Mitochondria and High Fat Diets

Lots of buzz over mitochondria of late.  So I thought I'd share this find:

High-fat diets cause insulin resistance despite an increase in muscle mitochondria

It has been hypothesized that insulin resistance is mediated by a deficiency of mitochondria in skeletal muscle. In keeping with this hypothesis, high-fat diets that cause insulin resistance have been reported to result in a decrease in muscle mitochondria.    In contrast, we found that feeding rats high-fat diets that cause muscle insulin resistance results in a concomitant gradual increase in muscle mitochondria.

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Free Fatty Acids and Cytokines Induce Pancreatic ß-Cell Apoptosis by Different Mechanisms

Free Fatty Acids and Cytokines Induce Pancreatic ß-Cell Apoptosis by Different Mechanisms

(I've scrubbed the distracting reference numbers from some excerpts of the introduction and I'm also going to try to cite only the information pertinent to T2)

Hypercaloric diets containing large amounts of fat, also called the Western diet, contribute to a major extent to the increasing prevalence of obesity and type 2 diabetes mellitus (T2DM). T2DM is characterized by peripheral insulin resistance, pancreatic ß-cell dysfunction, and decreased ß-cell mass associated with increased rates of ß-cell apoptosis. Elevated levels of circulating free fatty acids (FFAs) contribute to the pathogenesis of T2DM. High concentrations of FFAs lead to both impairment of insulin action and ß-cell dysfunction.  Moreover, FFAs have been shown to cause ß -cell death, mainly by apoptosis. 

Of note, increased adiposity is associated not only with increased FFA release but also with adipocyte secretion of a variety of cytokines and cytokine-like adipokines, such as TNF -α, IL-6, leptin, resistin, and adiponectin. TNF-α has direct cytotoxic effects on pancreatic ß -cells, especially in combination with other cytokines.

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Elevated Free Fatty Acids: Detrimental?

As many of my readers know, I've been challenged lately on my beliefs on NEFA.  So I thought I would summarize my thinking on this in a post rather than having several comments scattered amongst a few threads.   I'm not going to be referencing my post here at this time (it's too time consuming to do so at the moment).  If/when I have the opportunity to do so in the future, I'll do a bumped update.  

These are my thoughts based on extensive research of the peer review literature on this topic, in almost all cases, considered review of full text articles including reading as many supporting citations in major reviews as possible.  Over the past year I have read at least a hundred such articles.

Elevated non-esterified or free fatty acids (NEFA/FFA) are a symptom associated with insulin resistance, Metabolic Syndrome (aka Syndrome X) and Type II diabetes.  The overwhelming evidence in the literature points to elevated NEFA being more than just associated with these conditions, but rather the initiating step in their development..   

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Fat Futile Cycling ~ From Carb Excess??!!

This post may well make heads spin.  It sure did mine.

A common claim in LC circles is that we "waste" excess fats through futile cycling.  Although this has not been demonstrated in any significant amount in humans except in massive fat overfeeding, it is still incorporated into books and blog posts by the likes of Dr. Mike Eades (futile cycling to "blow off" low carb excesses is stated as if fact in The 6 Week Cure).

Inherent in these statements is the implication that this only happens for excess dietary fat when carbs are low ... insulin would be trapping the fats in the cells as the theories go.  Well ...

Substrate cycling between de novo lipogenesis and lipid oxidation: a thermogenic mechanism against skeletal muscle lipotoxicity and glucolipotoxicity  (2004 Review Article,  PDF)

Life is a combustion, but how the major fuel substrates that sustain human life compete and interact with each other for combustion has been at the epicenter of research into the pathogenesis of insulin resistance ever since Randle proposed a ‘glucose–fatty acid cycle’ in 1963. Since then, several features of a mutual interaction that is characterized by both reciprocality and dependency between glucose and lipid metabolism have been unravelled, namely:

(i) the inhibitory effects of elevated concentrations of fatty acids on glucose oxidation (via inactivation of mitochondrial pyruvate dehydrogenase or via desensitization of insulin-mediated glucose transport),

(ii) the inhibitory effects of elevated concentrations of glucose on fatty acid oxidation (via malonyl-CoA regulation of fatty acid entry into the mitochondria), and more recently

(iii) the stimulatory effects of elevated concentrations of glucose on de novo lipogenesis, that is, synthesis of lipids from glucose (via SREBP1c regulation of glycolytic and lipogenic enzymes).  

