ALPHA-LINOLENIC ACID Postprandial Lipid Metabolism and Enzymatic Interesterification of Triacylgylcerols

An important indicator of the metabolic capacity of humans is the ability to
regulate plasma triacylglycerol levels and to clear triacylglycerol-rich lipoproteins
(TRLs) from the circulation after a meal. This is crucial since most of the day is
spent in the postprandial state. High concentrations and long circulation times of
TRL remnants may be detrimental since these are considered to be highly
atherogenic.

Postprandial effects of alpha-linolenic acid (ALA) in men and women are poorly
characterized. A new ALA-rich oil was produced from rapeseed and linseed oil by
enzymatic interesterification. The postprandial effects of 3 meals containing 35 g
of this new ALA-rich oil, olive oil, or butter were compared in two randomized
crossover studies (26 men and 19 premenopausal women). Blood samples were
drawn at regular intervals up to 7 h after the meals. We hypothesized that the
postprandial lipid response might be attenuated by a preferential oxidation of ALA
compared to other long chain dietary fatty acids.

Premenopausal women showed lower postprandial lipemia and were less sensitive
to variations in dietary fat than men. Butter resulted in lower postprandial lipemia
than the oils in men, whereas no such difference was seen in the women. The ALA
oil and olive oil meals induced similar plasma triacylglycerol concentrations.
Women showed significantly lower NEFA responses after the olive oil and butter
meals than men. The ALA-rich oil had significant effects on the different plasma
lipid fractions and decreased the n-6:n-3 ratio in plasma several hours
postprandially.

ALA levels remained high in plasma triacylglycerols and NEFA even after 5-7 h.
This late high concentration of ALA in NEFA is indicative of spill-over NEFA
and/or preferential release of ALA by the adipose tissue into the circulation.
In summary we did thus not find evidence that ALA has a beneficial effect on
postprandial lipids by a selective partitioning to oxidation. This does not exclude
the possibility that ALA over a longer time period may have health effects not
only as precursor of longer chain n-3 fatty, primarily docosahexaenoic acid, but
also because it is sorted out for oxidation.


The enzymatic interesterification of triacylglycerols using immobilized
Thermomyces lanuginosus lipase (Lipozyme TL IM) as catalyst has also been
investigated. Three different reaction systems were studied: rapeseed oil + butter,
rapeseed oil + linseed oil (ALA oil), and trilaurin + 1,3-palmitin-2-olein. The ALA
oil (35% ALA) was the same as that used in the meal studies. All reactions were
followed by reversed-phase HPLC and the triacylglycerol peaks were tentatively
identified by calculating equivalent carbon numbers. The triacylglycerols in the
rapeseed oil + butter mixture and products were also identified by HPLC-electrospray
tandem mass spectrometry.

In ideal sn-1,3-specific lipase-catalyzed interesterification, the fatty acid
composition in the sn-2 position remains constant. In practice, however, slight
changes are observed in the sn-2 position and, under certain conditions, a
completely randomized fatty acid distribution can be obtained. Randomization is
slower than interesterification. Uncontrolled hydrolysis should, however, be
avoided as it lowers the TAG yield. Different triacylglycerol mixtures, i.e.
products originating from 1,3-specific interesterification as well as totally or
partially randomized products, can be produced by varying the reaction time.
Enzymatic interesterification could be used as a method of designing dietary oils
with new properties regarding fatty acid composition, susceptibility to oxidation
and effects on blood lipids. The future of enzymatic processes relies on efficient,
flexible, and easy-to-use systems that ensure high stability of the enzyme
preparation and stable output of high-quality products at a reasonable cost.