The maintenance of body weight appears to be increasingly difficult for a large portion of the world’s population. Reports of attempts at weight loss abound in the literature (9), as do reports of failed weight maintenance (21,30). Because the prospect of long-term weight loss continues to elude most people, the prevention of further weight gain assumes a greater importance. In this regard, recent evidence suggests that exercise may offer protection against weight gain, both in nonobese, and formerly obese, individuals (21).
Data from several studies suggest that those individuals who are more active are more likely to maintain body weight (14,21,31). A recent study by Wier et al. (31) of NASA employees demonstrated that both body weight and activity level at baseline are predictors of weight maintenance over a mean period of approximately 5 yr. Additionally, an analysis of an NHANES III cohort (14) found that individuals involved in higher occupational and leisure time physical activity were less likely to be obese than their sedentary counterparts. In a post weight loss study, Schoeller et al. (21) followed a group women for 1 yr and reported that those who had a physical activity level (PAL) greater than 1.75 or about 1–2 MJ·d−1 were best able to maintain their weight loss.
Although these studies provide promising consistency, they do not identify the mechanism of weight control. There has been much speculation in the literature about the long-term effects of exercise on weight control, including the influences of exercise to decrease appetite (26) or increase energy expenditure either during or post exercise (18). Perhaps another aspect of physical activity is its effect on substrate, particularly fat, utilization.
By definition, obesity is a condition of altered fat balance. In particular, the partitioning of dietary fat between storage and oxidation may be a key in the difference between staying lean and becoming obese. Li and Bjorntorp (16) reported that trafficking of dietary fat in rats was altered when animals were fasted rather than fed ad libitum. When fasted, dietary fat was trafficked to the muscle, presumably for oxidation, rather than to adipose tissue for storage. This finding suggests that interventions that lead to a greater negative energy balance may alter dietary fat trafficking.
Limited evidence of this has been reported in humans (8,27). Exercise at various intensities has been shown to decrease postprandial lipemia after a meal the next day. These studies, however, are not directly able to trace the metabolism of the dietary fat given in a specific meal. This study was designed as a noninvasive study to test the hypothesis that a bout of exercise after an overnight fast would increase oxidation of dietary fat from a meal given shortly after the end of exercise. Exercise sessions used in this study were 1250 kJ above rest, which is equivalent to the increase in energy expenditure required for weight maintenance in postobese women (21). We chose to perform this study in normal weight, moderately active subjects to establish a baseline level of the effect of exercise on dietary fat oxidation. Additionally, tracers of two of the most common fatty acids in the U.S. diet, [1-13C]oleate and [d31]palmitate, were used to analyze dietary fat oxidation (1).
A group of seven healthy female subjects (age 26 ± 1 yr, BMI 21 ± 1; mean ± SEM) were recruited to participate in the study. Further subject characteristics are available in Table 1. The subjects were thoroughly briefed on the experimental protocol, and a signed consent form was obtained. Subjects were also required to pass a physical exam and fill out a physical activity readiness form before participation (15). The Human Subjects Committee of the University of WI–Madison approved the protocol. The studies were performed at the General Clinical Research Center (GCRC) at the University of Wisconsin Hospital and Clinics (UWHC). The pulmonary function laboratory of the UWHC performed the exercise stress test. No medication, smoking, alcohol, or caffeinated beverages were allowed during the test days of the protocol. Subjects participating in the study were allowed to take birth control pills.
Subjects who were very sedentary or who regularly participated in >3 h·wk−1 of vigorous physical activity were excluded from the study. Additionally, those individuals with a physical disability, including metabolic disease, which would interfere with the performance of the exercise or the data measurements, were excluded. Any subject not approved for physical activity by a physician or with an abnormal response to the exercise stress test was excluded. Nine subjects started the tracer study but only seven were used in this analysis. One subject withdrew from the study due to time constraints. Another subject terminated her participation after becoming nauseated during her first GCRC stay. Three additional subjects participated in the study without receiving the fatty acid tracers to serve as a measure of background 13C (19).