This paper first revisits the physiological significance of these mutual interactions between glucose and lipids in skeletal muscle pertaining to both blood glucose and intramyocellular lipid homeostasis. It then concentrates upon emerging evidence, from calorimetric studies investigating the direct effect of leptin on thermogenesis in intact skeletal muscle, of yet another feature of the mutual interaction between glucose and lipid oxidation: that of substrate cycling between de novo lipogenesis and lipid oxidation. It is proposed that this energy-dissipating substrate cycling that links glucose and lipid metabolism to thermogenesis could function as a ‘fine-tuning’ mechanism that regulates intramyocellular lipid homeostasis, and hence contributes to the protection of skeletal muscle against lipotoxicity.

The figure below describes the mechanisms by -- as the figure caption states -- nutrient OVERSUPPLY can lead to insulin resistance.

I like schematics like this because even though I'm scientifically-minded, sometimes reading about various reactions gets really confusing.  It helps to just see it.  The possible ways all three macronutrients can lead to IR in excess are described:

• The fatty acids are shown in the upper left, with the fats that are "burned" proceeding down the left arrow through ß-oxidation (this metabolic process is often referred to as the fatty-acid spiral).  An excess of FA's leads to accumulation of "lipid pools" in the cell:  that little cycle in the upper left including DG = diacylglycerol (2 FA's on a glycerol) and FA-CoA. In my posts on lipotoxicity and intramyocellular lipids (IMTG or IMCL), I've discussed the implications of this previously.  The metabolites DG and FA-CoA activate various enzymes that inhibit glucose uptake.  They also lead to cell dysfunction and/or death when they undergo conversion to ceramides and/or peroxidation (formation of ROS).
• Amino acids, surprisingly, can inhibit insulin mediated glucose uptake through the mTOR pathway.
• Glucose can inhibit its own uptake by glucosamine synthesis and its action.

There are two central molecules in the metabolic engines of the cells:  Pyruvate and Acetyl CoA.  Pyruvate is an intermediary coming from glycolysis (glucose breakdown) and some amino acids, whereas Acetyl CoA is the ultimate "end product" of the preliminary breakdown of many amino acids, glucose and lipids.  From Ac-CoA on, the rest of the "metabolic engine" is the same regardless of the source of this molecule.  

• The authors describe a state of gluco-lipotoxicity and/or proteo-lipotoxicity that could develop when excesses in either glucose or AA's or both stimulate the conversion of Acetyl CoA to Malonyl CoA that can suppress fatty acid oxidation leading to a vicious cycle of lipid accumulation in the cellular pools.

Malonyl CoA is usually considered a "commitment step" in de novo lipogenesis - fatty acid synthesis - but has previously been ignored in skeletal muscle as DNL wasn't thought to occur at any significant rate in these cells.  This is the groundbreaking revelation of this paper ... it DOES occur!   The discussion describes how there are two different enzymes, Acetyl CoA Carboxylases (ACC's) that convert Acetyl to Malonyl CoA.  ACC-1 expressed primarily in liver and adipose tissue, and ACC-2 in skeletal muscle and other "non-lipogenic" tissues.  The two ACC's are compartmentalized in cells so that:

• ACC-1:  Generates the cytoplasmic (cellular) pool of Malonyl CoA used for DNL
• ACC-2:  Associated with mitochondria and governs this pool of Malonyl CoA to control ß-oxidation

Excerpt:  

In skeletal muscle, the suppressive effect of an elevated concentration of glucose on fatty acid oxidation is now recognized to occur ... but the question of whether a cytoplasmic pool of malonyl-CoA might be utilized for fatty acid synthesis has rarely been invoked most probably because of the long-held assumption that skeletal muscle is not an organ where de novo lipogenesis occurs. ... Consequently, the importance of skeletal muscle substrate metabolism in the homeostatic control of blood glucose has been viewed entirely from the reciprocal nature of interactions between glucose and lipid metabolism.  Recent evidence, however, suggests otherwise. As discussed below, de novo lipogenesis can indeed be shown to occur in muscle cells and to be modulated by factors influencing nutritional status, thereby revealing another facet of interactions between glucose and lipid metabolism that shows dependency between these two fuel substrates in skeletal muscle.

