International Journal of Obesity (2000) 24, 1158±1166
ß 2000 Macmillan Publishers Ltd All rights reserved 0307±0565/00 $15.00
www.nature.com/ijo
Endogenous fat oxidation during medium chain
versus long chain triglyceride feeding in healthy
women
AA Papamandjaris1, MD White1, M Raeini-Sarjaz1 and PJH Jones1*
1
School of Dietetics and Human Nutrition, Faculty of Agricultural and Environmental Sciences, McGill University, Macdonald Campus,
Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9
OBJECTIVE: To compare the effect of medium chain triglycerides (MCT) vs long chain triglycerides (LCT) feeding on
exogenous and endogenous oxidation of long chain saturated fatty acids (LCSFA) in women.
SUBJECTS: Twelve healthy female subjects (age 19 ± 26 y, body mass index (BMI) 17.5 ± 28.6 kg=m2)
DESIGN AND MEASUREMENTS: In a randomized cross-over design, subjects were fed weight maintenance diets
providing 15%, 45% and 40% of energy as protein, carbohydrate and fat, respectively, with 80% of this fat comprising
either a combination of butter and coconut oil (MCT) or beef tallow (LCT). Following 6 days of feeding, subjects were
given daily oral doses of 1-13C labelled-myristic, -palmitic and -stearic acids for 8 days. Expired 13CO2 was used as an
index of LCSFA oxidation with CO2 production assessed by respiratory gas exchange.
RESULTS: No difference in exogenous LCSFA oxidation was observed as a function of diet on day 7. On day 14, greater
combined cumulative fractional LCSFA oxidation (16.9 2.5%=5.5 h vs 9.1 1.2%=5.5 h, P < 0.007), net LCSFA oxidation
(2956 413 mg=5.5 h vs 1669 224 mg=5.5 h, P < 0.01), and percentage dietary LCSFA contribution to total fat oxidation (16.3 2.3%=5.5 h vs 9.5 1.5%=5.5 h; P < 0.01) were observed in women fed the MCT vs LCT diet. With the MCT
diet, but not the LCT diet, combined cumulative fractional LCSFA oxidation (P < 0.03), net LCSFA oxidation (P < 0.03),
and percentage dietary LCSFA contribution to total fat oxidation (P < 0.02) were increased at day 14 as compared to
day 7. Day 14 results indicated increased endogenous LCSFA oxidation during MCT feeding.
CONCLUSION: The capacity of MCT to increase endogenous oxidation of LCSFA suggests a role for MCT in body
weight control over the long term.
International Journal of Obesity (2000) 24, 1158±1166
Keywords: medium chain fatty acids; long chain saturated fatty acids; fat oxidation;
Introduction
A growing body of literature suggests metabolic
discrimination of fatty acids for oxidation vs storage
based on chain length. Both human and animal data
indicate that medium chain triglycerides (MCT), containing medium chain fatty acids (MCFA) composed
of chains of 8 ± 12 carbon atoms, are preferentially
oxidized as compared to long chain triglycerides
(LCT) containing long-chain fatty acids (LCFA),
composed of chains of 14 or more carbon atoms.1 ± 14
Observations at the whole body level have shown that
animals overfed and refed MCT- vs LCT-containing
diets had decreased body weight gain.1,3,4,7 Additionally, animal studies of substrate utilization have
shown more extensive and rapid oxidation of MCT
in comparison to LCT.6,8,13 In humans, greater propensity of MCT vs LCT for oxidation has also been
*Correspondence: PJH Jones, School of Dietetics and Human
Nutrition, Faculty of Agricultural and Environmental Sciences, 21
111 Lakeshore Road, McGill University, Macdonald Campus,
Ste-Anne-de-Bellevue, Quebec, Canada H9X 3V9.
E-mail: [email protected]
Received 1 September 1999; revised 23 February 2000; accepted
29 March 2000
13
C breath test; humans
observed.2,5,9 ± 12,14 Yet it still remains to be determined whether the presence of MCT in the diet can
alter the oxidation of other meal fatty acids. In
addition, the potential capacity of extended MCT
feeding to affect endogenous oxidation, whether by
changing the composition of the adipose tissue pool
through altered endogenous availability, or through
induction of enzymes and metabolic processes, has
not been determined. The capability of MCT to
increase endogenous fat oxidation could have implications for reduction of adipose tissue mass. Given the
recent evidence indicating impaired oxidation of LCT
yet normal oxidation of MCT in the obese,15 substitution of MCT for LCT may provide a means of
reducing adipose tissue deposition and increasing
adipose tissue mobilization.
