Elliot Danforth

Dietary-induced alterations in thyroid hormone metabolism during overnutrition.

Diet-induced alterations in thyroid hormone concentrations have been found in studies of long-term (7 mo) overfeeding in man (the Vermont Study). In these studies of weight gain in normal weight volunteers, increased calories were required to maintain weight after gain over and above that predicted from their increased size. This was associated with increased concentrations of triiodothyronine (T3). No change in the caloric requirement to maintain weight or concentrations of T3 was found after long-term (3 mo) fat overfeeding. In studies of short-term overfeeding (3 wk) the serum concentrations of T3 and its metabolic clearance were increased, resulting in a marked increase in the production rate of T3 irrespective of the composition of the diet overfed (carbohydrate 29.6 +/- 2.1 to 54.0 +/- 3.3, fat 28.2 +/- 3.7 to 49.1 +/- 3.4, and protein 31.2 +/- 2.1 to 53.2 +/- 3.7 microgram/d per 70 kg). Thyroxine production was unaltered by overfeeding (93.7 +/- 6.5 vs. 89.2 +/- 4.9 microgram/d per 70 kg). It is still speculative whether these dietary-induced alterations in thyroid hormone metabolism are responsible for the simultaneously increased expenditure of energy in these subjects and therefore might represent an important physiological adaptation in times of caloric affluence. During the weight-maintenance phases of the long-term overfeeding studies, concentrations of T3 were increased when carbohydrate was isocalorically substituted for fat in the diet. In short-term studies the peripheral concentrations of T3 and reverse T3 found during fasting were mimicked in direction, if not in degree, with equal or hypocaloric diets restricted in carbohydrate were fed. It is apparent from these studies that the caloric content as well as the composition of the diet, specifically, the carbohydrate content, can be important factors in regulating the peripheral metabolism of thyroid hormones.

Thermic effect of infused glucose and insulin in man. Decreased response with increased insulin resistance in obesity and noninsulin-dependent diabetes mellitus.

The thermic effect of infused glucose and insulin was measured by combining the hyperinsulinemic euglycemic clamp technique with indirect calorimetry, in 10 normal weight volunteers (group I), 7 obese subjects with normal glucose tolerance (group II), and 13 obese subjects with abnormal glucose tolerance or noninsulin-dependent diabetes mellitus before (group IIIa) and after weight loss of 10.8 +/- 0.4 kg (group IIIb). During hyperinsulinemia (760-1,100 pmol/liter), total glucose disposal from combined endogenous production and glucose infusion was 545 +/- 49, 441 +/- 70, 233 +/- 35, 231 +/- 31 mg/min and energy expenditure changed by + 0.476 +/- 0.080, +0.293 +/- 0.095, -0.114 +/- 0.063, and +0.135 +/- 0.082 kJ/min in group I, II, IIIa, and IIIb, respectively. The increased energy expenditure correlated with glucose storage (measured cost of processing the glucose: 1.33 kJ/g). In group IIIa there was no increase in energy expenditure in response to glucose and insulin infusions. After therapy (group IIIb) there was a significant recovery (P less than 0.05) of the thermic effect of infused glucose although total glucose disposal was unchanged. It is proposed that the recovered thermic effect of infused insulin/glucose is due to the different contributions of gluconeogenesis in the fasting state and during the glucose clamp before and after weight loss. In addition we hypothesize that some of the lower thermic effect of food reported in obese noninsulin-dependent diabetics may be explained by decreased energy expenditure due to a greater suppression of hepatic gluconeogenesis as well as by lower storage rate.

Evidence that insulin resistance is responsible for the decreased thermic effect of glucose in human obesity.

