The most important nutrient in your diet is not usually considered a nutrient at all. Ask your friends to name one and they will likely mention a vitamin or a mineral, or maybe say protein. And they'd be correct. But if you asked said friends to name the most important nutrient, or the one whose presence and quantity in the diet goes far in indicating the quality of the diet, who in their right minds would say fiber?
The nutrient that is not a nutrient, probably because it is calorie free and traditionally thought to be useless as an energy source. But it just so happens that the foods with the highest amounts of fiber (we're talking whole foods here, not processed junk in boxes and jars like cereals and powders) are without exception the healthiest on the planet.
First, what is fiber? Textbooks will tell you that fiber is the indigestible portion of plants. Because the body cannot absorb fiber, it remains in the digestive tract, moving through the 30-foot-long intestinal tube and acting like a rake to sweep out debris, adding bulk and softening the stool. Fiber is therefore a natural prevention for constipation, hemorrhoids, anal fissures and the like. There are two types of fiber, soluble and insoluble, and both types are usually found in fiber-rich foods. Soluble fibers absorb water, soften stool, and lower cholesterol, while insoluble fibers exert a rake-like influence and move food through the gut.
But is fiber really nondigestible? Maybe for humans, but not for the bacteria lining our intestines. These naturally-occurring, beneficial bacteria which are part of the normal gut flora - they are also known as probiotics - break down fiber into short chain fatty acids (SCFAs), which are the preferred energy source of the colon (that long tube that begins by your appendix and ends at your anus). SCFAs, and the fiber from which they are derived, not only exert a protective influence against colon cancer. They also stimulate blood flow and assist with fluid and electrolyte uptake, as shown by a study published in the American Physiological Society. All good things.
And just which foods contain fiber? If fiber is the indigestible/resistant portions of plants...you guessed it: plants.
Most rank fiber-rich foods on the basis of grams of fiber per cup of food, but this can be misleading, as concentrated foods (grains) contain way more calories per cup than water-rich foods (fruits, vegetables, beans). Consider that a cup of black beans contains 15 grams of fiber and 225 calories, while a cup of cauliflower, at 30 calories, contains 3.5 grams of fiber. By the old system (fiber per cup), beans would seem to be the better source. However, if you evaluate these fiber-rich foods by calorie, you'll find that while beans have about 6 1/2 grams of fiber per 100 calories, cauliflower has nearly double that amount (11 1/2 grams). For the calorie conscious, a better system evaluates foods based on amount of fiber per 100 calories.
So instead of counting calories, or carbs, or fat, all of which are restrictive methods which may leave you feeling limited and deprived, count fiber. Try to eat more fiber, as much as you can. In fact, aim to eat 100 grams a day. Why 100 grams? If the average fruit, vegetable, bean or seed contains about 5 grams of fiber per 100 calories (and some, like raspberries, contain as many as 14 grams of fiber per 100 cals), shooting for 100 grams of fiber per day will ensure that you eat 2,000 calories of these ultra-nutritious foods. For some, 2,000 cals per day may constitute the daily intake, done. For bigger/more active folks, this will leave an extra 500 or so calories for less nutritious foods such as grains, nuts, and (heaven forbid) animal products (eggs, dairy, meat).
To practice eating 100 grams of fiber per day, focus on fruit, especially berries, which are highest in fiber. Apples, bananas, and citrus fruits are other good choices. (Dried fruits - prunes, raisins, etc. - are inferior options as they are divorced from the water that aids digestibility, which may result in stomach upset for some.) For lunch enjoy a big green salad with fruit or beans, and for dinner work up to a cup or two of your favorite legume, along with vegetables, both cooked and raw.
Use a calorie counter such as fitday or nutridiary to verify that you are reaching your goal. You won't have record food consumption for very long. A couple/few days of eating fiber rich foods will give you a good idea of the quantities you should aim for.
Some say that adding fiber to the diet should be done incrementally. While this may be true with beans - to which your body must adjust by increasing the amount of beneficial bacteria in the gut - fruits and vegetables, especially raw varieties, can be eaten in superabundance without any digestive difficulty whatsoever.
And we know how tempting it is to meet the fiber requirement by eating mostly fruit. Fruit is sweet and easy, but make sure to include a large quantity of vegetables at lunch and/or dinner. A recent study published in the American Journal of Clinical Nutrition found increased consumption of the fiber found in vegetables to be associated with a lower risk of breast cancer. This same association was not found in fibers from other foods.
Vive le fibre!