Pharmacokinetics of Vitamin D in Multiple Sclerosis and in Health

Inclusion Criteria: * Female * Healthy or multiple sclerosis * Aged 18 to 60 * Body mass index is between 18 kg/m2 and 30 kg/m2 * Screening 25-hydroxyvitamin D level ≤ 75 nmol/L (30 ng/mL) * White race * Non-Hispanic ethnicity * Willing to use birth control during study * Willing to not use tanning bed during study If subject has multiple sclerosis: * Relapsing-remitting MS, as defined by McDonald 2005 criteria * Screening Expanded Disability Status Scale score ≤ 3.0 * Using no medication for MS, or taking Copaxone, (glatiramer acetate), interferons, or natalizumab Exclusion Criteria: * Pregnant or nursing * Taking multivitamin \& unwilling to remain off it during study * Taking cod liver oil \& unwilling to remain off it during study * On a fat-restricted diet * History of renal disease or nephrolithiasis (kidney stones) * History of liver disease * Taking thiazide diuretics * History of hyperthyroidism * History of infection with Mycobacterium species * History of sarcoidosis * History of cancer * History of cardiac disease * History of HIV * History of gastrointestinal disorder * Taking medications that interfere with gastrointestinal absorption * Cigarette smoker in past month * Use of illicit drugs in past month * Use of steroids in past month * History of hypercalcemia, and screening serum calcium ≤ 10 mg/dL (UCSF) or ≤ 10.7 mg/dL (Johns Hopkins) * History of hypercalciuria * Evidence of anemia (Hgb \<11.0 g/dL) * History of other serious medical conditions * Taking medications that involve the P450 system or may interact with vitamin D (digoxin, diltiazem, verapamil, cimetidine, heparin, or low-molecular weight heparin) * Other concerns about safety from the perspective of the treating physician If subject has MS: -History of major heat sensitivity (leading to sun-avoidant behaviors)