Why We Get (Sick) Fat (and Sick Livers) - Lessons from a Cafeteria Rat

Since we're talking about fructose and the liver of late, I thought I'd bump this post.  In this study groups of rats were fed one of four diets.  The "low fat" diet is better described as a high sucrose diet as 35% of the diet was sucrose.  This replaced 35% of the fat in the 45% "high fat" diet.  While the LF and HF rats gained a little more weight than the standard (also LF at 12%) chow rats, it is clear that the high fat has rather more negative metabolic effects.  I don't think the 35% sucrose diet was beneficial, rather the contrary, but that level of sucrose consumption, every single day for 10-15 weeks (which is quite a long time for a rat) is also hardly indicative of even SAD consumption.

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Original Posting:  3/8/11

It seems fairly generally accepted that whatever the cause or progression, the so-called Metabolic Syndrome, Syndrome X and Type 2 Diabetes are associated with a dysregulation of adipose tissue metabolism, and fat tissue that is infiltrated with macrophages and secretes excessive amounts of inflammatory molecules called adipokines (e.g. TNF-α, IL-6).   A term has been coined, adiposopathy, to describe this "sick fat".  

I've recently discussed the "Cafeteria Rat" study  as pertains weight gain in general.  But the other thing about this study is that it looked at the fat tissue with the different dietary interventions.  To recap, four groups of rats were fed ad libitum different diets:

1.  SC control (estimated <5% simple sugars, 12% fat)
2.  HF chow - 45% fat, lard
3.  LF - low fat (10% lard & soybean oil) matched to HF chow with sucrose substituted for fat content
4.  CAF = SC + 3 human snack foods varied daily from a list

Caloric consumption was  HF (559) < LF (660) ≈ SC (663) << CAF (908) and the weight gains were as shown below:

Of note here is that the HF rats and the LF (high sucrose) rats gained about the same amount of additional fat mass compared to the SC controls but the morphology of their fat differed considerably. Below are the slides of the eWAT (epididymal white adipose tissue).  That "eye" in slide "j" is called a crown-like structure, CLS and is a macrophage surrounded by dying adipocytes (fat cells) and/or the lipid droplet remnants of dead cells.  

The average CLS content of the eWAT was:

I found this rather striking.  It seems any degree of excess adiposity is associated with some macrophage infiltration and inflammatory state.  But it seems that adiposity induced by a high fat diet is particularly associated with the formation of this CLS-riddled "sick fat".  Despite comparable fat gains, the high fat rats had about twice the CLS as the low fat (high sugar) rats.  This is especially interesting with all the literature fingering fructose these days as "causing" insulin resistance, MetS and diabetes.  The LF rats actually consumed by far the most highly refined carbs and sugars:  more than 6.5X that of SC, over 2.5X that of HF, and about 1/3rd more than even the CAF rats.  They got as fat on average as the HF rats, but with only about half the CLS formation.  They got about half as fat as the CAF rats but only developed about 1/3rd of the CLS's of that group.  

One reasonable explanation for this might be that fat cell function as a buffer for dietary fat can easily be overwhelmed with high fat intake.  So, in this study, the high fat diet didn't make the rats fatter than a high sugar diet, but it made their fat sicker!  Perhaps the increased demand for uptake and esterification of dietary fat leads to upsetting the regulation?   Maybe ASP isn't such a good trapper of dietary fats after all leading to excess extracellular (outside the cells) fatty acid content in the fat tissue that the immune system cannot distinguish from lipids released by dying fat cells so macrophages move in?  Or does the higher lipid turnover in these cells required for high fat intake causes them to lose their sensitivity to hormonal signals regardless of size?  

Comparing the CAF to HF, we just see that at some point, excess adiposity just makes matters a whole lot worse.  If you eat a diet that causes a significant increase in ad libitum consumption (CAF ate 35% more than standard chow!) things will go awry faster.  

