The Dark Side of Fiber (Part I of III)

What is Fiber?

Fiber is the indigestible part of plant-based foods that consist of structural components such as the cell walls of fruits, vegetables, nuts and legumes, the tough outer layers of grains, any other non-digestible carbohydrates as well as lignin, a substance found in plants that binds cellulose fibers together. The quick list of dietary fibers includes:

• Fructans (chains of fructose)
• Pectin
• Polydextrose
• Gums
• Stachyose
• Raffinose
• Verbascose
• Hemicellulose
• Cellulose
• Lignin

But what good is all this fiber if we can’t digest it? How is it good for us and is there any down side?

Even though we humans can’t digest fiber, bacteria (the predominant type of microbe in our guts) that live in our large intestine can, and we can utilize some of the bacterial end products for energy. We get about 25% more energy out of a meal, thanks to microbes. How is that for boosting our effective mileage? Having this symbiotic relationship with microbes provided a survival advantage throughout our evolution. This advantage, allowed our ancestors to avoid starvation when food was scarce. Sound pretty good so far?

Fast forward to modern times. Today, we find ourselves surrounded by dietary excess. We can eat pretty much whatever we want and whenever we want. The problem is that our survival instincts are like little voices telling us to eat as much as possible whenever we get a chance. In the Paleolithic part of our brains, it may be the last meal we have for days. In our Neolithic times we are overeating and overeating across all of the food groups often including fiber.

But do we really need to eat all this fiber if we are not starving?

Like most people, I grew up thinking fiber was healthy for me. Companies that market foods that contain fiber certainly publicize many reported benefits including relief of constipation, lowering cholesterol, even protecting us from heart disease and cancer. But when I looked for the scientific evidence supporting this assumption, I was surprised to find very little.

Can Fiber Help with Constipation?

High fiber foods and fiber supplements are often recommended for people suffering from chronic constipation, sometimes referred to as constipation-predominant irritable bowel syndrome or IBS-C. The goal is to use fiber to accelerate intestinal transit which seems to be slowed in these patients. But in a clinical study looking at the use of bran to reduce constipation, 20 grams of bran per day did not improve constipation symptoms compared to the placebo[i].

Along the same lines, treating 275 IBS patients with psyllium or bran fiber for three months did not improve their quality of life, though psyllium, the soluble fiber, did offer some improvement [ii]. The bran group actually suffered a high dropout rate because the participant’s symptoms including IBS symptoms and heartburn kept getting worse. A systemic review (17 studies were included) of different fiber types for treating constipation in IBS patients found that the benefits of fiber were marginal for treating constipation and in some cases, worsened IBS symptoms in general[iii]. Finally, a 2012 study showed that actually reducing or eliminating (instead of increasing) fiber was highly effective in relieving constipation [vii].

Can Fiber Lower My Cholesterol?

A small study reported that 60 grams of oat bran or oat meal per day lowered cholesterol levels by roughly 3% (that’s all!) after 12 weeks when combined with a low fat diet[iv]. However, another study found that fiber had no cholesterol-lowering effect in men with high cholesterol who did not follow a low-fat diet[v]. A third study, at Brigham and Women’s Hospital in Boston, Massachusetts, found that oat bran and low-fiber wheat had the same moderate effect on cholesterol levels, suggesting that fiber itself is not responsible[vi].

Read Part II and Part III on fiber.