Insulin resistance (IR), known as a characteristic trait of type 2 diabetes (24), is associated with obesity (19) and aging (13). Weight loss and increased regular physical activity is currently the first line of standard care. The combination of a moderate weight loss with physical activity has been shown to be effective in reducing the incidence of type 2 diabetes (11,18) and to improve insulin sensitivity (16). However, weight loss- and exercise-induced improvements in IR are variable; some individuals experience robust enhancements in insulin sensitivity, whereas others do not. In a previous study (16), we found that 8% of subjects did not improve insulin sensitivity after a weight loss and exercise program.
Subclinical hypothyroidism (sHT) is essentially a biochemical diagnosis with an elevation in serum thyroid-stimulating hormone (TSH) with normal levels of thyroid hormones (10). The worldwide prevalence of subclinical hypothyroidism ranges from 1% to 20% depending on age, sex, and iodine intake. In a large cross-sectional study in Colorado (7), the mean prevalence was reported as 9% but was higher in women. Aging is also associated with an increased prevalence of subclinical hypothyroidism (5,7,14). In subjects older than 60 yr, the prevalence was approximately 15% for women and 8% for men (7). Although the exact cutoff point for the definition of subclinical hypothyroidism and its management are still controversial among endocrinologists (21,26-28), many adverse associations between subclinical hypothyroidism or a TSH level in the upper part of the reference range and components of the metabolic syndrome have been found. These include an association with dyslipidemia (4,8,23), with systolic and diastolic high blood pressure (3), and with increased insulin resistance measured by the HOMA index (23). Altered substrate metabolism in sHT patients, has also been described during an incremental bout of exercise (9).
Although thyroid hormone status is related to IR, it is not clear whether a TSH in the upper part of the reference range may help to explain the variability in weight loss- or exercise-induced improvements in IR. The purpose of this study was to examine whether thyroid hormone status was associated with the improvement in insulin sensitivity and physical fitness after 4 months of weight loss and exercise training in impaired glucose-tolerant (IGT) obese subjects. We hypothesized that subclinical hypothyroidism may undermine the beneficial adaptations to lifestyle modifications, blunting the improvement of insulin sensitivity by weight loss and exercise training.
Study design, subjects, and intervention.
All subjects included in this nested case-control analysis were enrolled in our ongoing clinical trial, which is a 16-wk pre/post intervention that includes exercise and weight loss. Volunteers were included if they were in good health, were without recent illnesses, were currently not engaged in a regular program (≤1 d·wk−1 of continuous physical activity), were aged 30 to 75 yr, had impaired glucose tolerance (2 h Oral Glucose Tolerance Test (OGTT) >140 mg·dL−1 but <200 mg·dL−1), were overweight or obese (body mass index (BMI) = 25-38 kg·m−2), and were weight stable (±3 kg) for at least 6 months before the study. Subjects were excluded if they had a history of type 2 diabetes, coronary heart disease, peripheral vascular disease, uncontrolled hypertension, and were taking chronic medications known to affect glucose homeostasis. TSH was measured among other screening tests and volunteers were excluded if they had overt hypothyroidism (TSH >8 μIU·mL−1). The protocol was approved by the University of Pittsburgh Institutional Review Board. All volunteers gave written informed consent.
The exercise training protocol consisted of 16 wk of moderate-intensity supervised aerobic exercise. Subjects were asked to engage in three to five sessions per week with at least three sessions supervised in our facility. Each session was 30-45 min in duration. Subjects could walk, bike, or row. The intensity of their workout was monitored by the use of HR monitors (Polar Electro Oy, Kempele, Finland). Moderate intensity was defined as 75% of their peak HR. The exercise prescription was based on the subject's individual peak HR achieved during graded exercise tests conducted at baseline and adapted at the midpoint of the intervention with a submaximal ergometer test. Exercise logs and HR monitors were also used for the unsupervised sessions to quantify exercise. Adherence to the exercise program was evaluated using the average duration of the exercise sessions, average HR per session, and average sessions per week. These measures allowed us to estimate the average energy expended per session during the 16 wk of intervention as previously described (22).
The weight loss program consisted in a deficit of 500 kcal·d−1 of their habitual diet with less than 30% of calories from fat. Subjects met weekly with our registered dietitian. All subjects were maintained weight stable for the last 2 wk before the postintervention measurements.
Definition of cases and controls.
Cases and controls were defined by transforming the baseline TSH level in a dichotomous variable as follows: sHT cases >3-8 μIU·mL−1 and controls 0.5-3 μIU·mL−1. Cases were found, then controls were matched on gender, BMI (±2.5 kg·m−2), and age. This process was performed by the first author who was blinded to the outcome data until the selection of cases and controls was complete.
