by  Robert Lustig, M.D.

Fiber—Half the "Antidote"

Sujatha is a thirteen-year-old Indian girl who has just been diagnosed with type 2 diabetes. At a height of 5 foot 4 inches, she weighs 170 pounds. According to her BMI, she is technically obese, but she doesn't look it. Her mother is a nurse in a local hospital and also a type 2 diabetic. She told me, "I don't understand how this can happen. We are Indian, we are vegans at home." However, the family consumes large volumes of "white foods" such as naan, rice, potatoes, and processed starches. Almost completely lacking from their diet are "brown foods" such as lentils, garbanzo beans, and whole grain products. Like many teenagers, Sujatha refuses to eat her vegetables. Beverages consist of soda and juice, and virtually no water. The fiber content of their diet is close to zero.

The Stealth Nutrient

Fiber, also known as roughage or bulk, is the most misunderstood weapon in our nutritional arsenal. The common belief, promulgated by countless TV commercials playing to the over-seventy crowd, is that fiber is important for our bowels and little else. Fiber makes you "regular" (as if constipation makes you "erratic"?). These commercials suggest that you should start eating fiber to make those golden years a little smoother. Meanwhile, gas-troenterologists have impressed upon us the value of fiber to prevent both colon cancer and diverticulitis. All that's true—but fiber is oh so much more. As you will see, fiber is half of the "antidote" to the obesity pandemic. But how can something that we don't even absorb be so darn valuable?

Unlike the other foodstuffs previously discussed—fats, proteins, and carbohydrates—fiber isn't digested or absorbed by your body. It travels through and out of your stomach, small intestine, and colon with minimal alteration. The USDA does not classify fiber as an essential nutrient; indeed, most people consider fiber a waste product of food and a waste of time. Despite this, the Dietary Reference Intake for fiber suggests a total of 14 grams per 1,000 calories, or essentially 25 grams of fiber per day.1 Paleo-biologists have performed DNA footprint analyses of three-to-ten-thousand-year-old stool samples from caves in Texas, allowing them to determine what our ancestors ate based on the bacterial makeup of their intestines. They estimate that these cave dwellers consumed about 100 grams of fiber per day,2 yet our median consumption of fiber is currently 12 grams. Does this matter? Why should we care about fiber? Why does processing a food remove its fiber? Aside from our current "irregularity," what might our lack of fiber be doing to us? After all, it's just to make us poop "regular," right?


Dietary fiber—found in fruits, vegetables, whole grains, and legumes—is the part of the plant that the human gut is unable to digest. So, you can't use it for energy. Therefore it has one destination: the toilet. 

There are two types of fiber: soluble, which dissolves in water, and insoluble, which does not (see table 12.1). This difference determines each type of fiber's impact on your body, health, and stool. Soluble fiber slows digestion and absorption, and is fermented by the bacteria of your colon into gases (read: socially unacceptable emissions except for teen boys at summer camp). This is one reason we haven't missed the removal of fiber from our diet. It consists of strings of glucose molecules such as pectins (found in fruit and used to make jelly) that absorb water to become a gelatinous, viscous substance. Insoluble fiber consists of polysaccharides (non-glucose carbohydrates) such as cellulose, the stringy stuff in celery. They are not digested at all. Because they do not dissolve in water, they have a laxative effect and speed up the passage of food and waste through your gut.

