Obese persons are known to have elevated levels of parathyroid hormone (PTH) and low levels of vitamin D. These hormones are important in regulation of the body's calcium stores and bone health. We would like to investigate these abnormalities and the accuracy of our current diagnostic tests by comparing results of standard assays for vitamin D and PTH to more specific tests, in obese subjects at baseline and as vitamin D is replaced. We will also compare two standard vitamin D replacement regimens to determine if one is more effective.

This is a pilot study with two parts: Part 1 will compare levels of PTH and vitamin D using two different assays in obese subjects who have normal vitamin D and those who do not. We plan to enroll 20 subjects who have normal vitamin D levels and 40 subjects who have vitamin D insufficiency. All subjects will fill out questionnaires about the amount of calcium and vitamin D in their diet, and their recent sunlight exposure. We will ask for blood samples so that we can measure levels of calcium, vitamin D, albumin, creatinine, glucose, insulin and the different forms of PTH.

Subjects who have vitamin D insufficiency will then be randomized to receive Vitamin D2 or Vitamin D3 in standard doses for eight weeks, in an open label trial. At four and eight weeks, these subjects will fill out the above questionnaires and have the blood tests repeated. For safety purposes, urine calcium will also be monitored.

Inclusion Criteria:

• Obese subjects with body mass indices greater than 40 kg/m2 or 35 kg/m2 with other medical conditions secondary to obesity
• Willingness to take vitamin D supplements
• If baseline vitamin D level is >25 ng/ml subjects will be eligible for the control group
• If baseline vitamin D level is <25 ng/ml subjects will be eligible for one of the two vitamin D replacement groups

Exclusion Criteria:

• Hypercalcemia
• Kidney disease
• Liver disease
• Malabsorption
• Prior diagnosis of bone disease
• Medical conditions requiring daily use of calcium, antacids, or medications known to affect bone metabolism or interact with vitamin D
• Hypersensitivity to any formulation of vitamin D