Cholesterol is a double-edged sword. On the one hand, you can’t survive without it: It’s present in every membrane of every cell in your body and does a wide range of jobs.
On the other hand, if you have higher levels of certain forms of cholesterol in your blood, you have an increased risk of heart problems.
Specifically, higher levels of low-density lipoprotein, or “bad” cholesterol, are linked to greater heart disease risk.
By contrast, increased levels of high-density lipoprotein, or “good” cholesterol, are associated with lower cardiovascular disease risk.
Because heart disease is the leading cause of death globally, identifying ways to improve blood cholesterol levels is vital.
Experts know that being more physically active and replacing saturated fats with healthy fats, like mono- and polyunsaturated fats, can quickly reduce levels of "bad" cholesterol.
And medications, like statins, are often very effective.
Still, around 40% of people in the United States and United Kingdom have high cholesterol levels. So, figuring out which factors impact blood cholesterol levels is important.
To this end, some scientists are looking into the role of gut bacteria.
Cholesterol and gut bacteria
Your gut microbiome consists of trillions of microorganisms, including bacteria. And everyone’s population of bacteria is unique.
Over the years, scientists have identified links between gut bacteria and a range of health outcomes, including cholesterol levels.
We also found that less healthy gut microbiome profiles were associated with forms of cholesterol linked to negative health outcomes.
Although these links are associations rather than causes, scientists now understand how some gut bacteria might influence blood cholesterol levels.
How do gut bacteria alter cholesterol levels?
At first glance, it’s difficult to imagine how levels of cholesterol in your blood and the bacteria in your gut might be related.
But we must remember that your body’s systems don’t operate alone, they're interrelated.
So, is there any evidence that your gut microbiome can influence cholesterol levels in your blood?
One fascinating study showed that if you transfer gut bacteria from a human with high blood cholesterol into a mouse, the mouse develops high blood cholesterol.
Now, as you know, what happens in a mouse doesn’t always relate to what happens in a human, but it suggests that the gut microbiome might be involved.
And scientists have outlined a few ways that your gut bugs might influence your cholesterol levels. To explain them, we’ll get into some interesting, if a little complicated, biology.
So, if you’re ready, let’s jump in. First, let’s look at a difficult to pronounce compound called "coprostanol."
Certain species of gut bacteria can convert cholesterol into coprostanol. In fact, scientists have known about this since the 1930s.
Why does this matter? Well, if cholesterol is converted into coprostanol in your gut, it can’t be absorbed into your blood. So, it exits in your poop.
And if it’s leaving your body and not entering your blood, this is one way that gut bacteria can potentially influence your blood cholesterol levels.
This is partly because not all gut bacteria can convert cholesterol into coprostanol. In line with this, studies comparing low- and high-converting individuals found differences in their microbiomes.
And there’s some older evidence, from the 1980s, that people whose gut bacteria convert less cholesterol to coprostanol have higher blood cholesterol levels.
At this stage, although it seems likely that gut bacteria and coprostanol play a part in blood cholesterol levels, many questions remain.
A quick tangent
Scientists use coprostanol as a marker for human waste. By checking the environment for this compound, experts can tell whether human waste has been leaching into its surroundings.
And because coprostanol is a very stable molecule, archaeologists can use it to work out whether humans lived in an area in the distant past. They can even use it to estimate population changes over time.
OK, back to the main event.
As we mentioned earlier, cholesterol is a vital building block for the membranes of your cells. In general, though, bacterial cell membranes don’t contain cholesterol.
However, when cholesterol is around, some bacteria absorb it. This is called, rather dramatically, cholesterol entrapment.
In laboratory studies, scientists have shown that several strains of Lactobacillus bacteria — common gut bacteria — entrap cholesterol from their surroundings and incorporate it into their membranes.
Scientists aren’t sure exactly how or why this happens. But some think that when bacteria add cholesterol to their membranes, it might help protect the bacteria from being broken down.
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Interestingly, there’s evidence that even dead or resting bacterial cells can entrap cholesterol.
Regardless of why it happens, sucking up cholesterol from your gut is another way that gut bacteria might affect your blood cholesterol levels — if bacteria take it up, cholesterol can’t get into your blood.
