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Sunday, 1 February 2015

The Guinea Pig Model of Atherosclerosis

This is the kind of research that doesn't get as much attention as it used to.
Animal models are vital ways of testing theories that it would be unethical to test on humans, and when theories or novel chemicals are tested on unsuspecting humans, we refer to those humans as "guinea pigs". Not mice, rabbits, rats or any other rodent, but cavia porcellus. The guinea pig was used by Lavosier to demonstrate, by melting snow around his calorimeter, that respiratory gas exchange is a combustion, and by Pasteur, Roux, and Koch in their germ experiments, but fell into neglect at the end of the 20th century: only 2% of laboratory animals in the USA are currently guinea pigs.

In the early history of the lipid hypothesis, the rabbit model of atherosclerosis was developed by Anitschkow in 1911:

"When fed fat and cholesterol, rabbits develop high TC levels and subsequent fatty deposits in their blood vessels. When cholesterol is taken out of their diet, TC levels generally reduce and the fatty deposits may regress. If not used as conclusive evidence as to the process in humans, such experiments are said to be supportive of the theory that under conditions of high TC, cholesterol is more likely to be deposited in human arteries."[1]

The amount of cholesterol fed in these experiments - 0.2% or 0.25% of dry matter - probably exceeds what a human could consume, and of course rodents in nature have a minimal exposure to dietary cholesterol. The lesions seen do not correspond exactly to human atherosclerosis, and the role of saturated and unsaturated fats, or of foods like butter, with regard to progression in rabbits does not always match the lipid hypothesis predictions.

In 2006 Maria Luz Fernandez and Jeff Volek published a paper which should have stirred things up:
"Carbohydrate restricted diets have been shown to reduce plasma triglycerides, increase HDL cholesterol and promote the formation of larger, less atherogenic LDL. However, the mechanisms behind these responses and the relation to atherosclerotic events in the aorta have not been explored in detail due to the lack of an appropriate animal model. Guinea pigs carry the majority of the cholesterol in LDL and possess cholesterol ester transfer protein and lipoprotein lipase activities, which results in reverse cholesterol transport and delipidation cascades equivalent to the human situation. Further, carbohydrate restriction has been shown to alter the distribution of LDL subfractions, to decrease cholesterol accumulation in aortas and to decrease aortic cytokine expression. It is the purpose of this review to discuss the use of guinea pigs as useful models to evaluate diet effects on lipoprotein metabolism, atherosclerosis and inflammation with an emphasis on carbohydrate restricted diets."[2]

Rats and rabbits, on the other hand, don't resemble humans in the way they disburse lipids. The LDL fraction is tiny, they lack CETP and lipoprotein lipase, and generally diverge from human measurements in ways guinea pigs, it seems, don't. Even though rabbits and guinea pigs have pretty much the same natural diet - grasses, and their own poop. Guinea pigs and humans, unlike rats and rabbits, also can't synthesise vitamin C. Is there an orthomolecular connection here?
OMG put that pig on statins stat!
Long story short, guinea pig lipids react to lipid lowering drugs, PUFA and fibre in the same way and via the same mechanisms that produce effects in humans. If you feed a guinea pig cholesterol, fat, and carbohydrate it gets atherosclerosis. If you feed the guinea pig a very low carbohydrate diet (20%E SFA in these experiments - 10%CHO,65%FAT,25%PRO ) it's protected from cholesterol-induced atherosclerosis and inflammation. If you feed it a low-fat diet (55:20:25) the atherosclerotic syndrome progresses [3].

Most recently, the low carb diet reverses the metabolic alterations induced in guinea pigs by high-cholesterol feeding: the high-carb diet doesn't.

"Higher concentrations of total (P < 0.005) and free (P < 0.05) cholesterol were observed in both adipose tissue and aortas of guinea pigs fed the HC compared to those in the LC group. In addition, higher concentrations of pro-inflammatory cytokines in the adipose tissue (P < 0.005) and lower concentrations of anti-inflammatory interleukin (IL)-10 were observed in the HC group (P < 0.05) compared to the LC group. Of particular interest, adipocytes in the HC group were smaller in size (P < 0.05) and showed increased macrophage infiltration compared to the LC group. When compared to the H-CHO group, lower concentrations of cholesterol in both adipose and aortas as well as lower concentrations of inflammatory cytokines in adipose tissue were observed in the L-CHO group (P < 0.05). In addition, guinea pigs fed the L-CHO exhibited larger adipose cells and lower macrophage infiltration compared to the H-CHO group."[4]
Why the guinea pig is such a good model is explained by Maria Luz Fernandez in this 2001 paper, which predates her collaboration with Jeff Volek.[5]
Researchers at our laboratory and other investigators have found that guinea pig cholesterol metabolism does indeed have some analogies to human cholesterol metabolism that merit discussion.
Some of these similarities include the following:
  1. Guinea pigs have high LDL-to-HDL ratios (Fernandez et al. 1990a).
  2. They have higher concentrations of free than of esterified cholesterol in the liver (Angelin et al. 1992).
  3. They have plasma cholesteryl ester transfer protein (CETP)2 (Ha et al. 1982), lecithin-cholesterol acyltransferase (LCAT) (Douglas and Pownell 1991) and lipoprotein lipase (LPL) (Olivecrona and Bengsston-Olivecrona 1993) activities for intravascular processing of plasma lipoproteins.
  4. They exhibit comparable moderate rates of hepatic cholesterol synthesis (Reihner et al. 1990) and catabolism (Reihner et al. 1991).
  5. Similar to humans, the binding domain for the LDL receptor differentiates between normal and familial binding defective apolipoprotein (apo)B-100 (Corsini et al. 1992).
  6. Apo B mRNA editing in the liver is negligible (Greeve et al. 1993)
        7. They require dietary vitamin C (Sauberlich 1978).       
         8. Females have higher HDL concentrations than males (
Roy et al. 2000).
         9. Ovariectomized guinea pigs have a plasma lipid profile similar to that of postmenopausal women (
Roy et al. 2000).         
         10. During exercise in guinea pigs, plasma triacylglycerol (TAG) decreases and plasma HDL cholesterol (HDL-C) increases (
McNamara et al. 1993).
           11. Guinea pigs respond to dietary interventions (
Fernandez and McNamara 1992b1992a and 1995aHe and Fernandez 1998a) and drug treatment (Berglund et al.1989Hikada et al. 1992) by lowering plasma LDL cholesterol (LDL-C)

