* Physical Activity Sciences Laboratory, PEPS, Laval University, Ste-Foy, Québec G1K 7P4, Canada;
School of Human Kinetics, 125 University Street, Ottawa University, Ottawa, Ontario K1N 6N5, Canada; and
Division of Kinesiology, PEPS, Laval University, Ste-Foy, Québec G1K 7P4, Canada
Received August 1, 2001; accepted November 9, 2001
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ABSTRACT |
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Key Words: obesity; weight loss; organochlorines; PCB; thyroid hormones; energy expenditure.
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INTRODUCTION |
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Organochlorines may have many adverse effects in animals and humans. Among these effects, the impairment of thyroid function has been reported. Indeed, evidence of goiters and diminution in plasma concentration of thyroid hormones were observed in Coho salmon (Oncorhynchus kisutch) (Barsano, 1981) and in rats (Barsano, 1981
; Bastomsky, 1977
; Byrne et al., 1987
) exposed to PCBs. In humans, mothers displayed inverse correlations between PCB concentrations in the maternal milk and plasma level of triiodothyronine (T3) and thyroxine (T4). PCB concentrations in the maternal milk were inversely correlated to plasma T4 level in their infants (Koopman-Esseboom et al., 1994
). Many mechanisms seem to be implicated in the diminution of thyroid hormones concentration by organochlorines. Collins et al. (1977) reported ultrastructural lesions in thyroid follicular cells of rats fed with PCBs, altering the synthesis and secretion of T4. Organochlorines compete for the receptor (Cheek et al., 1999
) and for transport proteins of thyroid hormones (Bastomsky, 1974
). Moreover, organochlorines are microsomal enzyme inducers and can increase the catalytic activity toward T4 of the hepatic enzyme uridine diphosphate-glucuronyltransferase (UDPGT), which is responsible for the glucuronidation of thyroid hormones. They increase the glucuronidation of T4 and so its excretion in bile (Barter and Klaassen, 1992
, 1994
; Bastomsky and Murthy, 1976
; Van Birgelen et al., 1995
).
Serum concentration of thyroid hormones decreases during energy restriction (Danforth and Burger, 1984; Jung et al., 1980
; Vagenakis et al., 1977
; Wimpfheimer et al., 1979
). Thyroid hormones are implicated in the control of resting metabolic rate (RMR) (Danforth and Burger, 1984
; Jéquier, 1984
). Energy expenditure decreases during body weight loss through a reduction of fat-free mass (FFM), fat mass (FM), insulin, sympathetic nervous system (SNS) activity, and leptin (Doucet et al., 2000
; Van Gaal et al., 1992
). However, the contribution of all these factors does not explain the total reduction of energy expenditure in response to weight loss, suggesting that other factors might be involved. Recently, Rosenbaum et al. (2000) found a correlation between changes in thyroid hormone concentrations and energy expenditure in response to weight changes. Moreover, Toubro et al. (1996) reported that even small physiological differences in T3 concentration are implicated in the variation of 24-h energy expenditure among humans. The main aim of this study was to determine if an increase in plasma concentration of organochlorines may affect serum T3 concentration and RMR. However, the only ethical way (if any) to increase plasma organochlorine concentration in humans is weight loss, which is already known to decrease T3 concentration and RMR by various factors. The residuals of changes in serum T3 and RMR that were not explained by weight loss were associated with changes in plasma organochlorine concentrations to evaluate the potential effect of these compounds without the effect of changes in confounding variables.
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MATERIALS AND METHODS |
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It is important to note that following the withdrawal of fenfluramine and dexfenfluramine from the market because of their effects on myocardial valve function (Khan et al., 1998; Weissman et al., 1998
), all subjects (including placebos) were subjected to an echocardiogram. Following this measure, a detailed analysis of valve function was performed by cardiologists who detected no abnormalities in response to the use of fenfluramine under these conditions (Prud'homme et al., 1999
).
RMR and anthropometric measurements.
