DHA status of vegetarians

doi:10.1016/j.plefa.2009.05.013

Prostaglandins, Leukotrienes and Essential Fatty Acids

Volume 81, Issues 2–3, August–September 2009, Pages 137-141

https://doi.org/10.1016/j.plefa.2009.05.013 Get rights and content

Abstract

Background

Docosahexaenoic acid (22:6n-3; DHA) is absent from vegan diets and present in limited amounts in vegetarian diets.

Objective

To review DHA status in vegetarians and vegans.

Design

To identify published studies and review their findings.

Results

Dietary analyses show that vegan diets are devoid of DHA and vegetarian diets that included dairy food and eggs only provide about 0.02 g DHA/d. Vegetarian and especially vegan diets supply more linoleic acid (18:2n-6) than omnivore diets. The intake of α-linolenic acid (18:3n-3) also tends to be similar or greater but depends on culinary oils used. The proportions of DHA in plasma, blood cells, breast milk, and tissues are substantially lower in vegans and vegetarians compared with omnivores. The lower proportions of DHA are accompanied by correspondingly higher proportions of the long-chain derivatives of linoleic acid, indicating that the capacity to synthesize long-chain polyunsaturated fatty acids is not limited. Short-term dietary supplementation with α-linolenic acid increases the proportion of eicosapentaenoic acid (20:5n-3) but does not increase the proportion of DHA in blood lipids. Small amounts of preformed DHA (as low as 200 mg) result in a large increase in the proportion of DHA in blood lipids in vegetarians and vegans. There is no evidence of adverse effects on health or cognitive function with lower DHA intake in vegetarians.

Conclusions

Preformed DHA in the diet of omnivores explains the relatively higher proportion of this fatty acid in blood and tissue lipids compared with vegetarians. The pathophysiological significance of this difference remains to be determined.

Introduction

Vegetarianism has been practiced for centuries in Asia, particularly by followers of the Hindu, Buddhist, and Jain religions. In the USA, a high proportion of Seventh Day Adventists follow vegetarian diets, and their health and diets have been extensively monitored [1]. Vegetarian and vegan diets have become increasingly popular in many developing countries for health, animal welfare, and ecological reasons, and there are many published reports on their diet and health [2]. Furthermore, migration from India has resulted in a large increase in the number of vegetarians in several developed countries, notably the United Kingdom, and their health status has also been the subject of numerous epidemiological investigations [3].

The parent essential fatty acids linoleic (18:2n-6; LA) and α-linolenic (18:3n-3; ALA) are made by plants but undergo further desaturation and chain elongation to give rise to longer chain derivatives. There is clear evidence for a human dietary requirement for LA [4], but arguments for the essentiality of omega-3 fatty acids are based on the physiological role of DHA in the visual process and synaptic functioning [5]. The dietary essentiality of ALA thus hinges on its conversion to DHA. Competition exists between LA and ALA for conversion to long-chain polyunsaturated fatty acids. Diets with a high ratio of LA:ALA suppress the synthesis of DHA and favor the production of docosapentaenoic acid (22:5n-6; DPA-n-6), which replaces DHA in retinal and neural tissues. It appears that subtle differences in visual and neurological function occur when DHA is mainly replaced with DPA-n-6.

It has been suggested that synthesis of DHA from ALA is inefficient and that the consumption of DHA would augment the supply to the brain [6]. This is of particular relevance to vegans because DHA is absent from their diets and to vegetarians whose diets contain smaller amounts of DHA than meat/fish eaters. Ovo-vegetarian and ovo-lacto-vegetarian diets are likely to supply more DHA than lacto-vegetarians diets. Although food tables often list cows’ milk fat as not containing long-chain n-3 fatty acids, it does contain approximately 20 mg/30 g fat, and infant formula based on cows’ milk fat alone was sufficient to prevent essential fatty acid deficiencies [4], [7]. Hens’ eggs contain approximately 20 mg DHA/egg. However, the proportions of DHA in these animal foods are dependent on the ratio of LA:ALA and whether DHA is present in the feed. This paper is a systematic review of the published data on the dietary intake of polyunsaturated fatty acids by vegetarians and vegans and biomarkers of DHA intake in relation to health outcomes.

Section snippets

Methods

The PubMed database was searched using the terms “docosahexaenoic acid” and “vegan” or “vegetarian” and the terms “fatty acids” and “vegetarian”. A similar search was conducted using the ISI database (Thomson-Reuters). These searches were supplemented with hand searches of journals. Emphasis was placed on identifying dietary data derived from fatty acid intakes from chemical analyses of diets or calculated from 7-d weighed food diaries rather than less reliable methods such as food frequency

Dietary intakes

Table 1 shows the polyunsaturated fatty acid content determined by chemical analyses of vegan and omnivore control diets [8]. Neither EPA nor DHA is present whereas the amount of LA is considerably greater than that of omnivores. Intakes of ALA also tend to be greater than in omnivores, but this depends on the source of culinary oils. Rapeseed and soy bean oil contain approximately 10% and 7% α-linolenic acid, respectively, but this may be destroyed during partial hydrogenation. In Northern

Discussion

Rosell et al. [16] in a large cross-sectional study failed to show that the proportion of DHA falls according to the duration on a vegan or vegetarian diet. This would suggest that there is a basal rate of conversion of ALA to DHA. There was some evidence to suggest that the proportion of 20:5n-3 and 22:5n-3 was higher in subjects with a lower ratio of LA/ALA in their plasma lipids. These derivatives may undergo further conversion to DHA in the brain. Reddy et al. [22] showed that cord arterial

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DHA status of vegetarians

Abstract

Background

Objective

Design

Results

Conclusions

Introduction

Section snippets

Methods

Dietary intakes

Discussion

Am. J. Clin. Nutr.

Atherosclerosis

Am. J. Clin. Nutr.

Atherosclerosis

Am. J. Clin. Nutr.

Atherosclerosis

Am. J. Clin. Nutr.

Am. J. Clin. Nutr.

Atherosclerosis

Am. J. Clin. Nutr.

Prostaglandins Leukot. Essent. Fatty Acids

Am. J. Clin. Nutr.

J. Nutr.

Am. J. Clin. Nutr.

Am. J. Clin. Nutr.

Am. J. Clin. Nutr.

Am. J. Clin. Nutr.

Cortex

Health effects of vegetarian and vegan diets

Proc. Nutr. Soc.

The relation of linoleic acid to infant feeding

Acta Paediatr. Suppl.