**Alexander J. Michels \* and Balz Frei**

Linus Pauling Institute, 307 Linus Pauling Science Center, Oregon State University, Corvallis, OR 97331, USA; E-Mail: balz.frei@oregonstate.edu

**\*** Author to whom correspondence should be addressed; E-Mail: michelsa@onid.orst.edu; Tel.: +1-541-737-5085; Fax: +1-541-737-5075.

*Received: 11 October 2013; in revised form: 23 November 2013 / Accepted: 27 November 2013 / Published: 16 December 2013* 

**Abstract:** Research progress to understand the role of vitamin C (ascorbic acid) in human health has been slow in coming. This is predominantly the result of several flawed approaches to study design, often lacking a full appreciation of the redox chemistry and biology of ascorbic acid. In this review, we summarize our knowledge surrounding the limitations of common approaches used in vitamin C research. In human cell culture, the primary issues are the high oxygen environment, presence of redox-active transition metal ions in culture media, and the use of immortalized cell lines grown in the absence of supplemental ascorbic acid. Studies in animal models are also limited due to the presence of endogenous ascorbic acid synthesis. Despite the use of genetically altered rodent strains lacking synthesis capacity, there are additional concerns that these models do not adequately recapitulate the effects of vitamin C deprivation and supplementation observed in humans. Lastly, several flaws in study design endemic to randomized controlled trials and other human studies greatly limit their conclusions and impact. There also is anecdotal evidence of positive and negative health effects of vitamin C that are widely accepted but have not been substantiated. Only with careful attention to study design and experimental detail can we further our understanding of the possible roles of vitamin C in promoting human health and preventing or treating disease.

**Keywords:** vitamin C; ascorbic acid; cell culture; animals; human; study design
