Fabulous Fiber

We know we need it, but how do we get more of it? In our first story, our former senior research scientist Dr. Kristine Dennis explores how eating more fiber can make us absorb fewer calories.

Fiber seems so, well, boring. It’s not one of the nutrition topics commonly placed among cutting-edge research. By definition, fiber is indigestible––can’t be absorbed into the body. So, it just stays in our gut to bulk up our stool. This is not to belittle the importance of bowel regularity. Constipation is a global issue. In the United States, for example, chronic idiopathic constipation impacts 5% of people. If even half of the adult U.S. population ate three more grams of fiber a day—a quarter cup (50 – 60g) of cooked beans or a bowl of oatmeal—we could relieve enough constipation to save billions of dollars in medical costs annually. But it’s not like we thought fiber really did anything. In fact, you can get the same laxative effect with indigestible plastic particles. Feeding people a couple spoonsful of sliced polyvinyl tubing – not recommended – ends up increasing their stool bulk, frequency, and softening the consistency. So, for decades fiber was viewed as a similarly inert, indigestible substance.

We’ve since moved beyond fiber’s classic function as an anti-constipation agent to recognizing its other beneficial effects, including the many ways it can facilitate weight loss. The first major review, “Dietary fiber and weight regulation,” based on a dozen interventional studies where people were randomized into higher or lower fiber diets, found that the additional consumption of 14 grams of fiber a day led to an average weight loss of four pounds (1.8 kg) over 3.8 months. That’s only about a pound (0.45 kg) a month, but the weight loss was greater among those with excess weight. Study participants with overweight or obesity lost triple the weight, compared to lean individuals. And 14 grams of fiber isn’t a lot. That would even take the average U.S. diet up to the recommended minimum average adequate daily fiber intake of 38 grams for men and 25 daily grams for women.

The increased fiber intake appeared to lead to about a 10% drop in daily calorie intake. Why would more fiber mean fewer calories? Well, conventionally, fiber is considered to have zero calories; so, it adds bulk to food without adding extra calories, resulting in a lower “energy-to-weight ratio” in the food. To illustrate, let’s compare a food to its fiber-depleted equivalent. Consider a bottle of cold-pressed apple juice. That’s basically just an apple with its fiber removed. You could drink a regular bottle of juice (about 15 ounces, 445 ml) in a matter of seconds. But to get the same number of calories, you’d have to eat nearly 5 cups (550 – 600 g) of apple slices.

First, you’d have to chew all those apple slices. Fiber-rich foods require more chewing, which slows down eating rate, which itself can improve satiety. That also allows for more secretion of saliva and stomach juices. Researchers spread a barium paste onto slices of different kinds of bread––talk about research that would not pass human safety review today––and found that on x-ray, the stomach shadow was larger after eating whole wheat bread compared to white.

Five grams of a highly viscous fiber, like what’s in apples and oats, can hold approximately a quart of water (945 ml) as it passes through the stomach and small intestine. So, that’s having an extra two pounds (0.9 kg) of zero-calorie food mass filling you up. If you feed people the same meal, one with natural fiber (whole-wheat pasta with pureed veggies and fruits) and the other meal composed of the same foods with the fiber-depleted (white bread with fruit and vegetable juices), but both meals have the same volume, the stomach empties faster with the fiber-depleted meal, about 45 minutes faster.

It’s not what we eat, but what we absorb. So, we can lose more weight on a high-fiber diet eating the exact same number of calories, because some of the calories get trapped and never make it into our system. Those on a standard American diet lose about 5% of the calories they eat in their waste. A higher-fiber diet can double that. It’s not just that the calories in high-fiber foods are less available to absorb; they trap calories across the board. Feed people whole-meal bread versus white bread, and stool analyses find that we flush more calories on the whole meal bread. Eat a Twinkie on a high-fiber diet and absorb fewer Twinkie calories. It’s like every calorie label you read gets instantly discounted on a high-fiber diet.

Most nutrition classes will teach that a gram of protein has four calories, a gram of fat has nine calories, and a gram of carbs has four. But that’s only on a typical low-fiber diet. On a higher-fiber diet, up around the average of those eating strictly plant-based diets, the effective calorie counts per gram drop from 4-9-4 for protein, fat, and carbs to around 3.5-8.7- 3.8. That may not seem like a lot, but if Americans just reached the minimum recommended fiber intake, that might decrease calorie absorption by more than 100 calories a day. That may be enough to prevent that average gradual annual weight gain most experience through middle age. Even a small change in daily calorie absorption could have long-term significance for weight management.

In our next story, Dr. Dennis brings us research on the effect of fiber on our gut.

Fiber can literally help flush calories away, assisting with weight management. A review entitled “Food fibre as an obstacle to energy intake” summarized the four Ds by which dietary fiber results in reduced caloric intake. There’s the dilution of calories by expanding the volume of food, distention of the stomach through fluid absorption, delay in stomach emptying of the gelled mass, and dumping of calories by blocking the absorption of other macronutrients. That fourth D triggers a fifth phenomenon, known as the ileal brake. As Dr. Greger has covered, the ileum is the last part of the small intestine before it empties into the colon. When undigested nutrients are detected that far down, the body puts the brakes on further eating by curbing our appetite.