Here's where this gets really interesting.  The discussion continues:

The recent recognition that de novo lipogenesis might have relevance for lipid homeostasis in skeletal muscle stems from the realization that Sterol regulatory element binding protein-1c (SREBP-1c), a member of the family of transcription factors that regulate the expression of genes involved in lipid storage in liver and adipose tissue, is also present in skeletal muscle at a level close to that observed in the liver,41,42 and that its dysregulation might lead to increased lipid storage, and hence contribute to the pathogenesis of insulin resistance. There is now evidence both in humans and in rodents that SREBP-1c mediates insulin upregulation of genes encoding glycolytic and lipogenic enzymes in skeletal muscle,42–46 but most fascinating are the very recent demonstrations that glucose alone (in the absence of insulin) can stimulate de novo lipogenesis in skeletal muscle cells. 

Translation:  Lipid storage pathways thought previously to only be active in significant levels in liver/adipose tissues have now been shown to be active in muscle cells.  What they are seeing is that the DNL stimulated by glucose in this manner is NOT accompanied by the expected suppression of ß-oxidation.  They speculate on possible explanations for this, but the bottom line is summarized:

Whatever the explanation, it is clear that de novo lipogenesis, although low in skeletal muscle, can be markedly stimulated in muscle cells, particularly under conditions of high glucose (and/or high insulin) concentrations.

Hmmmmmm.....  The discussion goes on to speculate on the physiological significance for DNL in muscles, and that it may contribute yet another "sink" for temporary glucose excesses (e.g. after a carby meal), particularly when glycogen depots are full.  However there's a "glitch" because lipid accumulation is associated with insulin resistance.  IOW, synthesizing more would seem counter-productive to glucose clearance/disposal  unless the synthesized lipids were also "disposed of".  Well here's where the observation that ß-oxidation is not suppressed comes into play.

... recent work from our laboratory investigating the mechanisms by which leptin ... may interact with insulin to stimulate thermogenesis in skeletal muscle, suggests the possible existence of a thermogenic pathway of substrate cycling in which lipids derived from glucose... are subsequently oxidized.

So the carbs are thermogenic, not the fats?  Looks like it!  The discussion gets a bit complicated because it names enzymes and pathways, etc.  I'll let the more scientifically inclined amongst you read that part for yourselves.  

My summary of "Substrate cycling between de novo lipogenesis and lipid oxidation"

One role of leptin has been demontrated to be it's regulation of thermogenesis.  This hormone is recognized as being involved, in conjunction with insulin, in glycemic control and in preventing excessive IMCL accumulation.  Leptin has been shown to stimulate both glucose utilization and lipid oxidation.   This research group demonstrated that leptin can stimulate thermogenesis by a pathway requiring an enzyme known as P13K, and P13K is stimulated by insulin (insulin is described as a "potent activator" of P13K).   Leptin apparently also stimulates AMPK, an enzyme involved in lipid oxidation.  Therefore, these researchers have demonstrated a concurrent stimulation of glucose and fatty acid oxidation resulting in thermogenesis, and that evidence is consistent with DNL involved in the thermogenesis - experiments with DNL suppressing agents and leptin demonstrate this as w/o DNL, thermogenesis was not observed.  As the authors summarize:

Taken together, these studies suggest that the direct effect of leptin in stimulating thermogenesis in skeletal muscle could be mediated by substrate cycling between de novo lipogenesis and lipid oxidation, and that the orchestration of this substrate cycling requires both PI3K and AMPK signaling.

The discussion continues on the possible control mechanisms for this proposed futile cycle of DNL/ß-oxidation that are likely initiated with Acetyl CoA produced by both glycolysis and fatty acid oxidation "backing up" as it overwhelms the Krebs Cycle.    This futile cycle is established in brown adipose tissue.  I was surprised to learn that this thermogenesis has pretty much been established to involve DNL from glucose:

... analogous to the relation between substrate metabolism and thermogenesis in brown adipose tissue. In this tissue, whose primary function is to produce heat for thermoregulation ... it has long been known that much of the fuel for thermogenesis also derives from glucose being first converted to lipids before being oxidized.70–73  {They go on to describe mouse studies demonstrating that suppressing DNL in BAT leads to hypothermia.} 

Furthermore, this ‘dependency’ interaction between these two substrates and thermogenesis is well recognized at the whole-body level, and is attributed to activation of a neuroendocrine network (comprising insulin, leptin and the sympathoadrenal system), which plays a pivotal role in several overlapping regulatory systems: that of blood glucose, body temperature, body weight and more recently intramyocellular lipids.56–58

... The energy-dissipating substrate cycle that links glucose and lipid metabolism to thermogenesis in skeletal muscle (depicted in Figure 3) provides a novel molecular mechanism of thermogenesis through which this abovementioned neuroendocrine network operating through insulin, leptin and catecholamines overlaps in the regulation of body weight, blood glucose and intramyocellular lipids, and hence in the protection against obesity, hyperglycemia and lipotoxicity. 

