Effects of Niacin Therapy on Lipoprotein Composition and Function

Background: \- Niacin is a vitamin in many foods, including meats, fish, fruits, and vegetables. It is often used as a dietary supplement that causes many improvements in the body. Researchers think it can affect heart health. Objective: \- To better understand the good effects of niacin supplementation on cholesterol, fat metabolism, and vascular health. Eligibility: \- Adults 18 years of age and older with fasting good cholesterol (HDL-C) below 60 mg/dL. Design: * Participants will come to the clinic 4 times during the study. * They will complete a 7-day food journal before visits 1 and 3. * At visit 1, participants will be screened with questions about their diet and exercise, medical history, and any drugs and vitamins they take. Vital signs and body mass index will be measured. * They will have a Cardio-Ankle Vascular Index (CAVI) test of the arteries. Blood pressure will be taken in the arms and legs and the heart will be monitored. * Blood will be drawn. Participants will fast for 8 12 hours before this. * Women will have a pregnancy test. * Eligible participants will get a 2-week supply of niacin. They will take 2 tablets daily for one week, then 4 daily. * Visit 2 will be 2 weeks after visit 1 and the niacin dose will be increased. Visit 3 will be 16 weeks after visit 1, and participants will stop taking niacin. Visit 4 will be 4 6 weeks after stopping niacin. * During study visits, participants will repeat visit 1 tests.