Everyone knows you need fiber in your diet. Everyone knows fiber comes from plant foods. And everyone knows that fiber helps move food through the system – just like a lot fiber in grass and hay help move a horse’s food through its unusually long and tortuous bowel. Fiber in the human diet helps package waste in softer, bulkier bowel movements, and a high fiber diet reduces constipation, which in turn reduces the risk of hemorrhoids, diverticulitis and even colon cancer.  

More than a laxative

      What is not obvious is just how a diet high in fiber lowers cholesterol levels and improves the other cardiovascular risk factors associated with the metabolic syndrome (obesity, high blood pressure and diabetes). How can the indigestible component of food, which releases no energy and is not absorbed into the body, affect metabolism? And how does moving food through the body with greater ease and efficiency alter body chemistry? The answers are interesting and worth knowing, since they provide impetus for even the unconstipated to pay attention to fiber intake. 

What is fiber?

    By definition, fiber is the indigestible component of food. Both cooking and chewing break fiber-rich food down in size, but fiber is impervious to stomach acid and digestive enzymes. The stiff portions of the plants – the parts that that give them shape and cover– are carbohydrates called cellulose and lignins. Since this type of fiber doesn’t even dissolve in water, it is called “insoluble fiber.” Insoluble fiber is what most people think of when they read about the virtues of a high fiber diet. It is like the horse’s hay and grass. 

    The softer plant parts provide a different kind of fiber which does dissolve in water and is therefore “soluble.” Soluble fiber is made up of carbohydrates called pectins, mucilages and gums. Because it attracts water, soluble fiber helps ease the passage of food by making it softer and bulkier. So far, then, this kind of fiber seems like nothing more than a softer version of insoluble fiber. But along with water, soluble fiber attracts bile, making soluble fiber much more than a passive factor in the transit of food. The bile connection is the key component in the role of dietary fiber in cholesterol metabolism.   

What is Bile? 

    Bile is a solution of chemical compounds called bile salts that act like detergents in breaking down fat and making it ready for absorption from the intestine. Using cholesterol as the main building block, the liver makes about 4 cups of bile a day, storing it in the gall bladder until food arrives in the stomach. The gall bladder then squirts bile into the small intestine. Without bile we could not absorb necessary fats and fat-soluble vitamins. The liver also uses bile as a shipment device for the fat soluble debris and toxins it filters from the bloodstream, especially the breakdown products of hemoglobin. Bile is the trash hauling contractor for the liver. 

Elimination or recycling?

 Bile breaks down once it has completed its digestive work. Its pieces get absorbed in the last part of the small intestine and carried back to the liver via the blood – or it escapes the body via the waste in the colon (bile imparts the color to bowel movements). Like the oil in your car, which accumulates dirt and get sluggish, bile that is re-circulated concentrates more and more fat-soluble waste. Escaping bile takes the waste along with it. And the less bile returned to the liver for recycling, the more cholesterol the liver has to use in the bile manufacturing process – making less cholesterol available for clogging up arteries. 

    In the small intestine, soluble fiber also sops up other carbohydrates, slowing their digestion and the absorption of sugar into the blood stream. This function appears to improve insulin sensitivity, making soluble fiber beneficial to people who have type 2 diabetes. Slowing carbohydrate absorption indirectly improves fat metabolism as well. 

Soluble fiber and the colonic environment

    Once soluble fiber reaches the colon, it begins another phase of its work. The colon, unlike the sterile small intestine, contains numerous bacteria. Bacteria need to eat, and they take whatever they can from the food passing through. Soluble fiber, for bacteria, is eminently digestible. They chew it up and produce short-chain fatty acids, creating an environment favorable for the absorption of minerals like calcium and iron. Some researchers think the acid environment helps slow cancer development. 

Getting enough fiber

    Is it hard to get enough fiber in your diet? Yes. Not because it is not available, but because we opt for easy food – easy to get, easy to prepare, and easy to eat. On average adult Americans get about half the 25-35 grams of fiber a day that they need, and children only about 20%. Constipation is a cardinal sign of a fiber poor diet. Bowel movements that are hard and dry, with frequency of less than once every three days, and the regular need to strain to evacuate the bowels are all signs of constipation. Constipation is also made worse by inactivity. Other results of a fiber deficient diet are less visible and occur over the long term: development of outpouchings of the colon wall called diverticuli, inflammatory changes in the colon lining, cancerous changes in colon cells, and the possible contributions to the metabolic syndrome and heart disease. 

    The best sources of insoluble fiber are the plant foods with tough structures: vegetables and whole grains. Soluble fiber comes in the form of oat bran, fruits, nuts, beans, and peas. The most useful fiber shopping rules are to stay as far as possible from manufactured foods and to choose liberally from the fresh produce section of the grocery store. Fiber supplements? Studies of their metabolic effects are contradictory, but supplements such as psyllium, guar gum and pectin appear to do no harm. If they produce satisfactory results in terms of easing bowel symptoms, they are probably helpful.