Lymphatic absorption of structured triglycerides vs. physical
mix in a rat model of fat malabsorption
Patrick
Tso1,
Theresa
Lee2, and
Stephen J.
Demichele2
1 Department of Pathology,
University of Cincinnati Medical Center, Cincinnati 45267; and
2 Medical Nutrition Research and
Development, Ross Products Division, Abbott Laboratories, Columbus,
Ohio 43215
 |
ABSTRACT |
Comparison was
made between the intestinal absorption and lymphatic transport of a
randomly interesterified fish oil and medium-chain triglyceride (MCT)
structured triglycerides (STG) vs. the physical mix in rat small
intestine following ischemia and reperfusion (I/R) injury.
Under halothane anesthesia, the superior mesenteric artery (SMA) was
occluded for 20 min and then reperfused in I/R rats. The SMA was
isolated but not occluded in control rats. In both treatment groups,
the mesenteric lymph duct was cannulated and a gastric tube was
inserted. Each treatment group received 1 ml of the fish oil-MCT STG or
physical mix (7 rats/group) through the gastric tube followed by an
infusion of PBS at 3 ml/h for 8 h. Lymph was collected hourly for 8 h.
Lymph triglyceride, cholesterol, and decanoic and eicosapentaenoic
acids increased rapidly and maintained a significantly higher output
(P < 0.01) with STG compared with
physical mix in control rats over 8 h. After I/R, lymphatic triglyceride output decreased 50% compared with control. Gastric infusion of STG significantly improved lipid transport by having a
twofold higher triglyceride, cholesterol, and decanoic and
eicosapentaenoic acids output to lymph compared with its physical mix
(P < 0.01). We conclude that STG is
absorbed into lymph significantly better than physical mix by both the
normal intestine and the intestine injured by I/R.
lymph; fat absorption; enteral nutrition; long-chain triglycerides; medium-chain triglycerides; lymph-fistula rat
 |
INTRODUCTION |
AN EXCITING NEW APPROACH to optimize the metabolic
benefits of specific lipid mixtures has been the development of
structured triglycerides (STG). Considerable advances have been made
over the past 10 years that have broadened our knowledge base as to the
understanding of the basic chemistry, physiology, and metabolism of
these novel lipids. STG are a chemically interesterified mixture of
both medium- and long-chain fatty acids incorporated on the same
glycerol backbone by hydrolysis and random reesterification (3, 25).
These triglyceride molecules are chemically distinct and offer unique
advantages from their constituent medium (MCT)- and long (LCT)-chain
triglycerides. For example, STG that contain medium-chain fatty acids
may provide a useful vehicle for rapid hydrolysis and absorption due to
the smaller molecular size and greater water solubility in comparison
to LCT. Although STG retain some characteristics of MCT and LCT, they
may provide an alternative lipid source that could overcome the
gastrointestinal intolerance related to the sole use of MCT or LCT in
patients with malabsorptive diseases.
Numerous studies in animal models of trauma, burn injury, and endotoxic
shock have demonstrated that STG have metabolic benefits compared with
identical physical mixtures of oils that have not been interesterified
(10, 11, 24, 28, 33, 34). Rats receiving STG showed the greatest gain
in body weight and attenuation of the hypermetabolic and protein
catabolic response to injury over comparable groups of rats given
either LCT or MCT alone or a physical mixture of LCT and MCT. Metabolic
improvements have also been observed with STG compared with a physical
mix of oil in rat models of cancer (23, 27) and sepsis-induced liver injury and dysfunction (20). Recently, the first clinical testing of a
novel STG composed of fish oil and MCT was performed in patients undergoing major abdominal surgery for upper gastrointestinal malignancies (19, 32). Results showed that the enteral administration of a diet containing the fish oil-MCT STG during the postoperative period significantly reduced the total number of reported
gastrointestinal complications and infections as well as improved renal
and liver function through modulation of proinflammatory eicosanoid production.
The advantages of enterally fed STG may relate to differences in
absorption, chylomicron formation, and transport of the triglycerides. Enhanced absorption of linoleic acid [18:2 (n-6)] was
observed in cystic fibrosis patients fed STG containing long- and
medium-chain fatty acids (13, 26). In vitro lipase digestions and
absorption studies using isolated intestinal loops revealed rapid
hydrolysis and absorption of a STG containing linoleic acid in the
sn-2 position and medium-chain fatty acids in the
sn-1 and -3 positions (15). Jensen et al. (16) reported in
the lymph-cannulated dog that lymphatic absorption of medium-chain
fatty acids from STG was 2.6-fold higher (10:0 in excess of 8:0)
compared with its equivalent physical mix. Molecular species analyses
revealed that the medium-chain fatty acids in lymph were present on the
same triglyceride as long-chain fatty acids. Recently, we assessed the
intestinal absorption of STG containing two medium-chain fatty acids
(8:0) and one long-chain fatty acid [18:2 (n-6)] using a
lymph fistula rat model (35). Conclusions from this study indicate that
the chain length of the fatty acid on the STG molecule affected the
digestion, absorption, and lymphatic transport of the triglyceride.
