Today’s post reviews an interesting rat study conducted in South Korea and recently published in the World Journal of Gastroenterology. (1) This trial sought to determine whether probiotic supplementation, in this case Lactobacillus curvatus along with Lactobacillus plantarum, would lessen the ill effects of high-fructose feeding. By the way, both bacterial strains were derived from Korean fermented cabbage.
For those of you who have been living under a rock and are unaware of what fructose is, it’s the simple sugar that comprises half of the molecule making up table sugar. It’s responsible for the sweet taste that gets so many people in trouble when eaten in excess. It’s obviously also found in high-fructose corn syrup where it commonly comprises 55% of this sweetener. It’s also present in whole foods like fruit.
We, as a species, are well adapted (assuming no hereditary fructose intolerance or irritable bowel syndrome) to digest fructose in limited to moderate quantities without ill effect. This is especially true in the case of fruit, raw honey, and real maple syrup because of the antioxidant substances contained in these complete foods that temper the oxidizing properties of fructose.
Also, the fiber that accompanies fruit typically prevents its gross overconsumption. Eating four oranges at one sitting is difficult; drinking the equivalent amount of orange juice devoid of fiber isn’t.
It’s this lack of fiber that can make it quite easy to overindulge in refined sugars and if the person doing the overindulging is not engaging in exercise vigorous enough to use it for energy, it readily becomes converted to fat in the liver. In rodents, excessive fructose intake (between 30% to 70% of calories) causes high blood pressure, elevated insulin and glucose levels, elevated triglycerides, and increased production of low-density lipoproteins. (2) (3) (4)
It isn’t the purpose of today’s post to explore how large amounts of fructose, especially unbound fructose from high-fructose corn syrup, increases the formation of advanced glycation end products (AGEs), particularly in the presence of polyunsaturated lipids. However, I will briefly explain how fructose disrupts tight-junction proteins in the duodenum and jejunum.
Fructose is taken up by specific intestinal transporter cells known as Glut5 receptors located in the intestinal epithelium. Fructose is then metabolized by an enzyme known as fructokinase C (also known as ketohexokinase C and abbreviated as KHK-C).
This enzyme is found in the liver, kidneys and small intestine with the highest expression in the duodenum and jejunum. The metabolism of fructose by KHK-C generates fructose-1 phosphate, and this process results in a rapid depletion of cellular adenosine-5′ triphosphate (ATP).
ATP is the energy storehouse of the body. Liberating it allows for muscle contractions and the synthesis of many chemical compounds, for example.
However, the swift depletion of ATP produces abundant uric acid and inflammatory oxidants. Nature, in its ultimate wisdom, packages antioxidants with fructose when found in whole foods like fruit or maple syrup. Humans, being nowhere near as intelligent as Mother Nature, strip out these antioxidants when refining sugar.
While we are at the beginning of understanding how refined fructose affects the intestinal wall, there is no doubt in my mind that the rapid depletion of ATP and resulting oxidation causes increased intestinal permeability. In rats, the administration of refined fructose causes transient increases in endotoxemia. (7)
Refined fructose reduces the expression of the tight-junction protein claudin-4 in cultured intestinal epithelial cells. It also decreases gene expression for the tight-junction proteins occludin and zonula occludens 1. (8)
One of the beneficial effects of the diabetes drug Metformin is its ability to inhibit disruption of these tight-junction proteins. By doing so, it reduces intestinal permeability and endotoxemia (9). Probiotics and prebiotics do the same.
Suffice it to say that excess refined fructose consumption coupled with polyunsaturated fats and gluten—along with copious quantities of salt in the manufacture of most processed foods—is not healthy for rodents or humans.
Alright, let’s delve into this study and see whether probiotics were capable of mitigating some of this metabolic carnage.
The animals used in this trial were Male Wister rats. During the first three weeks (pretreatment phase), twenty-seven rats were fed a high-fructose diet to induce metabolic syndrome, while nine rats were fed a control chow diet. Of the 1,000 grams that composed the high-fructose rat pellets, a full 700 grams were fructose! Sweet-tea Jesus.
Be aware that people rarely ingest free fructose in these quantities without also eating/drinking glucose right along with it. However, for the purposes of quickly inducing metabolic derangement, this diet is more than adequate.
Now I would be somewhat derelict in my blogging duties if I didn’t comment on the composition of fat used in this formula. While it only comprises 4.5% of total grams, fatty acids probably had some effect on liver function and the markers seen in these metabolically challenged fructose fiends.
Assuming a fatty acid profile for the 25 grams of corn oil of 59% polyunsaturated fat (58% omega 6), 28% monounsaturated fat and 13% saturated fat and for the 20 grams of lard of 27% polyunsaturated fat, 39% monounsaturated fat and 34% saturated fat, we arrive at the following fatty acid profile: 20.15 grams polyunsaturated fat, 14.80 grams monounsaturated fat, and 10.05 grams saturated fat. Of the three fatty acid types, polyunsaturates would contribute the most to endotoxemia, liver inflammation and lipid peroxidation. Just thought I’d point that out.
OK, with that out of the way, the rats allowed to eat this sweet concoction had, by week three, developed metabolic syndrome as evidenced by elevated triglyceride levels, fatty liver, high liver cholesterol, raised plasma glucose, elevated insulin, and increased markers indicative of lipid peroxidation.
The treatment phase began on week four and lasted for two weeks. During this part of the trial, the high-fructose fed rats were divided into three groups of nine: a high-fructose group receiving a placebo, a high-fructose group receiving a probiotic formula consisting of 1 billion colony-forming units (CFUs) of Lactobacillus curvatus and Lactobacillus plantarum, referred to as the low-probiotic group (LP), and a last group receiving the same probiotic formula, but with 10 billion CFUs known as the high probiotic group (HP).
