1. Introduction

The polyunsaturated n-6 and n-3 fatty acids are essential for a variety of functions in the body, including the synthesis of prostaglandins, leukotrienes, cellular membranes, phospholipids, retinal photoreceptors (vision), cerebral gray matter (brain tissue), testes, and sperm. Linoleic (18:2n-6; LA) and arachidonic acids (20:4n-6; AA) are classified as n-6 fatty acids, and α-linolenic (18:3n-3; ALA), eicosapentaenoic (20:5n-3; EPA) and docosahexaenoic acids (22:6n-3; DHA) are n-3 fatty acids [ 1 ]. Of these fatty acids, LA and ALA are considered essential and must be provided by the diet.

The Dietary Reference Intakes (DRIs) are “nutrient reference values” developed by the Institute of Medicine of The National Academies. They are intended to serve as a guide for good nutrition and provide the scientific basis for the development of food guidelines in both the United States and Canada, and these nutrient reference values are specified on the basis of age, gender and lifestage groups” [ 2 ]. The United States DRIs are used to make informed judgements during the dietary planning process. New research or discoveries of physiological factors, inborn errors of metabolism, health risks, characteristics of the nutrient sources, and nutrient bioavailability may require adjustments in DRI nutrient values for planning individual and group dietary intakes. The present 2003 version of the DRIs states that the nutrients of concern for vegetarians and vegans (VGNs) are vitamin B12, vitamin D, calcium, iron, zinc, and protein [ 3 ]. Essential fatty acids in the VGN and vegetarian diets are not presently a DRI concern.

When the DRI concludes that there is inadequate information to set either LA or ALA Estimated Average Requirements (EAR) or the Recommended Dietary Allowance (RDA) for healthy individuals, the Adequate Intake (AI) is used. The present essential fatty acid AI is based on “the highest median intake of LA and ALA in United States adults, where a deficiency is basically nonexistent in non-institutionalized populations” [ 4 ]. The present n-6 AI requirement range is 12–11 g/day for men and 17–14 g/day for adult women (non-pregnant/lactating), and the AI n-3 fatty acid recommendations are 1.1 g/day of ALA for women and 1.6 g/day for men [ 4 ]. However, it is unknown if the AIs are beneficial or physiologically adequate because dose-response data studies are lacking, Essential fatty acid status is not usually clinically tested, and absence of deficiency symptoms is not necessarily evidence of adequacy.

The authors utilized the definition of ‘vegan’ as the following: a diet which is “devoid of all flesh foods. It excludes eggs and dairy products and may exclude honey” [ 5 ]. Frequently, the term ‘strict vegetarian’ or ‘total vegetarian’ is used instead of ‘vegan’ to describe their dietary pattern. Thus, the abbreviation ‘VGN’ is used in this manuscript as a description of the dietary pattern of total vegetarians/strict vegetarians and vegans.

People choose VGN or vegetarian diets for different reasons, which include but are not limited to health reasons, compassion toward animals, a desire to better protect the environment, to lower their risk of chronic diseases, or to therapeutically manage diseases; however, these diets require careful dietary planning. According to the Position Paper of the Academy of Nutrition and Dietetics regarding vegetarian diets, “Well designed vegetarian diets provide adequate nutrient intakes for all stages of the lifecycle and can also be useful in the management of some chronic diseases” [ 5 ].

Upon examination of the published articles that relate to ‘vegetarian’, the researcher’s definitions for the terms ‘vegan’ and ‘vegetarian’ are inconsistent, and some researchers consider plant-based diets to be ‘semi-vegetarian’. However, VGN diets are very different from omnivore diets and even diets that are considered to be ‘vegetarian’ (ex. ‘lacto-ovo’, ‘pesco-vegetarian’, ‘plant-based vegetarian’, ‘pollo-vegetarian’, or ‘semi-vegetarian’). These ‘vegetarian’ diets may also include animal-origin foods such as eggs, dairy, fish, or chicken. Thus, dietary patterns and levels of biochemical indicators of essential fatty acid status are very different between VGN and omnivore diets or even other types of vegetarian diets. This narrative review focuses on apparently healthy VGNs and comparisons of VGNs and ‘omnivore’ or ‘meat-eating’ diets, rather than the comparisons of VGN to vegetarian diets or other terms that include the word ‘vegetarian’.

7,8,9,10, Diet study results between VGNs and omnivores/meat-eaters are inconsistent. Additionally, interpretation or comparison is difficult when some of the information is not reported, or missing. For example, some studies report dietary total polyunsaturated fatty acids (PUFAs), total n-6, total n-3, or collected some, but not all lipid fractions [ 6 11 ], which limits interpretation of essential fatty acid biological indicators as compared to dietary intakes.

12,13,14,15, Additional fatty acids of interest are EPA and DHA. They are not considered to be essential since they can be converted from ALA; however, they may be of concern for VGNs, since VGN diets are typically absent of EPA and DHA unless VGNs consume supplements containing EPA and DHA [ 9 16 ].

Therefore, the purpose of this review was to report and summarize relevant findings of studies on dietary intake and biological indicator status of LA, AA, ALA, EPA, and DHA in plasma, serum, erythrocytes, platelets, breastmilk, and adipose tissue in VGNs, and discuss the reasons why VGN diets may need DRI Special Consideration. We also provide dietary suggestions as to how VGNs can ensure adequate dietary intakes of LA and ALA.