INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

AN ELEVATED RESPIRATORY EXCHANGE ratio (RER), indicative of increased reliance on carbohydrate, rather than fat oxidation, has been reported during the postabsorptive (fasting) condition in obese humans (3, 23). An increased postabsorptive RER was predictive of subsequent weight gain in Pima Indians (26). An elevated fasting RER has also been reported with weight loss compared with the condition before weight loss (obesity) (3, 4, 8, 23). These findings suggest a linkage between a reduced capacity to oxidize lipid, fat storage, and obesity (3).

Obese individuals after weight loss are hypothesized to represent the preobese state (1-3, 5-8, 11, 14, 18-20). A comparison of subjects after weight loss with matched controls may thus provide information on factors that predispose individuals to obesity. There is relatively little information, however, on how substrate utilization in weight-reduced (postobese) subjects compares with that in their weight-matched counterparts. Ranneries et al. (20) reported no differences in RER during and after exercise between postobese individuals and matched controls. Resting, postabsorptive fat oxidation was significantly depressed, however, in the weight loss group (20), as in other studies (6, 11). Conversely, others reported no differences in lipid oxidation during exercise (10) or at rest (2, 7, 19, 24) when comparing subjects after weight loss with weight-matched controls. Such findings make it difficult to determine whether the development of obesity is influenced by a predisposition to lipid storage over oxidation.

The purpose of the present study was to determine whether substrate oxidation is indeed different between individuals prone to obesity and weight-matched controls. We specifically focused on substrate use during exercise, inasmuch as defects in the rate-limiting steps of oxidative pathways in skeletal muscle would be potentially easier to detect during elevated energy demand. We therefore compared substrate utilization at rest and exercise in previously morbidly obese women who had lost significant amounts of weight with that in their weight-matched counterparts. Other exercise studies (10, 20) have compared responses in subjects after weight loss with responses in controls at identical relative workloads [percentage of maximal O₂ uptake (O_2 max)]; the exercise bouts may have thus been performed at different absolute (i.e., watts) workloads. To discern whether responses to exercise differ in individuals prone to obesity, we compared similar absolute and relative workloads. Although there are conflicting data (10, 20), it was our hypothesis that fat oxidation would be depressed in previously morbidly obese women compared with controls.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Subjects and experimental design. Subjects completed a health history questionnaire before participation. None were taking medications that would interfere with metabolism, and all were free from known cardiovascular disease and diabetes mellitus. Two groups (n = 8/group) were studied. One group consisted of previously morbidly obese [body mass index (BMI) >40 kg/m²] patients who had undergone gastric bypass surgery to induce weight loss (WL group). Characteristics of this type of patient population and the operative procedure have been described elsewhere (17). Briefly, the surgery produces weight loss of ~50 kg, with patients becoming weight stable at 8-12 mo after the procedure (17). BMI decreased significantly (P < 0.001) in the subjects examined in this study from 47.6 ± 2.4 to 34.8 ± 2.0 kg/m² and mass decreased from 134.8 ± 5.6 to 96.9 ± 4.1 kg with weight loss intervention. All patients in the WL group had undergone gastric bypass surgery 1 yr before participation and were weight stable (±2 kg) for the previous 6 mo. The control (C) group consisted of weight-stable women matched for age, race, and BMI. None of the C subjects had lost >4 kg body mass via intervention. All subjects were sedentary and not performing regular exercise. The experimental design consisted of comparing substrate oxidation during rest and submaximal and maximal exercise, energy intake and expenditure, and body composition in the WL group with the same parameters in the C group. Procedures were approved by the East Carolina University Institutional Review Board.

Maximal and submaximal exercise. O_2 max was determined with an incremental test to voluntary exhaustion on an electrically braked cycle ergometer (Lode, Diversified, Brea, CA). The initial workload was 0 W and increased 25 W every 2 min. Subjects had to meet at least two of the following criteria (16) for a valid test: 1) RER > 1.0, 2) a plateau in O₂ consumption (O₂), despite an increase in workload, and 3) attainment of 85% of age-predicted maximal heart rate. Expired gases were continuously analyzed (model 2900 Metabolic Measurement Cart, Sensor Medics, Anaheim, CA). Heart rate and electrocardiogram tracings were monitored at each stage. Data from the O_2 max test were used to 1) screen for evidence of cardiovascular disease and 2) discern the workload for submaximal exercise testing.

Energy intake and expenditure. Energy intake and expenditure were measured over 14 days. Energy intake was estimated from two 3-day dietary records, with foods consumed recorded over one weekend and two weekdays. Records were coded and entered into a computer program (Food Processor II, ESHA Research, Salem, OR), and energy intake was calculated. Instructions for estimating food volume and serving size were given, and subjects were instructed not to alter their diet.