So, there are lots of ways eating more fiber can help with weight regulation. Across more than 10 weight-loss studies, those randomized to consume higher-fiber diets lost more weight, even when calorie intake was fixed. So, more weight loss even when prescribed the same number of calories. If it wasn’t the “calories in” side of the equation, was it the “calories out”? Normally that means things like exercise, but in the case of high-fiber diets, there literally are calories out, as in out the other end and flushed down the toilet. But the same-calorie, higher-fiber groups were losing more weight than could be accounted for––even after taking into account the excess calorie dumping. Where were the calories going?

To solve the mystery of the missing calories, researchers fed people different amounts of fiber, and sealed them in an airtight chamber called a whole-body calorimeter to closely monitor their metabolic rate. Those with more fiber in their system were burning more calories even in their sleep! Only about 2% more, but that translated into burning almost 50 more calories a day without getting off the couch.

For people on long-term fiber-rich diets, they figured that all that fiber might bulk up their intestinal lining, which is a highly metabolically-active tissue. Why were they spontaneously burning off more energy even when they were asleep? It turns out all that extra fiber may have been giving their gut a workout.

Our intestines are muscular tubes. So, our small intestine is like 20 feet (6.10 m) of muscle, which contracts in waves to move food along. But fiber-depleted foods don’t offer much resistance. The apple juice and Wonder Bread get absorbed almost immediately, and don’t put up much resistance. It’s like weight lifting with a barbell made out of Styrofoam. Not very effective.

Researchers had people swallow long strings of electrodes to measure the electrical activity of the muscular contractions of the intestines of those eating low-fiber meals versus high-fiber meals. The individuals eating more fiber not only had stronger, faster, longer contractions, but reduced the periods of intestinal inactivity. Turns out our gut can be sedentary, too. But if we eat lots of fiber-rich foods, we can be asleep and our gut will still be exercising all night long.

Finally today, the surprising benefits of going beyond the recommended minimum fiber intake.

“Hippocrates, the father of Western medicine, believed that all disease begins in the gut.” Of course, he also thought women were hysterical because of their “wandering uterus.” So much for ancient medical wisdom.

But something like constipation can have a “major [negative] impact on physical, mental and social well-being.” Yet it’s “often overlooked in health care.” This may be because poop-talk is taboo, but it can have “a severe influence on…everyday living,” both psychologically and physically. Constipation can hurt, causing “abdominal discomfort and pain, straining, hard stool, infrequent bowel movements, bloating and nausea.”

No wonder “[l]axatives are among the most commonly used drugs. Most are safe when used judiciously [and] intermittently,” but because of the frequency of their use, they do end up being one of the most common causes of adverse drug reactions. Perhaps “treatment should [instead] address the underlying problem,” such as lack of dietary fiber. You probably don’t need a meta-analysis to demonstrate that “dietary fiber can obviously increase stool frequency.”

“Populations in most Western countries must be considered by world standards to be almost universally constipated.” Here, it’s an epidemic among the elderly, but it’s simply not a problem among those centering their diets around fiber-rich foods. Where is fiber found? This patient summary in the AMA’s journal sums it up with an illustration: whole unrefined plant foods. Now for those of us smug in our intake of fruits and vegetables, we need to realize that “fruits and leafy vegetables are the poorest whole food sources of fiber.” Why? Because they’re 90 percent water. Root vegetables have more, but the fiber superstars are legumes, which means beans, split peas, chickpeas, and lentils, as well as whole grains. And gram for gram, fruit fiber doesn’t seem to have the same effect. It may take 25 grams of fruit fiber to double stool output, something just 10 grams of whole grain fiber or vegetable fiber can do.

And that’s not all fiber can do. If you eat some whole grain barley for supper, by the next morning your good gut bacteria are having it for breakfast, releasing butyrate into our bloodstream—a compound that seems to exert broad anti-inflammatory activities, which could help explain why significant decreases in the prevalence of inflammation are associated with increasing dietary fiber intakes. So, what, if you have knee pain or something, you should eat more fiber-rich foods? We didn’t know…until now. “Dietary Fiber Intake in Relation to Knee Pain Trajectory.” Thousands of patients were followed and…”a high intake of dietary fiber,” which is to say just the minimum recommended intake, was “associated with a lower risk of developing moderate or severe knee pain over time.” And two Framingham studies found that higher fiber intake was related to a lower risk of having symptomatic osteoarthritis in the first place.

But wait; don’t a variety of diseases have an inflammatory component? How about fiber consumption and all-cause, cardiovascular, and cancer mortality? They found that, compared with those who consumed least fiber, those who consumed the most had 23 percent less cardiovascular disease mortality, 17 percent lower risk of dying from cancer, and 23 percent lower mortality from all causes put together. “Unfortunately, most persons in the United States consume less than half the recommended intake of dietary fiber daily.”

These researchers suggest all sorts of potential mechanisms for which fiber could be life-saving, from improving cholesterol, immune function, and blood sugar control, but there also may be more of a direct cause. If you ask people to bear down as if they’re straining on stool, you can get a rapid increase in intracranial pressure, pressure inside your skull, and indeed, if you look at trigger factors for the rupture of intracranial aneurysms, if you ask hundreds of people who had strokes—bleeds within their brains—one of the biggest trigger factors noted was straining for defecation, multiplying risk seven-fold.