*Geek Box: Sphingolipids & Ceramides
Lipids – fats – are one of the most complex groups of compounds in the body, and in the diet. Given that ceramides will be a term you read a lot in this Deepdive, it pays to expand on what exactly sphingolipids and ceramides are. So, let us begin.
Sphingolipids are synthesised in the body [in the endoplasmic reticulum of cells, to be precise]. They are used as structural lipids, i.e., they help to form cell membranes and form lipoproteins. Sphingolipids are a dizzying array of compounds: there are over 200 sphingolipid species. They are also used as signalling compounds, and used to synthesise bioactive compounds involved in lipid metabolism.
Ceramides are an important precursor in the synthesis of different sphingolipids [remember how many there are!]. Ceramides constitute around 3% of circulating plasma sphinoglipids, and ceramides are found on VLDL, LDL, and HDL. There appears to be a relationship between atherogenic lipoproteins, which express apolipoprotein-B [apoB], and ceramide levels. For example, people with a rare condition known as abetalipoproteinemia have no circulating apoB-containing lipoproteins, and have been shown to have up to 80% less circulating ceramides.
The relevance of ceramides for the liver is that it is thought that liver cells may produce ceramides to deliver to other tissues, although for what purpose this is not yet known. However, to date it would appear that the increase in ceramide production in the liver may precipitate de novo lipogensis of triglycerides in the liver, providing a potential explanation for the increases in liver fat levels which have been shown to correspond with increasing levels of circulating ceramides.