The thermic effect of glucose was investigated in nine obese and six lean subjects in whom the same rate of glucose uptake was imposed. Continuous indirect calorimetry was performed for 240 min on the supine subject. After 45 min, 20% glucose was infused (609 mg/min) for 195 min and normoglycemia was maintained by adjusting the insulin infusion rate. At 2 h, propranolol was infused (bolus 100 micrograms/kg; 1 microgram/kg X min) for the remaining 75 min. To maintain the same glucose uptake (0.624 g/min), it was necessary to infuse insulin at 3.0 +/- 0.6 (leans) and 6.6 +/- 1.2 mU/kg X min (obese) (P less than 0.02). At this time, glucose oxidation was 0.248 +/- 0.019 (leans) and 0.253 +/- 0.022 g/min (obese) (NS), and nonoxidative glucose disposal was 0.375 +/- 0.011 and 0.372 +/- 0.029 g/min, respectively. Resting metabolic rate (RMR) rose significantly by 0.13 +/- 0.02 kcal/min in both groups, resulting in similar thermic effects, i.e., 5.5 +/- 0.7% (leans) 5.4 +/- 0.9% (obese) (NS) and energy costs of glucose storage 0.35 +/- 0.06 and 0.39 +/- 0.09 kcal/g (NS), respectively. With propranolol, glucose uptake and storage remained the same, while RMR fell significantly in both groups, with corresponding decreases (P less than 0.05) in the thermic effects of glucose to 3.7 +/- 0.6% and 2.9 +/- 0.8% (NS) and the energy costs of glucose storage 0.23 +/- 0.04 and 0.17 +/- 0.05 kcal/g (NS) in the lean and obese subjects, respectively. These results suggest that the defect in the thermic effect of glucose observed in obese subjects is due to their insulin resistance, which is responsible for a lower rate of glucose uptake and hence decreased rate of glucose storage, which is an energy-requiring process.

Metabolic studies in human obesity during overnutrition and undernutrition: Thermogenic and hormonal responses to norepinephrine

Overfeeding increases the thermogenic response of norepinephrine (NE) in normal but not in certain genetically obese rodents. It has been suggested that human obesity may be associated with a similar thermogenic defect. To determine whether there are differences in the thermogenic sensitivity to NE in human obesity, energy expenditure in response to graded infusions of NE (0.05, 0.10, 0.15, 0.20 micrograms/min/kg fat-free mass) was measured in six lean and six obese subjects (9.5 +/- 1.8 v 36.3 +/- 3.8% body fat P less than 0.005). Resting metabolic rate (RMR), thermogenic response to NE, and thermogenic response to exercise were measured during weight maintenance and during the third week of feeding 1000 extra Kcal/d in the lean and obese subjects. These components of energy expenditure were also measured in the obese subjects during the third week of a 589 Kcal/d diet. Resting metabolic rate increased during overfeeding in lean (6.6%, P less than 0.05) but not in the obese subjects (2.7%, P = NS) and fell during underfeeding in the obese (-9.1%, P less than 0.02). There was a logarithmic increment above baseline in VO2 v plasma NE concentration during the NE infusions (r = 0.75, P less than 0.005) in lean subjects which was unaltered by overfeeding. The obese exhibited equivalent VO2 responses to NE to that measured in the lean. Supine plasma NE concentrations were lower but metabolic clearance rates (MCR) of NE were similar in the obese compared to lean subjects during both weight maintenance and overfeeding. Overfeeding minimally increased plasma concentration but not MCR of NE in both groups. The thermogenic response to exercise was similar in the lean and obese subjects and was unaltered by overfeeding or underfeeding. The increments in plasma glycerol and free fatty acid in response to the NE infusions were proportional to the total fat mass of each individual and were greater in the obese subjects. Overfeeding partially suppressed the lipolytic response to NE in both groups and underfeeding increased the lipolytic response in the obese. There are no differences in thermogenic responses to NE in human obesity to account for excessive fat deposition. Overfeeding does not increase the thermogenetic responses to NE in humans as has been reported in small mammals.

Effects of aerobic exercise on energy expenditure and nitrogen balance during very low calorie dieting

