Refining of flour causes the loss of the majority of its nutrients and phytochemicals (plant-derived substances believed to be beneficial to health). For example, over 75% of the fiber (nondigestible compounds, mostly complex carbohydrates) is lost when flour is refined. Although a high-fiber diet is associated with a reduced risk of many chronic diseases, it is not clear to what degree the fiber itself is responsible and to what degree it simply represents a marker for high plant food consumption. We do know, however, that it speeds up movement of digested foods through the intestines. This reduces stagnation and pressure buildup that can cause diverticulosis and hemorrhoids. High-fiber foods remain longer in the stomach, however. This is beneficial for people who have ulcers, and it also provides a feeling of satiety (fullness) that reduces the quantity of food we feel compelled to eat, thereby promoting maintenance of a normal weight and allowing the potential health benefits associated with a reduced caloric intake (see question on dietary restriction). High-fiber diets also reduce the levels of sex hormones such as estrogens in the body, an effect that may reduce the risk of certain hormone-dependent cancers, such as breast cancer. On the other hand, people who take thyroid replacement hormones may need to increase their dosage when they increase the fiber content of their diets.
A wholesale loss of vitamins occurs when flour is refined. Some of the B-vitamins are replaced. These include vitamin B-1 (thiamine), vitamin B-2 (riboflavin), vitamin B-3 (niacin), and folate. Of those that are not replaced, over 85% of the vitamin B-6 (pyridoxine) is lost, over half the pantothenic acid is lost, and there is no vitamin B-12 to begin with.
Whole wheat is a good source of vitamin E, an important antioxidant vitamin. The wheat germ fraction is a good source of the entire spectrum: it is rich in the alpha- and beta-tocopherol forms of vitamin E, contains moderate amounts of the gamma- and delta-tocopherol forms, and even contains some of the less common tocotrienol forms. Most of the vitamin E is lost when wheat flour is refined. In addition, 80% of the vitamin K, which is important for blood clotting and bone health, among other things, is lost.
The only mineral replaced after refining is iron. Whole wheat is an excellent source of magnesium, which we believe is important for bone health, maintenance of normal blood pressure and blood sugars, and prevention of arrhythmias (abnormal heart rhythms) and kidney stones. Over 80% of the magnesium is lost. Similarly, 75% of the zinc, 80% of the manganese, 40% of the copper, and half of the selenium is lost.
Important phytochemicals lost when whole wheat is refined include lutein and zeaxanthin, the carotenoids that give whole wheat its golden color, and phenolic acids like ferulic acid, which are antioxidants. Unbleached wheat flour does retain most of the lutein and zeaxanthin, but otherwise it is very similar to white flour and should not be confused with whole wheat flour. Be aware that "wheat bread" generally does not refer to whole wheat bread.
Although nutritional studies in the past often ignored the differences between whole and refined grains, things are changing. A study from Italy showed that people who infrequently consumed whole grains had five times the risk of soft tissue sarcomas (cancer) as those people who consumed whole grains frequently. There is now evidence of an effect on other kinds of cancer as well. Studies have also shown a lower risk of heart disease, stroke, and diabetes in people with high intake of whole grains. These were confirmed by the recent finding, published in 2006 in The American Journal of Clinical Nutrition, that the higher the intake of whole grains, the lower the risk of metabolic syndrome, a condition characterized by obesity, high blood pressure, elevated cholesterol and triglyceride concentrations, and insulin resistance, which often leads to diabetes.
This apparent protective effect of whole grains needs to be emphasized in light of recent studies suggesting that consuming foods with a relatively high glycemic index (tendency to raise blood sugar concentrations) increases the risk of heart disease and diabetes. It will surprise many people to learn that products made from whole wheat flour have just as high a glycemic index as products made from white flour and just as high a glycemic index as sugar! It is important to realize that the studies having to do with glycemic index do not prove a cause and effect relationship, and the fact that whole grain consumption appears to reduce the risk of many diseases is evidence against such a relationship. My feeling is that the average glycemic index of a diet may simply be a marker for "junk food" diets, which are typically high in sugars and refined grains. Such diets may indeed increase the risk of chronic disease, but healthful food choices such as whole grains should be encouraged, regardless of their glycemic indices.
The late Ernst Wynder, M.D., of the respected American Health Institute in Valhalla, New York, used to quip that refined grains should be made illegal. I wholeheartedly agree that there is no place for refined grains in an optimal diet. Whether you're eating bread, spaghetti, bagels, or rice, choose those that are 100% whole grain whenever possible, and certainly avoid those in which the majority of the grain ingredients are not whole grains.