Effects of isoenergetic, low-fat diets on energy metabolism in lean and obese women.

Whether changing from a high-fat diet to an isoenergetic, low-fat, high-complex-carbohydrate diet results in thermogenic benefits is controversial. Brief dietary interventions and failure to account for the potential influence of body-fat distribution on energy metabolism could have confounded the interpretation of previous studies. To address these issues, eight upper-body obese, seven lower-body obese, and eight non-obese premenopausal women underwent measurements of body composition, resting energy expenditure, overnight energy expenditure, and meal fat oxidation at the end of a weight-stabilizing, high-fat (42%) diet, and after 4 wk of an isoenergetic, low-fat (27%) diet. No change in body composition, resting energy expenditure, overnight energy expenditure, or meal fat oxidation occurred. We conclude that isoenergetic shifts from dietary fat to dietary carbohydrate within the generally recommended range have little or no effect on energy metabolism, and that body-fat distribution does not predict differences in energy expenditure.

Serum lipid responses to a eucaloric high-complex carbohydrate diet in different obesity phenotypes.

OBJECTIVE: To determine whether the eucaloric substitution of complex carbohydrates for dietary fat (15% of daily energy intake) affects plasma lipid concentrations differently in upper-body obese, lower-body obese, and nonobese women. DESIGN: We studied 23 premenopausal women before and after dietary intervention. MATERIAL AND METHODS: After the 23 subjects achieved weight maintenance on their usual high-fat diet (43% fat, 37% carbohydrates, and 20% protein), the 7 upper-body obese, 8 lower-body obese, and 8 nonobese women consumed a eucaloric, high-complex carbohydrate, low-fat diet (27% fat, 53% carbohydrates, and 20% protein) for 4 weeks in the Clinical Research Center. Before and after the high-carbohydrate diet, body composition and plasma lipids and apoproteins were measured. RESULTS: After the high-carbohydrate diet, fasting plasma triglyceride concentrations increased (from 1.50 +/- 0.14 mmol/L to 2.00 +/- 0.25 mmol/L; P = 0.04) in upper-body obese women but were not significantly changed in lower-body obese (1.37 +/- 0.28 mmol/L and 0.96 +/- 0.12 mmol/L) or nonobese (0.70 +/- 0.08 mmol/L and 0.73 +/- 0.08 mmol/L) women. The hypertriglyceridemia was present before the evening meal and throughout the night in upper-body obese women. Plasma cholesterol and high-density lipoprotein cholesterol were not significantly affected by the change in diet. No changes in plasma apoprotein concentrations or body composition occurred that could account for the dietary-induced hypertriglyceridemia in the women with upper-body obesity. CONCLUSION: The hypertriglyceridemic response to a high-complex carbohydrate, low-fat diet may be obesity phenotype specific. These findings suggest that further studies of this phenomenon should be focused on this obesity phenotype and further emphasize the importance of assessing body fat distribution when treatment outcomes are determined.

Effects of estrogen on free fatty acid metabolism in humans.

To determine whether estrogen directly affects effective adipose lipolysis, palmitate rate of appearances ([14C]palmitate) was measured in 15 postmenopausal women. Each volunteer was studied after > or = 2 mo of estrogen treatment and again after > or = 2 mo of estrogen deficiency. Plasma hormone concentrations were controlled and identical on the 2 study days with use of the pancreatic clamp technique, and the lipolytic response to epinephrine and epinephrine + phentolamine was assessed. Results showed that overall palmitate flux was greater (10-20%, P < 0.05) during the estrogen-deficient than during the estrogen-replete study. Adrenergic stimulation of lipolysis was not specifically influenced by estrogen treatment, and control of plasma hormone concentrations did not eliminate the difference in palmitate flux between the estrogen-deficient and estrogen-replete study days. We conclude that estrogen deficiency is associated with increased plasma free fatty acid availability and that estrogen likely has direct, albeit small, effects on adipose tissue lipolysis.

Postprandial free fatty acid kinetics are abnormal in upper body obesity.

Excess free fatty acid release (rate of appearance) is seen in overnight postabsorptive, upper body obese women and, if present postprandially, could contribute to glucose intolerance. These studies examine the antilipolytic effect of a mixed meal in upper body obese, lower body obese, and nonobese women and the contribution of meal triglyceride fatty acids to circulating free fatty acids. Eight upper body obese, 8 lower body obese, and 8 nonobese age-matched, premenopausal women were studied. Systemic oleate Ra ([3H]oleate) was measured before and after an evening meal that contained triolein labeled with [14C]triolein as the only source of fat. Premeal oleate Ra was greater in both upper body obese and lower body obese women than nonobese women. The nadir of total oleate Ra occurred 90-240 min postprandially and was less (P < 0.01) in nonobese and lower body obese women (63 +/- 10 and 87 +/- 17 mumol/min) than in upper body obese women (140 +/- 21 mumol/min). Meal oleate Ra contributed substantially to total oleate Ra. The nadir for endogenous oleate Ra in nonobese and lower body obese women was less (P < 0.01) than that observed in upper body obese women. We conclude that the antilipolytic effect of a mixed meal is reduced in upper body obese women and that meal triglyceride fatty acids contribute significantly to postprandial free fatty acid flux.

Assessment of body composition with use of dual-energy x-ray absorptiometry: evaluation and comparison with other methods.

Dual-energy x-ray absorptiometry (DEXA) is a relatively new method of assessing body composition in humans. In the current study, DEXA was analyzed for accuracy and precision by using both anthropomorphic phantoms and a combination of body composition techniques in humans. Satisfactory precision for measurement of total body fat, fat-free mass, and total body bone mineral could be demonstrated in vivo and in vitro. Predictions of lean body mass in humans on the basis of DEXA, total body water, and total body potassium were significantly different. The results of multiple regression analysis suggested that a component of total body water was related to body potassium, and another component was predicted by body fat. In addition, extracellular fluid volume, as measured by the bromide space technique, was significantly associated with both fat-free mass and fat mass as measured by DEXA. These findings have implications for the interpretation of body composition data in humans.

Role of gastric inhibitory polypeptide in postprandial hyperinsulinemia of obesity.

To assess the contribution of gastric inhibitory polypeptide (GIP) to the postprandial hyperinsulinemia of obesity, secretion rates of GIP (generated from kinetic analyses from infusions of porcine GIP) and insulin (from C-peptide applied to a validated kinetic model) to meals of 3 sizes were determined in 10 obese (5 male and 5 female) and 10 lean, sex- and age-matched healthy subjects. Although the postprandial secretion rates of GIP were greater in obese subjects (P = 0.03), postprandial concentrations of GIP were not. The latter may be explained by the greater volume of distribution of GIP in obese subjects (P = 0.036). Secretion rates and volume of distribution of GIP were correlated (r = 0.652, P less than 0.01). Despite excessive integrated postprandial (P = 0.010) insulin concentrations, insulin secretion was not significantly different between obese and lean subjects. We conclude that 1) although postprandial plasma GIP concentrations are normal, GIP secretion is increased in obesity, 2) the postprandial hyperinsulinemia of obesity is not due to excessive insulin secretion but is likely secondary to altered insulin clearance, and 3) GIP cannot account for the hyperinsulinemia of obesity through its insulinotropic action.