A comment I read on PaleoHacks led me to this article on Livestrong: Does Coconut Oil increase your appetite? It cites the following study: Ingested Medium-Chain Fatty Acids Are Directly Utilized for the Acyl Modification of Ghrelin .
Ghrelin, an acylated brain and gut peptide, is primarily produced by endocrine cells of the gastric mucosa for secretion into the circulation. The major active form of ghrelin is a 28-amino-acid peptide containing an n-octanoyl modification at serine that is essential for activity. Studies have identified multiple physiological functions for ghrelin, including GH release, appetite stimulation, and metabolic fuel preference. Until now, there has not been any report detailing the mechanism of ghrelin acyl modification. Here we report that ingestion of either medium-chain fatty acids (MCFAs) or medium-chain triacylglycerols (MCTs) increased the stomach concentrations of acylated ghrelin without changing the total (acyl- and des-acyl-) ghrelin amounts.
After ingestion of either MCFAs or MCTs, the carbon chain lengths of the acyl groups attached to nascent ghrelin molecules corresponded to that of the ingested MCFAs or MCTs. Ghrelin peptides modified with n-butyryl or n-palmitoyl groups, however, could not be detected after ingestion of the corresponding short-chain or long-chain fatty acids, respectively. Moreover, n-heptanoyl ghrelin, an unnatural form of ghrelin, could be detected in the stomach of mice after ingestion of either n-heptanoic acid or glyceryl triheptanoate. These findings indicate that ingested medium-chain fatty acids are directly used for the acylation of ghrelin.
So, while the MCT's in CO don't stimulate more ghrelin production, they do bind ghrelin and convert it to the active form that stimulates appetite. Short chain fatty acids like butyrate and long chain fatty acids like palmitic acid do not do this. The Livestrong article goes on to mention how CO can aid in weight loss, but I've not found peer review evidence of impact on metabolic rate, but I have seen one study where replacement of LCT's with MCT's has aided slightly in this regard.
I was wondering why this might be and then I was reminded that the only other food I can think of high in MCT's is human breast milk. Perhaps the MCT's activating ghrelin ensured babies would eat enough?
In any case, I just wanted to share this with my readers since CO is all the rage in both the low carb and paleo movements. Perhaps it's something to consider if you're not losing or gaining weight on an ad libitum diet that includes lots of CO. Those who take CO as a supplement of sorts (why?) might want to reconsider that unless they are trying to gain weight.