The goal of this study, therefore, was to examine
effects of MCT- vs LCT-enriched diets on exogenous
and endogenous oxidation of myristic (MA), palmitic
(PA) and stearic (SA) acids over 8 days, following one
week of prefeeding. The speci®c objective was to
determine whether consumption of a MCT-enriched
diet would affect oxidation of long-chain saturated
fatty acids (LCSFA) contained in the diet in a manner
different from an LCT-enriched diet. Myristic acid,
Fatty acid chain length and fat oxidation
AA Papamandjaris et al
PA and SA were chosen as they represent common
saturated fatty acids found in the North American diet
and were the major LCSFA in the experimental diets.
Methods
Subjects and study design
Twelve healthy, non-smoking female college students,
aged 22.7 0.7 y with a height of 1.62 0.01 m and
initial body weight of 56.2 1.5 kg, were recruited
from Macdonald Campus, McGill University. Subjects were screened for normal blood lipid levels (total
cholesterol < 5.3 mmol=l, triglycerides < 1.24 mmol=
l), absence of chronic disease, exercise frequency
and regular menstrual history.
A randomized cross-over design was used involving two 2 week feeding periods separated by a 14
day washout. An equal number of subjects per treatment were tested on each diet cycle and subjects were
blinded to diet sequence. All procedures were
approved by the McGill University Ethics Committee
and informed, written consent was obtained from all
subjects prior to their entrance into the study.
Experimental diets
Prepared solid food diets providing 15%, 45% and
40% of energy as protein, carbohydrate and fat,
respectively, were fed over each 14 day period.
Treatment fat, comprising of a combination of butter
and coconut oil for the MCT treatment, or beef tallow
for the LCT treatment, made up 80% of the dietary
fat. Diets (Table 1) were provided as a 2 day rotating
menu comprising three meals per day repeated every
second day which were isoenergetic both in total
energy and fat. The rotating menu was modi®ed to
ensure that the same meals were fed on pre-test days,
day 6 and 13, and on test days, day 7 and 14. Ingestion
of foodstuffs other than those provided was prohibited, except for water, which was permitted ad libitum. All ingredients were weighed to the nearest 0.5 g.
Energy intake was calculated to maintain energy
balance using the Mif¯in equation16 and an activity
factor,17 and diets were designed to meet the Recommended Nutrient Intakes for Canadians.18 Meals were
prepared and consumed under supervision at the Mary
Emily Clinical Nutrition Research Unit at McGill
University.
1159
Protocol
On days 7 and 14 of each diet cycle, following a 12 h
fast and overnight stay at the Unit, subjects were
awakened at 7:00 am and baseline breath samples
collected (Figure 1). Basal metabolic rate (BMR)
was measured over 30 min using a Deltatrac Metabolic Monitor (Sensormedics, Anaheim, California).
Subjects were then served a hot breakfast containing a
treatment-speci®c 10 mg=kg mixture of 1-13C labelled
MA, PA and SA (CDN Isotopes, Quebec, Canada)
blended into the meal. Labelled LCSFA were given at
levels proportional to the levels of tracees, MA, PA,
and SA, contained in each diet (Table 2). Thus the
fraction of label ingested appearing as 13CO2 represented oxidation of the majority of dietary LCSFA.
Over 5.5 h following the breakfast meal, the volume
of subjects' expired CO2 was obtained from respiratory gas exchange (RGE). Breath samples were also
collected at hourly intervals during this time. Subjects
were then fed lunch, delivered a post-lunch breath
sample, and a ninth breath sample was collected predinner, at approximately 18:00 h. From days 8 ± 13,
the same dose of isotope mixture was delivered with
breakfast, and breath samples were collected prior to
each meal. Throughout the entire study, subjects kept
diaries recording dates of menstruation and type and
duration of exercise.
Whole body respiratory gas exchange measurements
On days 7 and 14, following single and repeated
isotope doses, respectively, O2 consumption and
CO2 production for each subject were measured
using a Deltatrac Metabolic Monitor as described
previously.19 Brie¯y, a transparent ventilated hood
was placed over each subject's head, with a hose
connecting the hood to the analyser system. After a
minimum 30 min warm-up, reference gas standards
were used to calibrate the monitor. Respiratory gas
exchange measurements were collected for 0.5 h
prior to breakfast following a 12 h fast, and for
5.5 h following termination of consumption of the
scheduled breakfast.