What we have here is a story of good science that should be better known. Maria Luz Fernandez develops the modern guinea pig lipoprotein model, Jeff Volek recognises its value for testing the carbohydrate hypothesis of atherosclerosis and the safety of LCHF diets, and Fernandez sees the value of such testing in adding to our knowledge of cardiovascular disease and lipid and carbohydrate contributions to inflammation.

[1] Flaws, Fallacies and Facts: Reviewing the Early History of the Lipid and Diet/Heart Hypotheses, Elliot J 2014. Food and Nutrition Sciences. Vol.5 No.19, October 2014 link

[2] Guinea Pigs: a suitable animal model to study lipoprotein metabolism, atherosclerosis and inflammation. Fernandez ML and Volek J. 2006  Nutrition and Metabolism 3:17 doi:10:1186/1743-7075-3-17 link
[3] Low-carbohydrate diets reduce lipid accumulation and arterial inflammation in guinea pigs fed a high-cholesterol diet. Leite JO et al. 2009 Atherosclerosis. 2010 Apr;209(2):442-8. doi: 10.1016/j.atherosclerosis.2009.10.005 link

Cholesterol-induced inflammation and macrophage accumulation in adipose tissue is reduced by a low carbohydrate diet in guinea pigs. Aguilar D. et al. 2014 Nutr Res Pract. 2014 Dec;8(6):625-631  link
Guinea Pigs as Models for Cholesterol and Lipoprotein Metabolism. Fernandez, ML 2001 J Nutr. Jan 1 2001 Vol 131 no.1 10-20 link


Anonymous said...

Thanks for this, George. Apart from all the reading you've provided via Fernandez and co., you have me wondering if the mutant mice that ApoE researchers like using, are in fact the most appropriate animal to use.

Re human lipids - I've had difficulty in the past trying to figure out the mechanism(s) and evolutionary logic behind the reduction of plasma cholesterol after ingesting PUFAs.

The logic interests me most, I guess. It's fine to say that LDL receptor expression increases (or whatever), but nicer to know why.

I see Fernandez has some good stuff on this, and I am yet to read those papers properly, however I noticed that a few years ago you made a comment on Peter's blog suggesting it was the cholesterol being taken by cells (from the plasma) in order to improve membrane stability. Others suggested the body was simply trying to get rid of the PUFAs as quickly as possible - which I s'pose could be true but sounds a bit simplistic.

Has your opinion changed since then? I've got this nagging feeling I've asked you this before. Apologies if I have.

Puddleg said...

Keys of blessed memory said that human diets don't supply enough PUFA (at 3-7%E) to lower LDL. He thought it was all about SFA.
I don't really know why it happens, but getting rid of excess PUFA makes sense.
Cholesterol forms esters with PUFA so maybe the LDL-R is upregulated to provide extra cholesterol to sequester the PUFA. For this reason also (I think) much of the PUFA 2-carbon is turned into cholesterol.
So it can sequester the rest of the PUFA in neutral lipid form?

Anonymous said...

Sounds fair enough.

Not sure if Rod is a mate of yours, but perhaps you can clear one more thing up for me - is "professor of epidemiology" more or less prestigious than "professor of astrology"?

Or "economics". Or "political science" etc etc.

Puddleg said...

Well I like Rod, and he does know what he knows, he was there when it all went down and deserves to be listened to for that reason alone, but I think he's a bit wrong.
Good on him for drawing attention to Nina's book in NZ, and the comments are wonderful.

dr j said...

any idea why a pure potato diet for 6o days leads to a blood panel that is as impressive in trigs as a LCHF diet? He gives the raw data in 3 attachments

Puddleg said...

Well you can't really make trigs without dietary fat or FFAs in any great quantity - DNL alone won't support that.
So I expect after a few days on potatoes fat becomes relatively scarce and there simply isn't enough to make much in the way of TGs no matter how much glucose or insulin there is to catalyse TG synthesis.

raphi said...

I really like the historical perspective you provide. It's all the more valuable in a time where information overload (not darth) is mostly problematic.

Maybe this is worth making into a series?