Introduction

The total body burden of organochlorines (OCs) is greater in obese than in lean individuals because of their increased body fat stores and plasma and adipose tissue OC concentration.¹ In response to body weight loss, OC concentration increases in plasma^{2, 3} and they are more susceptible to exert their adverse effects on metabolism. In this regard, the increase in OC concentration induced by weight loss has been shown to be an independent predictor of the decrease in plasma tri-iodothyronine (T₃),⁴ resting metabolic rate,⁴ and skeletal muscle oxidative enzyme activity⁵ resulting from weight loss. Considering that OCs are also known to alter the functionality of mitochondria,⁶ it is realistic to hypothesize that the increase in plasma OCs is a factor implicated in the adaptive decrease in thermogenesis occurring with weight loss.^{7, 8} To investigate this issue, we measured the sleeping metabolic rate (SMR) and its deviations from predicted values before and after weight loss in obese individuals. This allowed an estimation of adaptive thermogenesis, defined as the difference between the expected decrease in SMR due to the decreases of fat mass (FM) and fat-free mass (FFM) and the decrease in SMR really measured. This difference was then used as the dependent variable in a stepwise multiple regression analysis to compare the contribution of OCs to variations in thermogenesis to that of other factors known to affect thermogenesis in the context of a weight-reducing program.

Methods

In all, 58 men and 28 women with a wide range of body weight (47.5–146.1 kg) were used as control subjects to establish a reference equation predicting the SMR from gender, age, FM, and FFM. Eight men and seven women who gave their written consent to participate in a weight-loss program approved by the Laval University Ethics Committee and for whom all the variables of interest were available before and after weight loss were analyzed in the present study. The weight-loss program, which was conducted between 1995 and 1997, has been described in detail elsewhere.⁴ Briefly, this 15-week weight-loss program consisted in a nonmacronutrient specific energy restriction of about 2930 kJ/day, associated with either fenfluramine (n=11) or a placebo (n=4). To prescribe the energy intake, the daily energy expenditure (DEE) was calculated by multiplying the measured resting metabolic rate by an activity factor of 1.4 and then 2930 kJ was subtracted from DEE. Macronutrient composition of the diet was assessed by a 3-day food record, and this composition was maintained during the weight-loss program.

SMR was measured in all subjects using a whole-body indirect calorimeter. The description and principles of operation of the metabolic chamber used in this study were described in detail by White et al.⁹ SMR was determined as the mean of the two consecutive hours of the night when the subject had the lowest oxygen consumption. Body composition was also determined in all subjects using the hydrostatic weighing technique, as described elsewhere.⁴

The concentrations of fasting plasma leptin, serum total tri-iodothyronine (T₃) and free thyroxine (T₄) and plasma OC (11 chlorinated pesticides (beta-hexachlorocyclohexane (-HCH), p,p'-dichlorodiphenyltrichloroethane (p,p'-DDT), p,p'-dichlorodiphenyldichloroethane (p,p'-DDE), hexachlorobenzene (HCB), mirex, aldrin, -chlordane, -chlordane, oxychlordane, cis-nonachlor, trans-nonachlor), 14 polychlorinated biphenyls (PCBs) (IUPAC nos. 28, 52, 99, 101, 105, 118, 128, 138, 153, 156, 170, 180, 183, 187), and one commercial mixture of PCBs (Aroclor 1260)) were determined before and after weight loss in obese subjects, as previously described.⁴ The concentrations of OCs are expressed in g/kg of lipids, to correct for the differences in total plasma lipids between individuals. Total plasma OC concentration was calculated as the sum of all detected compound concentrations.

Statistical analysis

Control and obese subjects (tested before the weight-loss program and in a reduced-obese state) were compared for age, weight, BMI, percentage of body fat, FM, FFM, and SMR by means of a one-way ANOVA, whereas values at baseline (obese) and post weight loss (reduced obese) were compared by means of a MANOVA for repeated measures to assess the effect of time, medication (fenfluramine or placebo), gender and their interactions on these variables as well as on plasma leptin, total T₃ and free T₄ concentrations, and on the total plasma OC concentration. As no interaction of time medication or time sex was observed (except for total T₃, for which a time medication sex and a time sex interactions were observed, P<0.05), all individuals were pooled together. Multiple regression analysis was performed in the control group with SMR as the dependent variable and gender, age, FM and FFM as predicting variables, and the following equation was retained to predict the SMR:

