Heaney RP Lessons for nutritional science from vitamin D. Am J Clin Nutr. 1999 May;69(5):825-6.
Holick MF The vitamin D deficiency pandemic and consequences for nonskeletal health: mechanisms of action. Mol Aspects Med. 2008 Dec;29(6):361-8. doi: 10.1016/j.mam.2008.08.008. Epub 2008 Sep 2. Review.
Leichtmann GA, Bengoa JM, Bolt MJ, Sitrin MD Intestinal absorption of cholecalciferol and 25-hydroxycholecalciferol in patients with both Crohn's disease and intestinal resection. Am J Clin Nutr. 1991 Sep;54(3):548-52.
Pappa HM, Bern E, Kamin D, Grand RJ Vitamin D status in gastrointestinal and liver disease. Curr Opin Gastroenterol. 2008 Mar;24(2):176-83. doi: 10.1097/MOG.0b013e3282f4d2f3. Review.
Interventional studies are often prospective and are specifically tailored to evaluate direct impacts of treatment or preventive measures on disease.
Observational studies are often retrospective and are used to assess potential causation in exposure-outcome relationships and therefore influence preventive methods.
Expanded access is a means by which manufacturers make investigational new drugs available, under certain circumstances, to treat a patient(s) with a serious disease or condition who cannot participate in a controlled clinical trial.
Clinical trials are conducted in a series of steps, called phases - each phase is designed to answer a separate research question.
Phase 1: Researchers test a new drug or treatment in a small group of people for the first time to evaluate its safety, determine a safe dosage range, and identify side effects.
Phase 2: The drug or treatment is given to a larger group of people to see if it is effective and to further evaluate its safety.
Phase 3: The drug or treatment is given to large groups of people to confirm its effectiveness, monitor side effects, compare it to commonly used treatments, and collect information that will allow the drug or treatment to be used safely.
Phase 4: Studies are done after the drug or treatment has been marketed to gather information on the drug's effect in various populations and any side effects associated with long-term use.
