A high fiber diet is good for you. Everyone knows that. But in recent years the proliferation of claim and counter claim about insoluble fiber, soluble fiber, oat bran versus wheat bran and fresh fruit versus wholemeal bread has made what once seemed a simple prescription for health a bit of a muddle. No longer is it clear just what the word “fiber” means.
For instance, most people think that fiber cannot be digested. It is true that fiber – the structural bit of a plant, cannot be broken down by ordinary digestive juices in the stomach and the small intestine. But it can be broken down, at least in part, in the large intestine, by the millions of bacteria which colonize that part of our bodies. Much of the bulk of feces is actually made up of the dead bodies of these bacteria.
Dietary fiber is a plant-based nutrient
Fiber is not a single entity. There are several kinds of fiber in food, and they each have different effects on the body during their passage through the digestive tract.
Insoluble fiber is cellulose. It is sometimes called crude fiber and it makes up only a small percentage of the total fiber in an average diet. The soluble fibers, such as the gums in oats and dried beans, and pectin, are completely digested by the bacteria in the large intestine, leaving acids in their wake which stimulate the wall of the intestine to contract.
One kind of dietary fiber, lignin, undergoes almost no digestion, but there is evidence that it helps the body get rid of cholesterol by stripping away bile acids (one of the major functions of cholesterol in the body is to make these acids).
Fiber helps the body eliminate toxins
A diet high in fiber is handy in treating and preventing constipation. The acids produced during the digestion of fiber stimulate and speed the movement of food through the body, and the fiber itself absorbs water, giving bulk to feces.
There is also evidence that fiber can help prevent hemorrhoids, hernias of the intestinal wall, colon cancer and possibly cardiovascular disease.
It is believed that by speeding up the passage of food through the body, fiber might help eliminate toxic nasties before they have a chance to cause disease, but there is also some evidence that the acids produced during the digestion of fiber could act as anti-cancer agents.
The picture is clouded somewhat by the fact that a high-fiber diet is also usually fairly low in fat. Since a high consumption of fat is known to increase a person’s risk of cancer, it is unclear how much of the benefit in a high-fiber low-fat diet is due to the fiber and how much to the low level of fats.
There is some evidence that a high-fiber diet, in conjunction with low intakes of fat and sugar, can help diabetics control their disease. The fiber found in some foods, such as oats and legumes, forms a gel on the surface of the stomach which helps slow the rate at which sugar enters the bloodstream.
Dietary fiber intake
Most people consume about 15g of dietary fiber a day. Nutritionists recommend that 30 to 40g would be a better target.
But there are dangers in eating too much fiber. Fiber absorbs iron and zinc as it passes through the digestive system, potentially robbing the body of these essential nutrients. Bran by itself will not cure constipation.
It needs water to make it swell and do its job, and in fact, if bran is eaten without adequate fluid to complete the cure, constipation can actually worsen. Large amounts of unprocessed bran can cause the cells lining the intestine to be damaged.
Anyone planning to embark on a diet higher in fiber should do so gradually. A sudden shift to a high fiber diet can cause the digestive system to revolt, resulting in wind, nausea and even vomiting.
Ideally, dietary fiber should be obtained from a variety of sources.
Foods which are very high in fiber include:
- baked beans (13g of fiber to a cup of beans)
- pumpernickel bread (5.6g a slice, compared with 2g for a slice of wholemeal bread)
- dried dates and figs
- split peas
- pumpkin and raspberries
Foods with surprisingly little fiber include:
- cherries (0.4g in 10 cherries)
- grapes (0.3g for the same number)
Flour is relatively high in fiber, which means that most white bread produced contains about 0.8g of fiber a slice (about the same as a tablespoon of sultanas). That is not great, but it means that white bread is not really the dietary baddie it is sometimes made out to be.
Variety is the key when it comes to fiber. It is much more pleasant to get your daily requirement from a selection of unpeeled fresh fruit and vegetables, along with some whole grain bread, than it is to grind your way through a few tablespoons of unprocessed bran at breakfast time. Look at it this way: a tablespoon of wheat bran has 2.2g of fiber. An unpeeled apple has 2.5g. Which would you rather eat?