Subjects were required to complete an exercise stress test at least 1 wk before participating in the GCRC overnight stays. After completing the exercise stress test, subjects were required to visit the GCRC on three separate occasions to participate in three sessions (rest, light, heavy). All subjects completed all three sessions, but the order was randomized for each subject. The GCRC visits were timed to be 3–4 wk apart and occurring during the follicular phase of their menstrual cycles. For 3 d before when the subjects entered the GCRC, they were provided with microwaveable meals for breakfast, lunch, and dinner as described below. Subjects were asked not to participate in any moderately heavy or vigorous physical activity on the day they entered the GCRC.
At 1800, subjects were admitted to the GCRC after a fast of at least 5 h. They were asked to provide a breath and urine sample for the measurement of baseline 13CO2 and 2H2O. Additionally, subjects were asked to fill out a physical activity recall for that day. On the first of the three visits, subjects were also given a dose of H218O (0.4 g·kg−1 estimated total body water) for the measurement of total body water (TBW) from urine 18O. During the first GCRC visit, subjects were also asked to fill out a food frequency questionnaire (3). At 1900, subjects were given a meal, which they were required to eat completely. During the exercise visits, but not the rest visit, subjects were given a 1250 kJ snack at 2100 to compensate for the energy expended during exercise the next day. Subjects were asked to sleep at the GCRC at 2300.
At 0630, subjects were awakened and allowed to shower. At 0730, subjects entered the human respiratory chamber. A schematic of the chamber protocol is given in Figure 1. Between 0800 and 1000, the exercise sessions took place. All exercise sessions ended at 1000. After returning to resting levels at 1030, subjects received a liquid replacement meal containing the labeled fatty acids: [1-13C]oleic acid (10 mg·kg−1) and [d31]palmitic acid (15 mg·kg−1).
Subjects were given research meals at 1300 and 1800. Subjects were required to provide breath samples for the measurement of 13CO2 hourly after the breakfast meal. Additionally, subjects were asked to provide urine samples every four hours from 0800 to 2200 for the measurement of 2H2O. Aliquots of all additional urine produced were collected as well. Total urine volume while subjects were in the chamber was noted for the measurement of nitrogen excretion. Subjects were allowed to exit the chamber at 2200 and were subsequently discharged from the GCRC.
A separate study with different subjects was done to measure [1-13C]acetate recovery after exercise. It has been established that a large percentage of the 13C label is temporarily sequestered in the TCA cycle during metabolism, leading to an underestimation of label recovery (25). We gave seven female subjects [1-13C]acetate after exercise/rest rather than the [1-13C]oleate. Each subject participated in trials involving rest, light, and heavy exercise that were identical in intensity and duration to the current study. The end recovery of [1-13C]acetate from the side study was used to correct the end cumulative recovery of [1-13C]oleate to more accurately compare oleate and palmitate values (25). The [1-13C]acetate recovery 7.5 h postdose was 49.2 ± 2.8%, 47.4 ± 3.3%, and 46.4 ± 2.8% for rest, light, and heavy exercise, respectively. A repeated-measures analysis of variance revealed that the cumulative recovery at 7.5 h postdose did not differ between exercise trials (P = 0.81). Recoveries were extrapolated to 11.5 h postdose by the area under the terminal elimination rate from 7.5 to 11.5 h. This resulted in correction factors of 51.3 ± 2.8%, 49.5 ± 3.5%, and 48.9 ± 3.1% for rest, light, and heavy exercise, respectively.
At the start of their enrollment in the study, subjects performed a V̇O2peak exercise stress test on a cycle ergometer. A three-lead ECG was used to monitor heart rate while subjects were cycling. The pedal resistance was increased 20–40 W every 2 min until exhaustion. Achievement of V̇O2peak was determined to be at a respiratory exchange ratio of greater than 1.1 and a heart rate within 90% of the predicted maximum heart rate (12).
