We’re consistently told to add more fiber to our diets. Increasing your fiber intake is healthy for you, and Western diets are notoriously low in it.
But what is fiber? Does it add calories? How is it healthy for me, and how much should I be eating?
In this post, we dive into some of these details, and more.
Fiber is sometimes grouped into the “carbs” category, but this can often be misleading. Here’s a better way to determine what is fiber: all of the polysaccharides that are not digested by human enzymes.
What does that mean?
“Poly” means “many,” and “saccharides” are sugars. The term is used to describe a molecular structure that essentially contains long strands of many sugar molecules joined together.
Some of these we can digest. Our bodies secrete enzymes that degrade these sugars and we can use them for energy.
But some of them our bodies can’t break down. These are the class of fibers. They can include resistant starches (as the name suggests, starches that resist digestion) and nonstarch polysaccharides like cellulose, hemicellulose, pectin, lignins, gums, alginates, carrageenans, and mucilages. Generally, you will find most of your dietary fiber in fruits and vegetables.
Insoluble fiber is one way to classify fibers. These fibers don’t mix well with water, and pass relatively quickly through the GI tract.
They are still helpful, though, primarily helping you stay regular. Cereal fibers tend to have relatively more insoluble fibers.
Soluble fiber is another way to classify fibers. These fibers mix well with water, although they don’t break down completely, forming colloids.
Soluble fibers are often associated with many of the healthy benefits we’ll discuss shortly. These pass much slower through the GI tract.
Fruits, vegetables, and nuts tend to have relatively more soluble fibers.
A shorthand estimate for caloric fiber is often put at 2 calories per gram. This can be a useful estimate, but the real story is more complex.
The definition of fiber, stated earlier, says it all. Humans, by themselves, cannot digest fiber. So, how do we get any calories from fiber?
Our gut microbiota can ferment some fiber into short-chain fatty acids. We can then extract calories thereafter.
The diversity of your gut microbiota can vary quite widely from one another, due in part to many factors such as diet, stress, physical activity, genetics, etc. And the diversity and composition of your gut microbiota determines how many calories you can actually extract from dietary fiber.
And according to an article00311-8?_returnURL=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS1550413114003118%3Fshowall%3Dtrue) from Cell Metabolism:
“Attempts to use measurements of soluble versus insoluble fiber as approximations of fermentable versus nonfermentable, respectively, have proven inaccurate.
When referring to the carbohydrates that can be metabolically used by gut microbes, we propose the term “microbiota-accessible carbohydrate”
So, from a terminology viewpoint, soluble vs. insoluble may not be the best way to distinguish which fiber provides calories and which do not. The better way appears to be which fiber is enzymatically accessible by the bacteria present in your gut. The amount of calories derived from those fibers, then, will also likely be dictated by the quantity and diversity of your resident gut flora.
Yes. Studies continue to show that adding fiber to our diets improves health and wellness outcomes.
So fiber has been shown to benefit long-term health and reduce the incidence of chronic diseases, but why is fiber healthy? How exactly does it help us?
Fiber is healthy because it can help lower your blood cholesterol levels.
Bile is made by your live and stored in your gallbladder. Bile helps with the digestion of fat. But in order to make bile, the liver needs to make cholesterol.
Bile salts, however, will bind to soluble fiber as it passes through your GI tract. This triggers your liver to make bile because your reserves will drop. This in turn requires the use of cholesterol to make bile. Thus, your cholesterol is lowered as soluble fiber removes bile from your GI tract.
Fiber is healthy because it can help you manage conditions of metabolic syndrome like blood glucose and insulin levels. Soluble fiber can delay the absorption of glucose during digestion, thus preventing glucose spikes and better managing insulin sensitivity.
Fiber is healthy because it can help you lose weight by increasing satiety. This means fiber can help you feel fuller, for longer. Soluble fibers pass slowly through your digestive system. This keeps you fuller for longer.
A high-fat/low-fiber “Western” diet can alter the gut microbiota, which in turn can influence the production of short-chain fatty acids and lead to gut inflammation, changes in immune regulation, and altered gut permeability.
Insoluble fiber decreases colonic transfer time, leaving less time for colonic cells to be exposed to carcinogens, while soluble fiber is fermented by the microbiota into short-chain fatty acids. Butyrate is the preferred energy source of normal colonocytes, and when normal cells are exposed to it, proliferation increases. In contrast, butyrate results in decreased cell proliferation and increased cell death in cancer cells (who use glucose as their main energy source).
In America, 96 percent of men and 87 percent of women have fiber levels below the Adequate Intake (AI), averaging around 10 grams per day.
“96% of men and 87% of women don’t eat the recommended amount of fiber in America.”
Below is an interactive chart you can adjust and analyze to see the average dietary fiber intake in America based on age and gender. Please feel free to copy and share - just cite us at Intake!
While the National Cancer Institute recommends 25 - 30 grams per day, many observational studies suggest eating much more than. Studies of ancient human diets are similar. For example, the Hadza, a hunter-gatherer tribe in Africa, eats approximately 100-150 grams of fiber daily.
Fiber will supply your gut microbiome with the fuel it needs to diversify and help keep you healthy. But be careful about switching to a fiber-rich diet too quickly. Gradually increase your fiber intake over time.
Many fruits and vegetables have plenty of fiber. So along with their rich stores of vitamins, minerals, and healthy fats, making sure to eat plenty of these daily will ensure you are meeting your dietary fiber intake needs.
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