Table 12.1: Sources of Dietary Fiber

Soluble Fiber (absorbs water)

Insoluble Fiber (doesn't absorb water)

oatmeal, oat cereal, lentils, apples, oranges, pears, oat bran, strawberries, nuts, flaxseeds, beans, dried peas, blueberries, psyllium, cucumbers, and carrots

whole wheat, whole grains, wheat bran, corn bran, seeds, nuts, barley, couscous, brown rice, bulgur, zucchini, celery, broccoli, cabbage, onions, tomatoes, celery, carrots, cucumbers, green beans, dark leafy vegetables, fruit, and root vegetable skins

Metabolically, the two together are an unbeatable pair.3 The insoluble fiber forms a latticework for the soluble fiber to sit on, while the soluble fiber bridges the gaps in the latticework to maintain its integrity—kind of like the hair catcher on your shower drain. Without it, the hair goes down the drain rapidly. But when the hair catcher catches the hair, now you'e got a stopped-up bathtub. In the case of fiber, however, inhibiting the rate of flux from the intestine crossing into the bloodstream is a good thing. It gives the liver a chance to fully metabolize what's coming in, so there's no "overflow." Unfortunately, the majority of the foods we are consume today lack fiber of any sort. Refined grains are stripped of both the bran and the germ in the process of milling. This gives them a finer texture and extends their shelf life while taking out various micronutrients (see chapter 14) and, in particular, fiber. Refined grains include white rice, white flour, pasta, potatoes, and many of the cookies, crackers, and cereals that stock your pantry. "Enriched" grains may replace some of the nutrients removed, but once the fiber is taken out you can't put it back in.


The Fallacy Surrounding Fiber

To get the full metabolic benefits of fiber, it needs to "coat" the starch granule on all sides (forming a sphere, or a "kernel") so that the digestive enzymes in the intestine must slowly strip it away. The starch (endosperm) is on the inside. The bran is on the outside. The whole kernel represents a source of insoluble fiber. Strip away the outside bran, and you are left only with the starch (glucose). When you ingest the whole kernel, your intestines will slowly strip away the outside bran, making the rise in serum glucose occur slowly and reach a lower peak concentration. But when the outside bran is removed by processing, your liver is hit with an influx of glucose and the rise occurs quickly, with a higher peak. And that means a higher insulin peak.

So, to derive maximal effects from fiber, you need to consume products with the unadulterated whole grain. Naan and white rice, the only grains in Sujatha's diet, ceased to be grains after being polished at the mill. But here's the problem: even "whole grain" doesn't always mean "whole grain." According to the IOM, food must meet at least one of the following criteria to be considered "whole grain": (1) contain at least 8 grams of whole grain per serving, (2) qualify for the FDA whole grain health claim (51 percent whole grain by weight), or (3) have a whole grain as the first ingredient by weight for non-mixed dishes (e.g., breads, cereals) or as the first grain ingredient by weight for mixed dishes (e.g., pizza, corn dogs). (So "whole grain" Lucky Charms is a misnomer as there are no "whole" grains in the cereal.) Manufacturers may mix in regular starch with the whole grain, but that's not a great idea if you're trying to keep insulin down.

There is nothing in the IOM's definition about the grain being "whole"—that is, uncracked, uncrushed, unadulterated. Plus, foods that list "whole grains" as the second or third ingredient may contain as little as 1 percent. Hence, the IOM definition leaves much to be desired.


The Jive on Juice

Whereas fruit does contain fructose, it also has inherent fiber. And that's not by accident. The reason the fructose in fruit doesn't cause significant health problems is that it's balanced by the endogenous fiber that makes up the solid part of the fruit. If you consume both together, as Nature intended, it reduces the rate of flux to the liver; the liver can keep up, which mitigates most of the negative effects of the sugar. In fact, the amount of fructose in most fruits is balanced nicely by the fruit's fiber content. Conversely, juice is devoid of the insoluble fiber found in whole vegetables and fruits. When "juicing," you keep some of the essential vitamins and minerals (but not all) inherent in the fruit or vegetable, but you discard perhaps the most important part: the fiber. Remember, it doesn't matter where the fructose comes from—fruit, sugarcane, beets—without the fiber, it has the same metabolic effect on your body. Our ancestors didn't have the health complications associated with fructose because they ate the whole fruit.