The cutoff point for the transformation of baseline TSH was chosen on the basis of a recent cross-sectional population-based study (3) and on the National Academy of Clinical Biochemistry Guidelines (21). Owing to the ongoing controversy in clinical endocrinology (27), we acknowledge the different opinions regarding the cutoff points and decided to opt for 3 μIU·mL−1, thus choosing the more statistically conservative approach. If a difference was to be seen with a lower cutoff, then indeed the difference would be robust. Also, for the clarity of the text, we named the cases as "subclinical hypothyroidism" and the controls as "euthyroid." However, we understand that the subclinical hypothyroidism group could be seen as a group with serum TSH in the upper level of the reference range.
Insulin sensitivity was determined by the glucose infusion rate (GIR) of the last 30 min of steady state of a 4-h hyperinsulinemic (40 mU·m−2·min−1) euglycemic clamp as previously described (16). On the evening before the clamp, subjects were admitted in the Clinical and Translational Research Center. They received a standard dinner (7.5 kcal·kg−1 of body weight; 50% carbohydrate, 30% fat, and 20% protein) and were then fasted until completion of the glucose and insulin infusion. For the postintervention assessment, the clamp was performed 36-48 h after the last exercise session to avoid the acute effects of exercise on insulin sensitivity. The GIR reflects mostly skeletal muscle insulin sensitivity because it is assumed that hepatic glucose production is nearly completely suppressed at this insulin infusion rate.
Weight was measured on a calibrated medical digital scale (BWB-800; Tanita Corporation, Tokyo, Japan) in undergarments. Height was measured at the same time with a wall-mounted stadiometer. Body mass index (BMI) was calculated as weight (kg) divided by square height (m2). Body composition, including fat mass (FM), fat-free mass (FFM), and percent of body fat (BF), was assessed by dual-energy x-ray absorptiometry (Lunar Prodigy and enCORE 2005 software version 9.30; GE Healthcare, Milwaukee, MI). FFM was used to express physical fitness and insulin sensitivity in relative units.
Physical fitness was determined by the peak aerobic capacity (V˙O2peak) measured using a graded exercise protocol on an electronically braked cycle ergometer (Ergoline 800S; Sensormedics, Yorba Linda, CA) as described previously (22). HR, blood pressure, and ECG were recorded before, during, and after the exercise test. Oxygen consumption was computed via indirect calorimetry (Moxus; AEI Technologies, Pittsburgh, PA).
TSH was obtained in the morning after an overnight fast and was processed through standard hospital-certified laboratory protocols at the Special Chemistry Laboratory (Department of Pathology, University of Pittsburgh, PA) with an automated platform using the chemoluminescence technique (Centaur; Siemens Medical Solutions, Tarrytown, NY). Plasma glucose during the screening OGTT and the glucose clamp were measured using an automated glucose oxidase reaction (YSI 2300 Glucose Analyzer; YSI Inc, Yellow Springs, OH).
Although nested case-control studies are considered strong observational studies (15) due to the fact that selection biases are reduced, we first explored the data with statistical tests specific for small sample sizes (nonparametric test). These include the Wilcoxon signed rank test (within-group comparison) and the Mann-Whitney U test (between-group comparison). After assessing the normality of the data (Lilliefors test of normality) and possible outliers, we carried out parametric tests. These included independent t-tests to assess baseline differences between groups and exercise adherence parameters. A 2 × 2 repeated-measures analysis of variance was performed on the dependent variables as a function of group (two levels: sHT and controls) and time (two levels: before and after intervention). The assumptions of compound symmetry were checked with the Box M and the Mauchly test.
Data are presented as mean and SEM. P values reported in the results are those of the parametric tests. Nonparametric tests are detailed in Figure 1. The α level was set a priori at 0.05. All the analyses were performed in a 2-tailed approach using Stata for Windows, version 9.2 (StataCorp, College Station, TX) and SPSS for Windows, version 13.0 (SPSS Inc, Chicago, IL).
Among the subjects that adhered to the exercise intervention (more than two supervised exercise sessions per week), eight cases were found, that is, five females and three males. Their mean TSH was 3.96 ± 0.56 μIU·mL−1, with a range from 3.06 to 7.66 μIU·mL−1. Controls (1:1) were matched as described above. The mean TSH of controls was 1.75 ± 0.31 μIU·mL−1, with a range from 0.71 to 2.79 μIU·mL−1. The baseline characteristics for the cases and controls can be found in Table 1. No significant differences were found between groups at baseline for any of the variables (weight, BMI, FM, FFM, BF, physical fitness, and insulin sensitivity).
Improvements with intervention.
No significant differences were found between groups on the average duration of exercise performed per session, the average intensity performed per session (percent of maximal HR), the average number of sessions per week, and the average calories expended per session (Table 2).
Percent changes in insulin sensitivity, weight, and V˙O2peak are presented in Figure 1. The improvement in insulin sensitivity was significantly greater in the controls compared to the cases (interaction effect, P = 0.037). Both the cases and the controls lost a similar and significant amount of BMI (and weight) with intervention (main effect of time, P < 0.001) without a significant difference between the two groups. Physical fitness tended to increase in the controls but not in the cases (interaction effect, P = 0.07).