The exercise sessions while in the respiratory chamber were calculated to expend 1250 kJ above rest. While in the chamber, subjects either sat quietly for 120 min, exercised lightly (25% V̇O2peak) for 120 min, or exercised heavily (85% V̇O2peak) for three bouts of 10–12 min with 10-min rest periods between bouts.
Before admission to the GCRC, subjects were provided with commercial microwaveable meals for 3 d to ensure that intake was the same before each of the three visits. Energy intake for free-living conditions was calculated as resting metabolic rate (RMR) as calculated from the WHO equation for women (10) times an activity factor of 1.8. All meals were calculated to be 30% fat, 15% protein, and 55% carbohydrate. The microwaveable meals were supplemented with Boost Plus and Boost Bars (Mead Johnson, Evansville, IN) to reach the desired energy levels. Subjects were asked to eat only to satiety and to provide a written record of what they consumed. The average intake during this time was 9490 ± 507 kJ·d−1 (54.9 ± 0.6% carbohydrate, 30.5 ± 0.6% fat, 14.9 ± 0.2% protein).
Upon admission to the GCRC, subjects were provided with research meals from the UWHC kitchens. All meals were designed to provide 30% fat, 15% protein, and 55% carbohydrate, and energy intake was determined as RMR times an activity factor of 1.35 for the inpatient period. The ratio of saturated to unsaturated fatty acids was 1:1. Subjects were given 30% of their energy intake at breakfast, 30% at lunch, and 40% at dinner. The breakfast meal in the chamber was a liquid replacement meal (Boost High Protein; Mead Johnson), which was heated to 40°C before serving to provide better mixing of the labeled fatty acids. While staying at the GCRC, subjects were required to eat all of the food provided to them.
The respiratory chamber at the UW-GCRC was modeled after the chamber in the Department of Human Biology at Maastricht University in The Netherlands (22). The chamber has a 12-m3 shell volume and the walls and ceiling are made of panels consisting of 10-cm–thick polyurethane foam insulation between two layers of 1-mm–thick embossed aluminum (Norlake Scientific, Hudson, WI). The floor panels are similar to the walls but contain a smooth galvanized steel interior surface. The chamber has three windows: two windows allow a view outside of the hospital, while one allows for contact with the study and medical personnel. An airlock, made of galvanized steel, is used for passing food, drink, and biological samples between subjects and study personnel.
The temperature of the chamber is kept at 23°C by passing chilled water (9°C) through copper dehumidification coils and radiator-style heating elements. A variable speed fan moves 3000–10,000 L·min−1 of air over the coils and heat exchangers. The system is designed to cool and dehumidify continuously, with the reheating of recirculated air controlled by an electronic PID controller (Omega, Stamford, CT). The air inlet and outlet of the chamber have mechanical ball valves, which allow outside air to be drawn through the chamber at a rate of 35–200 L·min−1 by an in-line ventilation blower. This produces a slight negative pressure inside the chamber of −5 to −7 mm of Hg. The composition of the air is measured by Hartman and Braun Uras-4 CO2 and Magnos-6 O2 gas analyzers (Applied Automation, Bartlesville, OK). The data acquisition is performed using LabView2 software (National Instruments, Austin, TX) and NB-TIO-10 data acquisition interface. Calculations of energy expenditure, V̇O2, V̇CO2, and RQ are performed with a spreadsheet macro program (Excel, Microsoft, Seattle, WA).
The proper functioning of the chamber is confirmed by a series of monthly checks (22). Methanol (99.9% pure; Fisher Scientific, Pittsburgh, PA) was burned for 8–16 h, and percent error in recovery was −0.13 ± 0.12% (mean ± SD) for CO2 and 0.90 ± 0.85% for O2. The ratio of observed L burned to predicted L burned was 1.00 ± 0.01 for CO2 and 0.99 ± 0.02 for O2. Infusion of CO2 into the chamber over a period of time also allows for checks of the accuracy of O2 and CO2 measurements. The CO2 infusion in our chamber has a percent error of −0.12 ± 0.88%, with an observed to predicted ratio of 1.00 ± 0.01 L.
Sample collection and analysis.
