We studied bone metabolism and density in adolescent girls with AN and compared them with those in age - and pubertal stage- matched controls. Bone density was significantly reduced in patients. IGF-1 is a nutritionally dependent endogenous bone trophic factor with a defining role in bone [18]. Moreover, IGF-1 acts on preosteoblasts and osteoblasts, resulting in potent stimulation of bone synthesis [19].
Previous studies on bone metabolism in adolescents with AN were limited by the lack of information about specific markers of bone formation. Longitudinal data from healthy girls show that gain in bone mass is most pronounced between 11 and 14 years of age and decreases significantly after 16 years of age and/or 2 years after menarche [20]. Studies in adult women with AN have shown decreased bone turnover, with dysregulation between markers of bone formation and markers of bone resorption [21]. However, these data cannot be generalized to adolescent girls who are in a stage of rapid bone growth.
Abrams et al. [22] found that patients with AN have decreased absorption of calcium and increased levels of its excretion in the urine. Saggese et al. [23] reported low levels of carboxyl terminal propeptide of type 1 procollagen, a bone formation marker, in adolescent patients with AN and did not investigate other bone formation markers. Bone formation markers change from early to late puberty. Furthermore, the levels of bone formation and resorption in adolescents are higher compared with adult levels [24]. Adolescents with AN showed decreased height and the reason for this reduced height was not studied in this cross-sectional study and will require a longitudinal study. Reduced circulating IGF-1 leading to low bone formation may be a cause [25]; however, significant height difference was not found in our group of patients when compared with controls.
Our data showed markedly reduced body composition in patients, but there was a non-significant difference in total BMD, in agreement with the results of Bachrach et al. [2]. The reasons for relatively normal BMD in underweight individuals may include environmental exposure (i.e. prepubertal exercise) or genetic factors such as vitamin D receptor polymorphisms [26], but these were not systematically evaluated in this study. Our data show that significant spinal osteopenia is present in patients with AN compared with healthy adolescent girls. The results of lateral spine BMD, which is more related to trabecular bone [27], showed even more severe osteopenia.
An important finding in our study was that low serum calcium was prevalent in our study population of adolescent girls with AN as shown in other studies of adolescent girls [28], and a reduction in dietary calcium intake is a cause [29]. Calcium supplementation increases bone density in healthy adolescents and the resulting effect of calcium is dependent on the pubertal stage [30].
The level of serum 25(OH) D was significantly lower in patients with AN compared with controls, indicating that most adolescent girls require these important nutrients from food.
Although 25(OH) D affects osteoblasts and osteoclasts [31], a previous study has reported that low levels of serum OC in children with kwashiorkor are improved during nutritional rehabilitation and this improvement was not related to the serum level of 25(OH) D [32].
GH is a bone anabolic hormone through both direct and IGF-1-mediated effects [33]. Girls with AN have lower levels of circulating IGF-1 compared with normal-weight girls of comparable age, despite higher GH levels, indicative of nutritionally acquired hepatic resistance to the effects of GH. Higher GH concentrations in girls with AN are a consequence of increased basal GH and also an increased frequency of GH secretory bursts. Although GH concentrations strongly predict the levels of bone turnover markers in normal weight girls, this association is completely lost in girls with AN, indicative of a resistance to the effects of GH at the bone level [4].
These data were further understood by a report of a randomized-controlled study in which the changes in levels of IGF-1 (bone formation marker) did not differ in adult women with AN who were randomized to supra physiological doses of recombinant human GH compared with those randomized to placebo. These data show that in severe under nutrition, there is a fixed block in the production of IGF-1 by the liver that cannot be overcome by increasing the levels of GH [34].
IGF-1 is a positive predictor of bone turnover in girls with AN, and in a study by Sokya et al. [35], changes in the levels of IGF-1 over a period of 1 year were associated positively with changes in the levels of bone turnover markers over the same period. In adult women with AN, IGF-1 levels are associated independently with bone architectural parameters such as trabecular volume, thickness, and number, and inversely associated with trabecular separation, even after controlling for BMI. In addition, short-term administration of recombinant human rhIGF-1 in doses of 30–40 mg/kg/dose twice daily is successful in significantly increasing the levels of bone formation markers in both girls [36] and adult women with AN [21].
In our study, the bone formation index, OC, was significantly lower in adolescent girls with AN compared with healthy adolescents. In controls, there were high levels of bone formation, whereas low bone formation was found in patients. Formation marker in adolescents (OC) reflects bone mineralization. We suggest that in patients with AN, remodeling continues, whereas bone formation is reduced.
We found a high negative correlation between IGF-1 levels and BMI, indicating that undernutrition reduces the levels of IGF-1 in adolescents with AN. This could be because of the fact that with the decrease in BMI, serum levels of IGF-1 decrease in a BMI-dependent manner. These data are in agreement with the findings of Loud et al. [37], who found low levels of serum IGF-1, and these low levels were correlated with low BMI in their patients with AN.
In our study, correlation of bone formation in patients with AN with significantly reduced levels of IGF-1 was found. Studies on nutritional causes of reduced IGF-1 in normal men showed that fasting for 5 days causes serumIGF-1 levels to decrease up to 36% of prefasting values and refeeding leads to an increase in serum IGF-1 levels, indicating that food intake increases serum IGF-1 [38].
Primary or secondary amenorrhea is commonly present inpatients with AN with resulting reduced estrogen levels. Reduced estrogen is hypothesized as a factor contributing toward low bone density in patients with AN. In adolescent girls with amenorrhea, lower lumbar bone density is present compared with control normal menstruating girls. These changes are reversible even before the return of menses during recovery of patients from AN [39].
Misra et al. [36] showed that rhIGF-1 therapy in patients with AN significantly increases serum levels of OC within 1 week of rhIGF-1 therapy before a significant increase in body weight and only after an increase in the levels of serum IGF-1. This suggests that bone formation marker production increases rapidly in response to the increase in the level of serum IGF-1 [40]. Patients with AN have disturbance in perception of body weight or shape and marked fear of gaining weight, which makes it difficult to advise them to increase their weight. They have to be informed that they are at a risk of developing osteoporosis, which is one of the most severe complications of AN.