Answer: What is dietary fiber?

 It seems obvious...  

.. but as I said, I suddenly realized that I didn't really understand what "fiber in my diet" really meant.  

This happens all the time. There's a kind of skill of recognizing that something isn't lining up--of knowing that you don't quite know what something really is. 

As you might have noticed, a lot of these posts start with "I didn't know..."  

In this particular case, I thought I knew was "dietary fiber" was.  Isn't fiber just the indigestible part of your food--you know, the little threads, husks, hulls, and tiny fragments that just pass straight through without much digestion seeming to take place?  

But as I noticed when I let my cereal sit for too long in the milk, things that say they have fiber in them (like my whole grain cereal) often don't look like there's anything fibrous within.  There's nothing like the kernels of corn or bits of apple skin that (as far as I can tell) are untouched by my digestive juices.  How could there be any "fiber" there?    

So... what is fiber really?  It's obviously not just undigested bits of food.  

Can you help me understand what's going on here?  Today's Challenges are: 

1. So what, really, is dietary fiber?  Is it something more than indigestible bits like wheat bran, corn kernels, and rice husks?  

I wanted to start with the basics.  First query: 

   

     [ define fiber ] 

This starts to untangle part of my confusion. I was thinking of dietary fiber as a kind of textile-like fiber.  That is, my mental model of "fiber in food" was based on what I thought of as a fiber that I might see in clothing or the husk of a seed.  This is very much along the lines of that word you see at the bottom of the definition, "roughage."  

But reading carefully, I learned that "fiber" (or "fibre") is also a "dietary material containing substances such as cellulose, lignin, and pectin that are resistant to the action of digestive enzymes." 

And THAT is a surprise. It means that the fiber in my diet can be pretty non-fibrous.  

Just a few weeks ago I had way too many plums on the tree at my house, so I spent a few happy hours making jelly and jam.  One of the steps in the process is to put pectin into the mix.  But when I do that, I can see that the pectin dissolves into a clear liquid... there aren't any obvious fibers that look like roughage.  

I was curious about lignin and cellulose, so I looked up those terms as well.  Are they fibrous in the way I'd expected? 

Answer: NO!  If you look at images of lignin, cellulose, or pectin, it's pretty clear that all of those come in powdered form--and if you look for [ liquid cellulose ] or [ liquid pectin ] it's pretty obvious that those don't have any roughage in them. 

So dietary fiber must mean something other than the roughage you get from (say) celery.  

My next query was for: 

     [ fiber in food ] 

and I quickly learned that fiber is, indeed, not just the rough indigestible bits, but is actually all the long-chain molecules that make up those indigestible bits!  

As the Wikipedia entry on dietary fiber tells us "Dietary fiber consists of non-starch polysaccharides and other plant components such as cellulose, resistant starch, resistant dextrins, inulin, lignins, chitins (in fungi), pectins, beta-glucans, and oligosaccharides."  

Okay.  I now know that "dietary fiber" is really all of those complex (long-chain) molecules in the food I eat.  So, "high fiber foods" have a lot of those chemicals AND roughage. 

But wait... there's more!  

I also learned by reading some of the links from the SERP that there are two kinds of dietary fiber!  There's "soluble" and "insoluble," that is, fiber that dissolves, and fiber that doesn't.  

     [ soluble insoluble fiber ] 

The first result is to the Medline page about dietary fiber, which includes this: 

"There are 2 different types of fiber -- soluble and insoluble. Both are important for health, digestion, and preventing diseases.

Soluble fiber attracts water and turns to gel during digestion. This slows digestion. Soluble fiber is found in oat bran, barley, nuts, seeds, beans, lentils, peas, and some fruits and vegetables. It is also found in psyllium, a common fiber supplement. Some types of soluble fiber may help lower risk of heart disease.

Insoluble fiber is found in foods such as wheat bran, vegetables, and whole grains. It adds bulk to the stool and appears to help food pass more quickly through the stomach and intestines." 

Ah ha!  So there really are two kinds of fibers in our food.  The soluble kind and the insoluble kind.  

Insoluble fiber:  This type of fiber does not get dissolved in the body and is also known as “roughage”, helping to clear out the intestinal tract. Since it's not broken down in the body, insoluble fiber has effectively zero calories. This is why a high fiber diet is recommended for weight loss as insoluble fiber will fill you up and pass through your body unchanged. 

Soluble fiber: By contrast, this kind of fiber is a bit more complex than the other. Unlike insoluble fiber, soluble fiber dissolves making a gel like substance, which is incredibly handy for slip/sliding your gut contents along.  

2. What is the current recommendation for dietary fiber in my diet?  Is 5 grams of fiber in a serving a lot?  Or is it a little?  

A quick search for: 

     [ dietary fiber recommendations ] 

generated a lot of hits.  Interestingly, when you compare the top 5 reputable sources, you see this: 

Mayo Clinic – Women:  21 to 25 grams / day; men: 30 to 38 grams / day

UCSF – everyone: 25 to 30 grams / day from food, not supplements 

NIH -everyone:  25-29 grams / day (more than 30 grams would be better) 

Harvard – people < 50 years, 25 grams for women and 38 grams for men. 

                For people > 49, Women and men should have 21 and 30 daily grams

Oddly, none of the sources break down the amount of fiber by soluble vs. insoluble.  But it's clear that "fiber from food" solves a lot of the soluble/insoluble question by providing both in roughly equal quantities.  

Basically, if you can dissolve the food in water, it's got soluble fiber.  If it doesn't dissolve, it's insoluble fiber.  You need both.  

Bottom line: Women need slightly less fiber than men, but everyone needs roughly 30 grams / day.  

If you look up various foods (after doing a query like [ data set food fiber amount ]), you'll probably end up on the USDA's list of foods with the amount of fiber listed for each.  In that list you'll see a lot of high fiber foods that you probably won't eat in large amounts (who eats 100 grams of cinnamon?), but you'll be able to look up your favorite foods and estimate the amount of fiber you're getting.  

US folks: Note that the table is given in "amount of fiber in 100 grams of food."   So you'll have a vivid image--100 grams is around 3.5 ounces, or to make it visually memorable, small cans of cat food are 3.5 ounces (100 grams). 

And you'll see that my favorite cereal has 10 grams of fiber in 100 grams of cereal.  But take note--most people I know don't eat 100 grams of cereal in a serving at breakfast.  My bowl of morning cereal usually has 36 grams of cereal, which means I'm getting 4 grams of fiber in each bowl. 

Apple with 2.8 grams of fiber

That's okay, I guess, but an average apple has 2.8 grams of fiber, and a regular serving of plain old oatmeal has 4 grams of fiber and 6 grams of protein.  (And it's about half the cost per serving.)  

SearchResearch Lessons 

1. Search out the things you don't understand, even sometimes fairly obvious things.  This was just one of those little things that struck me while reading: I couldn't actually define dietary fiber.  I make it a practice to question what I'm reading at a very basic level.  This is deeply important when you're reading something that's complicated or has an intricate back story.  Do you really understand all of the parts and pieces that are involved?  

Being a skilled searcher is, in large part, having enough background knowledge to know when something you just read (or heard) doesn't fit in with the other things you've read.  In this case, it was a small observation about my breakfast cereal.  For you, it might be something else.  Follow up those small questions--they could be deeply important.  

2. Look for multiple sources.  As I did with searching for multiple sources of fiber recommendations, it's easy to find them.  Be aware of differences between measurements! 

Keep Searching!