RMR was determined by indirect calorimetry after an overnight fast. Following a 15-min resting period, expired gases were collected through a mouthpiece for 15 min while the subject had his nose clipped. A nondispersive infrared analyzer (Uras 10 E, Hartmann & Braun, Frankfurt, Germany) was used to measure the oxygen and carbon dioxide concentrations. The pulmonary ventilation was determined with an S-430A measurement system (Ventura, CA). The energy equivalent of oxygen volume was calculated by the Weir formula (Weir, 1949).
Body density was measured by the hydrostatic weighing technique, and the Siri equation (Siri, 1956) was used to determine the percentage of body fat. The pulmonary residual volume required for this measurement was determined by the helium dilution technique (Meneely and Kaltreider, 1949
). The percentage of body fat was multiplied with body weight to calculate total fat mass, which was subtracted from body weight to obtain fat-free mass. All measurements were performed before and after weight loss.
Determination of plasma organochlorine concentrations.
The concentration of 11 chlorinated pesticides and metabolites (ß-hexachlorocyclohexane (ß-HCH), p,p'-DDT, p,p'-DDE, hexachlorobenzene (HCB), mirex, aldrin, -chlordane,
-chlordane, oxychlordane, cis-nonachlor, trans-nonachlor), 14 PCBs (IUPAC nos. 28, 52, 99, 101, 105, 118, 128, 138, 153, 156, 170, 180, 183, and 187) and one commercial mixture of PCBs (Aroclor 1260) were determined in plasma before and after weight loss at the Québec Toxicology Center. Blood samples were centrifuged to extract plasma (2 ml), which was cleaned up on deactivated Florisil columns. Samples were eluted with methylene chloride:hexane (25:75, v/v) and concentrated to a final volume of 100 µl. Samples were then analyzed on an HP-5890 series II gas chromatograph with dual-capillary columns and dual 63Ni electron detectors. Peaks were identified by their relative retention times obtained on the two columns using a computer program developed by the Québec Toxicology Center. Depending on the compounds, detection limits varied from 0.02 to 0.2 µg/l. Plasma lipid concentration was also determined. This measurement was performed by enzymatic methods on a Technicon automatic analyzer (RA-500) with test packs: Randox for total cholesterol (TC) and triglycerides (TG), BMC for free cholesterol (FC), and Wako for phospholipids (PL). Plasma total lipids were calculated with the equation recommended by Patterson et al. (1991):
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Plasma leptin, serum total T3 and free T4 concentrations.
Fasting plasma leptin concentrations were determined with a highly sensitive commercial double antibody RIA (Human Leptin Specific RIA Kit, Linco Research, Inc., St. Louis, MO), which has a low detection limit of 0.5 ng/ml and does not cross-react with human insulin, pro-insulin, glucagon, pancreatic polypeptide, or somatostatin. Serum total T3 and free T4 concentrations were determined by heterogeneous competitive immunoassay (Bayer Immuno 1TM System, Bayer Corporation, Tarrytown, NY). Detection limits were 0.09 nmol/l and 1.3 pmol/l for total T3 and free T4, respectively.
Statistical analysis.
The deltas were equal to the values post-weight loss minus pre-weight loss. A two-way ANOVA for repeated measures was used to evaluate the impact of time, treatment (fenfluramine or placebo) and their interaction on weight, BMI, fat mass, fat-free mass, RMR, leptin, total T3, free T4 and organochlorine concentrations. Pearson correlation coefficients were calculated to measure the associations between changes in plasma organochlorine concentrations and those in T3 concentration and in RMR, not adjusted and adjusted for body weight loss, during the weight-reducing program. Pearson correlation coefficient was also calculated between changes in T3 concentration and those in RMR during weight loss. As many factors are known to affect RMR, a stepwise multiple regression analysis was used to determine which factors contribute to variation in RMR before, during, and after weight loss. Results are presented as means ± standard deviation and were considered statistically significant at p 0.05.
