As promised, I want to discuss what role, if any, dietary fat plays in metabolic endotoxemia, especially in relation to cardiovascular disease. It appears from both animal and human studies that fat indeed increases the translocation of inflammatory lipopolysaccharides (LPS) from the gut into systemic circulation. What needs to be determined is whether all or only some fats do this and if so how. Let’s begin with what the research shows.
In a pioneering study that appeared in 2007, feeding mice a high-fat diet increased endotoxemia 2.7-fold in comparison to mice fed regular rodent chow. 72% of energy in the high-fat rodent diet came from a mixture of corn oil and lard. Levels of LPS were consistently higher in these animals. They also became fat and developed insulin resistance. (1)
Let me clarify that corn oil and lard are two different types of fat. Corn oil is a long-chain, mostly polyunsaturated fat, and lard is a mostly monounsaturated long-chain fat. Yes Virginia, the predominant fat in lard is monounsaturated oleic acid, the same fat in olive oil, not saturated fat.
Also observed were changes in the intestinal microbiota including reduced levels of bifidobacteria. As I’ve already blogged, this is important because bifidobacteria produce short-chain fatty acids that promote a healthy gut wall thereby decreasing gut permeability (2) However, in mice fed a diet containing the same composition of fat but comprising only 40% of energy, the increase in endotoxin levels was a modest 1.4-fold higher.
In the same study, LPSs were given orally to wild-type mice with either oil or water. Blood levels of LPSs were elevated in the mice fed the oil mixture again implicating fat ingestion with endotoxemia.
To see if LPS alone was enough to cause the identical symptoms seen in high-fat fed mice, some animals were fed standard chow and infused continuously with LPS to reach the same levels seen in the high-fat fed rodents. These mice gained the same amount of weight and showed similar levels of dysregulated glucose control and insulin resistance. Both the high-fat fed and LPS infused mice experienced increases in inflammation in both muscle and fat tissue.
Finally, the researchers used mice lacking CD14 immune receptors to see if these mice would develop obesity and insulin resistance when infused with either LPS or fed the same high-fat diet. CD14 is a molecule that senses the presence of LPS and initiates an immune cascade. These CD14 knock-out mice were completely immune to both the LPS infusion and the high-fat feeding. These types of mice are also naturally hyper-sensitive to insulin when fed a normal diet suggesting that the CD14 receptor is involved in insulin sensitivity.
Studies done with mice given antibiotics to kill their gut flora show that they are totally resistant to the ill effects of a high-fat diet. In these animals, fatty liver and markers of inflammation went down while insulin sensitivity went up after elimination of gut bacteria. (3) As in the case of both non-alcoholic and alcoholic fatty liver disease, translocation of gut bacteria is a necessary part of the equation.
These elevated LPS levels have also been observed in humans. Endotoxemia is 2-fold higher in BMI-, sex-, and age-matched type 2 diabetics than in non-diabetics. Fasting insulin also significantly correlates with metabolic endotoxemia even when controlling for sex, age and BMI. (4)
What research exists to implicate fat intake with raised LPS levels in humans?
One very small study sought to answer this question using twelve “healthy” men as participants. (5) Healthy is in quotes because all 12 men self-identified as occasional smokers. Granted, the researchers were also trying to determine whether endotoxins from smoking affected systemic LPS levels. Nevertheless, this is a wee bit of a confounding variable knowing what we know about smoking’s effect on oral and respiratory flora.
Another confounder that left me chuckling and shaking my head (I’m easily amused) was that they fed these men their fat (LPS enriched butter) on toast. Hmm, the last time I checked, gluten-grains have some really negative effects on intestinal permeability.
What were the results? The researchers noted a significant transient ten-minute increase in plasma endotoxin levels from baseline after the fatty meal. The highest concentrations were found in the “healthy” men who had the high-fat meal and smoked three cigarettes in a four-hour period.
However, the researchers also observed a very rapid clearance of these same bacterial toxins. They also didn’t see any increase in the cytokine tumor necrosis factor or an increase in C-reactive protein, a marker for inflammation. However, the authors cautioned that the four-hour duration of the study may not have given them enough time to witness any increases in inflammatory markers.
