Positional distribution of DHA in triacylglycerols: natural sources, synthetic routes, and nutritional properties

Docosahexaenoic acid (DHA, 22:6 n-3) is a long-chain polyunsaturated fatty acid (PUFA) present in high quantities in the mammalian brain and is a precursor of several metabolites. Clinical trials have demonstrated the benefits of dietary DHA in infants and adults. Triacylglycerols (TAGs) are the most abundant components of many natural oils, and in specific oils (e.g., fish, algal oils, etc.), they represent the main molecular form of dietary DHA. The positional distribution of DHA in the TAG glycerol backbone (sn-2 vs. sn-1/3) varied among different sources. Recent studies have shown that in human breast milk, DHA is mainly esterified at the sn-2 position (∼50% DHA of the total DHA), thus attracting research interest regarding the nutritional properties of sn-2 DHA. In this review, we summarize the different sources of TAG in natural oils with high amounts of DHA, including fish, algae, and marine mammal oils, with a focus on their positional distribution. Methods for analyzing the distribution of fatty acids in TAG of high-PUFA oils are discussed, and the lipase-catalyzed synthetic routes of specific triacylglycerols with sn-2 DHA are summarized. Furthermore, we discuss the recent research progress on the nutritional properties of DHA associated with its positional distribution on TAGs.

Influence of Dietary Triacylglycerol Structure on the Accumulation of Docosahexaenoic Acid [22:6(n-3)] in Organs in a Short-Term Feeding Trial with Mildly Omega-3 Deficient Rats

SCOPE

To study the effect of positional distribution of docosahexaenoic acid (DHA) in dietary triacylglycerols (TAG) on the tissue fatty acid content and composition of mildly (n-3) deficient rats.

METHODS AND RESULTS

In a 5-day feeding trial, mildly (n-3) deficient rats received 360 mg daily structured TAGs: sn-22:6(n-3)-18:0-18:0, sn-18:0-18:0-22:6(n-3), sn-18:0-22:6(n-3)-18:0, or tristearin. A fifth group receives standard (n-3) adequate feed AIN-93G from birth till the end of the trial. The DHA-fed groups show significantly higher DHA levels in the liver and visceral fat compared to the tristearin or normal feed groups showing that the dose and the short feeding period of DHA were sufficient to restore the DHA content in the organs of (n-3) deficient rats. Feeding sn-1 DHA resulted in higher levels of DHA in the liver TAG compared to sn-3 DHA feeding, although the difference did not reach statistical significance.

CONCLUSION

These findings indicated a possible difference in the tissue accumulation and/or metabolic fate of DHA from the sn-1 and sn-3 positions of TAG.