Glyceroneogenesis Is the Dominant Pathway for Triglyceride Glycerol Synthesis in Vivo in the Rat

Glyceroneogenesis Is the Dominant Pathway for Triglyceride Glycerol Synthesis in Vivo in the Rat*

OK ... yes, this is a rat study, but the body of work by Hanson's group has demonstrated that the results obtained for the rat correlate well with human metabolism.  These studies utilized radiolabeling "tracer" methods to track the substrate source for G3P.  Three dietary groups were compared:
1.  Controls - regular chow fed (removed 7am study morning)
2.  48 hour fasted (food removed 48 hrs prior)
3.  Lipogenic (high sucrose) diet (5 day sucrose water in addition to regular chow and glucose infusion during testing to "maintain the lipogenic state").

The abstract is long so I'll let y'all readers just read it at the source if you like.  I'll focus this post on excerpts from the discussion of the results.

Plasma:  The plasma concentration of triglyceride were not different in the three groups (Table 1). The fraction of plasma triglyceride glycerol derived from glyceroneogenesis was ∼60% and not different among the three groups (Table 1). Approximately 15% of triglyceride glycerol was derived from glucose in the control group (Table 1). The contribution of glucose was lower in 48-h fasted rats (∼11%) and was higher in sucrose-supplemented animals (∼28%). Although sucrose supplementation resulted in a higher contribution of glucose to plasma triglyceride glycerol when compared with controls, the contribution of glucose was less than that of glyceroneogenesis (Table 1).
The highlighted  statements say it all.  No difference in triglycerides or the fraction of those triglycerides formed using glyceroneogenesis-derived G3P between the groups.
Glyceroneogenesis and Glycolysis as G3P source in Adipose Tissue:  There was no significant (p = ns) difference in the triglyceride concentration in the adipose tissue of controls and 48-h-fasted animals. In the sucrose-supplemented animals, the concentration of triglyceride was significantly higher than controls (Table 2).

Note:  It is important to remember that one way human and rat metabolisms differ considerably is in the rate of de novo lipogenesis (aka converting excess carbs to fat) and it's contribution to fat stores - that being that DNL is not a major pathway in humans for fat storage (see my DNL label for some posts on this) but is in rats.  It is also important to note the use of the word "supplemented":  e.g. the sucrose rats were getting an excess.  

This section continues:
... The rate of glyceroneogenesis in the control animals was ∼600 nmol/g/h in the epididymal adipose depot and ∼800 nmol/g/h in the mesenteric depot (Table 2). Glyceroneogenesis did not change in response to fasting for 48 h. In contrast, sucrose-supplementation resulted in a significant increase in this pathway in both adipose tissue depots. Glyceroneogenesis was higher in mesenteric adipose tissue as compared with epididymal adipose tissue in all groups; however, a statistically significant difference was only observed in the sucrose-supplemented group (Table 2).
... 48-h fast caused a significantly lower incorporation of total glucose carbon into triglyceride glycerol as compared with controls. In contrast, sucrose supplementation resulted in a higher total contribution of glucose carbon to triglyceride glycerol (Table 2). The direct contribution of glucose to triglyceride glycerol was ∼80 nmol/g/h in control animals in both adipose depots (Table 2). In the 48-h-fasted animals the direct contribution of glucose via glycolysis was negligible. In contrast, sucrose supplementation resulted in a doubling of the direct contribution of glucose to triglyceride glycerol in both adipose tissue depots.
We confirmed the predominance of glyceroneogenesis, as compared with glycolysis,.... We examined the mesenteric adipose tissue of sucrose supplemented rats because glyceroneogenesis was highest in the adipose tissue of this group. ... [the results show] ... a greater contribution of glyceroneogenesis relative to the direct contribution of glucose via glycolysis to triglyceride glycerol synthesis (Fig. 3). 
Fatty Acid Synthesis in Adipose Tissue:  The incorporation of 14C of glucose into fatty acids was negligible in 48-h-fasted animals and high in controls in both the epididymal and mesenteric adipose tissue (Table 4). Furthermore, fatty acid synthesis in the sucrose-supplemented group was significantly higher as compared with control animals in both adipose tissue depots examined.
Note: epididymal and mesenteric adipose depots are types of visceral fat.  Interesting, no?  Glyceroneogenesis rates in the fat generally considered to be the more metabolically active are not changed by fasting vs. normal feeding.  They increase in response to sucrose supplementation.  So excess carbs caused an increase in G3P production from pyruvate/lactate.  So much for the view of GlyNG as a minor alternative path.  Sucrose did increase the absolute contribution from glycolysis, but the contribution of GlyNG was also stimulated.  Again, it should be remembered that DNL is a more significant pathway for triglycerides in the rat.  So those rats made fatty acids from the excess glucose, used some of the glucose to make G3P, but made the additional required to esterify the synthesized fatty acids predominantly from GlyNG.   

My take-away message from this discussion is that the body gets the G3P it needs to esterify the fat it needs to store from the available substrates.   Since humans use less glucose for DNL than rats, it is possible we use more to make G3P when it is "lying around", but GlyNG occurs at considerable rates continually, and is at the ready to "step up" when needed.  The body has better uses for glucose apparently under normal conditions, and it has a ready supply of other substrates to make G3P in adipose tissue.