Table 1 Sample menu used during the dietary interventiona
Breakfast
Vegetable omelette*
Fruit and ®bre cereal
1% milk
Orange juice
Blueberry muf®n*
Raisins
a
Lunch
Grilled ham and
cheese*
Diet soda
Grapes
Oatmeal cookie*
Dinner
Spaghetti with meat
sauce
Parmesan cheese
Garlic toast*
Celery
Fruit juice
Date square*
Food items to which treatment fat was added are denoted by*.
Figure 1 Experimental protocol for each 14 day dietary treatment phase.
International Journal of Obesity
Fatty acid chain length and fat oxidation
AA Papamandjaris et al
1160
Table 2 Fatty acid pro®le and percentage of 1-13C myristic (C14 : 0), palmitic (C16 : 0), and stearic acids (C18 : 0)
delivered in treatment dose on medium chain and long chain triglyceride dietary treatmentsa
MCT
Fatty acid
C8 : 0
C10 : 0
C12 : 0
C14 : 0
C16 : 0
C16 : 1
C18 : 0
C18 : 1
C18 : 1
C18 : 2
a
o7
o9
o7
o6
Percentage of
total fatty acid
spectrum
3.9
4
17.7
13.3
25.4
1.6
8.1
19.3
0.6
4.8
LCT
13
C labelled fatty
acid (g=100 g of total
C14 : 0, C16 : 0, C18 : 0)
Percentage of total
fatty acid spectrum
0
0.4
1.4
4.4
27.3
2.9
16.9
35.9
0.8
8
28.4
54.2
17.3
13
C labelled fatty
acid (g=100 g of total
C14 : 0, C16 : 0, C18 : 0)
9
56.2
34.8
LCT ˆ long chain triglyceride; MCT ˆ medium chain triglyceride.
Collection, puri®cation and analysis of carbon dioxide
in breath samples
Subjects collected breath samples by exhaling into
sealed bags through a sealed 1.3 cm diameter tube
equipped with a stopcock. At each collection time,
subjects ®lled the bags twice, with only the second
exhalation being used as sample. During RGE measurement periods, subjects delivered breath samples
while under the hood. A syringe was used to extract
180 ml of air which was then bubbled through a glass
condenser, 1.0 m in total length, containing 10 ml
1 M NaOH, to trap the CO2. The samples were then
transferred to vials and stored at 720 C until analysis.
To release the CO2, 2.5 ml of the NaOH ± CO2
solution at room temperature was injected into vacutainers containing 2.5 ml of o-phosphoric acid 85%.
The vacutainers were shaken for 90 s to release the
CO2. A SIRA isotope ratio mass spectrometer (SIRA
12, Isomass, Cheshire, UK) was used to determine the
13C enrichment of the samples in delta per mil (%).
The mass spectrometer was corrected for 17O
and calibrated daily using CO2 gas and standards
of known isotopic composition. Enrichments were
corrected for background 13C abundance by subtraction of baseline enrichment, the pre-isotope dose
enrichment on either day 7 or day 14.
split ratio was 100 : 1. The total run time was 57 min.
Individual fatty acids were identi®ed against authentic
standards (Sigma Chemical Company, St Louis, MO)
using retention times.
Mean percentages of MA, PA and SA of total fatty
acid spectrum for each dietary treatment were determined. The total combined percentage of MA, PA and
SA was then calculated and each of these FA were
expressed as a percentage of this total (Table 2). This
proportion of these individual fatty acids in the diet
was used to establish the mixture of 1-13C labelled
MA, PA and SA delivered to the subjects in a
10 mg=kg body weight dose.
Data analysis
Fractional oxidation rates of breakfast meal mixture of
1-13C labelled LCSFA, based on the percentage of the
total label ingested appearing in the breath, were
determined using the following equation:14
Fractional LCSFA oxidation …%h† ˆ
mmol excess 13 C=mmol CO2
mmol 13 C administered
…mmol CO2 excreted=h† 100 1:35
…1†
Fatty acid composition of test meals
Replicate portions of all test meals were homogenized
using a commercial blender. Fatty acid content was
determined using gas ± liquid chromatography after
lipid extraction20 and boron tri¯uoride methylation.21
The gas chromatograph (Hewlett Packard 5890 Series
II, California) was equipped with an autosampler and
¯ame ionization detector. Separation was achieved on
an SP 2330 capillary column, 30 m 0.25 mm 0.2 mm. Samples were injected at 100 C. Oven temperature was then increased to 190 C at a rate of
3 C=min and was held at that level for 25 min. The
International Journal of Obesity
where mmol excess 13C=mmol CO2 ˆ (13C enrichment above baseline) R Pee Dee Belemnite Limestone (RPBD) 1073,12 RPBD ˆ 0.011273,22 and the
factor of 1.35 was added to account for bicarbonate
retention of 13C in body pools.23,24 Amount of CO2
excreted (mmol) was determined using RGE. The
mmol 13C administered was determined as the total
number of mmol delivered in the mixture of the 13C
labelled MA, PA and SA. Cumulative fractional
LCSFA oxidation was determined as the total fractional LCSFA oxidation over 5.5 h.
