AHS(?) Bumpity Bump Bump: When is Fat a Fat?

Soooo ... I've been following the #AHS12 hashtag on Twitter, and I'm not sure this came directly from there or from me clicking on an ID on one of the tweets and then checking out other tweets ... but anyway, I stumbled across this exchange between Aaron Blaisdell and Robb Wolf:

9 AugRobb Wolf ‏@robbwolf

Ruminant fermentation converts cellulose to fats. Cows actually run on a 60-70% FAT diet.

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16hAaron Blaisdell ‏@AaronBlaisdell

@robbwolf Paul Jaminet wrote the same thing in his book Perfect Health Diet

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7hEvelyn ‏@CarbSane

@AaronBlaisdell @robbwolf SCFA are not metabolized as "fats" . Ruminants convert the bulk of these via DNL to LCFA.http://carbsanity.blogspot.com/2011/02/when-is-fat-fat.html

FWIW, Robb retweeted this.  Now I'm pretty sure since he's at AHS12, this tweet came from a statement someone made in their presentation.  This is a claim I hear made from time to time -- that pretty much every animal being on the planet somehow consumes a high fat diet because when one eats the "right" carbs, gut bacteria convert them into short chain fatty acids.  This is a highly misleading concept as I attempted to outline in this post.   I hope that in revamping and adding to the book, the Jaminets will clarify the section on chain length to distinguish long, short and medium chain FA's (and leave out the part about SCFA qualifying as "fat burning" entirely ... it is generally the other way around in ruminants).

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Original Publish Date:  Feb. 24, 2011

Something has not been sitting quite right with me for a while when it comes to the classification of molecules.  I think it's how we define them, or better yet from which discipline/angle we characterize them from.  Is it simply a matter of structure?  Or, is it also about the chemical (e.g. reactivity) and physical (e.g. solubility, "feel") properties?  Or ... is it how the molecule is metabolized by some living creature, humans in particular.  

Behold the common palmitic acid molecule:

The points on the sawtooth "chain" are carbon atoms with 2H's attached, 3H's on the terminal C on the right.  This is a hydrocarbon, which as the name implies is made up of carbon and hydrogens.  The actual atoms shown on the left, however,  compose the "functional group" giving this molecule it's "acid" designation.  That is the carboxyl group often written as ~COOH in chemical formulas, from which the hydrogen can be ionized (dissociated, knocked off) leaving behind the palmitate ion with the ~COO^-.   

So molecules of this makeup from a chemistry point of view are called carboxylic acids.  But the structures below are also carboxylic acids:

Left-to-right these are acetic acid (acetate), propionic acid (propionate), and butyric acid (butyrate).  All share the same basic structure - unbranched hydrocarbon "chain" of different lengths and a carboxyl end-group - as palmitate and other long-chain saturated (no double bonds in the hydrocarbon chain) fatty acids.

But there can be no clearer example of how not all acids are "fatty" than to look at acetic acid - the acid in vinegar!  And we know how oil and vinegar don't mix.  But what of concentrated acetic acid?  Surely it's "oily" because this is a short chain fatty acid after all.  As you can see below, pure acetic acid isn't called glacial acetic acid for nothing.

All three of these acids are water soluble something we don't associate with "fats" from a physical/chemical standpoint.  What we consider a "fat" from a physical standpoint comes from the concept of polarity in molecules.  Atoms like oxygen (O) and nitrogen (N) have what's called a high electronegativity or electron affinity (these two concepts are related and often used interchangeably, if you're interested in the nitty gritty see here).   What that means to the layperson is that they are electron "magnets" in molecules and draw the negatively charged electrons in a molecule in their direction.  This results in an uneven charge distribution creating what's called a polar molecule, often referred to as a dipole.  Water, good ol' H2O, is our quintessential polar molecule as shown below:

Now carbon (C) and hydrogen (H) have low electronegativities and therefore charge distribution in long hydrocarbon chains is relatively uniform.  We call these types of compounds non-polar.  Polar compounds are soluble (dissolve in or are miscible with) in other polar compounds.  Since water is our most common polar solvent, we often call such compounds hydrophilic.  The short chain carboxylic acids have considerable polar character due to the two oxygens at the terminal end.  The CH part is non-polar but not a sufficient fraction of the molecule to influence its overall behavior.  Therefore these are water soluble.  Once the chain length reaches 6C in length (common name caproic acid - a fat present in goat's milk) the non-polar chain becomes of sufficient size so as to dominate the behavior and it is no longer water soluble.  We call non-polar molecules hydrophobic.  These tend to have that oily feel to them that we associate with fats.  The longer the chain, the oilier they feel as the molecule as a whole becomes more and more dominated in character by its hydrophobic "tail".   Consumers of coconut oil or refined MCTs especially will note these "thinner" oils seem less fatty than, say, olive oil.

Bottom line, unbranched carboxylic acids containing 5 or fewer carbon atoms are not "fatty" acids to a chemist.  They are not "fats" in a physicochemical sense.

But surely they are still fats metabolically ... right?   The best known of these "short chain fatty acids" (SCFA) is the 4C butyrate (and we commonly refer to the "ate"  base so I'll use that form from here out).  Butyrate is found in butter and is a metabolic byproduct of fermentation of fiber by bacteria in our guts.  So it is not at all uncommon to hear that bacteria convert carbs to fat in our guts.  But not only are they not physicochemically fats, they are also not biochemically/metabolically "fats" in the way we recognize long chain fatty acids (LCFA) to be absorbed and metabolized (or even MCT's for that matter).   The fatty acids formed from fermentation of fibers are simply not metabolized in the same manner as the 9 cal/g common dietary fat that are esterified/hydrolyzed and undergo beta-oxidation in the fatty acid spiral, etc.  Fiber is generally approximated to generate on average 2 cal/g because about 40% is converted to SCFA's and 15% to lactate, and may well be considerably less.

What happens to the short chain carboxylic acids?   They are not triglycerides or esterified into them and packaged up in chylomicrons to be shipped off and eventually undergo beta-oxidation.  According to this text

• 90-95% of SCFA's are absorbed
• The majority of butyrate is used by the cells of the colon for maintenance energy
• All propionate is metabolized by the liver
• Acetate serves as a metabolic substrate for the liver (50-70%) and peripheral tissues (mainly muscles)
• It is estimated that ~50% of the lactate is absorbed and reaches systemic circulation. 

The metabolism of the SCFA's and lactate:

• Cells lining the colon use butyrate as a major substrate for energy production in metabolic pathways that are not fully understood.  The caloric production is estimated by the authors at ~1.4 cal/gram
• All proprionate and residual butyrate is metabolized by the liver via gluconeogenesis
• Muscle cells oxidize acetate
• Lactate is likely utilized as a substrate for gluconeogenesis or lipogenesis.

So IMO, I think it might be a good idea to drop the term "fatty" in the designation of short chain carboxylic acids.  They are not fats by any definition or means of classification.  

They do not behave chemically/physically like the molecules we would define as fats.  Neither do they behave metabolically as the molecules we generally recognize as dietary fats.

Fiber carbs converted to short chain "fatty" acids are not in any way, shape or form equivalent to dietary LCFA or endogenous LCFA released from adipose tissue.