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Short-term mixed calorie overfeeding increases basal energy expenditure in man but its effects on the thermic effect of a meal (TEM) are unclear. The thermogenic and hormonal responses to an 800-kcal liquid mixed meal were measured in six lean and six obese subjects during weight maintenance, during 18 d of overfeeding 1000 kcal/d, and during 18 d of a 589 kcal/d diet (obese subjects only). There was no change in the TEM in lean subjects between weight maintenance and overfeeding. In the obese group the TEM was lower during both overfeeding (p less than 0.05) and underfeeding (p less than 0.05) compared with weight maintenance. Overfeeding increased rates of net postprandial glucose oxidation and decreased lipid oxidation in the lean subjects only. Alterations in glucose oxidation rates and the insulin response to meals may contribute to an impaired TEM in human obesity during overnutrition.

Strong support has been gathered for a defect in insulin-glucose-mediated thermogenesis in obese and particularly obese insulin-resistant and/or diabetic subjects compared to lean subjects. This defect appears to be reversible with weight loss and improved insulin sensitivity. The mechanism for this blunted thermogic response is directly related to the decreased glucose storage in the obese and obese diabetic subjects.

Continuous respiratory-exchange measurements were performed on ten moderately obese and ten lean young women for 1 h before, 3 h during, and 3 h after either parenteral (IV) or intragastric (IG) administration of a nutrient mixture infused at twice the postabsorptive, resting energy expenditure (REE). REE rose significantly from 0.98 +/- 0.02 to 1.13 +/- 0.03 kcal/min (IV) and from 0.99 +/- 0.02 to 1.13 +/- 0.02 kcal/min (IG) in the lean group; from 1.10 +/- 0.02 to 1.27 +/- 0.03 kcal/min (IV) and from 1.11 +/- 0.02 to 1.29 +/- 0.03 (IG) in the obese group. These increases resulted in similar nutrient-induced thermogenesis of 10.0 +/- 0.7% (IV) and 9.3 +/- 0.9% (IG) in the lean group; of 9.2 +/- 0.7% (IV) and 10.1 +/- 0.8% (IG) in the obese. Nutrient utilization was comparable in both groups and in both routes of administration, although the response time to IG feeding was delayed. These results showed no significant difference in both the thermogenic response and nutrient utilization between moderately obese and control groups using acute IV or IG feeding.