Fat Tissue Expansion: Part I ~ Terminology

One of the things that irks me about discussions of various obesity related topics is the inappropriate use of terminology.  I would like to give the benefit of the doubt and presume that for most who do this, it is inadvertent.   Often this is due to not having a complete understanding of human metabolism and physiology (cough ... ahem ... Mr. Gary Taubes) , but at some point, when speaking from a presumed position of authority, this excuse doesn't cut it.  To be fair, the peer review literature and higher level texts are rife with inconsistencies of their own.  Most authors are likely simply using the term they are most familiar with not realizing that those terms mean different things in different contexts.  Still, a careful reading of said literature is all that is needed to understand how they are using the terms and the process to which they are referring.  

This has been briefly addressed here previously.   As with insulin resistance, I think the "fat formation" realm is in dire need of some more clear definitions and applications of the terminology.   The terms adipogenesis and lipogenesis are often used interchangeably (even considered synonymous).  But I would like to propose that -- although it's probably not going to happen -- a revised and expanded terminology should be agreed upon and used consistently.   Expanded?  Yes, because the conversions between types of lipids -- the cyclic conversion of --  fatty acids + glycerol ↔ triglyceride -- is not a "genesis" of anything, it is merely a conversion of one form of lipid to another.  On that note ...

The prefix lipo refers to molecules that are classed as lipids -- in the context of human physiology and dietary components, when we discuss lipids we are mostly discussing long chain fatty acids (LCFA's).  As just discussed, the term lipids includes both the free fatty acid form and the esterified or acylated form, triglycerides  (to keep things simple, we will not discuss mono and diglycerides).   So let's start with making fats.

Lipogenic - Lipogenesis

One need not be a Latin scholar to understand that the "genisis" part of the term refers to the generation or creation of something, in this case lipids.  This implies that we are forming lipids from something else.  In other words, I propose that the term lipogenic apply to something that promotes lipogenesis, and only lipogenesis, and that lipogenesis be applied solely to those processes by which lipids are formed from other biochemical classes of molecules.  In terms of physiology, we're really talking one thing:  de novo lipogenesis, DNL.  In humans DNL is largely the synthesis of the saturated fatty acids palmitic and stearic acids from acetyl-CoA produced from oxidation of glucose.  In other animals, ruminants for example, DNL often refers to elongating short chain fatty acids into longer chain fatty acids.  In any case, these processes essentially produce LCFA's from carbohydrates in some manner.   (I've discussed before how not all "fatty acids" are necessarily lipids)

Adipogenic - Adipogenesis

Here's the definition of adipogenesis from Answers.com:  "The formation of fat or fatty tissue."   Right there folks, we see the problem.  Indeed in many cases, even in "medical dictionaries" such as here and  here  the terms adipogenesis and lipogenesis are said to be synonymous.  This simply is not true.  The prefix adipo refers to fat tissue/fat cells (aka adipocytes) and therefore I think it is high time for adipogenesis to be recognized separately from lipogenesis as the formation/growth of fat tissue.  

Because ...

• lipogenesis does not necessarily leads to adipogenesis
• adipogenesis can occur in the absence of lipogenesis

Now even with this more precise definition of adipogenesis, there is confusion.  This is because fat tissue grows by two means:  increased adipocyte number (e.g. the generation of new fat cells), and increased adipocyte size (the "generation" of more fat tissue).  By the most strict definition, adipogenesis should probably be used only to describe the former, because more fat tissue per se is only generated when more fat cells are produced. And yet, at some point, most of the growth and increased mass of fat tissue is attributable to growth of the fat cells themselves.   Looking at fat tissue as sponges, the cells of which form the matrix of the "tissue", we need more sponges to technically have more tissue.  And yet you can buy thin compressed sheets of sponges -- perfect for crafts and making stamps because you can cut them with scissors.  One could generate the same volume of sponge by layering several sheets of the thin dry stuff, or by merely adding water to one thus causing its expansion.  Since it is the latter that seems to be what happens when most of us "get fat" -- particularly in adulthood -- in my opinion the definition of adipogenesis as the growth (or expansion) of fat tissue (by either means) seems a functional working definition.  Thus something that is adipogenic would be any factor leading to the growth of fat tissue or partitioning of lipid storage to fat tissue.

Acylglycerogenic - Acylglycerogenesis

As mentioned previously, even if we clearly define lipogenesis and adipogenesis, there is still something involved in the metabolism and ultimate storage of lipids that is missing.   It does not fall under either umbrella, although it is often associated with adipogenesis.  This process is the fatty acid → triglyceride conversion.  I propose that since the formation of triglycerides involves the acylation or esterification to produce acylglycerols (triglycerides are also called triacylglycerols), we call the formation of triglycerides acylglycerogenesis.  (The geek in me likes this better than triglycerogenesis).  The formation of triglycerides is no more lipogenesis than would the lipolysis of triglycerides to fatty acids be, because triglycerides and NEFA are both lipids. 

Triglycerides are the form in which lipids are stored in cells.  They are also the major form by which the bulk of fatty acids are transported in circulation in various lipoproteins (chylo, VLDL, LDL, etc.).  By contrast, free fatty acids, my preferred acronym for which being NEFA,  are the lipid form that can be transported across cell membranes, and oxidized "burned" for energy by what is called ß-oxidation.  There are also relatively small amounts of fatty acid transported in the bloodstream -- however even these are not technically free, as they are almost entirely bound to albumin.    In any case, triglycerides are the storage form of lipids, and the major transport form in circulation.  However, in a preview of what is to come,  every time your body makes a triglyceride, it is not necessarily storing fat in your adipose tissue for the long term.

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In Part II, we'll discuss the possible mechanisms by which fat tissue can expand.  Then subsequent parts of this series with discuss which of these mechanisms contribute considerably to fat tissue expansion (large amounts of which are otherwise called obesity).