as parathyroid hormone in uence on broblast growth factor-23-mediated bone absorption of calcium and phosphorus. 49In this study, we found a trend toward a higher likelihood of phosphorus below 2.9 mg/dL in those with vitamin D de ciency.Additionally, there was evidence of moderation between weight on admission and serum phosphorus nadir for individuals with higher levels of 25-hydroxy vitamin D. Finally, low levels of serum calcium on admission were signi cantly associated with lower nadir phosphorus levels.Together, this suggests that further research is needed to clarify the negative feedback loop that promotes calcium and phosphorus homeostasis in the context of RH in eating disorders.Future work should focus on further understanding exactly what level of 25-hydroxy vitamin D, if any, marks a protective benchmark against the development of RH.
This work is not without limitations.First, the study population is unique in its severity and may not represent mild-moderate illness.The sample was overwhelmingly female and therefore comparative analysis between sexes was not powered in this cohort.This sample did not represent transgender individuals who may require additional consideration in the context of hormonal treatment as seen in a recent case report of recurrent and persistent RH. 50

Conclusions
This study describes the prevalence of vitamin D de ciency and its associations with RH in severely malnourished inpatients with AN and ARFID.Over 35% of individuals in this cohort had hypophosphatemia over the course of admission with no differences between groups.Nadir phosphorus was associated with lower weight, BMI, % IBW, serum potassium, calcium, and prealbumin and higher hemoglobin on admission, longer length of stay, and greater number of days receiving tube feedings.Individuals with ARFID had signi cantly lower 25-hydroxy vitamin D levels on admission than individuals with AN.Higher, but not lower, levels of 25-hydroxy vitamin D on admission signi cantly moderated the relationship between admission weight and serum phosphorus nadir.
This study highlights that a substantial proportion of individuals with AN and ARFID admitted for medical stabilization may be de cient or insu cient in vitamin D, and that individuals with ARFID may be more susceptible to both vitamin D de ciency and RH than those with AN.

Figures
Figures

Figure 1
Figure 1 SD) admission 25-hydroxy vitamin D level was 40.6 ng/mL(17.6) in the sample overall.Admission 25-hydroxy vitamin D levels differed signi cantly between diagnoses (F=6.79,p=.001; η 2 =0.044); post-hoc analyses showed signi cantly higher 25-hydroxy vitamin D levels in those diagnosed with AN-R (mean 44.1 ng/mL) compared to individuals with AN-BP (mean=39.6ng/mL; p=.009), as well as signi cantly higher levels in those with AN-R compared to ARFID (mean=36.1 ng/mL; p=.004).There was no signi cant difference in 25-hydroxy vitamin D levels comparing AN-BP and ARFID (p=.64).Additionally, when combining AN-R and AN-BP into one category, individuals with both subtypes of AN had a signi cantly higher 25-hydroxy vitamin D level compared to those with those with ARFID (F=4.53,p=.03, η 2 =0.015).Approximately 1/3 of individuals with ARFID (35%) were either de cient or insu cient in vitamin D, and 29% of individuals with AN were de cient or insu cient (x 2 =0.93, p=.34, ϕc=0.05).
Additionally, 89% (n = 275) of the sample had a starting caloric prescription of 1400, with only 24 individuals receiving 1800 kcal on admission, 2 individuals receiving 2000 kcal on admission, and 6 receiving 2100-4000 kcal.Therefore, the nonsigni cant relationship between admission kcal and RH should be interpreted with caution.Future prospective work should include higher caloric variability in a similarly medically fragile, very lowweighted cohort.The present sample was comprised of individuals admitted to a medical critical care unit with a mean average admission weight lower than what may be found in other institutions; however, individuals with substantial weight loss, as in atypical AN, exhibit similar physiologic outcomes to those with AN.48Research on vitamin D in individuals with atypical AN may be of importance in further classifying risk of RH.Finally, two key components of this negative feedback loop, parathyroid hormone and broblast growth factor-23, were not available in existing medical records and thus were not included in analyses.Further work is necessary to investigate the role of parathyroid hormone and broblast growth factor-23 in maintaining phosphorus levels during nutritional restoration.

Table 2 .
Screening for 25-hydroxy vitamin D on admission may be an important component of clinical care for malnourished individuals with AN and ARFID.Further work is needed to create clinical scoring systems to facilitate identi cation of risk of RH and prevent development of RH and refeeding syndrome.Phosphorus and vitamin D on admission by ARFID, AN with 3-way post-hoc analyses

Table 3 .
Correlation between variables of interest and nadir serum phosphorus during hospitalization

Table 4 .
Odds of hypophosphatemia by associated risk factors (N=307)