Relative contribution of the main tissues and organs to body fatty acid synthesis in the rat

Abstract

Tritiated water was used to measure the rate of fatty acid synthesis in the main tissues and organs of 7-week old Wistar male rats in order to determine the relative contribution of each tissue to body fatty acid synthesis. We reached the following conclusions: (a) the liver is the main site of fatty acid synthesis, it alone synthesizes 42% of the newly synthesized fatty acids in the body. (b) The dissectable white adipose tissues synthesize 27% of the fatty acids in the body. This group of tissues is heterogeneous because the mesenteric adipose tissue alone contains 40% of the labeled fatty acids present in the white adipose tissues. (c) Besides the intestines, organs other than the liver play a negligible role (2% of the total) in fatty acid synthesis. (d) The skin contributes 7% of the body fatty acid synthesis. (e) The rest of the carcass, essentially composed of the musculature and the skeleton, contributes 18% of body fatty acid synthesis and accounts for 33% of the extrahepatic tissue fatty acid synthesis.

The Regulation of Lipogenesis by Thyroid Hormone and Its Contribution to Thermogenesis*

“…The tissue location of the 2-fold stimulation in the transition from hypo-to hyperthyroidism is not known, but it may in large part be attributed to other fat depots. Extrapolation from epididymal fat to all other adipose tissue is not possible because rates of lipogenesis differ markedly between different adipose sites (33). Furthermore, we know of no data comparing thyroid effects on lipogenesis in these tissues or, indeed, the influence of thyroid hormones on the size of different adipose depots.…”

“…This would leave a further approximately 500 jumol H incorporated into fatty acids in extrahepatic nonadipose tissues, in a manner unresponsive to thyroid hormone. Presumably, these tissues would include skin, lung, brain, and intestines (33).…”

“…Previous estimates of the contribution of the liver to total body lipogenesis have only been made in the euthyroid state. Estimates have varied from 16-50% (33), with values in the higher range derived from animals fed a high carbohydrate fat free (lipogenic) diet.…”

The growth hormone-dependent decrease in hepatic fatty acid synthase mRNA is the result of a decrease in gene transcription

“…Likewise growth hormone reduces hepatic FAS mRNA in pigs and rats (Clarke 1993); however, these changes are less dramatic than for adipose tissue (S S Donkin & T D Etherton, unpublished data). In pigs, adipose tissue is the main site of fatty acid synthesis (reviewed by Clarke 1993) whereas, in the rat, liver is responsible for 42% and adipose tissue for 27% of newly synthesized fatty acids in vivo (Gandemer et al 1983). Consequently, depending on the species, growth hormone may act either directly or indirectly to alter carcass lipid composition; however, both situations involve a reduction in FAS mRNA.…”

CONFERENCE CHEVREUL Les lipides dans notre alimentation : comment leur accorder une juste place sans les diaboliser ni les encenser

“…Par deux méthodes, l'une qui mesurait l'activité enzymatique de l'AcCoAC et l'autre utilisant de l'eau tritiée pour mesurer la synthèse lipidique, nous avons ainsi réussi à montrer que ce qui se disait jusqu'alors était inexact. Chez les rongeurs, l'essentiel de la synthèse de novo des acides gras se fait dans le foie [3]. Pourquoi une telle différence de résultats ?…”