The CAF rats in this study were the only group that achieved a high enough and sick enough degree of adiposity to exhibit the signs of Metabolic Syndrome.    Below are the NEFA levels:

The inefficient trapping of dietary fatty acids is a possible explanation for the more elevated NEFA seen for HF v. LF given the same degree of adiposity.  Also possible that the dying adipocytes in those CLS's release fatty acids.  

Below is the liver tissue:

If you enlarge, you can see CV = central veins and PT = portal triads identified.   The 2nd column from the right is the healthy liver of the SC fed rats.  They note "macro-vessicular steatosis" in the LF rats and "micro-vessicular steatosis" in both the CAF and HF rats.  These are defined as:

• Macrovesicular steatosis is characterized by engorgement of the hepatocyte (liver cell) by a large fat globule that displaces the nucleus.
• Microvesicular steatosis "refers to a variant form of hepatic fat accumulation whose histologic features contrast with the much more common macrovesicular steatosis".  This is usually attributed to mitochondrial dysfunction or toxicity.

Two patterns of hepatic steatosis are recognized: (1) microvesicular steatosis: the cytoplasm is replaced by bubbles of fat that do not displace the nucleus; and (2) macrovesicular steatosis: the cytoplasm is replaced by a large bubble of fat that displaces the nucleus to the edge of the cell.

I'm not sure either form is worse or better to have, but clearly we would want to avoid this entirely.   A 35% sugar diet is NOT a good idea, but we don't really need to be told that now, do we??   But high sugar diets clearly lead to a different liver dysfunction than do high fat diets.  Is the microvessicular form one of lipotoxicity?

The researchers tallied up the number of inflammatory loci in the liver as shown below.

The pattern is similar to that seen for the fat tissue so we are offered no clues really as to progression.  The inflammation seems to correlate with NEFA release from the fat tissue as well.

In any case the inflammation in the fat tissue and liver seems to correlate with both fat content in the diet, and degree of adiposity achieved.  With modestly elevated NEFA, glucose tolerance is retained.  Dietary fat seems to cause inflammation as adiposity increases (moreso than sugar!).  Directly?  Seems more not than so.  But indirectly, perhaps as simply as by overtaxing the triglyceride/free fatty acid regulatory cycle, it sure seems so.

The high fat diets only - HF & CAF - altered the pancreatic β-cells.

LFD- and SC-fed rats displayed normal pancreatic islets with defined borders, whereas HFD-fed rats contained slightly hypertrophic and disfigured islets. Islets from CAF-fed rats were much larger in size, with dramatically distorted architecture.

FWIW, I'm not seeing much if any difference in the HF vs the LF or SC, but I have no reason to question these researchers' description of their findings.

The authors stated that only the CAF rats became prediabetic, but I see degrees/shades in the results.  One can see a progression, but only the CAF rats demonstrated a statistically significant difference - this explains the authors' statements.  The high fat and high sugar diets did appear to have a "glucose metabolism" more akin to that of CAF than the SC rats.

The first test was an intraperitoneal (IP = inject into abdominal cavity) glucose tolerance test, IPGTT.  The results are shown below:            

Note:  This test done at 7 weeks

Yes, there's no statistical significance between, but the response is similar to what we see for fat-CLS, liver-IL and NEFA.   They also conducted an IP insulin tolerance test, IPITT.  Here insulin is injected and blood glucose measured to determine insulin resistance.  The results are below:

Note:  This test done at 11 weeks

From the paper:

In line with weight gains, IP-GTT at 7 weeks on diets demonstrated that glucose intolerance and insulin resistance increased from SC < LFD < HFD < CAF (least resistant to most resistant) (Figure 3a,b), and insulin levels were significantly elevated for all diets relative to SC (Figure 3c). IP-ITT performed at 11 weeks on diets demonstrated that the insulin-mediated drop in glucose was not significantly different between diet exposures (Figure 3d,e).

I don't know about y'all, but this study is sobering to me.  I'm going to redouble my efforts to get hubby off the junk food, that's for sure!!