Take away message here?  Well, the futile cycle is lipids.  But what stimulates and/or is required for this futile cycle to "waste" energy?  Who'da thunk it.  CARBS!!!

Comparative Fatty Acid Toxicity on Macrophages

Comparative toxicity of fatty acids on a macrophage cell line (J774)

In the present study, the cytotoxicity of palmitic, stearic, oleic, linoleic, arachidonic, docosahexaenoic and eicosapentaenoic acids on a macrophage cell line (J774) was investigated. The induction of toxicity was investigated by changes in cell size, granularity, membrane integrity, DNA fragmentation and phosphatidylserine externalization by using flow cytometry. Fluorescence microscopy was used to determine the type of cell death (Acridine Orange/ethidium bromide assay). The possible mechanisms involved were examined by measuring mitochondrial depolarization, lipid accumulation and PPARγ (peroxisome-proliferator-activated receptor γ ) activation. The results demonstrate that fatty acids induce apoptosis and necrosis of J774 cells. At high concentrations, fatty acids cause macrophage death mainly by necrosis. The cytotoxicity of the fatty acids was not strictly related to the number of double bonds in the molecules: palmitic acid>docosahexaenoic acid>stearic acid=eicosapentaenoic acid=arachidonic acid>oleic acid>linoleic acid. The induction of cell death did not involve PPARγ activation. The mechanisms of fatty acids to induce cell death involved changes in mitochondrial transmembrane potential and intracellular neutral lipid accumulation. Fatty acids poorly incorporated into triacylglycerol had the highest toxicity.

 I could C&P the entire introduction to this paper but don't want to do that, so please go read it.  Summary:

• The FFA/NEFA are generally implicated in having toxic effects in non-adipose tissues, aka lipotoxicity
• Saturated fatty acids tend to be more lipotoxic
• Ectopic (non-adipose) triglyceride storage is somewhat protective but this remains under consideration as metabolites/intermediates of triglycerides (ceramides) are implicated in toxicity.
• Cell death contributes to the inflammatory properties of FA's
• PPAR-ɣ increases reduce inflammatory cytokines like IL's and TNF-α

Macrophage infiltration into adipose tissue in obesity is implicated in the inflammatory state associated with obesity.   It should be noted that this study was in vitro (e.g. culture dish) on a non-human (murine to be exact) derived cell line.   But the elevated NEFA associated with insulin resistance and T2 Diabetes would produce a "toxic" state within adipose tissue leading to macrophage death (and adipocyte death?). 

Cell death types:  

• Apoptosis, aka programmed cell death:  When functioning properly, this is the "natural" death of cells for cellular turnover in tissues, etc.  In cancer, apoptosis is short circuited leading to so-called immortal cells.  Some toxic conditions lead to disruption of the normal signals and pre-mature apoptosis.  
• Necrosis:  Premature cell death due to some - always detrimental - external source.  Necrosis initiates a greater immune response as dead cells must be engulfed and removed, and such cells rupture and "spill" more "stuff" into the surroundings than cells undergoing PCD.

Some exerpts:

In the present study, we evaluated whether the induction of cell death could be a mechanism by which FAs modulate macrophage function. Indeed, treatment with different concentrations of FAs was toxic to the macrophage cell line J774, as assessed by loss of membrane integrity and DNA fragmentation.  In most cases, the lowest concentration that caused loss of membrane integrity was the same that induced DNA fragmentation (Table 3), and the percentages were similar. These findings are indicative that necrosis and apoptosis occurred concomitantly.