But we still don’t know enough about it to understand how much of a role this process might play.
Bacteria and bile
Your gut bacteria's interaction with bile might also influence your cholesterol levels. To understand this interaction, here’s some quick info about bile and bile acids.
Bile helps break down fats in your diet. Without it, most fats would pass right through and come out in your poop.
Your liver continuously produces bile, and this is stored in a pouch-like organ called the gallbladder. You make around 600 milliliters (2.5 cups) of this important stuff each day.
In the gallbladder, bile becomes more concentrated before getting released into your intestines when a fat-containing meal arrives.
Bile has several ingredients, but bile acids are the active ingredients for breaking down fats.
And, importantly for this article, one of bile’s main components is cholesterol.
After they’ve done their digestive job in your gut, most bile acids make the journey back to your liver to be recycled. But about 5% stay in your gut, where they feed gut bacteria.
Gut bacteria break down bile acids into so-called secondary bile acids. And these compounds (there are more than 20) appear to have a wide range of activities.
Some bile acids send signals to the liver, telling it to reduce bile synthesis. And because making bile requires cholesterol, if the liver makes less bile, there’s more cholesterol around, and blood cholesterol levels might rise.
It’s a convoluted system, though: Not all bile acids and secondary bile acids send signals of the same strength, so they have varying influences on bile synthesis.
Secondary bile acids seem less likely to reduce bile synthesis, so having more of them around is likely to improve your cholesterol levels.
And because these are produced by gut bacteria, experts believe that having an “unbalanced” gut microbiome might have a negative effect on cholesterol levels.
Scientists are still figuring out exactly what’s going on. Needless to say, it’s complicated.
But it seems likely that ratios of bile acids to secondary bile acids have some sway on your blood cholesterol levels.
Overall, researchers are still working out the details. But coprostanol, entrapment, and secondary bile acids all seem to alter our cholesterol levels. This leads us to ask:
Could probiotics lower cholesterol?
Probiotics are live microorganisms that, when consumed in large enough amounts, provide health benefits.
Probiotics exist in a range of fermented foods, like natural yogurt, some cheeses, kimchi, and sauerkraut. They’re also available in capsules and powders.
So, if gut bacteria can influence cholesterol levels, could probiotics help manage these levels? Over the past decade or so, some scientists have investigated.
Several animal studies have hinted that some probiotic strains might help manage cholesterol levels. And a smaller number of human trials have also provided a glimmer of hope.
For instance, one study included 114 participants with high cholesterol. Twice a day for 6 weeks, the researchers gave participants either "placebo" yogurt or yogurt containing a species of Lactobacillus bacteria.
Compared with the placebo group, the probiotic yogurt group had an almost 5% reduction in overall cholesterol and a nearly 9% reduction in “bad” cholesterol.
A review from 2018 looked at 32 relevant randomized controlled trials that together included almost 2,000 participants.
Overall, the researchers concluded that probiotics could reduce total blood cholesterol levels. They also found that probiotics were most effective for people who had higher cholesterol levels at the start of the study.
A mini-review from 2019 also noted that probiotics seemed to reduce levels of overall and “bad” cholesterol. But they found that not all probiotics seem to work.
How long people take probiotics and how much they take are likely to make a difference, too.
What should you do?
It’s becoming increasingly clear that gut bacteria play a role in a wide range of systems in your body. And this includes maintaining blood cholesterol levels.
By producing coprostanol, trapping cholesterol, and breaking down bile acids into secondary bile acids, gut bacteria seem likely to influence these levels.
Although it’s not 100% clear whether probiotics help reduce cholesterol levels, at ZOE, we know that adding probiotic fermented foods to your diet is a healthy choice.
Whether probiotic foods reduce cholesterol or not, they’re likely to benefit your health, so it’s worth including them in your diet.
ZOE co-founder Prof. Tim Spector recommends having small portions of fermented foods every day. You might try the four K’s: kefir, kombucha, kimchi, and 'kraut. Or you might prefer plain yogurt with live cultures and smelly cheeses.
If you're worried about your cholesterol levels, make sure you speak with a doctor. And if you’d like to learn more about how to eat the right foods for your body, take our free quiz.
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