North American blacks tend to have low blood levels of vitamin D because pigmentation blocks vitamin D production in the skin. They also have higher rates of developing type 2 diabetes and higher rates of complications from the disease compared with whites. Although there is compelling evidence that adequate vitamin D may reduce the risk for type 2 diabetes in whites, recent evidence from a national survey demonstrated an association of vitamin D with diabetes in whites but not in blacks. However, the central hypothesis of this study is that providing enough supplemental vitamin D to blacks (raising their blood levels higher than that of most participants in the survey) will improve blood measures related to diabetes risk. The proposed study is a 12-week randomized, double-blind, placebo-controlled experiment designed to examine the effect of vitamin D supplementation (100 μg/d ) on insulin secretion, insulin sensitivity and glucose control in pre-diabetic black men and women aged 40 and older.

Inclusion Criteria:

• African-American by self designation
• Glucose intolerance defined as FPG ≥ 100 mg/dl or A1c ≥ 5.8%
• BMI 25.0-39.9
• Age 40 or older

Exclusion Criteria:

Medical Conditions

• Diabetes potentially requiring pharmacotherapy, defined as A1c > 7%

• Uncontrolled thyroid disease

• Current parathyroid, liver or kidney disease

• Renal stone within 5 years

• Sarcoidosis, current pancreatitis, active tuberculosis, hemiplegia, gout

• Inflammatory bowel disease, colostomy, malabsorption

• Cancer other than basal cell skin cancer within 5 years

• Uncontrolled arrhythmia in past year

• Albinism or other condition associated with reduced skin pigmentation

• Pregnancy over the last 1 year

• Intent to become pregnant

• Menopause onset within 1 year

• Any other unstable medical condition Laboratory Tests

• Fasting plasma glucose < 100

• Hemoglobin A1c > 7%

• Laboratory evidence of liver disease (e.g. AST > 70 U/L or ALT > 72 IU/L)

• Laboratory evidence of kidney disease (e.g. estimated glomerular filtration rate < 60 ml/min/1.73 m2).

• Elevated spot urine calcium to creatinine ratio > 0.38 mg/dl*

• Abnormal serum calcium (serum calcium > 10.5 mg/dl)

• Anemia (Hematocrit < 36% in men, <33% in women) Medications (use in past three months)

• Estrogen or testosterone

• Prescription vitamin D

• Lithium

• Oral corticosteroids

• Anti-seizure medications

• Unstable doses of psychotropics or phenothiazines

• Cholestyramine Supplements (current use - may discontinue after screening)

• Vitamin D supplements, cod liver oil, calcium supplements Other

• Body mass index less <25 or > 39.9

• Consumption of more than 14 alcoholic drinks per week

• Inability to attend all three study visits as scheduled

• Inability to provide written informed consent

• age < 40 years

• not African-American (by self-designation)

• Participation in another research intervention study

  • corresponds to a 24-hour urinary calcium excretion > 400 mg