The effect of bed rest on bone turnover in young women hospitalized for anorexia nervosa: a pilot study

Abstract

Context: Malnourished adolescents with anorexia nervosa (AN) requiring medical hospitalization are at high risk for skeletal insults. Even short-term bed rest may further disrupt normal patterns of bone turnover.

Objective: The objective of the study was to determine the effect of relative immobilization on bone turnover in adolescents hospitalized for AN.

Design: This was a short-term observational study.

Setting: The study was conducted at a tertiary care pediatric hospital.

Study participants: Twenty-eight adolescents with AN, aged 13-21 yr with a mean body mass index of 15.9 +/- 1.8 kg/m(2), were enrolled prospectively on admission.

Intervention: As per standard care, all subjects were placed on bed rest and graded nutritional therapy.

Main outcome measure: Markers of bone formation (bone specific alkaline phosphatase), turnover (osteocalcin), and bone resorption (urinary N-telopeptides NTx) were measured.

Results: During the 5 d of hospitalization, serum osteocalcin increased by 0.24 +/- 0.1 ng/ml . d (P = 0.02). Urine N-telopeptides reached a nadir on d 3, declining -6.9 +/- 2.8 nm bone collagen equivalent per millimole creatinine (P = 0.01) but returned to baseline by d 5 (P > 0.05). Bone-specific alkaline phosphatase exhibited a decline that was strongly age dependent, being highly significant for younger subjects only [age 14 yr: -0.42 +/- 0.11 (P = 0.0002); age 18 yr: -0.03 +/- 0.08 (P = 0.68)]. Age had no effect on other outcome measures.

Conclusion: Limitation of physical activity during hospitalization for patients with AN is associated with suppressed bone formation and resorption and an imbalance of bone turnover. Future interventional studies involving mechanical stimulation and/or weight-bearing activity are needed to determine whether medical protocols prescribing strict bed rest are appropriate.

Figure 1

Trends in weight and bone turnover markers in 28 young women hospitalized for AN. Estimated mean and 95% confidence interval from repeated-measures ANOVA. Solid line, Adjusted for age, baseline percentage of ideal weight, spinal aBMD, duration of amenorrhea, regular exercise, and family history of osteoporosis; dotted line, additionally adjusted for daily weight during hospitalization. See Table 2 for statistical assessment of trends.