Fatty acid chain length and fat oxidation
AA Papamandjaris et al
The total amount of combined MA, PA and SA in
the breakfast meal was calculated for individual subjects based on the following formula:25
Breakfast meal fatty acid content …mg† ˆ
Breakfast meal fat …mg† concentration of
fatty acid in the breakfast meal …percentage
…2†
fatty acids of the total fatty acid content†
Net oxidation rates of combined dietary MA, PA, and
SA were calculated as follows:25
Net LCSFA oxidation …mg=5:5 h† ˆ
breakfast meal fatty acid content …mg†
cumulative fractional LCSFA oxidation
…3†
…%=5:5 h†
Percentage contribution of combined dietary MA, PA
and SA to total postprandial fat oxidation was calculated as:26
Percentage dietary LCSFA contribution to total fat
oxidation …%=5:5 h† ˆ net LCSFA oxidation
1161
Results
Study subjects
…mg=5:5 h†=total postprandial fat oxidation
…mg† 100%
…4†
where total postprandial fat oxidation was determined
using RGE and the equations of de Weir.27
Approach to calculations
For all dependent variables, results were calculated for
day 7 using day 7 baseline values, and two sets of
results were generated for day 14 using either day 7 or
day 14 baseline values in the calculations. Day 7
results represent exogenous LCSFA oxidation, as
subjects had only received one isotope dose and had
no stored labelled LCSFA to release during endogenous oxidation. Enrichment values for day 7 were
obtained by subtraction of the day 7 baseline values
from subsequent breath enrichments. On day 14, one
set of enrichment values was generated by subtraction
of each subjects' initial baseline as determined on day
7. These values are taken to represent combined
exogenous and endogenous LCSFA oxidation, as
subjects had received repeated isotope doses and
therefore could produce label through both exogenous
and endogenous oxidation. The other set of day 14
enrichment values was generated by subtraction of the
day 14 baseline value, determined prior to breakfast
and delivery of isotope on day 14. These values are
taken to represent exogenous LCSFA oxidation, as
oxidation from endogenous sources has been
accounted for through baseline correction.
Statistical analysis
24), and diet by meal interaction was used to compare
overall appearance of 13CO2 in the breath over 8 days.
Fractional LCSFA oxidation was assessed using
repeated measures ANOVA with factors of sequence
(1 or 2), diet (MCT or LCT), hour (0.5 ± 5.5), and diet
by hour interaction for each set of results, day 7, day
14 exogenous, and day 14 combined. For the dependent variables cumulative fractional LCSFA oxidation, net LCSFA oxidation, and percentage dietary
LCSFA contribution to total fat oxidation, repeated
measures ANOVA models with factors of sequence (1
or 2), diet (MCT or LCT), day (7 or 14), and diet-byday interaction were employed. This approach was
applied twice, once using the day 14 exogenous
enrichment data for the day 14 factor, and once
using the day 14 combined enrichment data for the
day 14 factor. Post-hoc comparisons were made using
contrasts. Level of signi®cance was set at P < 0.05.
All values are expressed as means s.e.m.
A repeated measures ANOVA with factors of
sequence (1 or 2), diet (MCT or LCT), meal (1 ±
During both dietary cycles, subjects were observed to
have consumed all food provided. Two subjects'
energy intakes were increased based on reported
hunger over the ®rst 3 days of the ®rst dietary cycle.
There were no signi®cant changes in body weight
between start (56.2 1.5 kg) and end (56.7 1.4 kg)
of the study, either between or within study groups.
Subject diaries indicated regular menstruation, with
mean cycle length of 28.5 0.6 days. No abnormalities in health or large ¯uctuations in activity patterns
were reported.