Breath samples for the collection of 13CO2 were taken hourly after the labeled fatty acid dose by having subjects blow through a straw into a 15-mL No-Additive VacutainerTM (Becton Dickinson, Franklin Lakes, NJ), which was then capped. Breath CO2 was sampled directly from the VacutainerTM with a syringe, and 13CO2/12CO2 was measured with continuous flow isotope ratio mass spectrometer (IRMS) (Delta S, Finnigan MAT, Bremen, Germany). CO2 was introduced into a helium stream onto choromosorb-Q to separate CO2 from the less polar gases and directed into the source of the IRMS (20). Each sample was injected twice for isotope ratio analyses. The average standard deviation for all injections of these samples was 0.15 permil (°/°°; (δ°/°°) = (RU/RSTD − 1) 1000). Excess 13C was calculated relative to baseline breath CO2 before label administration and was corrected for natural fluctuations in 13C-breath content that occurs as a function of meals (19).
The 2H was measured in urinary water samples collected every 4 h while subjects were in the chamber. Urine was decolorized with carbon black, and deuterium was measured with IRMS. Urine samples were analyzed for deuterium content as a ratio of 2H/1H by using the Delta Plus IRMS (Finnigan MAT), after reduction on chromium powder held at 850°C. Each run included three injections of the sample with independent analysis, with an average standard deviation of less than 0.50 permil. Data were corrected for H3+ and expressed relative to standard mean ocean water (SMOW) (20).
For the measurement of 18O, 1 mL of urine was allowed to equilibrate with CO2 at 25°C for 48 h. 18O enrichment was measured by continuous flow IRMS, and body water was calculated by dilution (20). Each sample was run on 2 separate days, with three injections per run. The average standard deviation of the runs was 0.17 permil.
Five-milliliter aliquots of all urine produced in the chamber were saved and acidified with 250 mg of citric acid (Acros Organics/Fisher, Chicago, IL) to prevent the volatilization of the nitrogen compounds. The volume of all urine produced while subjects were in the chamber was recorded. Samples were kept at −5°C until their dilution for analysis of nitrogen content. For each subject, the urine aliquots were combined at 1% of the total volume to obtain a representative sample of urine output for the entire chamber stay. The combined urines were then diluted 1 to 100 in distilled water for analysis. Nitrogen analysis was performed using the Antek 9000N chemiluminescence nitrogen analyzer (Antek Instruments, Inc., Houston, TX). Nitrogen concentration was measured in parts per million against a calibration curve with known concentrations of urea (10–100 ppm).
Recovery of 13C-oleate was calculated hourly after ingestion of the label. For the labeled subjects, two baseline breath samples were taken: one upon admission to the GCRC (1800) and one the following morning (0730). The average baseline value was subtracted from the subsequent dose values for each subject, so that each time point is expressed as a permil increase relative to the subjects’ own baseline (Δδ). The 13CO2 values of the control subjects were subtracted from those of the labeled subjects to correct for background variation. This resulted in values that were adjusted for each individual’s baseline as well as meal related natural background variation. At a 10 mg·kg−1 dose, peak enrichment ranged from 5.9 to 11.8 permil.
Total CO2 production was obtained from the respiratory gas exchange measurements and total excess 13C expressed as amount of dose recovered during each hour. EQUATION
Where RSTD = 13C/12C of standard CO2; P = 13C isotope atom percent; n = number of labeled atoms per molecule (1); MW = molecular weight ([1-13C]-oleic acid = 282); D = dose (mg); and V̇CO2 = carbon dioxide production ratio in M·h−1. Cumulative recovery was calculated using the trapezoid rule.
Cumulative recovery of deuterium from palmitic acid oxidation was measured in urine samples, a technique that we have validated against acetate corrected 13CO2 recovery for 13C-palmitate (29). When oxidized, the deuterium appears as water and mixes with the body water pool. Except for a loss of about 4%/12 h as urine and insensible water, the rest of the label accumulates in body water and provides a cumulative record of fat oxidation. As with the breath samples, baseline urine samples were obtained upon admission to the GCRC as well as upon entry into the respiratory chamber. Recovery was calculated as excess 2H times TBW divided by the dose of 2H administered:EQUATION
Where RSTD = 2H/1H of SMOW; P = 2H isotope atom percent; n = number of labeled atoms per molecule (31); MW = molecular weight (d31-palmitic acid = 287); and D = dose (mg). The hours postdose for 2H recovery in urine were assumed to equal the midpoint between sequential voids.