Although these observations are very important, few studies have
assessed the potential absorptive benefits of STG in malabsorptive syndromes. Christensen et al. (9) showed that defined triglycerides with specific fatty acids in the sn-2 position on the
glycerol moiety had a higher lymphatic transport of polyunsaturated
fatty acids compared with a similar physical mixture of oils in rats with pancreatic deficiency. However, the benefits of STG consisting of
random distributions of long- and medium-chain fatty acids on the
glycerol backbone have not been explored in a rat model of fat
malabsorption. The goals of this study were to compare the digestion,
absorption, and lymphatic transport of a fish oil-MCT STG vs. its
physical mix in a normal lymph fistula rat model and in a rat model of
lipid malabsorption caused by ischemia and reperfusion (I/R)-induced injury of the small bowel (12).
 |
MATERIALS AND METHODS |
Conditioning of animals.
Male adult Sprague-Dawley rats weighing 300-350 g were used for
the study. When the animals first arrived at the vivarium, they were
housed in quarantine for 1 wk and fed Purina rat chow. The light in the
room was regulated to give 12:12-h light-dark cycle.
Lymph-fistula rat model and ischemic injury.
Approval of this study was granted by the Animal Care Committee of the
University of Cincinnati in accordance with guidelines set forth in the
National Institutes of Health Guide for the Care and Use of
Laboratory Animals.
Rats were fasted overnight before surgical procedures. Under halothane
anesthesia, a laparotomy was performed. The superior mesenteric artery
(SMA) was occluded for 20 min with a microbulldog clamp. At the end of
the ischemic period, the clamp was released and three drops of
lidocaine were applied directly on the SMA to facilitate perfusion
(12). In the control (sham) animals, the SMA was isolated in a similar
fashion but was not occluded. In both groups of animals, the intestinal
lymph duct was cannulated according to the method of Bollman et al.
(6). In addition, a second soft silicone gastric tube (1.6 mm outer
diameter) was introduced into the fundus of the stomach. The tubing was
secured in the stomach by closing the fundal incision with a
purse-string suture. Postoperatively, the rats were infused
intragastrically at a rate of 3 ml/h with a 5% glucose-saline solution
containing 145 mM NaCl, 4 mM KCl, and 0.28 M glucose. The animals were
allowed to recover for at least 24 h in restraining cages maintained at a temperature of 30°C before lipid infusion.
Experimental plan.
Four groups of rats were studied: two groups of sham-operated controls
and two groups of rats with small bowel I/R injury. Seven rats were
studied in each group. In the first two groups of sham-operated
controls, rats were randomized to gastrically receive either 1 ml of
fish oil-MCT STG or its physical mix equivalent. The fish oil-MCT STG
was produced by mixing 55% by weight fish oil and 45% by weight
fractionated MCT followed by chemical interesterification using sodium
methoxide as a catalyst. This commercial process of interesterification
leads to randomization so that predominantly all possible new
triglyceride structures are found including LML, LMM, LLM, MLM (where L
and M represent long- and medium-chain fatty acids, respectively, at
the sn-1, -2, and -3 positions on the triglyceride molecule)
with lesser amounts of the starting lipids MMM and LLL (3). The
physical mix was prepared by mixing the same weight proportions of fish
oil and MCT as in the STG without interesterification (Fig.
1). The fatty acid composition of the starting oils, STG, and the physical mix is outlined in Table
1. The 2-monoglyceride fatty acid
composition and molecular species determination of the fish oil-MCT STG
and the physical mix are shown in Tables 2
and 3, respectively. Similarly, 1 ml of
either STG or physical mix was given by gavage in the rats injured by
I/R.

View larger version (34K):
[in this window]
[in a new window]
|
Fig. 1.
Drawing of synthesis of structured triglycerides (STG) by chemical
interesterification. Manufacture of STG used in study is composed of
mixing fractionated medium-chain triglycerides (MCT) and fish oil
(long-chain triglycerides, LCT) in specific proportions, allowing for
hydrolysis of triglycerides to form free fatty acids and glycerol.
These components are then reesterified to form new triglycerides
containing randomized mixtures of medium (MCFA)- and long (LCFA)-chain
fatty acids. There are 6 possible fatty acid combinations on
triglyceride molecule. Individual triglyceride molecules may contain 2 MCFA and 1 LCFA or 2 LCFA and 1 MCFA, as well as smaller quantities of
pure MCT and LCT, with amount of each form being dependent on
proportion of initial MCT and LCT oils.
|
|
Experimental procedure.
Lymph was collected into precooled conical graduated centrifuge tubes
for 2 h before lipid infusion. This sample was analyzed as the fasting
lymphatic output of lipid. Additional lymph samples were collected
hourly for 8 h and between 8 and 24 h after the beginning of lipid
infusion. After the lymph volume was determined, the samples were
centrifuged for 15 min at 700 g at
room temperature to remove blood cells. Lymph lipid was extracted by
the method of Blankenhorn and Ahrens (5), and aliquots of the extract were taken for determination of triglyceride (4), phospholipid (29),
and cholesterol (30). Aliquots were also taken from a subset of three
rats for the analysis of fatty acid composition of the lymph triglycerides.