Physical activity and moderate weight loss are advocated for the treatment of obesity and insulin resistance in subjects with impaired glucose tolerance at risk for type 2 diabetes. Unfortunately, improvements in insulin resistance are variable; although most individuals experience robust enhancements in insulin sensitivity, some do not. In our study, insulin sensitivity significantly improved in the euthyroid group but not in the subclinical hypothyroid subjects. Physical fitness determined by V˙O2peak tended to improve in the euthyroid group but not in the hypothyroid subjects. This was despite the fact that both groups lost a significant amount of weight with the intervention. The findings of this nested case-control study support the hypothesis that subclinical hypothyroidism may in part explain the variability in improvements in insulin resistance and physical fitness that typically occur with weight loss and increased physical activity.
The differential response in the improvement in insulin sensitivity was not explained by the degree of weight loss or FM. Although not measured in this study, the possibility remains that changes in regional fat distribution, particularly changes in abdominal adipose tissue, could explain part of this difference. Subjects exercised at a similar intensity, with a similar duration of exercise per session and similar number of sessions per week. The estimated energy expenditure per exercise session was similar in both groups. Therefore, exercise adherence was similar, and we do not have evidence that thyroid status influenced the overall energy expenditure during exercise. The lack of significance in the improvement in physical fitness (V˙O2peak) is likely due to the rather large variability in this response to moderate increases in physical activity. Although in the larger study we did not observe a relationship between the change in physical fitness and the change in insulin sensitivity, in this small nested case-control sample, a significant positive relationship was found: R2 = 0.30, P = 0.03. Thus, the blunted response in V˙O2peak may be one factor for the blunted response in insulin sensitivity.
This efficacy-oriented study was not designed to take into account any generalizability element because of the relatively small sample sizes. The purpose was only to examine the internal validity of our hypothesis on the basis of physiological markers. Thus, we only included subjects that were adherent to the exercise regimen and defined the cases and the controls using solely the dichotomous transformation of the baseline TSH level without taking into account any prior diagnosis or treatment of thyroid disease. TSH measurement in our healthy and asymptomatic volunteers was performed only for screening purposes (12). In individuals with an intact hypothalamic-pituitary axis, serum TSH is more sensitive than free T4 for detecting sHT (21). Thus, we did not measure serum thyroid hormones levels or thyroid antibodies. We acknowledge this as a limitation, as well as the ongoing controversy among endocrinologists regarding the exact definition (serum TSH cutoff point) and treatment of subclinical hypothyroidism (2). Our study is not intended to show that a lower cutoff point is needed to define subclinical hypothyroidism; it supports the hypothesis of an association between thyroid status and exercise-induced changes in insulin resistance.
The potential mechanisms that could explain this blunted effect on muscle metabolism are not yet clearly identified. Prior studies in animal models showed that thyroid hormones play a role in the regulation and activation of signaling pathways, such as AMPK (6), on insulin receptors (20) and glucose transporter proteins (29), and affect the expression of different isoforms of skeletal muscle myosin heavy chains (1). In hypothyroidism, skeletal muscle structural, biochemical, and electromyographic changes have been reported (17) as well as mitochondrial alterations (25). It is not known if these skeletal muscle alterations already exist in subclinical hypothyroidism; if present, these may potentially limit exercise-induced changes in skeletal muscle metabolism and peripheral insulin sensitivity. Further investigations are clearly needed to identify mechanisms underlying the attenuation or elimination of improved insulin sensitivity and physical fitness with weight loss and exercise associated with thyroid status.
In summary, despite similar amounts of exercise and weight loss, insulin sensitivity and physical fitness improved in euthyroid but not in subclinical hypothyroid subjects. Therefore, subclinical hypothyroidism may interfere with beneficial adaptations on metabolic risk factors that typically occur with weight loss and increased physical activity. Due to the important prevalence of this condition particularly in women and in the aging population, this observation may have considerable clinical implications. Thyroid status could explain why some individuals do not seem to respond to traditional lifestyle modification programs that include diet and exercise. Further studies are needed to corroborate this observation, to explore potential mechanisms for this effect, and to investigate whether treatment with thyroid hormone replacement may restore the ability to respond to lifestyle modifications.
Sources of support: ADA clinical Research Award (B.H.G.), NIH R01 (AG20128), NIH GCRC (5M01RR00056), and Obesity Nutrition Research Center (1P30DK46204).
The authors thank the volunteers for the participation in this study. The authors also acknowledge the valuable contributions of Krista Clark for directing the diet-induced weight loss programs.
Disclaimers: Part of this work has been presented at the annual meeting of the Mid-Atlantic Regional Chapter of the American College of Sports Medicine in November 2006 in Harrisburg, PA, and at the annual scientific meeting of the American Diabetes Association in June 2007 in Chicago, IL.
The authors have nothing to disclose. The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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