... high FA concentrations cause macrophage death mainly by necrosis. This effect was also observed by others after treatment of different cell types, such as melanoma, leukaemia cell lines, lung carcinoma and fibroblasts, with high concentrations of FA [49–51].  The results of both loss of membrane integrity and/or DNA fragmentation shown in the present study suggest the following rank of toxicity on J774 cells: PA>DHA>SA=AA=EPA>OA>LA 

... The relationship between lipid accumulation and apoptosis has been demonstrated through a series of experiments with different cell lines [63–65]. Accumulation of excess FAs into the TAGpool has been postulated to divert these molecules from pathways that lead to toxic effects and, thus, lipid bodies may serve as buffers against lipotoxicity [19]. Treatment with all FAs led to an increase of lipid bodies inside J774 cells (Figure 2B).  Cells treated with non-toxic concentrations of the FAs exhibited higher granularity, indicating accumulation of lipid droplets that was observed by fluorescence microscopy. 

A look at Figure 1 indicates a threshold behavior for the toxic effects.

I have not addressed everything in this article and this post is sort-of half book marking, half just putting this out there as I had not seen a discussion of this nature before.

Separating Fats & Carbs

Just some musings on carbohydrates and fats, but with a scientific basis so put it here.

Personally I believe the obesity epidemic can be blamed primarily on two phenomena:

1.  The abundance of high calorie foods high in fats & carbs (I'll call them CF) in ever larger portions, and

2.  Liquid calories loaded with sugar and/or fat

To prevent obesity my solution is simple:  Keep the fats and carbs separate.  If you're going to eat carbohydrate, eat it with lean protein and/or in whole form so you get sufficient fiber.  Go easy on the fat.  If you're going to eat fat, chances are it is attached to protein, forgo the carbs.  If you simply must eat CF foods, rely on portion control/calorie counting and not on satiety to determine how much you eat.  Keep the total caloric load low, perhaps in the 2-300 cal range, to keep the unnatural assault on your metabolism to a minimum.  

My reasons for this are twofold:

1.  Our paleolithic ancestors, from whom we differ very little genetically, did not have access to foods that were rich in both lipid and carb content simultaneously.  I don't envision Paleo dude saved up his tubers to cook in rendered wild boar fat to plate tuber fries with his boar ribs and a side of some veggie also cooked in boar fat.  It seems far more likely that  Paleo dude ate the tubers if in abundance perhaps even delaying the need for a hunt, or saved those tubers for a rainy day when a kill was available for the eating.   Paleo dude was mostly an opportunistic eater in a scarce world.  Obesity was not a problem nor did Paleo chick worry over a little belly roll.  We are programmed to store energy in its most efficient form (lipid) in amounts that are seemingly unlimited.  There would have been no evolutionary advantage to not being able to store energy, or not wanting to partake in this energy source during times of abundance to save for times of scarcity.    There is no physiological reason to limit lipid intake on any given day.  I see no reason to doubt Eaton's work indicating that Paleos ate a relatively low fat diet.  Further underlining the need for taking it while they could get it -- e.g. overconsumption one day if necessary -- and weak signaling at best.  Bottom line, our metabolisms seem designed to switch between fuels depending which was more available, not deal with being bombarded by mixed fuels.

(2) As outlined in Nutrient Fates After Absorption  dietary intake of protein and carbohydrate share the following in common:

(a) they invoke an insulin response

(b) they are on the order in terms of quantity with the body's storage capacity (nitrogen "pool" and glycogen)

(c) their intake stimulates their metabolism (protein synthesis, oxidation)

(d) their absorbed form is as metabolic substrate (amino acid, glucose)

As such, our hormonal signaling is tightly attuned to intake of these macronutrients to maintain levels in a relatively narrow window.  Dietary fat, OTOH, is

(a) once absorbed, packaged as triglycerides in chylomicrons and transported mostly to the adipose tissue for immediate storage.

(b) as chylos, the absorbed form is not the metabolic substrate for lipids, that being free fatty acids (NEFA/FFA).

(c) in quantity, orders of magnitude less than total stored lipid even in the leanest of humans

(d) can virtually be stored without limit thus eliminating any need to limit intake in one feeding

Circulating NEFA levels are controlled indirectly by release from storage.  Dietary fat does not significantly contribute directly to the levels of this energy source.  My take-away message from the post/article is that our appetite/satiety signals are finely attuned to intakes of carb and protein out of necessity to maintain structure and storage/circulating levels within a relatively narrow range.  Our metabolisms change remarkably within 24-48 hrs of deprivation.  Lipid storage, even on a lean person, can last weeks (or more).  Even gorging on fat is a drop in the bucket of the amount of lipid we store (again, even in the lean), so there's no need to limit this in the short term.  Fat mass regulation seems almost independent of dietary fat when you think about it.