Meal compositional analysis
Mean fatty acid composition pro®les of all meals for
each diet are shown in Table 2. The percentage of
MA, PA and SA differed between diets. The relative
proportions of MA, PA and SA calculated for the
dosing mixture were 28.4 0.3%, 54.2 0.2% and
17.3 0.2% for the MCT diet and 9.0 0.2%,
56.2 0.3% and 34.8 0.2% for the LCT diet,
respectively.
Substrate utilization as measured using respiratory gas
exchange
A full description of data obtained using RGE can be
found elsewhere.19 Diet fat treatment did not have a
signi®cant effect on total fat or carbohydrate oxidation
on either day 7 or 14. In the postprandial period on
day 7, repeated measures ANOVA showed an overall
greater19 rate of fat oxidation on the MCT diet.
Overall appearance of label in the breath
Comparison of pre-meal 13CO2 enrichment in breath
above baseline over the 8 day isotope delivery period
International Journal of Obesity
Fatty acid chain length and fat oxidation
AA Papamandjaris et al
1162
showed a main effect (P < 0.001) of dietary treatment,
with values of 13C in breath on the MCT diet higher
than those on the LCT diet (Figure 2).
Fractional LCSFA oxidation
On day 7, the ®rst day of isotope delivery, there was a
main effect (P < 0.02) of diet on fractional LCSFA
oxidation, with greater oxidation on the MCT diet at
4.5 h (Figure 3A). Similarly, on day 14, a main effect
(P < 0.001) of diet on combined fractional LCSFA
oxidation was observed, with increased oxidation on
the MCT diet at 1.5 and 3.5 h (Figure 3B). On day 14,
a main effect (P < 0.006) of diet on exogenous fractional oxidation was also observed, with greater oxidation on the LCT diet at 2.5 and 5.5 h (Figure 3C).
The interaction between diet and hour for day 14
exogenous fractional oxidation was not signi®cant
(Figure 3C).
Cumulative fractional LCSFA oxidation on day 7
on the MCT diet (10.7 2.4%=5.5 h) did not differ
from oxidation on the LCT diet (6.1 1.8%=5.5 h,
Figure 4A). Comparison of combined cumulative
fractional oxidation between diets over 5.5 h on day
14 showed a main effect (P < 0.007) of diet with
greater
oxidation
from
the
MCT
diet
(16.9 2.5%=5.5 h) than from the LCT diet (9.1
1.2%=5.5 h; Figure 4A). Within-diet comparison
between day 7 and combined values for day 14
showed an increase (P < 0.03) in oxidation on the
MCT diet by day 14 (Figure 4A). On day 14, exogenous cumulative fractional oxidation was not different between diet types (MCT, 5.3 2.8%=5.5 h;
LCT, 10.6 3.6%=5.5 h); however, the interaction
term of diet by day approached signi®cance
(P ˆ 0.07) in the ANOVA model using day factors
of 7 and 14 exogenous (Figure 4A).
Net long-chain saturated fatty acid oxidation
The net LCSFA oxidation from the breakfast meals is
shown in Figure 4B. The results were similar to those
found for both between and within diet comparisons
for cumulative fractional LCSFA oxidation. Betweendiet comparison of net LCSFA oxidation for day 7
showed no signi®cant effect of diet (MCT,
1836 426 mg=5.5 h vs LCT, 1135 330 mg=5.5 h).
On day 14, there was an effect (P < 0.01) of diet on
combined net oxidation, with greater net oxidation on
the MCT diet (2956 413 mg=5.5 h) than on the LCT
diet (1669 224 mg=5.5 h). Exogenous net LCSFA
oxidation on day 14 was not affected by diet type
(MCT, 932 497 mg=5.5 h vs LCT, 1926 671 mg=
5.5 h); however, as with the cumulative fractional
oxidation data, the interaction term of diet by day
approached signi®cance (P ˆ 0.08) in the ANOVA
model using day factors of 7 and 14 exogenous.
Within the MCT diet, there was greater (P < 0.03)
combined net LCSFA oxidation on day 14 as
compared to day 7.
