Newsletter #311: Fiber, Fermentation, and the "Second Meal Effect”
You are probably already well aware that fiber is good for your health.
But you might not fully realize just how beneficial it is.
A recent meta-analysis, including a total of more than a million participants, found that consuming up to 40 grams of fiber per day was linked to a 45% reduction in risk of all-cause mortality.
Why? Well, one likely reason is that fiber can affect blood sugar regulation.
We have known for decades that when you consume fiber, your glycemic response is modulated. However, what you might not realize is that not all forms of fiber have this effect.
Odd as it may sound, water-soluble fiber specifically is able to lower blood sugar after meals because it forms a thick gel when it is mixed with fluid, like in your stomach.
(A good illustration of how this works is pectin, a soluble fiber found in fruit, which is used to make jelly because of this physicochemical property.)
Anyway, this gel sits in your gastrointestinal system, and digestive enzymes have a hard time penetrating and doing their job. This, in turn, slows the breakdown of carbohydrates and triggers the release of the hormone GLP-1, which helps your cells take up more glucose. In this way, consuming food high in soluble fiber helps tamp down glycemic “spikes” after that meal.
But here’s where things get interesting. Certain forms of fiber can also lower blood sugar responses to subsequent meals that are consumed hours later!
This has been dubbed the “second meal effect,” and it works a bit differently. As you probably know, some fibers are fermentable in the gut. When these fibers reach the large intestine, beneficial gut microbes devour them and produce metabolites, including short-chain fatty acids (SCFAs). Some very clever experiments in human subjects have revealed that these SCFAs are likely responsible for the chronic reduction in blood glucose that we see with the second meal effect.
Let’s take a look at one of the seminal studies that demonstrated this phenomenon, conducted by all-time great nutrition scientist David Jenkins, and then we’ll dig a little deeper into underlying mechanisms which have since been elucidated.
This Week’s Research Highlight
Consuming a meal high in fermentable fiber for breakfast significantly lowers the glycemic response to that meal — and that effect carries over to lunch four hours later.
In a previous trial, researchers at the University of Toronto found that supplementation with soluble fiber acutely lowered total blood sugar concentrations in response to an oral glucose tolerance test. To examine potential chronic effects of fiber on glucose tolerance, Jenkins et al recruited seven healthy volunteers and had them consume breakfasts of either lentils or whole wheat bread on separate days over a 3-month period.
Importantly, the calorie, protein, carbohydrate, and even fiber content of these two meals was almost exactly the same. But the form of fiber occurring in these foods was quite different. Whole wheat is high in insoluble fiber, which is not readily fermentable in the gut. Meanwhile, lentils are one of the best sources of prebiotic fibers, which gut microbes love to munch on.
As you can see, when the participants ate the lentils, there was a 71% reduction in total blood glucose (area under the curve or AUC). This, in turn, led to a reduced need for insulin to regulate the glucose load.
Next, the researchers had the subjects consume a standardized lunch, four hours after their breakfasts.
Here, too, we see a divergence. After consuming lentils for breakfast, the subjects exhibited a significant decrease in blood sugar levels in response to the standardized lunch, versus when they had the whole wheat bread. The total blood glucose level was reduced by 38%, and the peak glucose rise was attenuated by 34%. This impact on glucose tolerance appeared despite 1) the lentils contained roughly the same dose of fiber and carbs as the whole wheat bread, and 2) the lentils had been consumed four hours ago! Notably, a subsequent trial has shown that consuming fermentable fiber for dinner affects the blood sugar response to breakfast the next day, suggesting that this second meal effect may extend for much longer than four hours.
Now, this study was conducted more than forty years ago, and at the time they were not able to explain precisely what was going on. But we now know that this effect is being driven by gut metabolites released from fiber fermentation.
In order to understand how short chain fatty acids could enhance insulin sensitivity, we need to step back and look at some of the cellular mechanisms that underlie insulin resistance.
People with insulin resistance, and type 2 diabetes, tend to have very high levels of free fatty acids (FFAs). FFAs are fatty acids that are released from fat tissue and circulate in the blood (they’re known as "free" because they're not bound to proteins). Elevated free fatty acids can cause all kinds of havoc with respect to blood sugar control. For one thing, they cause your liver to release more glucose, which obviously makes your blood sugar go up. But free fatty acids also compete with glucose for the transporters that help shuttle sugar out of circulation and into cells. In effect, they are stealing the seats that rightfully should be occupied by glucose, leaving no room on the bus. Consequently, less glucose is able to enter your muscle cells, where you want it to be, and more of the sugar stays out in the blood. In fact, if you take healthy young people and intravenously infuse a lipid emulsion to induce high FFAs in their blood, their insulin sensitivity is dramatically reduced.
Okay, back to short chain fatty acids. As I said earlier, when you consume fermentable fibers, you feed gut bugs that generate short-chain fatty acids. When these SCFAs are released into circulation, they suppress the release of free fatty acids from fat tissue, thus indirectly enhancing insulin sensitivity. We know this because supplementation with a highly fermentable fiber has been shown to simultaneously increase serum short-chain fatty acids and reduce free fatty acids. Furthermore, directly administering SCFAs to the rectum (yikes) leads to a drop in free fatty acids.
From a practical standpoint, this mechanism matters because not all fibers are highly fermentable. To take full advantage of this phenomenon, you would want to make sure to consume some of these prebiotic fibers at least once a day. Fortunately, they’re not hard to find. Examples of fermentable fibers include pectins (found in fruit and peas), inulin (vegetables, legumes, alliums), beta-glucans (oats, barley, mushrooms), as well as some supplemental fibers like acacia and guar gum.
Random Trivia & Weird News
During a BBC news bulletin in 1930, the evening report simply said, “There is no news tonight.”
In an era of 24-hour news channels, not to mention relentless global information exchange via the internet and social media networks, the notion of there being “no news” is unthinkable for most of us — for better and for worse. Something is always happening somewhere, and we are never fully disconnected from the wider world around us.
But on April 18, 1930, listeners were informed that there was no news that evening. Instead, the BBC played piano music for fifteen minutes, then broadcast a performance of Wagner opera Parsifal.
Of course, that date was not truly devoid of events, based on other historical sources. This odd announcement probably reflects an evolution in what journalists deem newsworthy, as well as the limitations of communication technology back in the day.
From: https://www.bbc.com/news/entertainment-arts-39633603
Podcasts We Loved This Week
- Nicholas Burd: Protein science — intake, timing, and sources for muscle growth. Via The Proof Podcast.
- Luc van Loon: Optimizing muscle protein synthesis — the crucial impact of protein quality and quantity, and the key role of resistance training. Via The Peter Attia Drive.
Products We Like
Acacia Fiber
If you need some help getting enough fermentable soluble fiber in your diet, acacia is a really good option. Unlike some supplemental fibers like inulin, acacia ferments slowly, so it’s much easier on your gut. In fact, acacia fiber is routinely recommended for people who suffer from IBS or who are sensitive to FODMAPs for this very reason. It’s also very neutral-tasting and easy to add to oatmeal, yogurt, smoothies, etc.
humanOS Catalog Feature of the Week
Fiber — from The Mediterranean Diet
This week, we'd like to highlight one of the lessons from our new-and-improved Mediterranean Diet course. This lesson delves into some of the surprising benefits of dietary fiber, particularly in the context of a traditional Mediterranean diet.
Wishing you the best,