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So, whenever I hear the query "Why do we overeat", in many cases it is a passive process.  Our bodies were not made to handle the caloric punch of CF foods, so we tend to eat more calories before the stop signals go up were we consuming just carbs (usually with lots of fiber) or fats (usually with lots of protein).

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Over on the personal blog ( When to Eat ), Helen wondered about separating carbs and fats on a daily basis -- e.g. alternating high fat day(s) with high carb day(s).  This was actually the impetus for this post because my reply became too lengthy for the comments feature here to handle.  In any case, here are my thoughts on that:

Let's say during the day I have a fatty breakfast and a carby dinner or a carby breakfast and a fatty dinner.  Either way, if I'm consuming basically maintenance-caloric levels of these foods, my metabolism will do a bit of switching up within its normal mode.  After carbs, lipid oxidation will be down-regulated and the carbs burnt off or converted to glycogen (see that Nutrient Fate link), but as the glucose is "cleared", lipid oxidation ramps up again.  After fats, lipid oxidation remains as it was.  If there's no to minimal carb in the meal, insulin is likely low so NEFA are released from the fat cells to replenish the IMCL being "burnt".  The metabolism is "normal".  

It takes a few days, however, for the body to transition to a "fat burning" (low carb) metabolism.  Our bodies are inefficient during this transition -- spilling ketones, etc.  This can probably be used to our advantage to get a little more out of weight loss, but is it healthy?  I don't have the answer to that and I'll try to put it on my "to do list" to look into.  My educated guess is that so long as you're not overdoing it caloriewise this is probably OK as diacylglycerols and ceramides shouldn't build up.  It takes a while for IMCL to accumulate anyway on an HF diet.  I don't know if it will accumulate if one's "average" diet is HF, even if some days are LF.

From a weight loss perspective, I do feel this switching up was probably responsible for the whooshes I would experience upon returning to LC after carb cheats.  Was this healthy?  Who knows.  I look at it as a trade-off in the end.  Whatever I did to get here, I'm way better off for it now.

I worry more, however, over the transition from a HF day to a HC day.   The switch gets flipped back almost immediately.   I've posted that as little as a single high fat meal can induce IR, and impaired glucose tolerance (IGT) the following day.  If one is not efficiently burning lipids in short HF stints, this could potentially turn the HC day into a "diabetic day".  I've played with a glucose meter to see about this for myself.  Perhaps the fact that I don't eat particularly high fat (as a % or on a gram basis) version of LC, I've not had issues with tolerating carbs.   Meters are cheap, and you can get strips relatively inexpensively too.  Rather than guess, or look to studies to see, testing one's own response to this is probably better.    But I do think alternating days like this has a greater potential for creating issues than mixing it up "separately" throughout the day.  If one experiences a degree of IGT, I think a good bout of exercise between the last high fat meal and the first high carb meal might be all that's needed.

Companion post to follow ....

Fat storage in pancreas and in insulin-sensitive tissues in pathogenesis of type 2 diabetes

Fat storage in pancreas and in insulin-sensitive tissues in pathogenesis of type 2 diabetes

Obesity is associated with increased storage of lipids in nonadipose tissues like skeletal muscle, liver, and pancreatic b cells. These lipids constitute a continuous source of long-chain fatty acyl CoA (LC-CoA) and derived metabolites like diacylglycerol and ceramide, acting as signalling molecules on protein kinases activities (in particular, the family of PKCs), ion channel, gene expression, and protein acylation. In skeletal muscle, the increase in LC-CoA and diacylglycerol translocates and activates specific protein kinase C (PKC) isoforms, which will phosphorylate IRS-1 on serine, preventing its phosphorylation on tyrosine and association with PI3 kinase. This interrupts the insulin signalling pathway leading to the stimulation of glucose transport. In pancreatic b cells, short-term excess of fatty acids or LC-CoA activates PKC and also directly stimulates insulin exocytosis. Longterm exposure to free fatty acids (FFA) leads to an increased basal and blunted glucose-stimulated insulin secretion by affecting gene expression, increase in KATP channel activity, and uncoupling of the mitochondria. In addition, the saturated FFA palmitate increases cell death by apoptosis via increase in ceramide synthesis.

