Vitamin D, Glucose Control and Insulin Sensitivity in African-Americans
Vitamin D, Glucose Control and Insulin Sensitivity in African-Americans
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:
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