In nature, molybdenum is found as part of other metal complexes. In the soil, it serves as a catalyst to the nitrogen-fixing process; thus, decreased soil molybdenum can lead to deficient plant growth.

The body contains minute amounts, about 9 mg., of molybdenum. It is found mainly in the liver, kidneys, adrenal glands, bones, and skin, but it is present in all tissues. It is important to several enzyme systems, most significantly that of xanthine oxidase, which supports many functions, including uric acid metabolism and mobilization of iron from the liver for body use. Molybdenum is fairly easily absorbed from the gastrointestinal tract, though it competes with copper at absorption sites. It is eliminated through the urine and the bile.

As with chromium, depletions or deficiencies of molybdenum are common, and its availability in foods is decreased through soil depletion and food technology. This mineral has come to the nutritional forefront in the last decade with the recognition of its essential nature and the concern about deficiency.

Sources: The food levels of molybdenum depend largely on soil content. The amount in food may be increased a hundredfold with molybdenum-rich soil; in certain areas, hard water may contain some molybdenum. Soft water and refined foods contain hardly any. Whole grains, particularly the germ, usually have substantial amounts; oats, buckwheat, and wheat germ are some examples of grains containing molybdenum. Many vegetables and legumes are also good sources; these include lima beans, green beans, lentils, potatoes, spinach and other dark leafy greens, cauliflower, peas, and soybeans. Brewer's yeast also has some, and liver and organ meats are often fairly high in molybdenum.

Functions: Molybdenum is a vital part of three important enzyme systems—xanthine oxidase, aldehyde oxidase, and sulfite oxidase—and so has a vital role in uric acid formation and iron utilization, in carbohydrate metabolism, and sulfite detoxification as well. In the soil and possibly in the body, as the enzyme nitrate reductase, molybdenum can reduce the production or counteract the actions of nitrosamines, known cancer-causing chemicals, especially in the colon. Found more in molybdenum-deficient soils, nitrosamines have been associated with high rates of esophageal cancer.

Xanthine oxidase (XO) helps in the production of uric acid, an end product of protein (purine) metabolism. Though an excess of uric acid is known to cause gout, recent studies show that, in proper concentrations in the blood, it has antioxidant properties and helps protect the cells and tissues from irritation and damage caused by singlet oxygens and hydroxyl free radicals. This protection may prevent tissue wear and aging, in addition to other free-radical diseases discussed throughout this book. Thus, uric acid has a new image as being an important part of balanced human function and not just a waste product. With its different effects, uric acid is somewhat like cholesterol in its biochemistry. As with cholesterol, it is both made in the body and obtained through the diet; some people are genetically inclined to elevated levels; and, whereas the right amount is essential to important functions, excesses can lead to problems (cholesterol appears to be much more of a concern on this count than uric acid). Xanthine oxidase may also help in the mobilization of iron from liver reserves. Aldehyde oxidase helps in the oxidation of carbohydrates and other aldehydes, including acetaldehyde produced from ethyl alcohol. Sulfite oxidase helps to detoxify sulfurs in the body, particularly sulfites, which are used to preserve food. These potentially toxic and harmful substances can cause nausea or diarrhea and precipitate asthma attacks in sensitive individuals. The "salad bar" syndrome is caused by sulfite sprays used on vegetables to keep them "fresh" longer. It is possible that adequate tissue levels of molybdenum keep the sulfite oxidase activity levels high enough to counteract this chemical and reduce potential symptoms; molybdenum deficiency may be a factor in those people who are more sensitive to sulfites. Luckily, though, the use of sulfites in food preservation is being made illegal.