It lowers cholesterol, helps prevent constipation, decreases risk of cancer, and is relatively inexpensive, yet is absent in many of the fad diets that we follow. I’m talking about fiber. Understanding how it has become the forgotten stepchild in the American diet may be key to help your long-term health.

Fact: Americans consume less than half of the recommended amount of fiber every day. Most of us do not know exactly what fiber is, or how it can help.

Fact: Dietary fibers are ingredients in plants, and are a kind of complex carbohydrate. Soluble fiber is the edible part of a plant, which is partially digested and absorbed by our bodies. Sources include vegetables such as carrots, broccoli, onions and artichokes. Insoluble fibers, such as those found in stringy celery, are not absorbed by our bodies at all, but can still help us by bulking up stools.

Fact: Complex carbohydrates such as fiber have multiple health benefits and are very different from simple carbohydrates. Simple carbs — found in sodas, fruit drinks, white flour, breads, white rice and many processed foods — are digested by the body very quickly. The body treats them just as it does sugar — by raising insulin levels and increasing blood sugar, leading to diabetes and weight gain. Recent studies have shown that simple carbs are so bad for you that you are better off eating high-cholesterol foods than foods full of sugar.

Take-home message: Simple carbs lead to health problems, but complex carbs rich in fiber have tremendous health benefits.

This important difference was highlighted in a recent study that looked at the popular Paleo diet. This high-fat, low-carb diet tries to duplicate the way that cavemen ate. It has helped with short-term weight loss, but this study suggested that the elimination of grains from the diet resulted in a change in the kind of beneficial bacteria in the gastrointestinal tract, leading to long-term health problems.

Further evidence that the choice of carbs really matters comes from a study in the medical journal Lancet, which showed that both low-carb and high-carb diets were associated with higher mortality. The optimal risk was associated with a moderate-carb diet of 50% to 55% consisting of carbohydrates. While plant-based diets favoring carbs, protein and fat intake from vegetables, nuts, peanut butter and whole-grains breads were linked to longer lives, the opposite was true of low-carb diets rich in animal-based proteins. In short, it is all about the type of carbs that you are eating.

Dietary fiber has protective effects against chronic diseases other than those of the heart. Diabetes, metabolic syndrome, inflammatory bowel disease, diverticular disease, obesity and colorectal cancer rates are decreased when fiber is added. The average American daily fiber intake is only 15 grams a day. The American Dietetic Association recommendations are to increase this to as much as 30 to 35 grams, less for children and the elderly. There is no upper limit of fiber that can be good for you, but tolerance can be a problem. Gas and belly discomfort can occur if you increase your fiber intake too quickly.

Foods that have lots of fiber include grains (corn bran 60g/cup, barley 32g/cup, wheat 24g/cup, brown rice 6.7g/cup, and whole grain pasta 9g/cup), proteins (kidney beans 46g/cup, soybeans 30g/cup, peas 43g/cup, almonds 16g/cup, peanuts 13g/cup), fruits (dried peaches 13g/cup, dried plums 12g/cup, avocado 10g/cup, oranges 7g/cup and apples 3g/cup), and vegetables (mashed potatoes 14g/cup, mixed edamame 8g/cup, and tomatoes 6g/cup).

For the juicer fans who are reading this, a word of caution. Straining out the pulp from juice clearly eliminates fiber, but it also has been suggested that processing fruits and vegetables through juicing, rather than eating whole produce, can degrade fiber quality, and worsen blood sugar control.

Increasing fiber content in your diet may be one of the most important things that you can do for long-term health. But add fiber over a period of weeks or your stomach and friends may pay the price. If you absolutely cannot get fiber intake up, then supplements can help, but sticking with whole foods is best.

David Becker, M.D., is a frequent Inquirer contributor and a board-certified cardiologist with Chestnut Hill Temple Cardiology in Flourtown, Pa. He has been in practice for more than 25 years.