SUBSTRATE OXIDATION AND THYROID-HORMONE RESPONSE TO THE INTRODUCTION OF A HIGH-FAT DIET IN FORMERLY OBESE WOMEN

OBJECTIVE: To investigate the adaptation in substrate utilization to a sudden change in dietary composition from a medium fat to a high fat diet, during a three day period in formerly obese and never obese wome n. METHODS: Energy expenditure (EE) and substrate oxidation rates were measured in eight healthy formerly obese women and eight never obese controls, during four consecutive days in a respiration chamber. The f irst day and the day prior to the experiment, the subjects consumed a diet with 30 energy-% fat, whereas the diet had 55 energy-% fat on the subsequent three days. RESULTS: The rate of adjustment of oxidative s ubstrate partitioning expressed as 24 h non-protein respiratory quotie nt (RQnp) was similar in the two groups. RQnp on each of the days was also similar between the two groups, after accounting for a group diff erence in energy balance, caused by a non-significantly lower EE in th e formerly obese women. However, the formerly obese subjects, demonstr ated a greater suppression of postprandial fat oxidation after supper, which was unrelated to energy balance. Furthermore, the formerly obes e subjects, in contrast to the controls, exhibited a reduction in plas ma triiodothyronine/thyroxine ratio (T-3/T-4) following the high fat d iet. A positive correlation between T-3/T-4 and EE was found in the 16 subjects. CONCLUSIONS: The formerly obese subjects did not show a slo wer adaptation rate of substrate utilization when challenged with a hi gh fat diet, but exhibited an enhanced suppression of fat oxidation an d a lower T-3/T-4 ratio after supper, when fed a high fat diet.