ASE is a good general-purpose database that includes both scholarly and popular material covering virtually all subject areas. Some items are available in full text and others are indexed only. Utilize the Subject Terms button in the top bar for more precise results.
This database is a listing of controlled trials identified by contributors to the Cochrane Collaboration. This bibliography is part of an international effort to hand search worldwide medical journals and create an unbiased source of data for systematic reviews. Keyword searching and refining by content type, scope, etc. will serve you best here.
This medical database contains some full text articles, many of which compare and synthesize the results of multiple other studies (this is called meta-analysis). Most studies will focus on "medical interventions," such as nutrient supplementation.
From the National Library of Medicine, MEDLINE contains a wealth of authoritative information about the health sciences and the health care system. Be aware that many of these will need to be accessed via interlibrary loan. Use the MeSH terms button in the top bar for more precise results.
SAGE Journals contains a wealth of information in a wide array of disciplines. You may need to use the "advanced search" feature (available by hovering over the "search" tab) to get more precise results. Many articles are available in full-text. Articles that are not available in full text, may be requested through interlibrary loan.