Analysis of fatty acid composition.
Total lipids were extracted according to the official method of the
American Official Analytical Chemists' Association (1). Fatty acid
methyl esters were prepared and analyzed by gas chromatography as
previously described (2, 21). A Hewlett-Packard 5890 gas chromatograph
and a fused silica capillary column (Omegawax 320R, 30 m length, 0.32 mm internal diameter, 0.25 micron film thickness; Supelco, Bellefonte,
PA) were used throughout the study. A known amount of an internal
standard triheptadecanoin was used to quantitate the amount of fatty
acids in each sample. The results of the fatty acid composition were
expressed as grams per 100 grams fatty acids (relative weight percentage).
Analysis of triglyceride profile by supercritical fluid
chromatography.
The quantitative determination of triglyceride molecular species using
carbon dioxide supercritical fluid chromatography was performed as
previously described (22). Triglyceride species were separated
according to their equivalent carbon number (ECN), defined as the
"sum of the total carbon number in the acyl side-chains of the
triglyceride molecule." A known amount of the triglycerides was
dissolved in chloroform-methanol (95:5, vol/vol) solvent and analyzed
directly using a supercritical fluid chromatograph (Dionex 602 series;
Dionex, Sunnyvale, CA). An SB-100 methyl capillary column (10 m length,
100 µm internal diameter, and 0.25 µm film thickness) with frit
restrictor and a flame ionization detector were used for the
separation, detection, and quantitation of the triglyceride species.
Supercritical fluid carbon dioxide was used as the mobile phase. A
known amount of the monoacid triglyceride standards was used for
instrument calibration.
2-Monoglyceride analysis.
2-Monoglycerides were analyzed using a method similar to that described
by Jensen et al. (16). The triglycerides were hydrolyzed using a
freshly prepared lipase solution (Rhizopus arrhizus, Sigma, EC 3.1.1.3, 1 × 105 U/ml).
Then 2-monoglycerides were extracted from the reaction mixture and
separated using TLC. A solvent mixture containing chloroform-acetone
(85:15, vol/vol) was used to separate the triglyceride, diglyceride,
2-monoglyceride, and 1 (3)-monoglycerides (16, 36). The TLC zone for
2-monoglycerides was isolated, and the fatty acid composition was
analyzed in the same manner as previously described.
Statistical methods.
All values are expressed as means ± SE. A two-way repeated-measures
ANOVA was used to determine whether differences existed among groups
for each hour of lipid infusion for each dependent variable. If a main
effect of group or time was significant, Tukey's studentized range
test was carried out to determine where the difference occurred. When a
significant interaction was present, a one-way repeated-measures ANOVA
was conducted for each group and a one-way ANOVA was conducted at each
time. Significant findings were then subjected to the Tukey's test to
determine where the differences occurred. Results were considered to be
statistically significant at P < 5%.
 |
RESULTS |
Lymph flow.
As shown in Fig. 2, the mean fasting lymph
flow for all four groups of rats, two I/R (STG and physical mix) and
two controls (STG and physical mix), varied between 2.5 and 2.8 ml/h.
In all groups, the lymph flow increased significantly as a result of lipid infusion and reached a maximum flow rate between 3.3 and 4.0 ml/h
during the 3rd h after feeding the oil by gavage. After peaking at the
3rd h, lymph flow declined slowly and reached a steady output of about
3 ml/h during the 7th and 8th h following lipid feeding. There were no
statistically significant differences in the lymph flow rates between
the STG and physical mix rats in either the control or I/R groups.

View larger version (16K):
[in this window]
[in a new window]
|
Fig. 2.
Lymph flow expressed as milliliters per hour. Lymph flow was measured 1 h before (fasting) and hourly for 8 h after lipid infusion. The 4 groups of animals studied included
1) sham-operated controls infused
with physical mix (PM), control-PM;
2) sham-operated controls infused
with structured triglycerides, control-STG;
3) ischemia and reperfused
rats infused with PM, I/R-PM; 4)
ischemia and reperfused rats infused with STG, I/R-STG. There
are 7 rats in each group; values are means ± SE.
|
|
Lymphatic triglyceride output.
Figure 3 shows the lymphatic triglyceride
output during the first 8 h after the gastric feeding of either STG or
physical mix. The fasting lymphatic triglyceride output varied between 3.8 and 4.4 µmol/h in all four groups of animals. There
was a marked difference in the lymphatic triglyceride output following the feeding of either STG or physical mix in the control animals. Lymphatic triglyceride output in the STG-fed rats increased rapidly and
maintained a significantly higher output overall and for all time
points during the 8 h after lipid feeding
(P < 0.01) compared with physical
mix. The absorption of STG in control rats reached a steady-state
output much earlier following the lipid meal compared with the physical
mix (3-4 vs. 5-6 h, respectively). In addition, control STG
rats had a higher maximal triglyceride output compared with physical
mix. As we have previously observed, the lymphatic triglyceride output
decreased 50% following I/R injury compared with control animals.
