Relationship between Free Fatty Acid Turnover and Total Body Oxygen Consumption in the Euthyroid and Hyperthyroid States*

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Abstract
The effectiveness of intra-venously administered nicotinic acid in blocking almost completely the rise in free fatty acid (FFA) levels and turnover induced by catechol-amies was confirmed in normal young adults. It was shown to be effective also in triiodothyronine-treated subjects. The calorigenic response to intravenously administered catecholamines was also reduced, although not completely abolished, by prior administration of nicotinic acid. On the other hand, nicotinic acid did not alter basal metabolic rate or the respiratory quotient either in normal subjects or in triiodothyronine-treated subjects with elevated basal metabolic rate. Administration of glucose to control and to triiodothyronine-treated subjects reduced FFA levels to very low values, but again did not reduce metabolic rate. Although the hyperthyroid state is associated with elevated FFA levels and turnover, the present results demonstrate that the hypermetabolism persists unaltered when FFA levels and turnover are reduced, at least for short time periods, by administration of nicotinic acid or glucose. Under the influence of nicotinic acid, there was a large disparity between total calorie consumption and total FFA turnover. It is suggested that stored tissue lipids provide a "buffering reservoir" of substrate in this and similar situations, i.e., that the tissues call on endogenous substrate to support respiration at an unimpaired rate for some time, even though the rate of delivery of FFA from the depots is temporarily reduced to low values.