One current fad is to juice the entire fruit into a "smoothie." Juice bars have popped up all over the West Coast, ostensibly because juicing is healthy. The problem is that the shearing action of the blender blades completely destroys the insoluble fiber of the fruit. The cellulose is torn to smithereens. While the soluble fiber is still there, and can help move food through the intestine faster, it now does not have the "latticework" of the insoluble fiber to help form that intestinal barrier. The sugar in the fruit will be absorbed just as fast as if the juice were strained with no fiber at all. You need both types of fiber to derive the beneficial effects.

Chapter 4 showed us that insulin is the bad guy in terms of weight gain and that keeping insulin down is a priority to combat obesity. The amount and rapidity at which energy arrives at the mitochondria trigger the disease associated with metabolic syndrome (see chapter 9). In other words, the two elements to keep in mind are: the dose of carbohydrate (to keep the insulin down) and the flux of carbohydrates (to keep the liver happy and functioning properly). Fiber takes care of both.

A Waste Product or a "Waist" Product

As you saw in chapter 11, the glucose in sugar drives the insulin up, while the fructose brings a huge dose of energy straight to the liver for immediate processing, both of which drive obesity and metabolic syndrome (one of the reasons Sujatha developed diabetes). Fiber possesses five different properties that assist in fighting obesity and metabolic syndrome by keeping insulin down and reducing the energy hitting the liver.

1. The Annals of Absorption

Once fiber (soluble and insoluble) is consumed with a meal, it forms a gelatinous barrier between the food and the intestinal wall. This delays the intestines ability to absorb glucose, fructose, and fat. By slowing glucose absorption, the blood glucose rise is attenuated, which limits the peak glucose. In return, the pancreas, sensing the slower and lower rise in blood glucose, limits its response and reduces the amount of insulin released. Less insulin means less shunting of energy to fat. When patients with type 2 diabetes ate a high-fiber diet, blood sugar was cut by one third, thereby reducing the total insulin load of the body.4 The same thing happens with the absorption of fructose.5 Not only does fiber reduce the dose, it reduces the "flux" - that is, the rate at which fructose is absorbed and arrives at the liver cell for processing. The liver then has a chance to "catch up" and is able to process the fructose molecules to acetyl-CoA at essentially the same rate that new ones are being introduced. This allows them to burn in the mitochondrial Krebs cycle (see chapter 10), instead of overwhelming the mitochondria, to be shunted out and turned into fat, causing subsequent insulin resistance. So, consuming fruit, despite its fructose content, is not nearly as big a problem because the fructose is for the most part mitigated by the presence of fiber.

2. Calories and Cholesterol

Lower blood cholesterol levels are associated with lower rates of heart disease in large populations. One purpose of cholesterol is to aid in the production of bile acids (which help absorb fats in the intestine), some of which are excreted in your stool. So, if you manage to get rid of the bile acids, you lower your cholesterol. Because soluble fiber binds to bile acids, it can help lower LDL ("bad cholesterol"). Insoluble fiber also decreases cholesterol and helps lower blood glucose.

3. Speed and Satiety

You eat a whole plate of macaroni and cheese, yet you're still hungry. Why? Food in the stomach reduces ghrelin levels, which should tell the hypothalamus that you're not hungry anymore. But you still are. The reason is that lack of hunger isn't the same phenomenon as satiety. After food moves through the small intestine, a hormone called peptide YY(3_36) (PYY) is released into the bloodstream, which binds to receptors in the hypothalamus and tells you that you're full. PYY is the satiety signal.6 The problem is that there are twenty-two feet of intestine that the food has to traverse before the PYY signal is generated. That takes time. So anything that will move the food through the intestine faster will generate the satiety signal sooner. Insoluble fiber does just that; it increases the speed of transit through your gut in order to generate the PYY signal earlier. Soluble fiber forms a sticky gel, delaying the emptying of your stomach, making you feel full faster. Both types of fiber can cut down on the need for consuming second portions, helping to prevent further weight gain.7

4. Fat or Fart

With the presence of fiber, some of the dietary fat will be delayed from absorption in the small intestine. Instead, these fiber-delayed dietary fats will make it all the way to the colon, where they won't be absorbed, thus keeping insulin low.8 Although controversy remains, it is thought that insoluble fiber contributes more significantly to the effects on obesity and insulin resistance than does soluble fiber. The downside of this process is that this fiber will generate a lot of nitrogen, carbon dioxide, methane, and a little hydrogen sulfide in the process. In other words, it's fat or fart.