Although I/R injury resulted in a significant reduction in the
lymphatic absorption of both STG and physical mix, gastric infusion of
STG significantly improved triglyceride transport by having a
significantly higher triglyceride output overall and for all time
points during the 8 h following lipid feeding
(P < 0.01). The triglyceride output
from I/R rats fed STG was similar to the output observed for control
rats fed the physical mix.

View larger version (19K):
[in this window]
[in a new window]
|
Fig. 3.
Lymphatic triglyceride output expressed as micromoles per hour.
Triglyceride content was measured in lymph collected 1 h before
(fasting) and hourly for 8 h after lipid feeding. The 4 groups of
animals studied are the same as in Fig. 2. There are 7 animals in each
group; values are means ± SE.
* P < 0.01 vs. I/R-PM and
** P < 0.01 vs. control-PM
using repeated-measures ANOVA.
|
|
When the sum of the total number of micromoles of triglyceride
transported in lymph during the 8 h following lipid feeding was
calculated, the differences between STG and physical mix were obvious.
The overall lymphatic triglyceride output during the 8 h after lipid
feeding in the control STG group [268.7 ± 24.6 (SE)
µmol] was significantly higher than control physical mix (171.1 ± 14.3 µmol, P < 0.001), as
well as the I/R STG group (157.7 ± 11.6 µmol,
P < 0.001), having an overall
significantly higher triglyceride output compared with I/R physical mix
(77.6 ± 6.5 µmol).
Lymphatic phospholipid output.
Figure 4 shows the lymphatic phospholipid
output in micromoles per hour. In all four groups of rats, lymphatic
phospholipid output increased as a result of lipid feeding. Although
the lymphatic phospholipid output appeared to be higher in the control
STG group compared with the control physical mix rats, the overall
differences were not statistically significant
(P = 0.09, repeated measures ANOVA).
I/R injury to the small bowel significantly reduced lymphatic phospholipid output compared with the control animals regardless of the
type of lipid infused. However, lymphatic phospholipid output was
significantly higher in the I/R STG group compared with I/R physical
mix (P < 0.001; repeated measures
ANOVA and for all time points from 2 to 8 h). Similar differences like
those above were observed when the cumulative lymphatic phospholipid in
the four groups of rats was calculated. The total lymphatic phospholipid output during the 8 h following lipid feeding in the
control STG group was 28.7 ± 1.7 µmol and not statistically different from control physical mix (25.3 ± 1.4 µmol,
P = 0.105). In contrast, the overall
lymphatic phospholipid output was significantly higher in the I/R STG
rats (21.1 ± 1.2 µmol) compared with I/R physical mix (14.8 ± 1.6 µmol, P < 0.01).

View larger version (21K):
[in this window]
[in a new window]
|
Fig. 4.
Lymphatic phospholipid output expressed as micromoles per hour.
Phospholipid content was measured in lymph collected 1 h before
(fasting) and hourly for 8 h after lipid feeding. The 4 groups of
animals studied are the same as in Fig. 2. There are 7 animals in each
group; values are means ± SE.
* P < 0.01 vs. I/R-PM using
repeated-measures ANOVA.
|
|
Lymphatic cholesterol output.
As shown in Fig. 5, lymphatic cholesterol
outputs recapitulated the pattern observed with the lymphatic
triglyceride outputs. Similar to lymphatic triglyceride output, I/R
injury significantly reduced lymphatic cholesterol outputs in rats fed
with either STG or physical mix at all time points during the 8 h
following lipid feeding (P < 0.01 for comparisons at all time between control STG and I/R STG or control
physical mix and I/R physical mix). Lymphatic cholesterol transport was
significantly higher overall (P < 0.01) with STG feeding compared with physical mix feeding in both the
control and I/R rats (P < 0.01 for
comparisons at all time points between control STG and control physical
mix or I/R STG and I/R physical mix animals).

View larger version (19K):
[in this window]
[in a new window]
|
Fig. 5.
Lymphatic cholesterol output expressed as micromoles per hour.
Cholesterol content was measured in lymph collected 1 h before
(fasting) and hourly for 8 h after lipid feeding. The 4 groups of
animals studied are the same as in Fig. 2. There are 7 animals in each
group; values are means ± SE.
* P < 0.01 vs. I/R-PM and
** P < 0.01 vs. control-PM
using repeated-measures ANOVA.
|
|
The cumulative lymphatic cholesterol outputs showed that during the 8 h
following lipid feeding, the control STG rats transported 39.6 ± 1.8 µmol of cholesterol into lymph as compared with 27.1 ± 1.4 µmol in the control physical mix group
(P < 0.001). In the I/R group, rats
fed STG transport 24.4 ± 1.8 µmol of cholesterol in lymph
compared with 14.9 ± 1.5 µmol of cholesterol in the physical mix
animals, which was highly significant
(P < 0.001).
Lymphatic decanoic acid and eicosapentaenoic acid outputs.
The fatty acid composition of lymph triglycerides was measured to
assess whether differences exist between rats fed either STG or
physical mix in the absorption and lymphatic transport in the
individual fatty acids of interest, i.e., decanoic acid (10:0, a
medium-chain fatty acid) and eicosapentaenoic acid [20:5 (n-3), a
long-chain n-3 fatty acid]. As shown in Figs.
