Food Lipids: Chemistry, Nutrition, and Biotechnology

Figure 1 General structure of structured lipids: S, L, and M: short, medium, and long chain
fatty acid, respectively; the positions of S, L, and M are interchangeable.
iologic functional foods, pharmafoods, and nutritional foods. The nomenclature is
still confusing and needs to be worked out by scientists in this field. SLs can be
designed for use as medical or functional lipids, and as nutraceuticals.
B. Rationale for Structured Lipid Development
Over the past 39 years, long chain triacylglycerols (LCTs), predominantly soybean
and safflower oils, have been the standard lipids used in making fat emulsions for
total parenteral nutrition (TPN) and enteral administration. The emulsion provides
energy and serves as a source of essential fatty acids (EFAs). However, long chain
fatty acids (LCFAs) are metabolized slowly in the body. It was then proposed that
medium chain triacylglycerols (MCTs) may be better than LCTs because the former
are readily metabolized for quick energy. MCTs are not dependent on carnitine for
transport into the mitochondria. They have higher plasma clearance, higher oxidation
rate, improved nitrogen-sparing action, and less tendency to be deposited in the
adipose tissue or to accumulate in the reticuloendothelial system (RES). One major
disadvantage of using MCT emulsions is the lack of essential fatty acids (18:2n-6).
In addition, large doses of MCTs can lead to the accumulation of ketone bodies, a
condition known as metabolic acidosis or ketonemia. It was suggested that combining
MCTs and LCTs in the preparation of fat emulsions enables utilization of the benefits
of both TAGs and may be theoretically better than pure LCT emulsions. An emulsion
of MCTs and LCTs is called a physical mixture; however, a physical mixture is not
equivalent to an SL. When MCTs and LCTs are chemically interesterified, the randomized
product is called an SL. SLs are expected to be rapidly cleared and metabolized
compared to LCTs.
For an SL to be beneficial, a minimum amount of LCFA is needed to meet
essential fatty acid requirements. With the SL, LCFAs, medium chain fatty acids
(MCFAs) and/or short chain fatty acids (SCFAs) can be delivered without the associated
adverse effects of pure MCT emulsions. This is especially important when
intravenous administration is considered (2,3). TAGs containing specific balances of
medium chain, n-3, n-6, n-9, and saturated fatty acids can be synthesized to reduce
serum low density lipoprotein (LDL) cholesterol and TAG levels, prevent thrombosis,
improve immune function, lessen the incidence of cancer, and improve nitrogen
balance (1,4). Although physical mixtures of TAGs have been administered to patients,
an SL emulsion is more attractive because of the modified absorption rates of
the SL molecule. Figure 2 shows the difference between a physical mixture of two
triacylglycerols and SL pairs of molecular species.
SLs can be manipulated to improve their physical characteristics such as melting
points. SLs are texturally important in the manufacture of plastic fats such as
margarines, modified butters, and shortenings. Caprenin, a structured lipid produced
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.