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 women.

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 first 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 substrate partitioning expressed as 24 h non-protein respiratory quotient (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 difference in energy balance, caused by a non-significantly lower EE in the formerly obese women. However, the formerly obese subjects, demonstrated a greater suppression of postprandial fat oxidation after supper, which was unrelated to energy balance. Furthermore, the formerly obese subjects, in contrast to the controls, exhibited a reduction in plasma triiodothyronine/thyroxine ratio (T 3/T 4) following the high fat diet. 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 slower adaptation rate of substrate utilization when challenged with a high fat diet, but exhibited an enhanced suppression of fat oxidation and a lower T 3/T 4 ratio after supper, when fed a high fat diet.