All statistical analyses were completed using StatView Version 5.0.1 (SAS Institute Inc, Cary, NC). Each subject that participated in the three trials served as her own control. Therefore, a repeated-measures analysis of variance (RM-ANOVA) was used to analyze the various endpoints. The level of exercise was taken to be the dependent variable. When a significant interaction was shown by the RM-ANOVA, a Fisher’s PLSD test was used to correct for multiple comparisons. All values are presented as mean ± SEM, and a P-value of less than 0.05 was considered significant.
The seven young female subjects who participated in the study were of healthy weight and moderately active, though not physically trained (Table 1). By self-report (food frequency questionnaire) these subjects typically ate around 7900 kJ·d−1 and their macronutrient intakes follow dietary guidelines relatively closely (54.4 ± 2.0% carbohydrate, 31.8 ± 1.3% fat, and 14.9 ± 0.8% protein).
The values for energy expenditure and energy intake during the entire GCRC stay, including the stay in the chamber, are given in Table 2. The estimation of total energy expenditure is from admission to the GCRC (1800 day 1) to discharge (2200 day 2). The total amount of time subjects spent in the chamber was 14.5 h. Chamber energy expenditures during the light exercise (+1570 kJ) and heavy exercise (+1412 kJ) stays were greater than rest (P < 0.0001 for both), but the two exercise trials did not differ from each other. To match total energy intake during the entire GCRC stay to energy expenditure, we estimated that the energy expended the night (2200–0630) after discharge as equivalent to that of the night spent in the GCRC, resulting in an estimated energy expenditure of 10,786 ± 210 kJ, 12,356 ± 359 kJ, and 12,198 ± 272 kJ for the rest, light, and heavy trials, respectively.
The chamber results were divided into two periods: exercise (including prebreakfast recovery: 0800–1030) and the 11.5 h postdose. As previously mentioned, energy expenditure during exercise was similar during both exercise sessions, and this was greater than during rest. RM-ANOVA shows no significant interaction (P = 0.48) for energy expenditure in the postdose, suggesting no lingering effects of exercise on energy expenditure.
Table 3 shows the NPRQ, substrate use, and macronutrient intake during the chamber stay. NPRQ was higher in both the rest and heavy exercise trial than the light trial (P < 0.001 and P < 0.01, respectively). Over the 14.5 h, no difference existed across trials in protein utilization. Subjects utilized 27% more fat as fuel during the light versus the heavy exercise trial (P < 0.01). Carbohydrate use during the entire chamber stay was 17% greater during the heavy exercise than the light exercise trial (P < 0.05).
Dividing this into exercise and postexercise demonstrates a significant effect of exercise on total fat utilization (Fig. 2a;P < 0.001). Fat used during the light exercise session was 23 g greater than rest (P < 0.0001) and 20 g greater than during heavy exercise (P < 0.0001). During the postdose period, both exercise trials resulted in greater whole-body fat oxidation than rest (P < 0.01). In both cases, the increase in oxidation was ∼12 g.
As expected, carbohydrate use during exercise was greatest during the heavy exercise session (Fig. 2b). Additionally, light exercise required 33 g more carbohydrate for fuel than rest (P < 0.05). During the postdose period, the rest trial resulted in greater carbohydrate use than the exercise trials, which appeared to be compensating for the exercise session. Protein utilization both during the exercise period and the postdose period did not differ between trials (Fig. 2c).
Stable isotope recovery.