Another study showed that ingesting a breakfast containing various types of emulsified (liquid), and non-emulsified fats resulted in transient increases in plasma LPS with higher levels seen when emulsified liquids like olive and safflower oil were eaten in contrast to butter. But again, this study used French bread (it was a French study after all) as a vehicle for the fat. (6)
What explains these findings? There are two ways that gut pathogens can cross the gut wall. They can enter the bloodstream between the cells lining the digestive tract due to increased intestinal permeability or “leaky gut”; alternatively, they can be incorporated into vehicles that transport fat from absorptive cells to systemic circulation. It is this latter route that some researchers believe may partly account for these findings and it’s what I want to cover now. So pardon me, dear reader, as I review the exciting subject of fat digestion.
Because fats are hydrophobic or “water fearing”, they are insoluble in blood and present a special problem for digestion. Of the fats present in the standard diet, 95% are triacylglycerols (triglycerides). This type of fat consists of three fatty acids attached to a glycerol molecule.
Fat digestion begins with the secretion of the enzymes lingual lipase produced by salivary glands in the mouth and gastric lipase produced in the stomach. These enzymes account for about 10% to 30% of fat digestion acting primarily on short- and medium-chain fatty acids.
Apart from their classification as either predominantly saturated, monounsaturated or polyunsaturated, fats or lipids can also be classified by how many carbon atoms form them. Short-chain fatty acids contain between four to six such atoms. All short-chain fatty acids, including the highly beneficial ones produced by friendly bifidobacteria in the colon, are saturated.
Medium-chain fats are between eight and twelve carbon atoms in length. Found in butterfat and tropical fruit oils like coconut oil, they are rich in antimicrobial substances that inhibit pathogen growth.
Next come long-chain fatty acids that range in length from fourteen to twenty-four carbon atoms. These can be either saturated, monounsaturated or polyunsaturated. Finally, we have very-long-chain fatty acids that have between twenty-four to thirty carbon atoms.
All fats require a certain amount of emulsification in order to be broken down and digested. To emulsify lipids is a fancy way of saying that larger globules of fat are converted into small droplets like what happens when you whisk or shake together oil and vinegar to create a vinaigrette. These smaller fat droplets are called micelles.
Short- and medium-chain fatty acids are partially emulsified by the action of stomach contractions and from sheer force when they are squirted through the pyloric sphincter into the duodenum. However, most emulsification of fat, including long-chain fats, occurs in the small intestine as a consequence of bile released from the gallbladder. Phase two of fat digestion begins when the pancreas releases lipase to complete the breakdown of triacylglycerols to diglycerides and finally to fatty acids and monoglycerides.
I need to interject here that in order for the gallbladder to eject bile and the pancreas to release digestive enzymes, cholecystokinin or CCK, a gut hormone produced by the L-cells of the small intestine, must be released first as a signal to do so. Therefore, any dysfunction in the L-cells of the small intestine will result in the maldigestion of dietary fat.
As I wrote in this post, opioids from whatever source suppress gut hormone secretion. I would imagine gluten-derived adenosine has the same effect. Small intestinal dysbiosis will also impair CCK release which is why protein and fat malabsorption are so common in those who have small intestinal bacterial and yeast overgrowth. And a gallbladder that does not contract regularly runs the risk of forming concentrated and hardened bile, i.e. gallstones.
CCK also has well-known satiating effects, making you feel full when fat and protein are passing through the first and middle sections of the small intestine. Any impairment in its production or release will also blunt these appetite-suppressing signals.
Once bile and pancreatic enzymes emulsify ingested fat, lipid micelles are sufficiently water soluble to enter the absorptive cells of the small intestine. Like digested carbohydrates and proteins, short- and medium-chain fatty acids pass directly from the enterocyte into the blood and continue on to the liver.
Longer-chain fatty acids, however, re-form into triacylglycerols within the enterocyte. Together with phospholipids, cholesterol and proteins, they form large particles within the absorptive cell. If there are lipopolysaccharides in the vicinity, these too hitch a ride on the newly formed lipoprotein vehicle or chylomicron. Here is a diagram of this process minus the addition of LPSs:
And here is what the newly formed chylomicron looks like:
See those protein fragments in the exterior or phospholipid layer? These are composed of both apolipoproteins and lipopolysaccharide binding protein or LBP. It is the LBP protein that potentially attaches to lipopolysaccharides and brings them along for the ride, so to speak.
Keep in mind that in a healthy small intestine, there should be negligible amounts of gram-negative bacteria, certainly none colonizing the gut wall or found in any large number within enterocytes. It’s the job of friendly gut flora to make the gut wall inhospitable to these types of bacterial pathogens as well as yeasts like Candida.