6 and 7, there
was a two- to threefold increase in decanoic and eicosapentaenoic acids
transported in lymph during the 8 h following STG feeding compared with
physical mix. As observed before with triglyceride output, lymphatic
transport of either fatty acid was reduced following I/R injury.
However, lymphatic transport results of both decanoic and
eicosapentaenoic acids were two- to threefold higher as early as 2 h
after feeding STG compared with physical mix. This difference was
maintained during the 8-h lymph collection.

View larger version (18K):
[in this window]
[in a new window]
|
Fig. 6.
Lymphatic decanoic acid output expressed in milligrams per hour.
Decanoic acid was measured in lymph before lipid infusion
(time 0) and at 2, 4, 6, and 8 h
after lipid feeding. The 4 groups of animals studied are the same as in
Fig. 2. A subset of 3 rats/group was used for decanoic acid analysis in
lymph; values are expressed as means.
|
|

View larger version (17K):
[in this window]
[in a new window]
|
Fig. 7.
Lymphatic eicosapentaenoic acid output expressed in milligrams per
hour. Eicosapentaenoic acid was measured in lymph before lipid infusion
(time 0) and at 2, 4, 6, and 8 h
after lipid feeding. The 4 groups of animals studied are the same as in
Fig. 2. A subset of 3 rats/group was used for decanoic acid analysis in
lymph; values are expressed as means.
|
|
 |
DISCUSSION |
Under steady-state conditions, we compared the absorption and lymphatic
transport of STG and its physical mix under normal conditions and
following I/R injury of the small bowel. We clearly demonstrated that
intestinal lipid absorption was markedly reduced 24 h after I/R injury.
The details of this model of fat malabsorption have been previously
described (12). A conscious lymph fistula rat model was used as an
established method for studying the digestion and transport of lipids.
By infusing a physiological quantity of either STG or physical mix into
the stomach of rats, the lymphatic triglyceride, phospholipid,
cholesterol, and fatty acid output can be collected and quantitatively
compared between the two groups.
The data presented in this study show that the amount of triglyceride,
cholesterol, and specific fatty acids (decanoic and eicosapentaenoic
acids) transported into lymph were significantly higher after STG
infusion compared with physical mix in the normal conscious lymph
fistula rat. When we examined the lymphatic output data for these
parameters, the steady-state outputs occurred earlier with
significantly higher maximal outputs for control animals fed STG
compared with physical mix. It is felt that these observations are very
important, especially the findings that enhance the absorption of lipid
and key essential fatty acids under normal conditions with STG.
Although I/R significantly reduced the lymphatic transport of
triglyceride, phospholipid, cholesterol, and specific fatty acids
(decanoic and eicosapentaenoic acids) in both groups of rats, STG
infusion resulted in similar absorptive benefits as seen in normal
animals. For all parameters measured, the difference between STG- and
physical mix-fed rats was greater in the I/R group compared with the
controls. The magnitude of this difference is evident from the fact
that the lymphatic transport of triglyceride, phospholipid,
cholesterol, and decanoic, and eicosapentaenoic acids from I/R rats fed
STG was similar to the output observed for control rats fed physical
mix. These data clearly demonstrate that STG were digested and
transported to lymph more efficiently than the physical mix under
normal and I/R injury conditions.
There are a number of possible explanations for these important
findings. The enhanced lymphatic absorption of the above lipids from
STG most likely reflects the unique molecular structure of its
triglycerides that results from interesterification of the fish oil and
MCT. Triglyceride molecular species analysis of the two oils was
performed to determine the relative distribution of medium-chain fatty
acids in the various triglycerides of both oils (Table 3). The
calculated ECN represents the sum of the acyl side chains of the
triglyceride. The triglyceride species containing a mixture of medium-
and long-chain fatty acids (ECNs 32-43) were abundant with STG but
were absent in the physical mixture. In contrast, the physical mix had
a higher proportion of triglycerides with an ECN number less than 30 or
greater than 50, indicating that the triglycerides contain exclusively
either medium- or long-chain fatty acids, but not mixed. Thus it is
likely that the novel triglyceride species (ECNs 32-43) in STG may
be responsible for the increased absorption and transport of
triglyceride in lymph for both control and I/R rats. The precise
mechanism of these absorptive benefits remains to be investigated but
may lie in the packaging and secretion of chylomicrons. Because we were
not able to radiolabel the abundance of different triglycerides in STG
and physical mix, we could not trace the luminal digestion and mucosal
uptake of the lipid digestive products of either oil. However, it is
not likely that the difference in the lymphatic transport of
triglycerides in rats given either the STG or physical mix is caused by
a difference in digestion and mucosal uptake of the lipolytic products.
Most likely, it is caused by a difference in the re-esterification and
packaging of the absorbed lipid into chylomicrons. The mechanism of why
the lipid digestion products from STG is packaged better into
chylomicrons vs. physical mix remains to be elucidated and is currently
being investigated in our laboratory. In support of the this theory,
more efficient packaging and either larger or more chylomicrons may be
formed as evidenced by the observed increases in lymphatic phospholipid and cholesterol outputs.