Figure 3 shows the hourly recovery of [1-13C]oleate in breath 13CO2 after the post- exercise dose. RM-ANOVA shows that a significant effect of time exists (P < 0.0001). During the rest trial, there is a plateau in recovery beginning at 4 h postdose, which corresponds to 1 h after lunch and lasts until right before dinner. This effect is not seen during either exercise trials. Peak recovery during the heavy exercise trial is at 4 h postdose and at 6 h postdose during the light exercise trial.
Total cumulative recovery of [1-13C] was 18.0 ± 1.8%, 21.2 ± 1.9%, and 24.0 ± 1.7% for rest, light, and heavy exercise (P < 0.05). When corrected for sequestration, the 11.5-h postdose oxidation of [1-13C]oleate was 34.2 ± 3.6%, 39.5 ± 3.6%, and 49.3 ± 3.6% for rest, light, and heavy exercise, respectively. As with the uncorrected results, heavy exercise results in greater 13CO2 recovery than rest (P < 0.005). Unlike the uncorrected recovery, the corrected recovery results in a significantly greater dietary oleate oxidation after heavy exercise when compared with light (P < 0.05). Dietary oleate oxidation after light exercise tended to be greater than rest (P = 0.17).
Recovery of the [d31]palmitate as 2H2O was measured at 4-h intervals while subjects were in the respiratory chamber (Fig. 4). Cumulative recovery at 11.5 h postdose was 10.1 ± 1.6%, 11.2 ± 1.1%, and 12.3 ± 1.3% for rest, light, and heavy exercise, respectively. These values were not significantly different from one another. Because most of the label is released before the acetate oxidation, no acetate correction is performed (29).
The novel finding of this study is that prior exercise has a positive effect on the oxidation of dietary monounsaturated fat, as evidenced by the recovery of [1-13C]oleate, but apparently not saturated fat, as evidenced by [d31]palmitate oxidation. Furthermore, this effect was seen at two exercise intensities that are known to rely more heavily on different fuel sources for energy. These results demonstrate that exercise alters the partitioning of dietary fatty acids when compared with rest.
Previous literature has established a relationship between physical activity and weight control or leanness in humans (21,31). Although it is evident that individuals who are more active are better able to maintain a healthy body weight than their sedentary counterparts, a mechanism for this phenomenon has not been clearly documented. Herein we demonstrate that at least part of the mechanism is related to short-term partitioning of dietary fat between oxidation and storage.
This finding concurs with the body of literature on animals in which interventions before the feeding of a particular fatty acid alters the partitioning of that fat between storage and oxidation (2,16). Fasting has been shown to result in oxidation, rather than storage, of fatty acids (16), and the extent of oxidation differs depending on fatty acid type (2). Most of these studies, however, have manipulated energy to alter the partitioning of fat. Indeed, previous work by Dionne et al. (6) has also shown that energy balance is key to the whole-body substrate oxidation after exercise. It is important to note that our findings, however, were not simply due to an energy imbalance resulting from the exercise session. When participating in an exercise trial, subjects received a 1250-kJ snack the evening before as compensation for the extra ∼1400 kJ expended during exercise. Therefore, by the time the fatty acid doses were given, the exercise and rest subjects were at equivalent energy balances for the previous 13 h and throughout the recovery period. Indeed, as mentioned in the results, including the estimated overnight energy expenditure for the night after leaving the chamber in our energy calculations, it is indicated that, over the 36-h period, subjects were in energy balance within 937 kJ, 471 kJ, and 723 kJ for the entire rest, light, and heavy visits.
In this study, we deliberately chose both light and heavy intensity exercise sessions for their contrasting effects on dietary substrate utilization. Previous studies have shown that light exercise utilizes mainly plasma free fatty acids for fuel, whereas heavy exercise is more dependent on glycogen stores (17). As expected, we did see a difference in whole body fat utilization during exercise (light > heavy) that confirmed the expected use for these exercise intensities, but we also expected that heavy exercise would result in a postexercise dietary substrate compensation secondary to sparing of glycogen after heavy exercise (7). Instead, our data show that dietary fat oxidation and whole-body fat utilization during the postdose period was similar between both exercise trials regardless of differences in whole-body substrate utilization during exercise.