The extensive small intestinal immune system or GALT complex, should also inactivate most of these pathogens before they enter enterocytes or cross the gut wall. Nevertheless, even in a healthy digestive tract, there will always be some gut pathogens that appear in the lumen so it’s likely that some translocation of LPS will always occur. The situation is worse, however, if there is an overgrowth of gram-negative pathogens due to dysbiosis.
After forming, chylomicrons exit the enterocytes, enter the lymphatic system, travel through the thoracic duct and are eventually released into systemic circulation. These chylomicrons can continue entering the bloodstream for up to 14 hours following a high-fat meal. Peak levels of fat in blood plasma occurs anywhere from 30 minutes to three hours after eating, returning to near normal within five to six hours. It is chylomicrons that accounts for the milky appearance of blood plasma after a meal containing fat.
As these chylomicrons access various non-liver tissue sites, an enzyme called lipoprotein lipase is released causing the triacylglycerols in these lipoproteins to break down where they are then absorbed by either muscle, including the heart, for use as fuel or conversely stored as fat in adipose tissue in periods of caloric excess.
Once chylomicrons are depleted of triacylglycerols, they return to the liver as a chylomicron remnant with their load of attached LPSs. Once in the liver, some of their components may be recycled for other uses while the bound lipopolysaccharides are excreted into bile and feces.
And that, dear reader, is how fat is digested and one way LPS enters systemic circulation.
So yes, long-chain fatty acids and the chylomicrons produced to transport them increase systemic LPS. But should this alarm you? Not in the least. Why?
Given the fact that human beings have been eating long-chain fatty acids for a very, very long time, it’s absolutely absurd to believe that we evolved without some mechanism to protect us from possible bacterial translocation when digesting fat. (7) To think otherwise is to believe that dietary fat is out to kill us and that nature or [insert deity here] made a horrible mistake.
So what would that defense mechanism be? The very vehicles that transport fat and cholesterol through the blood: lipoproteins.
Most people, and I include the overwhelming majority of medical professionals, researchers, health gurus, dietitians, nutritionists and health bloggers are clueless to the fact that all carriers of triacylglycerols and cholesterol are part of our innate immune system. How anyone can graduate with a medical degree in this day and age and not know this is absolutely mind-boggling to me.
All lipoproteins—chylomicrons, VLDL, IDL, LDL and HDL—bind to and i-n-a-c-t-i-v-a-t-e lipopolysaccharides. (8) They also protect against gram-positive bacterial pathogens, viruses and parasites.
And yes, that includes “bad” LDL cholesterol, the lipoprotein that has been vilified within an inch of its life. Do you honestly believe that nature designed our livers to produce a substance that is out to kill us? Really?
Take two sets of mice and give each of them a lethal dose of gram-negative E.coli. Give one set of mice an infusion of human-derived chylomicrons and VLDL and the survival rate of these mice is 100%. In other words, they don’t die from an otherwise deadly dose. Infuse them with human LDL and HDL and you get the same result. (9)
Want another example? OK, how about using some LDL receptor deficient mice. Because they have been genetically bred to lack the LDL receptor, they have unusually high levels of LDL coursing through their little arteries and veins. The blood work from these rodents would cause the typical cardiologist to suffer a conniption fit.
Now pair these LDL-engorged mice against normal wild-type mice. Once again, give both sets of mice an infusion of gram-negative E. coli. The mice with very high levels of LDL cholesterol will have a significantly increased survival rate. These LDL mice don’t begin dying off until their endotoxin levels are eight times higher than their American Heart Association-approved litter mates. Their levels of circulating inflammatory cytokines are also far lower.
Now, with the same set of mice, infect them with Klebsiella pneumoniae, another gram-negative pathogen that is commonly found in arterial plaque. Again, the LDL mice show increased survival rates. After acute infection, only 42% of the mice with higher LDL levels die as opposed to 67% of the controls.
But what about chylomicrons?
These lipoproteins are particularly potent inhibitors of LPS and have also been found to neutralize and detoxify gram-positive pathogens. (10) Remember Streptococus mutans? The oral pathogen that causes cavities and is often found in arterial plaque? It’s a gram-positive pathogen that would be inactivated by chylomicrons.
This chart shows what happens when you incubate E. coli with chylomicrons derived from healthy human volunteers. The levels of inflammatory cytokines, interleukin 8 represented by circles, and tumor necrosis factor alpha, represented by squares, are graphed against an increasing concentration of circulating chylomicrons. What you see here is that the higher the percentage of chylomicrons, the lower the level of inflammatory cytokines elicited by blood immune cells.
