Heat Production of Lean and Obese (Ob/Ob) Mice in Response to Fasting, Food Restriction or Thyroxine

Direct measurements of heat production were made in 3 to 4 month old lean and obese mice during 3 days of fasting and 2 days of refeeding, 6 weeks of dietary restriction, and 2 weeks of thyroxine (T₄) administration. Obese mice produced slightly more total heat per animal at this age but weighed 1.5 to 2 times as much as lean mice and consumed more diet ad libitum. Heat production of obese mice was not as responsive to fasting as that of lean mice. When fed equal amounts of diet, obese mice continued to produce at least as much total heat as lean mice. Heat production of obese mice was more sensitive to T₄ administration when compared to that of lean mice and unlike the lean mice, T₄-treated obese mice were unable to adjust food intake to maintain body weight during a T₄-induced increase in thermogenesis. These data suggest that metabolic responses of obese mice to changes in energy intake or output do not occur as rapidly as those of lean mice.