The investigators will be administering oral high dose interval vitamin D, concurrently when participants are receiving biologic therapy for their inflammatory bowel disease. The investigators will be collecting some additional bloodwork and questionnaires at the time of participants infusions.

The investigators will be administering oral high dose interval vitamin D3 concurrently when participants with inflammatory bowel disease (IBD) are receiving biologic therapy every 4-8 weeks. The investigators will collect additional bloodwork when participants are getting clinical labwork in order to assess markers of bone health and inflammation in response to vitamin D treatment. The investigators will serially assess with questionnaires associated measures, including dietary vitamin D and calcium intake, sunlight exposure, physical activity, fracture history, IBD disease activity, and overall health.

Inclusion Criteria:

• Existing diagnosis of IBD, including Crohn disease, ulcerative colitis, and indeterminate colitis
• Receiving treatment with Infliximab or Vedolizumab every 4-8 weeks
• Age 5-25 years old, at study entry
• Measured serum level of 25-OHD of less than 40 ng/mL in the last 4-8 weeks and no changes in vitamin D supplementation in the interim. Of note, 25-OHD levels are evaluated routinely as part of standard clinical care for IBD

Exclusion Criteria:

• History of any underlying kidney disease
• History of preexisting liver disease
• History of granulomatous disease
• Inability to take oral vitamin D3 as a pill
• History of hypercalcemia or hypercalciuria
• Currently, or within the past 3 months, taking an anti-epileptic medication or Lasix

See above. Enrolling all eligible participants, who wish to participate, until reach enrollment target.