As mentioned above, all lipoproteins neutralize endotoxins but none, including saintly HDL, holds a candle to chylomicrons.
The left bar labeled Blanc, represents LPS-rich blood plasma devoid of lipoproteins. As a result, it shows the highest concentration of the inflammatory cytokine tumor necrosis factor (TNF). Here you see that HDL binds some LPS but not much. VLDL and LDL cholesterol come next in inactivating LPS and reducing levels of TNF. But look at chylomicrons. When it comes to inactivating these bad boys, blunting the inflammatory response and escorting them to the liver for elimination from the body, they are truly the outstanding bacterial scavengers of the body.
Gee, you don’t think this has anything to do with the fact that the higher your total cholesterol, the less likely you are to die from an infection do you?
This chart is courtesy of Portuguese blogger Ricardo Carvallo or O Primitivo. Unfortunately, his site appears to be undergoing a transition so I pulled this graphic from the Perfect Health Diet Blog written by Paul Jaminet. Paul blogged about this here.
What I want to emphasize is something that goes unmentioned by the anti-cholesterol crowd. Please look at the U-shaped blue line on the chart. Those people with total cholesterol between 200 and 240 had the lowest level of death from all causes. Those with cholesterol above these levels experienced increases in mortality. However, look at the left side of the curve. Those with lower cholesterol levels also see a jump in all-cause mortality.
Cardiovascular disease, represented by the broken red line, showed a weak correlation with total cholesterol and was similarly U-shaped. Note that those with total cholesterol levels below 190 experienced “higher” levels of cardiovascular disease than those who had total cholesterol in the 190 to 220 range.
There is something else very interesting about this graph. See the green dotted line in the lower-left-hand corner? This tracks deaths from infectious and parasitic diseases. The lower the total cholesterol, the higher the mortality which makes perfect sense since cholesterol is part of the immune system. Lower it, and you compromise your defense against both infections and parasites. It’s also the case that infections and parasites lower total cholesterol which indicates that cholesterol is a marker for infection.
So does this mean cholesterol has nothing to do with heart disease? Well, let me ask you this. Do firefighters have anything to do with fire? I certainly have never seen a structure or wild fire without firefighters present at the scene, have you? So we can say that fires are highly associated with fireman. However, it would be absolutely absurd to jump from this common sense observation to the conclusion that fireman are the cause of fires and that if we reduced their numbers we would lessen the incidence of destructive conflagrations.
It isn’t the purpose of this post to debunk the cholesterol hypothesis of heart disease. That would require a book. If you’re interested in exploring this topic further I recommend Anthony Colpo’s The Great Cholesterol Con, although there are plenty of other people questioning this hypothesis.
Dysregulated cholesterol (and that includes oxidized LDL), cortisol and glucose levels are all symptoms of metabolic endotoxemia, not the cause. Last time I checked, lowering cholesterol had no magical ability to cure small intestinal dysbiosis. Statins work because they are anti-inflammatories and reduce the tendency to form clots. Unfortunately, because they reduce cholesterol and prevent the synthesis of important substances like CoQ10, they have a number of serious side effects. (11)
I hope I’ve made clear that chylomicrons are not the likely source of translocated pathogens that appear in arterial plaque. On the contrary, they protect you from these pathogens by inactivating them and rapidly removing them from the bloodstream. We need to look elsewhere for an explanation of how these pathogens are entering systemic circulation. That “elsewhere” is via increased intestinal permeability spilling contents of the gut lumen into blood flowing to the liver. It is the liver, after all, that receives the full frontal assault from a “leaky gut”.
I’ve previously discussed how impaired gastric-barrier function, excessive alcohol intake, stress and gluten grains increase intestinal permeability both directly and by their promotion of bacterial and yeast overgrowth. However, there is another common dietary component that appears to affect intestinal permeability for the worse: polyunsaturated vegetable oils. That’s the topic of the next post.
Enig, M. G. (2000). Know Your Fats : The Complete Primer for Understanding the Nutrition of Fats, Oils and Cholesterol. Silver Spring: Bethesda Press.
Gropper, S. R., Smith J. L., Groff J. L. (2009). Advanced Nutrition and Human Metabolism. Belmont, CA: Wadsworth Cengage Learning.