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RESULTS |
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DISCUSSION |
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The increase in organochlorine concentrations was also associated with the decrease in RMR during weight loss. Thyroid hormones are implicated in the control of energy expenditure, which agrees with our observation that serum T3 concentration explained a significant portion of the variance in RMR in weight-stable obese men before the weight loss program. However, during the weight-reducing program, changes in T3 concentration and those in RMR were not significantly correlated. This is consistent with the results of Rosenbaum et al. (2000), who found a significant correlation between these two variables only when they pooled the data from subjects who gained weight (increasing their T3 concentration and RMR) and those from subjects who lost weight (decreasing their T3 concentration and RMR). In the present study, the absence of association between changes in T3 concentration and those in RMR was predictable, as T3 did not contribute to the variation of RMR after weight loss. It has been reported by Wimpfheimer et al. (1979) that the sensitivity of RMR to T3 was reduced by starvation. Furthermore, beyond the reduction in T3 concentration, fasting also decreases the capacity of the nuclear T3 receptors to bind the hormone (Schussler and Orlando, 1978). These two findings could provide an explanation for the absence of relation between T3 concentration and RMR after weight loss. In spite of the absence of association between changes in T3 concentration and those in RMR, changes in plasma organochlorine concentrations were the best predictor of changes in RMR observed during weight loss. Changes in organochlorine concentrations explained a greater part of the variation in RMR than other factors known to affect RMR during weight loss, such as body composition and plasma leptin. This is not discordant with the results of Doucet et al. (2000), who documented in these subjects that changes in leptin were an important predictor of changes in RMR in response to weight loss. However, the percentage of variation in RMR during weight loss explained by leptin in the study by Doucet et al. (2000) (28%) was lower than the percentage of variation in RMR explained by changes in organochlorine concentrations in the present study (32%).
The clinical significance of the association between changes in plasma organochlorine concentrations and changes in RMR is as yet unknown. The release of these compounds from body fat to bloodstream could be associated with the weight regain which more often than not occurs after weight loss (Weinsier et al., 1995). Previous studies in animals a priori fed with contaminated food and then put on starvation demonstrated that organochlorines redistributed in many tissues after weight loss may exert their adverse effects (Bigsby et al., 1997
; Dale et al., 1962
; Ecobichon and Saschenbrecker, 1969
). As proposed by Chevrier et al. (2000) and Tremblay et al. (1999), body fat accumulation can be a means of protection against the adverse effects of organochlorines. Although speculative, the weight-loss-induced increase in organochlorine concentrations might facilitate positive energy balance through a decrease in RMR and consequently promote body fat accumulation. As a result of the high affinity of organochlorines for adipose tissue, body fat accumulation would increase the storage compartment of these compounds, keeping them away from their target organs. However, toxicity of the concentration of organochlorines released from fat stores during weight loss can not yet be assessed. Because weight loss reduces risks of cardiovascular diseases, diabetes, and hypertension, known benefits of weight loss treatments are still considered as greater than possible health risks that would be due to organochlorines in the reduced-obese state. However, even if not representing health risks, small physiological effects of organochlorines such as the relationships observed in this study could complicate weight loss treatment, and thus favor a smaller body weight loss or weight regain.
An evident limitation of this study is that the increase in plasma organochlorine concentration was promoted by weight loss. Many factors such as fat mass, fat-free mass, and leptin concentration are also changing during weight loss and are known to decrease serum T3 concentration and RMR. Precautions were taken in this analysis to eliminate the effect of body weight loss on serum T3 concentration and RMR. However, it is still possible that confounding variables may explain a part of the relationships observed in this study. The small sample size is another limitation of this study. The correlations obtained give preliminary results that suggest the existence of a relationship between changes in organochlorine compound concentrations, serum T3 concentration, and RMR that are promoted by weight loss. However, further studies are needed to verify whether these observations are causally related and to investigate the mechanisms by which organochlorines may affect RMR. Apart from the decrease in serum T3 concentration, organochlorines may also interfere with T3 receptor and thus alter RMR. The results of the present study also suggest that the release of organochlorines from body fat during weight loss should be considered when treating obesity.
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ACKNOWLEDGMENTS |
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NOTES |
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