This is particularly interesting because [1-13C]oleate oxidation differed by 12% between the two exercise sessions, whereas total fat oxidation did not. Thus, the dietary fat oxidation is not simply a reflection of total fat oxidation. Although we cannot rule out the role of the liver, we speculate that the increased oleate oxidation postexercise is due to muscle metabolism, based on a large body of literature on the effects of exercise on muscle fat metabolism. A recent study by van Loon et al. (28) showed that the decrease in total fat oxidation in heavy exercise resulted from a decrease in plasma free-fatty acid and oxidation of muscle and/or lipoprotein-derived triglyceride fat sources. The compensatory increase in total fat oxidation post high-intensity exercise is therefore likely occurring in plasma FFA as well as triglyceride sources, though evidence from human studies on the effects of exercise on postprandial lipemia studies suggests that the triglyceride fractions are particularly important (8,27). Increased oxidation of both muscle and lipoprotein triglyceride fractions can both be quickly replenished by dietary fat and potentially have an important role in the effect we see in this study.
Evidence exists in the literature suggesting that increased oxidation of both triglyceride fractions, but particularly muscle pools, occurs post high-intensity exercise and that replenishment of the pools is related to dietary fat intake (4,13). Muscle triglyceride stores that have been decreased as a result of moderate exercise are more quickly repleted by a high-fat (55% fat) versus low-fat (15% fat) diet (4). Kiens and Richter (13) exercised subjects at high intensity (70–90% V̇O2peak) until exhaustion and measured muscle glycogen, triglyceride, and lipoprotein lipase (LPL) activity 18 h postexercise. Glycogen levels that had been significantly decreased during exercise were repleted during the postexercise period. Muscle triglyceride was not affected by exercise but decreased during the postexercise period, implying increased fat oxidation to spare glycogen. Additionally, LPL activity increased during the postexercise period in comparison with preexercise levels.
The increase in LPL activity after exercise has been noted in skeletal muscle but not in adipose tissue (23), indicating that increased muscle triglyceride use and/or trafficking of lipoprotein-derived triglyceride to muscles may be responsible for the enhanced dietary fat oxidation after exercise. Seip et al. (24) showed a transient increase in LPL mRNA 4 h postexercise (cycling at 55–70% V̇O2peak for 60–90 min) followed by an increase in LPL mass at 8 h postexercise. Those increases corresponded to decreases in serum triglyceride concentrations as well as increases in catecholamines and decreased insulin. It is interesting to note that the peak [1-13C]oleate oxidation in our study occurs between 4 and 8 h postdose (4.5–8.5 h postexercise).
An additional interesting finding in this study is the differential response in oxidation of oleate versus palmitate. There is little precedence in the literature for a direct comparison of the oxidation of dietary oleate and palmitate after an intervention that has the potential to alter fat partitioning. Several studies in fasted versus fed rats used only labeled oleate (16), with the general result of increased oleate oxidation in the fasted compared with the fed state. Additionally, Bessesen et al. (2) found that stearate was retained longer in the liver and skeletal muscle pools than both oleate and linoleate, which were oxidized more rapidly.
The literature in humans demonstrates that differential oxidation and handling of fatty acids is based on saturation and chain length (5,11). These differences do not result from variable intestinal handling (11), as long as the fatty acids are given as part of a liquid emulsion (5). DeLany et al. (5) did note a difference in oleate and palmitate oxidation over 9 h, but it was not as large in magnitude as in our study. It is likely, therefore, that variances in oxidation that result from chain length and degree of saturation are amplified when the tracers are given after exercise.
In conclusion, we have shown that prior exercise can alter the pattern of dietary fat oxidation from one meal when compared with rest. Although further research is needed to elucidate the mechanisms involved, this is promising data in terms of weight maintenance, because exercise appears to be able to partition dietary fat to oxidation rather than storage. Current studies in our laboratory are examining the effect of later dose times postexercise on the use of both [1-13C]oleate and [d31]palmitate. Furthermore, future studies in obese and postobese individuals are warranted.
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