{...} 

In obesity, a situation of excess supply and/or decreased oxidation, fatty acid not only accumulate in adipose cells but also in other tissues like skeletal muscle, liver, and pancreatic b cells. Triglycerides are not harmful as such, but are precursors of signalling molecules like LC-CoA, diacylglycerol, ceramides, acting directly or indirectly in skeletal muscle on insulin signalling and glucose uptake, and in pancreatic b cells, on insulin secretion and cell viability. 

Going to leave this one just "out there" except to say that it ties in with a lot of the other research/studies I've been posting about lately.

The Progression of Insulin Resistance

Vascular function, insulin resistance and fatty acids  (I'll blog on the vascular focus of this paper shortly, but this post is focusing on the bolded statements in the abstract).

Abstract

Over the past 10 years it has become clear that intact vascular function, especially at the level of the endothelium {cells lining the blood vessels}, is paramount in the prevention or delay of cardiovascular disease. It has also become clear that insulin itself, in addition to its metabolic actions, directly effects vascular endothelium and smooth muscle.  Insulin, at normal physiologic concentrations, causes changes in skeletal muscle blood flow in healthy, insulin-sensitive subjects. Insulin’s effect on the endothelium is mediated through its own receptor and insulin signalling pathways, resulting in the increased release of nitric oxide. Insulin’s vascular actions are impaired in insulin-resistant conditions such as obesity, Type II (non-insulin-dependent) diabetes mellitus and hypertension, which could contribute to the excessive rates of cardiovascular disease in these groups.  Insulin-resistant states of obesity and Type II diabetes show a multitude of metabolic abnormalities that could cause vascular dysfunction. Non-esterified fatty acid levels increase long before hyperglycaemia becomes present. Raised non-esterified fatty acids impair insulin’s effect on glucose uptake in skeletal muscle and the vascular endothelium and thus could have detrimental effects on the vasculature, leading to premature cardiovascular disease.

If it is true that NEFA levels rise before blood glucose becomes elevated, then perhaps a screening for pre-pre-diabetes should involve measurement of this blood biomarker?  

What causes elevated NEFA?  It's largely not dietary fats as these are mostly transported as chylomicrons, although there's some indication that in an obese person more FFA's escape re-esterification in the fat cells.  However NEFA levels are largely regulated by their release from adipose tissue in the ever-present FFA/Triglyceride cycling.   The release of NEFA is policed by the inhibitory action of insulin, and this role of insulin has been described as protective.

So if elevated NEFA is the first symptom in the cascade, and an indication of impaired insulin inhibitory action on fat stores, then is the progression of IR proposed by Taubes totally wrong?  Taubes contends that peripheral tissues develop IR first followed by organs and finally adipose tissue.  This statement in this article would indicate that it's the other way around.  Elevated NEFA would indicate some degree of insulin resistance of the fat cells.  Insulin is not largely involved in storing fat, it is involved in its release.  But what causes this?  Hmmmm.... over-stuffed fat cells perhaps?  As circulating NEFA's rise these induce insulin resistance skeletal muscle and perhaps the liver as well so that it pumps out too much glucose.  

It seems more and more apparent to me that carbohydrate consumption per se has relatively little to do with the development of IR.  It naturally occurs in certain phases of life (puberty, aging) but most of us are able to compensate for mild IR by increasing insulin production.  To be fair, it's not dietary fat that necessarily causes it either, although there's still the question of higher IMCL just from eating a higher fat diet and the potential for IMCL derived diacylglycerol and/or ceramides to induce IR in skeletal muscle cells.   Using our insulin does not appear to cause us to become resistant to it.  Indeed the opposite seems to be closer to the truth as low carbers are advised to "carb up" for several days prior to taking an oral glucose tolerance test so as to restore their insulin responses to as normal as possible.

I propose that the fat accumulation leads to elevated NEFA leads to peripheral IR and other deleterious effects on the liver and pancreas.  Only  chronic carbohydrate overfeeding seems to contribute to increases in fat mass, but net fat accumulation will still largely be contributed by dietary fat.  IOW fat accumulation leads to IR leads to hyperinsulinemia.   Fat accumulation is, in the end, dictated by energy balance.

Exercise to lose weight and reduce lipotoxicity!