Let me begin with what didn’t change with probiotic treatment. Neither the low-probiotic nor high-probiotic treatment had any significant effect on food intake as seen in graph A. Interestingly, the high-fructose fed rats in all three groups ate less than their chow-eating cohorts. Body weight gain and epididymal fat mass (charts B and C) were not statistically different between any of the groups, although total weight gain was slightly lower in the high-probiotic group.
Chart D is where we begin to notice divergences. Both the high-fructose rats receiving a placebo and low-dose probiotics exhibited the most liver weight gain. However, the rats fed high-dose probiotics saw improvements in liver weight in comparison to the other two fructose groups.
This displays differences in plasma triglyceride levels. I do want you to notice that the chow control group experienced a slight worsening in triglyceride levels over the course of the study. As you’ll soon see in the charts that follow, they also experienced increases in plasma glucose and insulin resistance. Nowhere near what was seen in the fructose-fed rats, mind you, but deterioration nonetheless.
Pity the researchers didn’t mention what this control diet consisted of. My guess is that it contained omega-6 fatty acids and perhaps gluten.
Anyway, getting back to the fructose-fed rats, it’s obvious that fructose feeding dramatically increases plasma triglyceride levels. Nonetheless, note how both low-probiotic and high-probiotic supplementation significantly reduced these numbers.
Neither probiotic protocol, however, had much of an effect on total cholesterol levels nor HDL. Again, interestingly enough, HDL was higher in the high-fructose groups as seen in this graph:
Starting with graph A, the highest plasma glucose levels were seen in the high-fructose group not receiving a placebo. No surprise there, although again note how the control group had a harder time keeping their glucose under control. Both high-fructose fed probiotic groups experienced lower plasma glucose levels.
Plasma insulin levels were lower in both fructose groups fed probiotics with the most improvement seen in the HP group. Illustration C shows that HOMA-IR, a measure of insulin resistance, was lower in both probiotic groups in contrast to the fructose group receiving placebo.
Plasma C-peptide, another marker for insulin resistance, was again lower in the probiotic groups, but most especially in the high-probiotic cohort. Finally, plasma levels of TBARS was lowest in both probiotic groups, even when measured against the control group. Most improvement was seen in the HP group.
TBARS stands for thiobarbituric acid reactive substances and is a byproduct and marker of lipid peroxidation. The fact that this level was high in the control group leads me to suspect polyunsaturates in the chow of that group.
Both illustrations plot levels of liver triglycerides and cholesterol. Once again, supplementation of both low-dose and high-dose lactobacillus probiotics improved levels in the fructose-fed rats.
This graphic charts gene expression for beta oxidation proteins in the liver. Beta oxidation is a fancy way of saying fat burning. A liver better able to burn fat is a liver less prone to becoming marbled with it.
PPARa is a gene that encodes for peroxisome proliferator-activated receptor alpha, a major regulator of fat metabolism in this organ. Carnitine palmitoyltransferase I (CPT1) and carnitine palmitoyltransferase II (CPT2) work together to transport long-chain fatty acids into cell mitochondria to be utilized for energy. ACOX1 is a gene essential for the production of peroxisomal acyl-coenzyme A oxidase 1. This enzyme is also necessary for oxidation of long-chain fatty acids. Defects or reduced expression in any of these genes increases fat accumulation in the liver.
Compared to high-fructose fed rats given placebo, probiotics, but especially high-dose probiotics, were able to mitigate the down regulation of these genes. Significant improvements in gene expression were noted in PPARa and CPT2.
Lipogenesis is the synthesis of fat from non-fat sources like glucose and fructose. All three of these genes encode for proteins that increase fat formation. Less is definitely better when it comes to preventing fatty liver. Once again, both of these probiotic formulations were able to lower lipogenesis with the most benefit seen in the rats given high-dose probiotics.
The CYP7A1 gene encodes for cholesterol 7α-hydroxylase, an enzyme involved in the formation of bile from cholesterol. The higher the level of this enzyme, the more cholesterol is reduced to produce bile. Levels were significantly elevated in the high-probiotic/high-fructose group.
LDLR gene expression, however, was only slightly raised in the high-probiotic group. LDLR controls expression of the LDL receptor with higher amounts leading to lower LDL plasma levels. This isn’t a surprising outcome given that there were no significant differences noted in total plasma cholesterol between fructose-fed groups.
It should be clear that these two probiotic strains were together able to mitigate some of the damage caused by consuming ridiculous quantities of refined fructose in these rats. I suspect the mechanism for this reduction was mainly, if not entirely, due to lowering levels of endotoxemia by strengthening gut-barrier function. Doing so would reduce cortisol release from the adrenals, lower glucose production in the liver and reduce insulin secretion.
However, given that no indicators of lipopolysaccharide translocation or inflammatory cytokine responses were measured in these rats, this is pure conjecture on my part. That said, my earlier post on non-alcoholic fatty liver makes the case for the results witnessed here.
It would have been interesting to see what effect a combination of probiotics and prebiotics would have had in these rats. I would also have liked to see the effect of using a probiotic with more than two strains. I suspect we would have seen greater protection against fructose overconsumption.
Nevertheless, we can add this paper to the growing list of studies showing how beneficial gut microbes attenuate bad dietary choices. I don’t mean to conclude here that you can pop probiotics and ingest prebiotics and expect no ill consequences from eating a predominantly highly processed and nutrient-poor diet. Far from it.
That said, it’s clear these beneficial microbes are capable of reducing the damage many of us intentionally or unintentionally cause ourselves when engaging in questionable dietary practices. More confirmation that any hypotheses about nutrition or disease that fail to take gut flora into consideration are woefully incomplete.