This present investigation confirms previous STG studies demonstrating
the absorptive benefits of STG consisting of a random distribution of
long- and medium-chain fatty acids on the glycerol moiety. Enhanced
absorption of 18:2 (n-6) was observed in cystic fibrosis patients fed
STG containing mixed triglycerides (13, 26). Jensen et al. (16)
developed a canine model to compare the lymphatic absorption of fish
oil-MCT STG and its equivalent physical mix; similar oils were used in
this study. Results showed that lymphatic absorption of medium-chain
fatty acids from STG was 2.6-fold higher (10:0 in excess of 8:0)
compared with the physical mix. Molecular species analyses revealed
that the medium-chain fatty acids in lymph were present on the same
triglyceride as long-chain fatty acids. In one dog that received a
double crossover, a two- to threefold increase in the amount of lipid
in lymph was detected with the STG diet compared with the physical mix.
Rat absorption studies by Christensen et al. (7-9) and Jensen et al. (17) have shown that defined triglycerides with specific fatty
acids in the sn-2 position on the glycerol moiety may
provide a means to increase absorption of essential fatty acids in
syndromes having reduced pancreatic lipase and/or compromised bile
production. Recently, the first clinical testing of fish oil-MCT STG
was performed by Kenler and associates (19). They compared the safety,
gastrointestinal tolerance, and clinical efficacy of an enteral diet
containing STG to an isocaloric formula containing a physical mixture
of vegetable oil and MCT in patients undergoing major abdominal surgery for upper gastrointestinal malignancies. Results showed that patients fed fish oil-MCT STG experienced 40% lower total days with reported gastrointestinal complications (P = 0.036) and a 50% decline in the total number of actual reported
gastrointestinal complications (P = 0.004) compared with patients fed control diet.
In addition to the increased absorption of triglyceride, phospholipid,
and cholesterol, enhanced lymphatic absorption of decanoic and
eicosapentaenoic acids was observed with STG compared with physical
mix. Positional specificity of fatty acids on the glycerol moiety was
likely one of the key factors. Lingual, gastric, and pancreatic lipases
hydrolyze medium- or long-chain fatty acids in the sn-1 and
sn-3 positions. The remaining fatty acid in the sn-2 position (2-monoglyceride) is quickly absorbed,
reesterified with long-chain fatty acids or longer medium-chain fatty
acids, formed into chylomicrons, and transported by the lymphatic
system to the systemic circulation for utilization by peripheral
tissues (18, 31). The increase in lymphatic eicosapentaenoic acid output after STG feeding was due in part to its higher percentage in
the sn-2 position on the glycerol moiety compared with
physical mix (Table 2). The overall increase in triglyceride output
would also contribute to the increase in eicosapentaenoic acid delivery to lymph. The higher lymphatic transport of decanoic acid after feeding
STG, however, cannot be explained by its preferential placement in the
sn-2 position of the STG molecule. In fact, the physical mix
has a higher percentage of 8:0 and decanoic acid (10:0) in the
sn-2 position compared with the physical mix. One explanation may be that the digestion of the physical mix resulted in a
higher amount of medium-chain fatty acids being transported via the
portal route, resulting in a lower concentration in lymph compared with
STG. The increase in lymphatic decanoic acid with STG may be due in
part to its unique, mixed triglyceride species containing both medium-
and long-chain fatty acids, which are absent in the physical mix. These
triglyceride species have been associated with rapid hydrolysis and
absorption in both in vitro (15) and in vivo animal models (14, 16,
35). These data suggest that a randomized triglyceride mixture
containing either two medium- and one long-chain fatty acid or one
medium- and two long-chain fatty acids are absorbed and transported
into lymph more efficiently than a mixture of triglycerides containing
predominantly either medium- or long-chain fatty acids. These novel
triglycerides have been shown to have metabolic benefits in animal
models of trauma as described earlier (10, 11, 24, 28, 33, 34).
In summary, this study provides important evidence supporting the use
of randomly interesterified STG to increase the absorption of
triglycerides and essential fatty acids under normal, healthy conditions and following small bowel dysfunction. Provision of oil
blends in the form of STG may have improved tolerance and lower
gastrointestinal complications in malabsorption syndromes.
 |
ACKNOWLEDGEMENTS |
We thank Emil Bobik for expert analytic assistance and Kathy Dailey
for assistance in preparation of the manuscript.
 |
FOOTNOTES |
This work was supported in part by Ross Products Division, Abbott
Laboratories, and the National Institute of Diabetes and Digestive and
Kidney Diseases Grant DK-32288.
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: P. Tso, Dept. of
Pathology, Univ. of Cincinnati Medical Center, 231 Bethesda Ave.,
ML-0529, Cincinnati, OH 45267 (E-mail:
tsopp{at}emailuc.edu).
Received 5 February 1999; accepted in final form 7 May 1999.
 |
REFERENCES |
1.
American Official Analytical Chemists' Association.
Official Method of Analysis (14th ed.). Gaithersburg, MD: Am. Official Anal. Chem Assoc., 1984, sect. 16.064.
2.
American Official Analytical Chemists' Association.
