Introduction

It is now recognized that in humans a-linolenic acid (ALA 18:3w) an omega-3 (n-3) fatty acid, is an essential fatty acid (EFA) that cannot be synthesized by the body and therefore must be supplied by dietary sources.

However, this recognition has been long in gestation. In 1976, Cuthbertson reviewed the requirements for infant formula and claimed that alpha-linolenic acid was not essential and only linoleic acid was required to replace breast milk. His claim was challenged by Crawford et al. who argued that the evidence available at that time not only favored the essentiality of alpha-linolenic acid but also the independent need for arachidonic acid and docosahexaenoic acid for infants. The position of the essentiality of ALA was confirmed by the 1978 World Health Organization/Food and Agriculture Organization Expert Consultation on the Role of Dietary Fats and Oils in Human Nutrition. For many years there was little interest in ALA and issues were raised concerning the danger of consuming highly unsaturated fatty acids that were susceptible to peroxidation. Holman argued that omega-3 fatty acids, although susceptible to peroxidation, were in practice, themselves protective. His conclusions have recently been confirmed by the discovery of neuroprotectins, powerful antioxidants derived from docosahexaenoic acid[1].

Figure 1 Characteristics of Linolenic acid

Synthesis Methods

In the natural world, linolenic acid is synthesized by plants and certain algae via a series of enzymatic reactions that elongate and desaturate shorter-chain fatty acids. These reactions are facilitated by a family of enzymes known as fatty acid desaturases, which introduce double bonds into fatty acyl chains. Specifically, the synthesis of linolenic acid involves the conversion of linoleic acid (18:2 omega-6) through the action of the omega-3 desaturase, which adds a double bond to create the three characteristic cis double bonds of linolenic acid at the 9th, 12th, and 15th carbon atoms.

Industrially, linolenic acid can be synthesized through chemical methods, such as catalytic desaturation, or via the use of microorganisms that produce omega-3 fatty acids through fermentation processes. These microbial processes involve genetically modified bacteria or yeast, which are capable of converting sugars into fatty acids, including linolenic acid, under controlled conditions.

Main Constituents

Chemically, linolenic acid consists of a chain of 18 carbons with three cis double bonds, which are located at the 9th, 12th, and 15th positions. This unique structure not only defines its liquid state at room temperature but also enhances its chemical reactivity. Linolenic acid’s role in the body is multifaceted; it is a precursor to eicosanoids, which are signaling molecules that play critical roles in inflammation and immunity[2].

Additionally, linolenic acid is incorporated into cell membranes, altering their fluidity and functionality. The presence of double bonds in linolenic acid contributes to its susceptibility to oxidative damage, which is a significant consideration in food processing and storage.

Usage Limits

The intake of linolenic acid is regulated by health authorities worldwide due to its potent biological effects. For instance, the FDA suggests a daily intake of up to 1.6 grams for men and 1.1 grams for women as adequate, while EFSA provides similar guidelines with a slightly higher recommended intake due to its cardiovascular benefits. However, these limits are set to prevent the potential negative effects of excessive consumption, which can include increased blood coagulation times and possible impacts on immune response.

Toxicity

Although linolenic acid is generally considered safe within the recommended consumption levels, its high degree of unsaturation makes it prone to oxidation, which can lead to the formation of lipid peroxides. These oxidation products can be harmful if ingested in large quantities, potentially leading to cellular damage and contributing to conditions such as atherosclerosis. The stability of linolenic acid is thus a critical factor in both dietary applications and industrial usage, necessitating the addition of antioxidants in products rich in linolenic acid to prevent degradation.

References

[1]Stark A H, Crawford M A, Reifen R. Update on alpha-linolenic acid[J]. Nutrition Reviews, 2008, 66(6): 326-332.

[2]Burdge G C. Metabolism of α-linolenic acid in humans[J]. Prostaglandins, leukotrienes and essential fatty acids, 2006, 75(3): 161-168.