Aerobic exercise in addition to severe caloric restriction was studied for its effects on resting energy expenditure (REE), weight loss, and lean tissue preservation in adult women. A formula diet providing 1.5 g protein and 0.5 g carbohydrate (CHO) per kilogram of ideal body weight daily (mean intake 720 kcal/d) was given to 12 overweight inpatients for 4 to 5 weeks. Six subjects remained sedentary (group 1), while the other six subjects (group 2) performed supervised endurance exercise (a total of 27 hours at 50% of maximal oxygen uptake (VO2max) over 4 weeks). Lean tissue preservation was excellent in both groups and was unaffected by the group 2 exercise regimen. Weight loss over 4 weeks in the two groups did not differ (group 1, 6.9 +/- 0.7 kg; group 2, 6.5 +/- 0.7 kg). The VO2max was not increased after 4 weeks of exercise compared with controls. The resting oxygen consumption (rVO2) of both groups declined 10% (P less than .001) in the first seven days of dieting. Thereafter the rVO2 in group 1 remained stable, but a further 17% reduction occurred in group 2 (P less than .03) by the third week of exercise. The free triiodothyronine (fT3) concentration also fell more in group 2 (P less than .05), suggesting a relationship between fT3 and energy expenditure during severe caloric restriction. The ergometer exercise for up to two hours daily was well tolerated. The absence of either a training effect or accelerated weight loss in group 2 may be due to the limited duration (4 weeks) or intensity of the exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose Metabolism and the Response to Insulin by Human Adipose Tissue in Spontaneous and Experimental Obesity: EFFECTS OF DIETARY COMPOSITION AND ADIPOSE CELL SIZE

[1-(14)C]glucose oxidation to CO(2) and conversion into glyceride by adipose tissue from nonobese and obese subjects has been studied in vitro in the presence of varying medium glucose and insulin concentrations as functions of adipose cell size, the composition of the diet, and antecedent weight gain or loss. Increasing medium glucose concentrations enhance the incorporation of glucose carbons by human adipose tissue into CO(2) and glyceride-glycerol. Insulin further stimulates the conversion of glucose carbons into CO(2), but not into glyceride-glycerol. Incorporation of [1-(14)C]glucose into glyceride-fatty acids by these tissues could not be demonstrated under any of the conditions tested. Both adipose cell size and dietary composition influence the in vitro metabolism of glucose in, and the response to insulin by, human adipose tissue. During periods of ingestion of weight-maintenance isocaloric diets of similar carbohydrate, fat, and protein composition, increasing adipose cell size is associated with (a) unchanging rates of glucose oxidation and increasing rates of glucose carbon incorporation into glyceride-glycerol in the absence of insulin, but (b) decreasing stimulation of glucose oxidation by insulin. On the other hand, when cell size is kept constant, increasing dietary carbohydrate intake is associated with an increased basal rate of glucose metabolism and response to insulin by both small and large adipose cells. Thus, the rate of glucose oxidation and the magnitude of the insulin response of large adipose cells from individuals ingesting a high carbohydrate diet may be similar to or greater than that in smaller cells from individuals ingesting an isocaloric lower carbohydrate diet.The alterations in basal glucose metabolism and insulin response observed in adipose tissue from patients with spontaneous obesity are reproduced by weight gain induced experimentally in nonobese volunteers; these metabolic changes are reversible with weight loss. The relationships among adipose cell size, dietary composition, and the metabolism of adipose tissue are similar in spontaneous and in experimental obesity.

Endurance training with constant energy intake in identical twins: Changes over time in energy expenditure and related hormones

The effects of exercise training and of its interaction with the genotype on components of energy expenditure and related hormones were examined in young male monozygotic twins. Energy intake was maintained at the pretraining level for a 93-day training period. The estimated net energy deficit induced by training was 244 MJ and was associated with a 5-kg body weight loss that was almost entirely explained by body fat loss. Resting metabolic rate (RMR) was significantly decreased by 8% after training despite the preservation of fat-free mass (FFM). Accordingly, plasma norepinephrine (NE) concentrations, NE appearance rate, and plasma levels of triiodothyronine (T3), free T3, and total thyroxine (T4) were lower after training. The energy cost of standardized exercise was also reduced after the training program. A modest to significant within-twin-pair resemblance was observed for absolute changes in the RMR, thermic effect of food, energy cost of exercise, NE clearance, and plasma concentrations of thyroid hormones. These results suggest that when exercise training is associated with a substantial negative energy balance, energy expenditure and levels of related hormones are decreased, and this effect is partly accounted for by heredity.