Percentage dietary long-chain saturated fatty acid
contribution to total fat oxidation
Results for percentage dietary LCSFA contribution to
total fat oxidation (Figure 4C) re¯ected the pattern
observed for between and within-diet comparisons for
both cumulative fractional LCSFA oxidation and net
LCSFA oxidation. The only signi®cant between-diet
®nding occurred with the combined results on day 14,
with LCSFA from the MCT contributing to 16.3 2.3
%=5.5 h of total fat oxidation, which differed
(P < 0.01) from the 9.5 1.5 %=5.5 h contributed by
those fatty acids on the LCT diet. For within-diet
comparisons, the day 7 contribution on the MCT diet
(9.3 1.9 %=5.5 h) was smaller (P < 0.02) than the
combined contribution on the MCT diet on day 14
(16.3 2.3 %=5.5 h). Again, no differences were
observed in between-diet comparisons of day 7 and
Figure 2 Comparison of pre-meal % 13CO2 enrichment in breath over baseline for MCT vs LCT diets from day 7 to day 14 after
ingestion of 13C labelled LCSFA at each breakfast meal in women. Values were determined by subtraction of initial baseline at day 7.
Arrows indicate daily breakfast delivery of labelled LCSFA. Letters (A or B) indicate day of two day rotating menu cycle. Whole body
oxidation of labelled LCSFA was greater (P < 0.001) during MCT vs LCT feeding.
International Journal of Obesity
Fatty acid chain length and fat oxidation
AA Papamandjaris et al
1163
Figure 3 (A) Fractional oxidation of labelled LCSFA on day 7
after ®rst oral administration of labelled LCSFA at breakfast (time
0); there was a main effect (P < 0.02) of diet on fractional oxidation with greater oxidation on the MCT diet at 4.5 h. (B) Fractional
oxidation of labelled LCSFA on day 14, after daily oral administration of labelled LCSFA at breakfast (time 0) starting on day 7.
Values represent combined oxidation as they were calculated
using the day 7 baseline. There was a main effect (P < 0.001) of
diet on combined fractional oxidation with greater oxidation on
the MCT diet at 1.5 h and 3.5 h. (C) Fractional oxidation of
labelled LCSFA on day 14 after repeated consumption of labelled
LCSFA at breakfast (time 0) starting on day 7. Values represent
exogenous oxidation as they were calculated using the day 14
pre-dose baseline. There was a main effect (P < 0.006) of diet on
exogenous fractional oxidation with greater oxidation on the
LCT diet at 2.5 and 5.5 h. Signi®cant difference between diets
indicated by * (P < 0.05).
day 14 exogenous values, but the interaction term of
diet by day approached signi®cance at P ˆ 0.10.
Discussion
Repeated delivery of 1-13C labelled MA, PA and SA
and assessment of breath 13CO2 enrichment over a
period of 8 days permitted measurement of exogenous
and endogenous oxidation of LCSFA. The novel
®nding presently is that the acyl structure of fatty
acids within the diet other than those which were
labelled, speci®cally MCT vs LCT, affected the capacity of the body to retain and subsequently release
13C, obtained from 1-13C labelled LCSFA, through
oxidation. Therefore, rate of oxidation of fatty acids
Figure 4 Cumulative fractional oxidation over 5.5 h of labelled
LCSFA (A), net LCSFA oxidation (B) and percent dietary LCSFA
contribution to total fat oxidation (C) on both MCT and LCT
treatments on day 7, day 14 combined and day 14 exogenous.
Cumulative LCSFA fractional oxidation (P < 0.007), net LCSFA
oxidation (P < 0.01), and percent dietary LCSFA contribution to
total fat oxidation (P < 0.01) were different between dietary
treatments for the combined day 14 values. Day 14 combined
cumulative LCSFA fractional oxidation (P < 0.03), net LCSFA
oxidation (P < 0.03), and percent dietary LCSFA contribution to
total fat oxidation (P < 0.02) within the MCT dietary treatment
were greater as compared to day 7. Different letters indicate
signi®cant differences between diets (P < 0.05). Signi®cant
within diet comparisons are indicated by * (P < 0.05).
depends not only on their structure, but also on the
structure of other fatty acids present in the dietary
fatty acid mix.
The day 7 results indicate that total exogenous
oxidation of LCSFA, following the ®rst exposure to
isotope after 1 week of prefeeding, was not signi®cantly affected by dietary treatment. Other researchers11,12 who reported increased oxidation of 1-13C
labelled fatty acids in subjects consuming MCT
diets labelled the MCFA speci®cally and therefore
were not assessing the impact of MCFA on oxidation
of other fatty acids, as in the present study. In
addition, in those studies,11,12 the increase in oxidation
was observed following initial feeding of MCT, not
following a 7 day prefeeding period, as was employed
International Journal of Obesity
Fatty acid chain length and fat oxidation
AA Papamandjaris et al
1164
presently. Therefore, it would appear that the presence
of MCT in the present diet was not capable of
signi®cantly altering the exogenous oxidation of dietary LCSFA following 7 days of diet administration.
