Triacylglycerols (TAGs) are mostly chiral and they are metabolized in the chiral environment. They are not only a source of energy but also provide essential fatty acids (FAs). Long chain omega-3 polyunsaturated fatty acids [LC (n-3) PUFAs] are derivatives of essential FA α-linolenic acid. Dietary LC (n-3) PUFAs, especially docosahexaenoic acid (DHA), have been associated with normal retinal and neural development and prevention of cardiovascular and circulatory disorders among other biological effects. Various structures of lipids in food products can modulate the release and bioavailability of FAs during digestion and subsequent metabolic fate of FAs. The current knowledge of the bioavailability of these LC (n-3) PUFAs from different TAG structures lies on the fish oil to seal oil comparisons as LC (n-3) PUFAs, mainly EPA and DHA, are located predominantly in the sn-2 position in fish and squid oil, but in the sn-1 and sn-3 positions in seal oil. Despite of numerous biological functions of LC (n-3) PUFAs, very little is known about the absorption of DHA from the stereospecific forms of TAGs, especially from the enantiospecific TAGs. Before recent advances in the synthesis of enantiospecific structured TAGs, the investigation of the impact of dietary TAG structure on the absorption of FAs from sn-1 and sn-3 positions of TAGs has been limited.
The aim of the current work was to investigate the effect of mild (n-3) deficiency and the effect of TAG structure on the extent of accumulation of DHA in organs, the bioavailability of DHA in plasma and feces, as well as on body weight, and organ weight. The effect of short-term first generation (n-3) deficiency was studied in a 33 day feeding trial on the lipid composition of different organs, plasma, and feces in rats. The effect of TAG structure on the FA content and composition of the different organs, plasma, and feces was investigated in a 5 day feeding trial with 360 mg/d enantiopure (> 96%) structured TAGs [sn-DHA-18:0-18:0 (sn-1 DHA) and sn-18:0-18:0-DHA (sn-3 DHA)] and their regioisomer [sn-18:0-DHA-18:0, sn-2 DHA). The lipids were extracted from organs, plasma, and feces, transformed into fatty acid methyl esters and analyzed by gas chromatography.
The (n-3) deficiency resulted in lower DHA levels in the plasma, brain, testicle, visceral fat, heart, and lungs. The DHA level of the visceral fat of (n-3) deficient animals was 14% of the visceral fat of (n-3) adequately fed animals, being most affected by the (n-3) deficient diet feeding. The brain DHA was least affected with the DHA level of the (n-3) deficient animals being 86% of the (n-3) adequate animals. The DHA level of the organs was affected in the order of visceral fat > liver triacylglycerols > lung > heart > liver phospholipids > testis > eye > brain. The effect of feeding structured TAGs for 5 days following an (n-3) deficient diet resulted in less secretion of fecal DHA from the sn-2 position compared with the sn-1 and sn-3 positions suggesting superior absorption of DHA from the sn-2 position. A significantly higher level of DHA in the sn-1 DHA group compared to sn-3 DHA group in the liver TAG showed the difference in the absorption of DHA between the two primary positions of dietary TAG. The eye of the sn-1 DHA group led to a small but insignificant increase in DHA level compared to the sn-3 DHA group and stearic acid (18:0, STA) in liver PLs was found to be significantly higher sn-1 DHA group as compared to the sn-3 DHA group. This indicated a possible difference in the absorption of FAs from sn-1 and sn-3 positions of TAG. This work provides the information to evaluate the conditions needed to reach mild (n-3) deficiency in the first generation of rats and to evaluate the feasibility to collect data from a variety of organs. Also, the short term feeding of structured TAGs provides evidence on the possible differences in the absorption of DHA from the primary positions (sn-1 and sn-3) of TAG in the liver TAG fraction. Together, this work shed light on the influence of the positional distribution of DHA in dietary TAGs on plasma, fecal, and organ lipids and (n-3) deficiency on the bioavailability of DHA strengthening the current understanding of lipid biochemistry.