Glyceroneogenesis in the Skeletal Muscle: Our data are the first demonstration of glyceroneogenesis in skeletal muscles. In response to fasting as well as sucrose feeding, glyceroneogenesis was the main contributor to triglyceride glycerol formation, whereas the direct contribution of glucose was not measurable. .... Although we anticipated that glyceroneogenesis would be a functional pathway, due to the presence of PEPCK-C activity in skeletal muscle (14), the dominance of this pathway was unexpected. Even more surprising was the lack of a direct contribution of glucose to G-3-P synthesis, given that in response to a glucose load, skeletal muscle is responsible for the majority (∼85%) of insulin-mediated glucose uptake (62). Our data demonstrating a marginal contribution of glucose to triglyceride glycerol in skeletal muscle are consistent with the report of Guo and Jensen (15)...
Hepatic (Liver) Glyceroneogenesis:  The fractional contribution of gluconeogenesis to the glucose Ra changed as expected (about 30% lower in controls, which increased to ∼60% after a 48-h fast), whereas glyceroneogenesis remained constant at about ∼60% under all conditions studied. Furthermore, glyceroneogenesis, and not glucose metabolism via glycolysis, was the dominant pathway for hepatic triglyceride glycerol synthesis, even in sucrose-fed, glucose-infused animals. ...
The above are pretty self-explanatory.

I saved the opening paragraph of the discussion for last, as it summarizes the above nicely:
In the present study we have examined the relative contribution of glyceroneogenesis and glucose via glycolysis to triglyceride glycerol synthesis in the rat. Our data show that glyceroneogenesis is quantitatively the predominant pathway for triglyceride glycerol synthesis in white adipose tissue, skeletal muscle, and liver during extended fasting as well as during periods of glucose availability. Surprisingly, the highest rates of glyceroneogenesis in adipose tissue were observed in sucrose-supplemented animals, when fatty acid synthesis and triglyceride deposition were high.
The *surprising* high rates of glyceroneogenesis under lipogenic conditions - e.g. conditions under which the rats were making fatty acids - are consistent with what is expected to go on in a low-carb, high-fat fed state.  Only now the source of fatty acids is directly from the diet ("deposited" by chylos).  There is no reason to believe that GlyNG wouldn't be increased in response to the *need* to store those fatty acids.   

In conclusion the authors outline where the understanding of GlyNG regulation remains unresolved.

(Note:  I'll address the epinepherine part at some future date)

Last, but not least, I would be remiss if I didn't address the date and source of this paper.   It was submitted in June of 2008 and first published in July 2008.   This was after the initial release of GCBC, but Hanson (a co-author) has been identified by Taubes as having vetted his version of G3P in the book to ensure its accuracy.  GCBC was released relatively late in 2007 (September).  This current article was rather detailed and embodied a considerable amount of research.  Given the amount of work that would have gone into writing up the material, it is reasonable to assume that the research itself was mostly complete if not concluded months prior - e.g. though not compiled, no researcher ignores the results as they record data!  Most of the work may well have been completed in advance of Hanson's review of GCBC.   In other words, it seems unlikely that Hanson's view on the role of G3P and glyceroneogenesis was altered considerably from before the publication of GCBC to after as Taubes implies.  That the latest work wasn't published formally until 2008 is not evidence of some great sea change in the understanding of the role of GlyNG in esterification.  There's no evidence that this work did anything more than strengthen the mounting evidence of GlyNG's importance as a metabolic pathway (as summarized in the 2002 & 2003 papers).  I'm still left most befuddled by this aspect of the controversy.  At the very least, I think it was incumbent upon Taubes to follow this up instead of  repeating his G3P theory in lecture after lecture as if it were established fact.


Sanjeev said…
I'm unclear on some wording; Is this reference:
Wolfe and Peters (38) demonstrated that lipolysis continued to occur in humans when glucose was infused at 8 mg/kg/min and that the rate of appearance of fatty acids in plasma was significantly reduced due to enhanced intracellular TG-FA cycling
saying that insulin did not impair lipolysis within adipocytes at all?

And fatty acids were not released into the blood because the g3p systems were not close to saturation, and could handle the additional load without shunting or spillover to other pathways?

That the situation in humans is known to tilt toward g3p to begin with, and this tilt might be exaggerated on higher carbs?
CarbSane said…
Sanjeev, I found the abstract of that reference even a bit more confusing. Essentially it showed that glucose does decrease fat mobilization - as expected. Nobody doubts that. I think what that wording meant was that lipolysis isn't shut down completely, but W&P did see it suppressed in a dose-dependent manner. Plasma FFA's were reduced b/c of increased cycling within the fat cells themselves.

It has been shown long ago that glucose can stimulate esterification. This would keep the total available energy substrates constant. But the way glucose does this does not seem to be by making G3P from glycolysis, but rather increasing G3P synthesis from other substrates via glyceroneogenesis.

This is quite an interesting phenomena. Carb overfeeding in the form of sucrose enhances the GlyNG pathway.
Sanjeev said…
OK, that makes sense.

Where I'm getting hung up is that in plain english, their "enhanced cycling" and your "increased cycling" BOTH make me think "increased reaction rates in both directions", that is,

tg ---> fa
fa <--- tg

so it makes it sound as if in the adipocytes, net lypolysis INCREASES as extracellular glucose increases.
Sanjeev said…
The arrows in the above post actually both go the same way.

something that simple & I still got it wrong (!!!)

tg ---> fa
tg <--- fa
CarbSane said…
Yeah, I'm confused by the increased/enhanced cycling too. As you say, that would imply increasing both directions. Also, there seem to be two FA/TG cycles -- one where there's just turnover in the cell, and one where fatty acids go out into circulation with some taken back up by the adipocytes. There is some discussion of different types of adipocytes that I plan on posting separately on.
CarbSane said…
I thought I would add that the two fat depots analyzed were both analagous to visceral fat in humans. Rodents don't have a lot of subQ fat so perhaps this was the reason for that. They did look at other tissues, but we should keep in mind that visceral and subQ fat does tend to behave differently under certain circumstances. And visceral fat is known to have higher rates of lipolysis to begin with.
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