Effects of Chronic Beta Receptor Stimulation on Glucose Metabolism

The acute administration of a beta receptor-stimulating agent profoundly affects insulin-mediated glucose metabolism; however, little is known about the impact of chronic beta receptor stimulation on glucose metabolism and insulin sensitivity. We therefore investigated the effect of the chronic administration of a beta-2-agonist, terbutaline sulfate (TS), on glucose metabolism in 7 healthy, normal-weight, male volunteers between the ages of 21 and 30 yr. Studies were performed using the euglycemic, hyperinsulinemic (1.0 mU/min · kg) clamp technique before and after the oral administration of 5 mg of TS three times a day for 1 and 2 wk. Basal endogenous glucose production (EGP) (2.54 ± 0.11 versus 2.64 ± 0.14 mg/min · kg) and basal glucose oxidation (1.87 ± 0.16 versus 2.0 ± 0.2 mg/ min · kg) were unchanged by the chronic administration of TS. However, insulin-stimulated total glucose metabolism increased by 29% (7.0 ± 0.47 versus 9.05 ± 0.67 mg/min · kg; P < 0.02). Insulin-stimulated, nonoxidative glucose disposal increased by 45% (3.62 ± 0.42 versus 5.26 ± 0.48 mg/min · kg; P < 0.01), while insulin-stimulated glucose oxidation did not change significantly (3.38 ± 0.15 versus 3.79 ± 0.22 mg/min · kg). EGP was completely suppressed under both conditions. Mean basal plasma insulin concentration (41 ± 9 versus 49 ± 15 pmol/L) and insulin clearance during the clamp procedure was unchanged (477 ± 45 versus 474 ± 37 ml/min · m2). We conclude that chronic beta receptor stimulation with TS improves insulin-stimulated glucose disposal in man, mostly by improving nonoxidative glucose disposal, i.e., “glucose storage.” Mechanisms explaining these findings have not yet been elucidated. We suggest that an adaptive response of the sympathetic nervous system plays a role in this phenomenon.

Thyroid hormones and thermogenesis: the metabolic cost of food and exercise

To mimic plasma T3 levels observed in a previous overfeeding study, six lean healthy men received replacement amounts of L-thyroxine (200 micrograms/d) to block endogenous thyroid hormone production while consuming their habitual diet. After 4 weeks equilibration on T4, L-triiodothyronine (T3) was given (45 micrograms/d) in addition to T4, to produce mild T3-thyrotoxicosis, for another 2 weeks. At the end of this period T3 was discontinued but the subjects continued to receive T4 for another 2 weeks. Resting metabolic rate, exercise efficiency, and the thermic effect of food were measured using a ventilated hood, open circuit indirect calorimeter at the end of each phase of the experiment. There was a significant increase in the resting metabolic rate of 6% (P less than 0.01) while the subjects were mildly T3-thyrotoxic. The increase in energy expenditure however, during exercise on a bicycle ergometer or following a 500 kcal liquid-formula meal remained unaltered in the same situation. Thus, mild T3-toxicosis does not alter the efficiency of exercise or the thermic effect of food. These results suggest that the increased plasma T3 levels, observed in overfeeding, could explain corresponding increases in resting metabolic rate but not changes in the efficiency of exercise or the utilization of food.

The effect of diet on thermogenesis in acquired lipodystrophy.

Abstract We tested the effect of variation of intake of carbohydrate, fat, protein, and total calories on the metabolic rate and thyroid hormones in an 18-yr-old female with total acquired lipodystrophy and a 23-yr-old normal female control subject. The lipodystrophic subjects resting metabolic rates, when expressed as W/m 2 body surface area, were elevated and varied directly with the caloric intake. The metabolic rates were highest after 3 days of the protein-supplemented diet and lowest after a 3 day fast. Serum triiodothyronine (T 3 ) concentrations of the lipodystrophic subject were within the normal range but varied directly with the caloric content of the diet. T 3 was highest during the period of protein supplementation and lowest after the 3 day fast. The resting metabolic rate rose beyond the normal range in the control subject only after 3 days of the protein-supplemented diet and fell to low normal values after the 3 day fast. In contrast to the finding in the lipodystrophic subject, T 3 concentrations were stable after each 3–6 day dietary alteration. We conclude that there is relative metabolic lability in the lipodystrophic subject, and this may be related to the diminished capacity to store energy as fat. Metabolic rates, when calculated as W/kg estimated lean mass, were normal in the lipodystrophic subject, consuming reduced amounts of food (1800 cal/day). The role, if any, of T 3 in modulating these processes is unclear. The subject with lipodystrophy may demonstrate a form of dietary-induced thermogenesis.