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Endogenous fat oxidation during medium chain versus long chain triglyceride feeding in healthy women

Abstract

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.

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.1,2,3,4,5,6,7,8,9,10,11,12,13,14 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 observed.2,5,9,10,11,12,14 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 specific 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, 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 modified 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 Mifflin 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.

Table 1 Sample menu used during the dietary interventiona

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-specific 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 pre-dinner, 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.

Figure 1
figure1

Experimental protocol for each 14 day dietary treatment phase.

Table 2 Fatty acid profile 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

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 Briefly, 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.

Collection, purification 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 filled 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 −20°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.

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 trifluoride methylation.21 The gas chromatograph (Hewlett Packard 5890 Series II, California) was equipped with an autosampler and flame ionization detector. Separation was achieved on an SP 2330 capillary column, 30 m × 0.25 mm × 0.2 µm. 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 split ratio was 100:1. The total run time was 57 min. Individual fatty acids were identified 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

where mmol excess 13C/mmol CO2=(13C enrichment above baseline) × R Pee Dee Belemnite Limestone (RPBD) × 10−3,12 RPBD=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.

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)=

 Breakfast meal fat (mg)×concentration of (2)

 fatty acid in the breakfast meal (percentage

 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)=

breakfast meal fatty acid content (mg) (3)

× cumulative fractional LCSFA oxidation

(%/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)=net LCSFA oxidation

 (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

A repeated measures ANOVA with factors of sequence (1 or 2), diet (MCT or LCT), meal (1–24), 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-by-day 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 significance was set at P<0.05. All values are expressed as means±s.e.m.

Results

Study subjects

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 first 3 days of the first dietary cycle. There were no significant 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 fluctuations in activity patterns were reported.

Meal compositional analysis

Mean fatty acid composition profiles 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 significant 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 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).

Figure 2
figure2

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.

Fractional LCSFA oxidation

On day 7, the first 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 significant (Figure 3C).

Figure 3
figure3

(A) Fractional oxidation of labelled LCSFA on day 7 after first 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. Significant difference between diets indicated by * (P<0.05).

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 significance (P=0.07) in the ANOVA model using day factors of 7 and 14 exogenous (Figure 4A).

Figure 4
figure4

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 significant differences between diets (P<0.05). Significant within diet comparisons are indicated by * (P<0.05).

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. Between-diet comparison of net LCSFA oxidation for day 7 showed no significant 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 significance (P=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) reflected the pattern observed for between and within-diet comparisons for both cumulative fractional LCSFA oxidation and net LCSFA oxidation. The only significant between-diet finding 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 day 14 exogenous values, but the interaction term of diet by day approached significance at P=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 finding presently is that the acyl structure of fatty acids within the diet other than those which were labelled, specifically 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 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 first exposure to isotope after 1 week of prefeeding, was not significantly 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 specifically 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 presently. Therefore, it would appear that the presence of MCT in the present diet was not capable of significantly 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 findings 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 significantly greater than on day 7 within the MCT diet. These findings 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 significant 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 influence 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 significantly greater on the MCT diet as compared to the LCT diet. By day 14, despite a lack of statistical significance, there was 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 fluctuation contributed significantly 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, 13C 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 findings. The retention factor used (1.35) was based on research in similar populations under similar conditions.23,24,25,26 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 13C. 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 specifically 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 non-labelled 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 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 definite incorporation into adipose tissue. These pools may exist in order to maintain a specific fatty acid pattern within adipose tissue.34,35,36 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, specifically 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 profiles. 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.

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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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Correspondence to PJH Jones.

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Papamandjaris, A., White, M., Raeini-Sarjaz, M. et al. Endogenous fat oxidation during medium chain versus long chain triglyceride feeding in healthy women. Int J Obes 24, 1158–1166 (2000). https://doi.org/10.1038/sj.ijo.0801350

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Keywords

  • medium chain fatty acids
  • long chain saturated fatty acids
  • fat oxidation
  • 13C breath test
  • humans

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