This equation is similar to that established with another large cohort of subjects by Doucet et al.⁸ The equation was validated using the same approach as that of Doucet et al.⁸ Control subjects were divided on the basis of their BMI (<25, 25-30 or >30 kg/m²) and predicted SMRs calculated by the equation were compared to the measured SMRs within each class of BMI, by means of a paired t-test. No significant differences between predicted and measured SMRs were observed in any of the classes. The equation is thus equally applicable for subjects of varying adiposity. The equation was then used to calculate the predicted SMR at baseline and post weight loss in the obese subjects. Post weight loss values of predicted and measured SMR were compared to their respective baseline values by means of a paired t-test. Paired t-tests were also used to compare the predicted to measured values of SMR at baseline, post weight loss, as well as the change in SMR during weight loss. A stepwise multiple regression analysis was then performed to examine which factors were associated with the difference between predicted and measured changes in SMR (estimate of adaptive thermogenesis) during weight loss. Results are presented as meansstandard deviation, and were considered as statistically significant at P0.05.

Results and discussion

The characteristics of the subjects are shown in Table 1. At the opposite of what has been observed in other studies,^{8, 10, 11} no effect of the interaction time sex was observed, maybe due to the small number of subjects. As expected, both predicted and measured SMRs in obese individuals were significantly decreased by the weight-reducing program. Figure 1 shows that the mean measured SMR was significantly higher than the value predicted from the data of control subjects before the weight-reducing program. However, once obese individuals had reached a reduced-obese state, their measured SMR was no longer significantly different from their predicted value. The adaptive component of thermogenesis was estimated by calculating the difference between changes in predicted and in measured SMR induced by weight loss (Figure 1). This significant difference (399615 kJ/day) was further used as the dependent variable in a stepwise multiple regression analysis (Table 2) in which fasting plasma leptin, total T₃, free T₄, and total plasma OC concentrations after weight loss as well as their changes induced by weight loss were used as predicting variables. As the change in plasma OC concentration is associated with the loss of FM,² and since Dulloo and Jacquet¹² have reported that fat depletion (loss in FM as a % of initial FM) is a determinant of the adaptive fall in basal metabolic rate during weight loss, fat depletion was correlated with the change in plasma OC concentration. This correlation was significant (r²=0.37, P<0.05), and fat depletion was thus included as the predicting variable in the analysis. As shown in Table 2, the change in total plasma OC concentration was the main factor explaining deviations in SMR from predicted values in response to weight loss (r²=0.47). Entering the change in plasma leptin concentration in the model explained an additional 20% of the variance (r²=0.67). When the contribution of the concentration in plasma leptin after weight loss was added to these two factors, an additional 13% of the variance in the deviation of SMR was explained (r²=0.80). Thus, taken together, these three factors explained 80% of the variance of the adaptive decrease in thermogenesis during weight loss.

Figure 1.

Illustration of the changes in predicted and measured SMR during weight loss in the obese subjects. predicted SMR, measured SMR, ^*significantly different from baseline value (P< 0.001), ^†significantly different from predicted values (P<0.05); ^††P<0.001.

Full figure and legend (21K)

Table 1 - Characteristics of subjects.

Full table

Table 2 - Stepwise multiple regression analysis examining factors associated with the difference between predicted and measured changes in sleeping metabolic rate in response to weight loss.

Full table

The main preoccupation of this study was to examine the potential role of OCs in variations of thermogenesis in obese individuals. This was performed by measuring the SMR, which is a well-standardized phenotype whose variations are not affected by voluntary movement. This measurement allowed the characterization of an adaptive component of thermogenesis, which represented the difference between predicted and measured changes in SMR during weight loss. As indicated above, this component was substantial and was about equivalent to what was estimated by other investigators as a proxy of adaptive thermogenesis.^{7, 8} In addition, the variance of thermogenesis was also quantitatively important, which offered a relevant context of investigation of factors predicting the greater than expected decrease in energy expenditure during body weight loss. In this regard, it is interesting to note that the combination of variables considered in the stepwise multiple regression analysis explained 80% of the variance in adaptive thermogenesis, which provides a good indication of the consistency of the data obtained in the present study.

The main finding of this study was that the changes in plasma OC concentration were the main predictor of adaptive thermogenesis, and explained about 50% of its variance. As discussed above, this is not a surprise since these compounds produce significant alterations of mitochondrial activity^{6, 13} and of the thyroid function.^{14, 15} Therefore, OC pollution should be perceived among potential factors influencing adaptive thermogenesis in obese individuals experiencing a substantial weight loss.

References

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Acknowledgements

This research was supported by Servier Canada. Angelo Tremblay is partly funded by the Canada Research Chair in Physical Activity, Nutrition and Energy Balance.