Official Method of Analysis (14th ed.). Gaithersburg, MD: Am. Official Anal. Chem. Assoc., 1984, sect. 28.056-28.067.
3.
Babayan, V. K.
Medium chain triacylglycerols and structured lipids.
Lipids
22:
417-420,
1987[Medline].
4.
Biggs, H. G.,
J. M. Erikson,
and
W. R. Moorehead.
Manual colorimetric assay of triglycerides in serum.
Clin. Chem.
21:
437-441,
1975[Free Full Text].
5.
Blankenhorn, D. H.,
and
E. H. Ahrens.
Extraction, isolation, and identification of hydrolytic products of triglyceride digestion in man.
J. Biol. Chem.
212:
69-81,
1955[Free Full Text].
6.
Bollman, J. L.,
J. C. Cain,
and
J. H. Grindlay.
Techniques for the collection of lymph from the liver, small intestine or thoracic duct of the rat.
J. Lab. Clin. Med.
33:
1349-1352,
1948.
7.
Christensen, M. S.,
C. E. Hoy,
C. C. Becker,
and
T. G. Redgrave.
Intestinal absorption and lymphatic transport of eicosapentaenoic (EPA), docosahexaenoic (DHA), and decanoic acids: dependence on intramolecular triacylglycerol structure.
Am. J. Clin. Nutr.
61:
56-61,
1995[Abstract].
8.
Christensen, M. S.,
C. E. Hoy,
and
T. G. Redgrave.
Lymphatic absorption of n-3 polyunsaturated fatty acids from marine oils with different intramolecular fatty acid distributions.
Biochim. Biophys. Acta
1215:
198-204,
1994[Medline].
9.
Christensen, M. S.,
A. Mullertz,
and
C. E. Hoy.
Absorption of triacylglycerols with defined or random structure by rats with biliary and pancreatic diversion.
Lipids
30:
521-526,
1995[Medline].
10.
DeMichele, S. J.,
M. D. Karlstad,
V. K. Babayan,
N. Istfan,
G. L. Blackburn,
and
B. R. Bistrian.
Enhanced skeletal muscle and liver protein synthesis with structured lipid in enterally fed burned rats.
Metabolism
37:
787-795,
1988[Medline].
11.
DeMichele, S. J.,
M. D. Karlstad,
B. R. Bistrian,
N. Istfan,
V. K. Babayan,
and
G. L. Blackburn.
Enteral nutrition with structured lipid: effect on protein metabolism in thermal injury.
Am. J. Clin. Nutr.
50:
1295-1302,
1989[Abstract].
12.
Fujimoto, K.,
V. H. Price,
D. N. Granger,
R. Specian,
S. Bergstedt,
and
P. Tso.
Effect of ischemia-reperfusion on lipid digestion and absorption in rat intestine.
Am. J. Physiol.
260 (Gastrointest. Liver Physiol. 23):
G595-G602,
1991[Abstract/Free Full Text].
13.
Hubbard, V. S.,
and
M. C. McKenna.
Absorption of safflower oil and structured lipid preparations in patients with cystic fibrosis.
Lipids
22:
424-428,
1987[Medline].
14.
Ikeda, I.,
Y. Tomari,
M. Sugano,
S. Watanabe,
and
J. Nagata.
Lymphatic absorption of structured glycerolipids containing medium-chain fatty acids and linoleic acid, and their effect on cholesterol absorption in rats.
Lipids
26:
369-373,
1991[Medline].
15.
Jandacek, R. J.,
J. A. Whiteside,
B. N. Holcomebe,
R. A. Volpenhein,
and
J. D. Taulbee.
The rapid hydrolysis and efficient absorption of triglycerides with octanoic acid in the 1 and 3 positions and long chain fatty acid in the 2 position.
Am. J. Clin. Nutr.
45:
940-945,
1987[Abstract].
16.
Jensen, G. L.,
N. McGarvey,
R. Taraszewski,
S. K. Wixson,
D. L. Seidner,
T. Pai,
Y. Y. Yeh,
T. W. Lee,
and
S. J. DeMichele.
Lymphatic absorption of enterally-fed structured triacylglycerol versus physical mix in a canine model.
Am. J. Clin. Nutr.
60:
518-524,
1994[Abstract].
17.
Jensen, M. M.,
M. S. Christensen,
and
C. E. Hoy.
Intestinal absorption of octanoic, decanoic, and linoleic acids: effect of triacylglycerol structure.
Ann. Nutr. Metab.
38:
104-116,
1994[Medline].
18.
Kayden, H. J.,
J. R. Senior,
and
F. H. Mattson.
The monoglyceride pathway of fat absorption in man.
J. Clin. Invest.
46:
1695-1703,
1967[Medline].
19.
Kenler, A. S.,
W. S. Swails,
D. S. Driscoll,
S. J. DeMichele,
B. Daley,
T. J. Babineau,
M. B. Peterson,
and
B. R. Bistrian.
Early enteral feeding in postsurgical cancer patients: fish oil structured lipid-based polymeric formula versus a standard polymeric formula.
Ann. Surg.
223:
316-333,
1996[Medline].
20.