Thanks to reader Cody for finding a study I had come across previously regarding IMCL/IMTG.  Actually the study linked to was an update, but there's a secondary lesson, I believe, to be had from the results.  Since this was a study in older folks, there's a sub-message here:  it's never too late!

Study:  Exercise-induced alterations in intramyocellular lipids and insulin resistance: the athlete’s paradox revisited

We previously reported an “athlete’s paradox” in which endurance-trained athletes, who possess a high oxidative capacity and enhanced insulin sensitivity, also have higher intramyocellular lipid (IMCL) content.

The purpose of this study was to determine whether moderate exercise training would increase IMCL, oxidative capacity of muscle, and insulin sensitivity in previously sedentary overweight to obese, insulin- resistant, older subjects. Twenty-five older (66.4 0.8 yr) obese (BMI 30.3 0.7 kg/m2) men (n 9) and women (n 16) completed a 16-wk moderate but progressive exercise training program.  

Body weight and fat mass modestly but significantly (P 0.01) decreased. Insulin sensitivity, measured using the euglycemic hyperinsulinemic clamp, was increased (21%, P 0.02), with modest improvements (7%, P 0.04) in aerobic fitness (V˙ O2peak). Histochemical analyses of IMCL (Oil Red O staining), oxidative capacity [succinate dehydrogenase activity (SDH)], glycogen content, capillary density, and fiber type were performed on skeletal muscle biopsies.  Exercise training increased IMCL by 21%. In contrast, diacylglycerol and ceramide, measured by mass spectroscopy, were decreased (n 13; 29% and 24%, respectively, P 0.05) with exercise training.   SDH (19%), glycogen content (15%), capillary density (7%), and the percentage of type I slow oxidative fibers (from 50.8 to 55.7%), all P 0.05, were increased after exercise.

In summary, these results extend the athlete’s paradox by demonstrating that chronic exercise in overweight to obese older adults improves insulin sensitivity in conjunction with favorable alterations in lipid partitioning and an enhanced oxidative capacity within muscle. Therefore, several key deleterious effects of aging and/or obesity on the metabolic profile of skeletal muscle can be reversed with only moderate increases in physical activity.

Here's a link to the preliminary work I believe I was looking for (from the references in the above paper):
Skeletal Muscle Lipid Content and Insulin Resistance: Evidence for a Paradox in Endurance-Trained Athletes

So I've blogged a bit about lipid accumulation in non-adipose tissue, lipotoxicity and insulin resistance.  IMCL seems to correlate with IR, but the "athlete's paradox" is that insulin sensitivity accompanies increases in IMCL in athletes.  Therefore IMCL cannot be "toxic" in and of itself.  In this study we see that exercise decreases diacylglycerol and ceramide levels at the same time as IMCL's increase.  The negative effects of IMCL appear to be correlated to the build-up of metabolites rather than the stored triglycerides themselves and/or the turnover of  IMCL -- it's a secondary storage tank in the obese, but perhaps more like a gas tank for the athlete.  Perhaps ceramide is the sole culprit, insulin sensitivity, oxidative capacity and IMTG all increased by around 20%.  Ceramide and DAG both decreased, but only ceramide decreases correlated with insulin sensitivity improvements.

But ... in reading the originally referenced article, something else jumped out at me.  They took 25, mainly weight stable, obese, older (avg age ~66), sedentary people and, near as I can tell, did not change their diet.  One can presume most of these were eating a SAD before and after.  The participants were simply put on a moderate exercise regime.  The exercise was 45 min cardio (moderate by heart rate and/or perceived exertion), 4-5X/week -- mostly walking or stationary cycling.  You know ... the type of exercise often poo pooed in the low carb community that can only, according to Taubes, make you hungrier and cause you to eat more.  The subjects actually averaged 3.5X/week for the 16 weeks of the study.  

The result?:   In 4 months an average loss of almost 3-3/4 lbs of fat mass.  If continued for a year, this would translate into an average of 11 pounds in a year.  Not too shabby when compared to the weight losses reported by Shai et.al., but more importantly this counters to oft-repeated claim that you can't lose weight by exercise alone.  

And health-wise?  Insulin sensitivity improved >20%  (even as IMCL increased), as ceramide and diacylglycerol decreased.  IOW, whatever the cause of the IR, exercise alone improved this state.  

So exercise CAN improve health independent of diet.