The reason for doing this study is to learn whether raising a person's vitamin D level from below normal to normal levels will improve his or her body's ability to use sugar. Vitamin D is well known to be an important vitamin for the development and maintenance of bones. Recently, scientists have learned that vitamin D may have a role in the prevention of cancer, diabetes, and autoimmune diseases. The investigators are specifically interested in studying this question in the overweight/obese population as they are at greater risk for both vitamin D deficiency and impaired ability to metabolize sugar (glucose intolerance).

Primary Hypotheses:

Vitamin D repletion (increasing the serum 25(OH)D level from ≤ 20 ng/ml to ≥ 30 ng/ml) will improve insulin sensitivity in individuals who are overweight/obese and insulin resistant.

Secondary Hypotheses:

1.Vitamin D repletion will improve biomarkers of cardiovascular risk and inflammation (directly altering macrophage cytokine production and/or indirectly as a result of improvement in insulin sensitivity.) 2.30,000 IU (0.25 mg) weekly of cholecalciferol (vitamin D3) will raise serum 25(OH)D levels from ≤ 20 ng/ml to ≥ 30 ng/ml overweight/obese population.

Inclusion Criteria:

• Nonsmoking men and women, ages 18-65
• Body mass index (BMI) ≥ 28 kg/m² and within 10% of maximum weight
• Serum 25(OH)D level ≤ 20 ng/ml
• Serum insulin level ≥ 7.2 mU/l
• Willingness to maintain current body weight for the duration of the study
• Willingness to maintain baseline lifestyle activities and routines for the duration of the study
• Willingness to avoid the use of non-steroidal anti-inflammatory drugs (NSAIDS), including low-dose daily aspirin, for at least ten days prior to each admission (due to the potential effects of these agents on inflammatory markers.)
• Willingness to avoid the use of all over-the-counter or prescription vitamins, dietary supplements, and herbal products during the study, with the exception of a basic multivitamin supplement.
• If of childbearing potential, willingness to use highly effective contraception for the duration of the study.

Exclusion Criteria:

• Current tobacco smoking
• History of bleeding or coagulation disorders
• Gastrointestinal disease resulting in significant gastrointestinal dysfunction or malabsorption
• History of diabetes, or diagnosed during screening OGTT
• Current treatment with hypertensive medications
• History of cardiovascular disease
• Renal disease, as evidenced by a serum creatinine above the upper limit of normal on more than one screening visit
• History of kidney stones
• History of chronic hepatitis, or liver enzymes (AST or ALT) more than 2.5 times the upper limit of normal
• Primary hyperparathyroidism or baseline hypercalcemia from any cause
• Current treatment with over-the-counter or prescription weight loss medications, such as orlistat or sibutramine
• History of bariatric surgery
• Current treatment with any cholesterol-lowering medications, such as statins, niacin, fibrates, or ezetimibe
• Blood pressure ≥ 145/90 after ten minutes of rest on more than one screen visit
• Hyperthyroidism or untreated hypothyroidism. Subjects with chronic, treated, stable hypothyroidism may be included in the study at the discretion of the Principal Investigator.
• Pregnancy, desired pregnancy, or lactation within the study period.
• HIV, Hepatitis B and C
• Medical conditions requiring daily calcium supplementation or antacid use
• Use of medications known to interact with calcium or vitamin D metabolism (bisphosphonates, corticosteroids, thiazide diuretics, cholestyramine, colestipol, mineral oil, phenytoin, barbituates, dititalis glycosides, antacids)
• Subjects with known hypersensitivity to cholecalciferol
• Participation in an investigational drug study within one month of screening
• History, physical, or laboratory findings suggestive of any other medical or psychological condition that would, in the opinion of the principal investigator, make the candidate ineligible for the study