Vitamin D Deficiency in Adolescent Girls

Fibroblast growth factor 23 (FGF23) is a newly discovered hormone. Its primary function is
to regulate phosphate metabolism, which it does both by directly regulating phosphate
excretion through the kidney as well as by regulating vitamin D activity. What controls
FGF23 levels is still under investigation; potential factors include dietary phosphate
intake and vitamin D levels.

In this pilot study, we aim to investigate what the normal levels of FGF23 are in healthy
girls and how these are related to other measurements of bone and mineral metabolism. Since
vitamin D deficiency is very common in healthy girls, we expect a subset of the subjects in
this study to be vitamin D deficient. We will then randomize these girls to vitamin D
repletion or placebo and follow changes in FGF23 levels. Vitamin D deficiency has also been
implicated in insulin resistance which is a precursor to diabetes. We will therefore also
look at changes in insulin resistance with vitamin D repletion.

Inclusion Criteria:

- healthy girls aged 9-18 years

Exclusion Criteria:

- significant cardiac, hepatic, oncologic, or psychiatric disease

- a history of malabsorption, kidney stones, hypoparathyroidism, or growth hormone
deficiency

- pregnancy

- diabetes mellitus

- BMI>/= 99th percentile for age and sex

- fracture within the preceding 3 months

- hypogonadism (no pubertal development by age 12, absence of menarche by age 14)

- serum calcium <8 mg/dl or >11 mg/dl

- radiographic evidence of rickets

- use of medications know to affect serum phosphate levels including phosphate-binding
antacids, sodium etidronate, calcitonin, excessive doses of vitamin D (>1000 units
per day), excessive doses of vitamin D (>20,000 units/day), calcitriol, growth
hormone, or anti-convulsants.

- use of hormonal birth control