Body composition. All measurements were performed by the same individual. Height (±0.1 cm) and mass (±0.1 kg) were determined after an overnight fast, and BMI (kg/m²) was calculated. Waist (level of umbilicus) and hip (maximal hip circumference) girths were measured (±1.0 mm) in duplicate. Fat and fat-free mass (FFM) were measured by hydrostatic weighing. The head-above-water method (9) was used, inasmuch as the majority of subjects were apprehensive of total submersion. Each subject exhaled to residual volume with her body submerged to a predetermined point on the jaw line (9), and weight was recorded. Four trials were performed, with the average of the last two trials used to calculate body density (9). Residual volume was determined using the oxygen dilution method (25).

Statistics. Data were compared between the WL and C groups using a factorial ANOVA. Significant differences were accepted at P < 0.05. Values are means ± SE.

	RESULTS

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

Subjects and body composition. Subjects were matched for age, race, and BMI (Table 1). All women were premenopausal and Caucasian. There were no significant differences in indexes of overall or regional adiposity between the C and WL groups.

Maximal and submaximal exercise. There were no significant differences between the C and WL groups in maximal heart rate, RER, or absolute O_2 max (Table 2). O_2 max expressed relative to body mass was significantly (P  0.05) lower in the WL group by 19% (Table 2), despite no significant difference in body mass (Table 1). There was a trend (P = 0.09) for O_2 max to be lower in the WL group when expressed per kilogram of FFM (Table 2).

View larger version (19K): [in this window] [in a new window]   Fig. 1.   Respiratory exchange ratio (A) and energy derived from fat (B) during submaximal exercise at the same absolute (15 W) or relative [65% maximal O2 uptake (O2 max)] workload in controls and previously morbidly obese subjects after weight loss. *Statistically significantly (P  0.05) different from control.

1

1

1

1

Energy intake and expenditure. Self-reported energy intake did not differ between the two 3-day periods; averaged results are thus presented. The WL group reported a significantly (P < 0.01) lower energy intake, by ~40% (2,600 ± 193 and 1,604 ± 188 kcal/day in C and WL, respectively; Fig. 2). There were no significant differences between groups in the percentages of self-reported energy intake from macronutrients (52% carbohydrate, 32% fat, and 16% protein). Self-reported energy derived from each macronutrient was significantly (P < 0.05) lower in the WL group, by ~40% (1,284 ± 97 and 791 ± 120 kcal carbohydrate, 514 ± 74 and 911 ± 123 kcal fat, and 359 ± 19 and 235 ± 27 kcal protein for C and WL, respectively).

View larger version (14K): [in this window] [in a new window]   Fig. 2.   Self-reported energy intake from 3-day dietary records and energy expenditure using the doubly labeled water technique in controls and previously morbidly obese subjects after weight loss. *Statistically significantly (P  0.05) different from control. Statistically significantly (P  0.05) different from energy expenditure in weight loss group.

1

1

	DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS DISCUSSION REFERENCES

In the present experiment, substrate utilization was examined in morbidly obese individuals after weight loss (~30 kg) and compared with that in weight-matched controls. A similar experimental design has been used by others (1, 2, 5-7, 10, 11, 18-20, 24) to examine questions pertinent to obesity. Primarily, individuals after weight loss are thought to represent the preobese state (1, 2). In the present study, we examined morbidly obese subjects after massive weight loss (see METHODS). Our findings may thus be indicative of metabolic factors that predispose individuals to the development of morbid obesity.

The main finding of the present study was that substrate utilization during submaximal exercise differed significantly between WL subjects and the C group. As indicated in Fig. 1, the WL subjects oxidized significantly more carbohydrate and proportionally less fat during submaximal exercise at the same absolute and relative workloads. The exercise intensities studied were relatively low (Tables 2 and 3), which suggests that physical activity of virtually any sort would impede fat oxidation in WL subjects relative to the C group (Fig. 1). This defect in lipid oxidation would tend to promote fat mass deposition, particularly during positive energy balance.

These findings contribute to our understanding of the etiology of obesity/morbid obesity in several ways. First, WL subjects were examined in the post-morbidly obese condition, i.e., when they were theoretically predisposed to developing morbid obesity. The observed reduction in fat oxidation suggests that this defect may be a symptom that contributes to the development of morbid obesity. Second, weight loss has been reported to increase fasting RER, which has been hypothesized to promote weight regain (4). Examination of subjects after weight loss thus also offers an opportunity to study factors that may lead to weight regain or recidivism after weight loss intervention. Both of these interpretations indicate the potentially important role of fat oxidation in the development of obesity.