5. Bowels and Bugs

The human body contains about ten trillion cells. But your gut harbors about a hundred trillion bacteria. They outnumber us ten to one! For years, we thought they were just along for the ride, making gas at inopportune times and visiting upon us the occasional "traveler's revenge." But those bacteria are a big part of our energy metabolism. Most of the gut bacteria live in the large intestine and are anaerobic, which means they metabolize without oxygen and therefore waste more energy than oxygen burners. Well, if all our nutrients (including fat, glucose, and fructose) are absorbed in our small intestine, what do the large intestinal bacteria have left to eat? What we can't absorb—the fiber, and in particular, the soluble fiber. This is why so many fiber supplements, such as psyllium, give people so much gas. There are thousands of species of gut bacteria, but science has thus far focused on three: Bacteroidetes, Firmicutes, and Archae. Almost assuredly, the bacterial composition of the gut is one of the factors that promote weight gain in some people. And the fiber composition of the diet is one of the factors that determine the bacterial profile,9 because fiber delivers more nutrients farther down the intestine, where the bacteria can utilize them for energy.10 Taken all together, it would appear that altering the fiber content of the diet alters the bacterial content in the gut, allowing for "beneficial" bacteria to proliferate while keeping the "obesogenic" bacteria at bay.11

Fiber and Insulin Resistance

So does dietary fiber consumption promote weight loss? Here's where the design of the study makes a big difference. If you keep calorie intake constant, the addition of fiber does not demonstrate significant effects on weight. However, in a free-range situation, where people get to choose how much they eat, higher dietary fiber appears to limit total food intake, which likely results in decreased weight. High-fiber foods tend to be less "energy dense," so you are consuming fewer calories for the same quantity of food. Also they often require more time to chew, giving your body more time to receive its satiety signal, and they move the food through the intestine faster, generating the satiety signal sooner.

The role of dietary fiber in the prevention of metabolic diseases is complicated by which kind of fiber you are talking about and what kind of study you are referring to. In the Insulin Resistance and Atherosclerosis Study (IRAS), dietary analysis demonstrated only one item that correlated with insulin sensitivity: fiber.12 Yet the soluble fiber content did not correlate with improvement in diabetes risk.13 For the most part, this improvement in insulin sensitivity was conferred by insoluble fiber (the stringy stuff).14 So there goes taking soluble fiber supplements such as psyllium. It looks like you have to get your fiber in the food itself, not from a pill. And there's only one way to get both soluble and insoluble fiber: the source— and the closer it is to its original form, the better. This concept of food being better than its components will come up again in chapter 14.

What Comes Out Is Just as Important as What Goes In

It's very clear that fiber is a big deal. Not just for your bowels, but also for your metabolism. Fiber doesn't get absorbed. There's no blood level of fiber, as opposed to levels of micronutrients that improve your metabolic machinery. But by reducing both the dose and the rate of flux of glucose, fructose, and fatty acids entering your bloodstream, fiber keeps your insulin down. By delivering nutrients to the large intestine to allow for fermentation, fiber improves metabolic machinery and selects for the "good" bacteria, which help with energy loss from the colon. Finally, fiber limits total food consumption. But it has to be eaten in the form of the whole, intact food in order to get the full benefit, so you get both the soluble and insoluble fiber. Fiber alone won't mitigate all the negative effects of sugar, but it's a hell of a good start. Want to reverse your diabetes? Want to improve your metabolic health? Put fiber back on the menu.



Keith Hunt