High-Fiber-Content Foods: Fruits, Vegetables, and Cereals

What is fiber?

Fiber is plant matter that can't be digested by humans. It is basically the cell walls of plants. It makes up the structures that support the plant, gather water and sunlight, and protect its seeds. More or less, plants are made up of three things: water, carbohydrates that people can digest, and carbohydrates that people can't digest, namely fiber.

This fiber, or indigestible carbohydrate, is further classified as insoluble or soluble. Insoluble fiber is the stuff that wood is made out of: cellulose and lignin. Soluble fiber includes large sugar- and starch-like molecules, like polysaccharides and pectin.

Why eat fiber?

When modern civilizations process vegetable foods, they seek out the sugar and starch and protein: turning whole wheat and brown rice into white bread and white rice, peeling apples and potatoes, turning fruit into juice, turning beans into tofu. Along the way they remove and discard the fiber: the bran, the skins, the pulp, the gel, the big molecules that the human body never learned to break down into energy.

But these no-calorie, non-nutrient parts have health benefits of their own which are looking more important all the time. Not only does fiber regulate digestion, making people feel full and enabling the flow of digested matter through the body, but it absorbs toxins, feeds helpful microorganisms, reduces “bad” cholesterol, and fights diabetes, cancer, and heart disease.

Fiber content of different foods

The data below, from the USDA, includes many fruits, vegetables, and grain products that are high in fiber, plus a few that are less high in fiber but so low in carbohydrates or high in protein that they could be useful in a low carb/high protein diet.

Health benefits of fiber

In the last few decades, nutritionists have realized that populations with traditional high-fiber diets are less subject to “diseases of affluence” like heart disease and diabetes than people on the refined-carbohydrate 20th-century Western diet.

Fiber lowers cholesterol. Soluble fiber, found in oats and other grains, lowers “bad”, or LDL (low-density lipoprotein) cholesterol. It also lowers blood sugar levels.

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Fiber seems to reduce cancer. Alice Bender, a nutritionist for the American Institute for Cancer Research, says it is likely that fiber-rich foods--either the fiber itself, or something else in these whole vegetable foods--reduce some kinds of cancer. “We know that a plant-based diet rich in fruits, nonstarchy vegetables, legumes, and whole grains is associated with a lower risk of a number of the most common cancers.”

"We don't dine alone, we dine with trillions of friends - we have to consider the microbes which live in our gut”

Prof. Jeffrey Gordon, Washington University School of Medicine

Relationship between fiber and microflora

Fiber may provide its more mysterious benefits to health by means of the microflora (bacteria and other microbes) in the human gut. Fiber feeds “trillions” of microorganisms living inside us, creatures that provide us with services we are just now beginning to appreciate. We can contain three to five pounds of these passengers. In fact, of the cells contained in the human body, only 10% are actually human: the other 90% are microbes of many kinds. The mix of microbe species changes with age and with diet. Babies are born with no bacteria in their gut, but quickly acquire them. People who eat a lot of carbohydrates have a different mix of species than those who eat a lot of meat. Obese people have a different mix than slender ones.

Many of these microorganisms can digest the soluble fiber components that people cannot. They live off this extra energy and in turn provide nutrition to human organs like the colon and liver. It’s beginning to appear that gut bacteria provide chemical signals that can affect mood and the immune system. These microbes have coevolved along with us and the plants we eat over millions of years; they belong with us, just like whole plants belong in our diet.

How much fiber do you need to eat?

Nutritionists recommend 25-30 grams a day; Americans eat about half that much. For children, nutritionists recommend five grams a day plus the child’s age.

Even though fiber is good for you, people say not to make a drastic increase in your intake, but to add whole foods gradually, to avoid digestive disruption. No doubt our trillions of gut bacteria need a few lifetimes (a few weeks?) to adapt to change in their world, even beneficial change.

Fiber in a high-protein, low-carbohydrate diet

Some people control their weight by shifting their diet to include more protein and less carbohydrate. But such dieters risk not getting enough fiber, because animal proteins--meat, milk, eggs--do not contain fiber, and "meat substitutes" made from tofu and gluten may or may not contain much. Here are some fiber sources that may be useful in such a diet:

• Nuts, for example sunflower seeds and almonds (high protein, high fiber, relatively low carbohydrate).
• Non-starchy green (or greenish) vegetables like artichokes, broccoli, cauliflower, cabbage, brussels sprouts, and okra (low carbohydrate, high fiber).
• Juicy or leafy vegetables like celery, greens, and tomatoes (low carbohydrate, high in water and fiber)
• High-fat fruits: avocados, coconut (low carbohydrate, high fiber)

Unless a low-carbohydrate diet is very strict, it should include the following whole, high-fiber foods in moderation:

• Fruit (contains carbohydrate, but better than sweets or soda)
• Beans (high protein, though relatively high carbohydrate)
• Whole grains (relatively high carbohydrate, but some protein, and better than white bread or rice)
• People can add additional fiber to their diet with supplements like bran cereals or psyllium, although this runs the risk of missing micro-nutrients present in whole plant foods themselves. Or they can sprinkle ground flaxseeds on their food as a fiber supplement (the seeds are easier to digest if they are ground up).

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