Lack of difference in exogenous oxidation between
the two dietary treatments is supported by results
determined using RGE measurements.19 On day 7,
there was no effect of dietary treatment on the thermic
effect of food or on total fat oxidation.
Examination of exogenous oxidation following 2
weeks of feeding showed similar results. For day 14,
as for day 7, exogenous cumulative fractional LCSFA
oxidation, net LCSFA oxidation, and percentage dietary LCSFA contribution to total fat oxidation were not
different between dietary treatments. These ®ndings
are supported by the previously reported RGE results
which showed no effect of MCT vs LCT consumption
on the thermic effect of food or on total fat oxidation
on day 14.19
Endogenous oxidation of LCSFA after 8 days of
repeated isotope delivery was also assessed. The low
percentage dietary LCSFA contribution to total fat
oxidation and the low cumulative fractional LCSFA
oxidation determined on day 7 indicated that most of
the label delivered remained in the body, and was not
oxidized postprandially. This is in accord with other
researchers who observed that less than 1% of label
was oxidized postprandially26 and that breath 13CO2
enrichment was low following delivery of label.14,28
Consequently, ingested LCSFA were deposited in the
body and available for subsequent endogenous oxidation. As the same amount of labelled LCSFA had been
consumed each day, it may be hypothesized that the
exogenous component of 1-13C labelled LCSFA oxidation remained the same over that period. Thus, any
increase in combined exogenous and endogenous
oxidation of the 1-13C LCSFA seen by day 14, as
compared to day 7, indicates contribution from endogenous sources. By day 14, following repeated isotope
dosing, the combined amounts of cumulative fractional LCSFA oxidation, net LCSFA oxidation, and
percentage dietary LCSFA contribution to total fat
oxidation differed between the two diets, and were
also signi®cantly greater than on day 7 within the
MCT diet. These ®ndings support the hypothesis that
the 1-13C labelled LCSFA delivered in the MCT diet
on previous days had been deposited in metabolic
compartments and were subsequently released during
endogenous oxidation. Lack of a signi®cant difference
between days 7 and 14 within the LCT diet indicates
that endogenous oxidation of stored labelled LCSFA
did not occur to the same degree as on the MCT diet.
The presence of MCT in the diet thus had the capacity
to in¯uence both the deposition and subsequent mobilization of LCSFA in a manner that the presence of
LCT did not. The effect of dietary treatment on BMR
as measured using RGE supports in part these
results.19 On day 7, BMR was signi®cantly greater
on the MCT diet as compared to the LCT diet. By day
14, despite a lack of statistical signi®cance, there was
International Journal of Obesity
a trend towards an elevated BMR on the MCT diet.
An elevated BMR may indicate increased endogenous
fat oxidation, as seen on day 14 using in the present
experiment. The present protocol did not permit
assessment of endogenous oxidation on day 7. In
contrast, doubly labelled water (DLW) assessment
of average daily total energy expenditure (TEE) did
not show an effect of dietary treatment.29 However,
the DLW method of measuring TEE may not allow
enough accuracy and precision to detect the small
differences that may occur during MCT vs LCT
feeding.
The possibility that 13C background enrichment
¯uctuation contributed signi®cantly to the difference
seen in endogenous oxidation between diets on day 14
is unlikely. The prefeeding period and uniformity of
the diet ensured that the baseline 13C breath measurement taken on day 7 was representative of the baseline
during the entire dietary period. Any background
shifts away from this measured baseline would have
been systematic and constant for both diets as the
carbohydrate content was identical.12 Additionally,
13
C breath enrichment due to oxidation of labelled
fatty acids delivered at the present dose is measurably
higher than that due to natural 13C abundance,30
making the impact of potential background shifts in
13C enrichment negligible.
