What is the current evidence for alpha-linolenic acid conversion efficiency in adult vegans? A systematic review
By Elena Holmes - MSc in Nutrition Science and Practice
Abstract
Background
There is increasing evidence of the importance of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the human diet for the prevention of chronic diseases and for cognitive development and health. A vegan diet excludes animal-based seafood, and the dietary consumption of EPA/DHA-containing algae is significantly low, which suggests that people eating a vegan diet are excluding an important source of preformed EPA/DHA. Because of the growing number of people choosing a vegan diet, it is essential that current evidence is compiled and understood in order to inform clinical practice.
Aims
To identify and review all current available primary evidence concerning the conversion of alpha-linoleic acid (ALA) (plant-sourced omega-3; n-3) to docosahexaenoic (DHA) (long-chain, marine-sourced n-3) in adults eating a vegan diet.
Objectives
- To provide an evidence base supported by critically appraised findings in the literature regarding DHA conversion and its status in vegans, in order to inform clinical practice.
- To make recommendations for future primary research based on gaps identified in the literature.
Methodology
A systematic literature search (systematic review; SR) was carried out using the following biomedical databases: Cochrane Library, Science Direct, PubMed and ProQuest. Other sources, such as specialist websites and trial registers, were also searched. The publications found were assessed using the Critical Appraisal Skills Programme (CASP) cohort study appraisal tool. The data were analysed and presented using a narrative synthesis method. Meta-analysis was inappropriate because of the heterogeneity of the data.
Results
Of 1,797 identified studies, five met the inclusion criteria. All five studies confirmed previous research suggesting that vegans have low plasma, serum and red blood cell DHA levels, although two studies suggested that DHA levels in vegans were 'higher than expected'.
Conclusions
The small number of studies found in this SR, and limitations in the literature search methodology, such as the broad scope of the search terms, precludes wider conclusions and broader generalisations being drawn from the results. All five studies suggested that vegans have lower than recommended tissue DHA levels. Insufficient data were found regarding details of the ALA to DHA conversion efficiency in vegans. The overall findings indicate the need for more primary research, such as cohort studies, of the vegan population, and of the health implications arising from a vegan diet.