Lanza-Jacoby, S.,
H. Phetteplace,
and
R. Tripp.
Enteral feeding of a structured lipid emulsion containing fish oil prevents the fatty liver of sepsis.
Lipids
30:
707-712,
1995[Medline].
21.
Lee, T.
Quantitative determination of linoleic acid in infant formulas by gas chromatography.
J. Assoc. Off. Anal. Chem.
70:
702-705,
1987[Medline].
22.
Lee, T.,
E. Bobik,
and
W. Malone.
Quantitative determination of mono- and diglycerides with and without derivatization by carbon dioxide supercritical fluid chromatography.
J. Assoc. Off. Anal. Chem.
74:
533-537,
1991.
23.
Ling, P. R.,
N. W. Istfan,
S. M. Lopes,
V. K. Babayan,
G. L. Blackburn,
and
B. R. Bistrian.
Structured lipid made from fish oil and medium-chain triglycerides alters tumor and host metabolism in Yoshida-sarcoma-bearing rats.
Am. J. Clin. Nutr.
53:
1177-1184,
1991[Abstract].
24.
Maiz, A.,
K. Yamazaki,
J. Sobrado,
V. K. Babayan,
L. L. Moldawer,
B. R. Bistrian,
and
G. L. Blackburn.
Protein metabolism during total parenteral nutrition (TPN) in injured rats using medium-chain triglycerides.
Metabolism
33:
901-909,
1984[Medline].
25.
Mascioli, E. A.,
V. K. Babayan,
B. R. Bistrian,
and
G. L. Blackburn.
Novel triacylglycerols for special medical purposes.
JPEN J. Parenter. Enteral Nutr.
12, Suppl. 6:
127S-132S,
1988[Medline].
26.
McKenna, M. C.,
V. S. Hubbard,
and
J. G. Pieri.
Linoleic acid absorption from lipid supplements in patients with cystic fibrosis with pancreatic insufficiency and in control subjects.
J. Pediatr. Gastroenterol. Nutr.
4:
45-51,
1985[Medline].
27.
Mendez, B.,
P. R. Ling,
N. W. Istfan,
V. K. Babayan,
and
B. R. Bistrian.
Effects of different lipid sources in total parenteral nutrition on whole body protein kinetics and tumor growth.
JPEN J. Parenter. Enteral Nutr.
16:
545-551,
1992[Abstract].
28.
Mok, K. T.,
A. Maiz,
K. Yamazaki,
J. Sobrado,
V. K. Babayan,
L. L. Moldawer,
B. R. Bistrian,
and
G. L. Blackburn.
Structured medium-chain and long-chain triglyceride emulsions are superior to physical mixtures in sparing body protein in the burned rat.
Metabolism
33:
910-915,
1984[Medline].
29.
Parker, F.,
and
N. F. Peterson.
Quantitative analysis of phospholipids and phospholipid fatty acids from silica gel thin-layer chromatograms.
J. Lipid Res.
6:
455-460,
1965[Abstract/Free Full Text].
30.
Rudel, L. L.,
and
M. D. Morris.
Determination of cholesterol using O-phthaladehyde.
J. Lipid Res.
14:
364-366,
1973[Abstract/Free Full Text].
31.
Small, D. M.
The effects of glyceride structure on absorption and metabolism.
Annu. Rev. Nutr.
11:
413-434,
1991[Medline].
32.
Swails, W. S.,
A. S. Kenler,
D. F. Driscoll,
S. J. DeMichele,
T. J. Babineau,
T. Utsunamiya,
S. Chavali,
R. A. Forse,
and
B. R. Bistrian.
Effect of a fish oil structured lipid based diet on prostaglandin release from mononuclear cells in cancer patients after surgery.
JPEN J. Parenter. Enteral Nutr.
21:
266-274,
1997[Abstract].
33.
Teo, T. C.,
S. J. DeMichele,
K. M. Selleck,
V. K. Babayan,
G. L. Blackburn,
and
B. R. Bistrian.
Administration of structured lipid composed of MCT and fish oil reduces net protein catabolism in enterally fed burned rats.
Ann. Surg.
210:
100-107,
1989[Medline].
34.
Teo, T. C.,
K. M. Selleck,
J. M. F. Wan,
J. J. Pomposelli,
V. K. Babayan,
G. L. Blackburn,
and
B. R. Bistrian.
Long-term feeding with structured lipid composed of medium-chain and n-3 fatty acids ameliorates endotoxic shock in guinea pigs.
Metabolism
40:
1152-1159,
1991[Medline].
35.
Tso, P.,
M. D. Karlstad,
B. R. Bistrian,
and
S. J. DeMichele.
Intestinal digestion, absorption and transport of structured triacylglycerols and cholesterol in rats.
Am. J. Physiol.
268 (Gastrointest. Liver Physiol. 31):
G568-G577,
1995[Abstract/Free Full Text].
36.
Wood, R.,
and
F. Snyder.
Chemical and physical properties of isomeric glyceryl monoethers.
Lipids
2:
161-171,
1967.
Am J Physiol Gastroint Liver Physiol 277(2):G333-G340
0002-9513/99 $5.00
Copyright © 1999 the American Physiological Society