The decrement in lipid oxidation was, however, only evident during exercise (Fig. 1) and not at rest (see RESULTS). This suggests that a metabolic stressor, such as exercise, may be necessary to unmask perturbations in substrate oxidation with morbid obesity (Fig. 1). In support of this suggestion, other studies comparing subjects after weight loss with weight-matched controls reported a decrement in lipid oxidation only when postobese individuals consumed a high-fat diet or meal (2, 7, 18). A necessity for stressing the capacity of the oxidative pathways may explain why observations in the resting, postabsorptive (fasting) state are conflicting, inasmuch as no difference (see RESULTS; 1, 2, 18, 19, 24) or a decrease (2, 6, 11, 14, 20) in lipid oxidation has been reported in postobese subjects compared with weight-matched controls.

The present findings are at variance with the other minimal data examining exercise. Ranneries et al. (20) observed a decrement in lipid oxidation during rest but no differences in lipid oxidation during exercise (60 min at 50% O_2 max) or recovery in postobese subjects compared with weight-matched controls. Ezell et al. (10) also observed no differences in lipid oxidation during 60 min of exercise at 60-65% O_2 max in individuals after weight loss vs. matched controls. However, in this study (10), subjects were fed a relatively high-carbohydrate meal before exercise that may have concealed differences in substrate utilization due to the propensity to effectively match carbohydrate intake and oxidation (3).

A possible explanation for the differences between our exercise data (Fig. 1) and that reported by others (10, 20) may lie in the subjects studied. The patients in the present experiment, while losing large amounts of weight, were still obese (Table 1). It is not evident whether weight loss to nonobese levels would normalize the exercise response. Other studies comparing obese subjects who have lost weight to nonobese levels (BMI < 28 kg/m²), however, report the persistence of a decrement in fasting or feeding-induced (2, 3, 5-7, 20) lipid oxidation compared with weight-matched controls. The present study extends this observation to previously morbidly obese subjects after weight loss.

In relation to possible cellular mechanisms, a reduction in the activity of enzymes involved in lipid oxidation in human skeletal muscle has been reported with obesity (12, 13, 19). This defect persists with weight loss (12), which may explain our findings. Another factor could be increased carbohydrate availability during exercise in previously morbidly obese subjects. However, several studies (10, 20) have reported that previously obese subjects after weight loss mobilize equal, if not greater, amounts of fatty acids during or after exercise. Such findings suggest that defects in lipid uptake and oxidation in skeletal muscle, rather than impaired lipid availability, are responsible for depressing fat oxidation in formerly obese women (20).

A possible explanation for the differences in submaximal substrate utilization (Fig. 1) and O_2 max (Table 2) between WL subjects and the C group could be variations in physical activity. Others (11, 14, 22), however, reported no difference in physical activity between subjects after weight loss and matched controls. In support of these observations, we did not observe a difference between WL subjects and the C group in total energy expenditure or estimated energy expenditure on physical activity using the DLW method (Fig. 2; see RESULTS). Other factors that may influence substrate oxidation such as FFM, fat mass, and absolute aerobic capacity were matched between groups. We cannot, however, entirely exclude the possibility that dietary differences between the two groups may have influenced lipid oxidation during exercise, inasmuch as we did not supply a fixed diet before testing but, rather, studied the free-living condition. Self-reported relative macronutrient intake was, however, not different between the WL subjects and the C group (see RESULTS). These findings suggest that, despite similar diet and activity levels, WL subjects still oxidized proportionally less lipid.

Several groups (15, 19) reported that obese subjects underreport energy intake by ~30%. The present findings in WL subjects approximate this value (Fig. 2). It is logical to assume that the WL group was indeed underreporting, inasmuch as subjects were weight stable, indicating a matching of energy intake and expenditure. Had the self-reported energy intake data been valid, the WL subjects would have lost ~2 kg of mass over the 14-day recording period. Also, negative energy balance elicits a reduction in RER and increased lipid oxidation (3). This was not evident during exercise or the fasting condition in WL subjects.

It should be noted that, in the gastric bypass patients examined in this and other studies (8), energy intake is mechanically restrained with the surgery (17). Patients thus tend to remain in a weight-stable condition after weight loss because of an inability to substantially increase energy intake (17). This model of weight loss is unique, in that WL subjects can be studied in a weight-reduced and stable state to discern metabolic characteristics that predispose individuals to morbid obesity.

In summary, lipid oxidation was significantly depressed during submaximal exercise in WL subjects compared with the C group. This reduction in fat oxidation may be a defect that predisposes individuals to morbid obesity.