We've been told that fiber is good for us and that we should eat more and more of it.
Fiber feeds the good bacteria in your gut. It helps you poop. It has loads of other health benefits. And we don't get enough.
But is that really the case? Is fiber an essential nutrient that we should be getting?
That is the question I will try to answer.
Tackling the myths, misconceptions and science behind fibre isn’t practical for a single post, so this is just the first part.
Here I try to answer the question of whether fiber is really necessary for gut health, from the point of view of an energy source.
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“We need to eat fiber to feed the good microbes in our gut.”
Technically, it's true.
Fiber is a carbohydrate found in plant foods that is not digested by the body. It is divided into soluble fibre, which dissolves in water, and insoluble fiber, which does not dissolve in water and is said to help move food through the digestive system.
Fiber reaches our colon undigested, where it's food for our microbiome.
The microbiome refers to the vast community of microbes that live in our gut. Fiber and other carbohydrates are fermented by these microbes, producing by-products that have many functions, including nourishing the gut lining and supporting immune function. These by-products not only feed our gut cells, but also play a key role in regulating metabolism, enhancing nutrient absorption and maintaining a healthy gut barrier.
One of the main by-products of carbohydrate fermentation is short-chain fatty acids (SCFAs).
SCFAs produced from the fibre in the gut include acetate, propionate and butyrate.
Butyrate has received a lot of attention in recent years. Although it makes up only 10-20% of the SCFAs produced from fiber fermentation, it is the preferred energy source for colonocytes, the cells that line the intestine. It is estimated that around 70% of the colonocyte energy source is butyrate.
Butyrate seems to be a very important short-chain fatty acid for our gut health.
Many studies have focused on butyrate and its role in gut health. It has been suggested to:
help reduce inflammation in the gut
support the integrity and repair of the intestinal lining (intestinal barrier)
protect against bowel cancer
regulate the immune system
influence insulin sensitivity
In addition, there is a lot of literature that clearly states that the gut cannot function properly without butyrate, as it is deprived of its main source of energy.
At the same time, we know that butyrate can only be produced from fiber.
Gut cells need butyrate, which can only be made from fiber, so fiber is an essential nutrient, right?
No, it's not that simple.
There are people who live on a diet with zero fiber. As extreme as it may sound from the perspective of mainstream nutritional guidelines, these people often do it for health reasons. And more often than not, they thrive on such a diet.
But if you don't eat fiber, won't that damage your gut, make you sick and quickly lead to colorectal cancer?
Not necessarily. Here's why.
Butyrate feeds your colon but not your small intestine.
Butyrate and other short-chain fatty acids (SCFAs) serve as the primary energy source for the colon, not the small intestine, where the activity of gut microbes is significantly lower. The small intestine is primarily responsible for absorbing nutrients from digested food, a process that does not require fiber.
The cells of the small intestine derive their energy primarily from glucose, ketones and glutamate.
Fiber is not needed for nutrient absorption. In fact, it can make it worse.
As fiber is consumed with plant foods, it is often accompanied by other substances that can interfere with the absorption of minerals and vitamins, such as phytates, oxalates or tannins. There is evidence that these substances can bind to iron, zinc, magnesium or calcium, reducing their bioavailability and making the food less nutritious.
But let's get back to the production of butyrate for the colon.
If you don't eat fiber and limit other carbohydrates, your microbiome won't produce butyrate, but your colon cells will use a different energy source.
Your colon can use isobutyrate and ketones as a source of energy.
Yes, there are other lesser known sources of energy for your colon. Isobutyrate is a similar SCFA to butyrate, but it is produced by the fermentation of amino acids (protein), not fiber. Ketones are produced by your liver when you don't eat much glucose and your insulin levels are low, such as on a ketogenic or low-carb diet.
In the absence of butyrate in the gut due to a lack of fermentable carbohydrates, isobutyrate production is increased, which feeds your intestinal lining cells.
There is evidence, that isobutyrate, may be even more potent than butyrate in stimulating the same receptors as butyrate, which are responsible for mucus secretion or immune regulation. This suggests isobutyrate may have at least the same health benefits as butyrate.
But this is less well known. You will find much less literature on isobutyrate and other gut energy sources. Why?
Most human studies are conducted on a high-carbohydrate diet.
That's why they always show high butyrate production. It's expected that if you increase your intake of fermentable carbohydrates, you'll get more butyrate. On this basis, it's generally accepted that butyrate is the preferred energy source for the gut.
More research is needed on low-carbohydrate diets to get the full picture.
As for ketones - these are not produced by the bacteria in your gut, but by your liver, and are carried by small blood vessels to your intestinal cells. It's worth noting, however, that butyrate enters exactly the same metabolic pathway as ketone bodies to be used in the TCA cycle for energy within the intestinal cell.
But what is wrong with fiber?
Why not just eat the recommended daily intake?
Why go to so much trouble to avoid it?
These are fair questions to ask at this point.
Here I just wanted to show that the claim that you need to eat fiber and other carbohydrates in order for the gut bacteria to produce SFCA to feed your intestinal lining cells is not true.
And energy-wise, your colon does just fine without fiber.
Next week we'll look at other myths and claims about fiber, its health benefits and its supposedly essential role in our wellbeing.
We'll also look at why fibre can be bad for your health.
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