The retention factor for 13C within the bicarbonate
pool used in the oxidation calculations may be important in the interpretation of the ®ndings. The retention
factor used (1.35) was based on research in similar
populations under similar conditions.23 ± 26 Nevertheless, results may have been affected by a shift in 13C
retention in the bicarbonate pool between day 7 and
day 14. However, even if 13C retention was altered,
either similarly for both diets between day 7 and 14,
or disparately between diets, such results would indicate the presence of a effect on the metabolic fate of
13
C. It is also possible that not all the 13C oxidized on
day 14 came from stored FA; some may have come
from the 13C labelled CO2 pool. In any such scenario,
the data as presented suggest that MCT consumption
alters the utilization of 1-13C labelled LCSFA, but that
greater appearance of 13CO2 during the MCT phase
does not speci®cally indicate greater mobilization of
stored 13C fatty acids. Furthermore, differences in
fatty acid composition across diets may exist as a
contributing factor. Oxidation rates of other nonlabelled fatty acids could have decreased in response
to the increased oxidation of MA, PA and SA, resulting in no overall change in total fat oxidation.
Nonetheless, examination of the pattern of label
appearance offers insight into what may be occurring
during storage and oxidation of the labelled LCSFA.
Except for the initial rise and subsequent overall
higher level of appearance of label on the MCT diet,
the pattern of label appearance for the two diets was
similar. The peaks seen at each day's pre-lunch breath
sampling were most likely the result of exogenous
oxidation of labelled LCSFA delivered with the
Fatty acid chain length and fat oxidation
AA Papamandjaris et al
breakfast meal. This rise was similar to that typically
seen following ingestion of 13C labelled fatty
acids.12,23 The immediate rise in enrichment in the
breath on the MCT diet not present on the LCT diet
may have indicated rapid maximal enrichment of a
fatty acid storage pool following the initial dose with
isotope. This pool may then have been maintained
at maximal enrichment during subsequent isotope
delivery with the stored labelled LCSFA constantly
available as substrates for endogenous oxidation.
Enrichment of this pool on the LCT diet may not
have occurred to the same degree, resulting in less
endogenous oxidation. On the LCT diet, the labelled
LCSFA can be hypothesized to have been deposited in
a second adipose pool which was much less metabolically active, and therefore did not contribute in the
same way to the presence of label in the breath
through endogenous oxidation.
The existence of two storage pools for fat has been
postulated previously. In 1960, Hirsch et al31 hypothesized the existence of two separate metabolic compartments, one a caloric depot, exchanging slowly
with dietary fat, the other a small metabolically
active compartment, which may be in close metabolic
relation to dietary lipids. Such a two pool system has
also been proposed by other researchers.32,33 Beynen
et al32 theorized that the rapid turnover rate of plasma
free fatty acids originating from adipose tissue was
explained by the presence of a small, rapidly
exchangeable pool. Pittet et al33 observed continued
incorporation of label from 13-methyltetradecanoic
acid (13-MTD), a structurally labelled fatty acid,
into adipose tissue in humans following cessation of
delivery of 13-MTD. The authors suggested that the
continued appearance of label indicated the presence
of a buffer pool for temporary fatty acid storage prior
to de®nite incorporation into adipose tissue. These
pools may exist in order to maintain a speci®c fatty
acid pattern within adipose tissue.34 ± 36 Fatty acids not
optimal with respect to adipose tissue composition
and subsequent lipid synthesis may be shunted
towards the rapidly exchanging pool, to be oxidized
more readily rather than stored long term. If, as
supported by the present results, the presence of
MCT within the diet can direct more fatty acids
towards temporary storage in the metabolically
active pool, rather than towards more permanent
storage in the inert pool, less fat could be stored
during consumption of a diet containing MCT. Manipulation of dietary fatty acids to include more MCT
could be important in the context of the development
and control of obesity, as it may result in less adipose
tissue deposition.
Conclusion
These results demonstrate that MCT can measurably
affect endogenous oxidation of LCSFA following 2
weeks of feeding compared to LCT as determined
using a repeated 13C LCSFA dosing paradigm. In
addition, the results obtained support the existence
of both a metabolically active pool and an inert pool
of fatty acid storage. Acyl structure of dietary fatty
acids, speci®cally chain length, may determine in
which pool dietary fat is stored, and subsequently
how rapidly fatty acids can undergo endogenous
oxidation. Further research in this area could include
examination of effects of diets with greater proportions of MCT and similar amounts of LCSFA between
diets to control for possible differences in oxidation
and bicarbonate retention of 13C stemming from
disparate LCSFA pro®les. Such research, in combination with the present results, will help in the understanding of the capacity of MCT to increase
endogenous fat oxidation, and therefore to potentially
decrease adipose tissue pool size during extended
feeding.
1165
Acknowledgements
The authors would like to thank Jayne Rop for her
technical assistance. This work was supported by a
grant from the Dairy Farmers of Canada.
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