L-tyrosine and L-dopa are two substances that differ slightly in their chemical structure and biological function but are linked by the processes your body uses to convert one substance into another to meet its needs.

A 50-milligram dose of zinc exceeds the tolerable upper intake level for zinc for every age group, according to the Linus Pauling Institute. Unless your doctor has specifically instructed you to take this dose, you should not take 50-milligram zinc supplements.

Saw palmetto is a palm-like plant indigenous to the southeastern United States from South Carolina to Florida. Also known as shrub palmetto, this small, bushy tree produces purplish-black berries that are used in herbal medicine to treat enlarged prostate in men triggered by low testosterone.

Egg yolks are not a good source of dietary calcium. A good source of a nutrient provides 10 to 19 percent of its daily value, or DV, while an excellent source provides at least 20 percent of its DV, according to the Food and Drug Administration. A single egg yolk provides only 2 percent of the DV of calcium.

Too much soda can rob your body of the potassium it needs to function normally. Potassium plays a crucial role in muscle contraction, including your heart and digestive tract. A lower-than-normal level of potassium is called hypokalemia. A small drop in potassium can be asymptomatic, while a bid drop can be fatal.

While everyone experiences aching, painful feet from time to time, chronic pain in your feet may be a sign of an underlying medical condition or nutritional deficiency. There's not much evidence linking vitamin D deficiency specifically to foot pain.

Fish oil contains two brain chemicals that are commonly found in individuals suffering from depression: EPA and DHA. Although some studies have linked fish oil supplements with reduced symptoms of depression, the long-term effect of these chemicals on depression and other mood disorders remains largely unknown.

Plant foods such as fruits, vegetables and whole grains contain compounds that are beneficial to human health. They include antioxidants that prevent disease-causing oxidative damage that can result from chemical exposure and natural body processes.

Turmeric, a rhizome related to ginger root, has been a staple of traditional Chinese and Indian medicine and cuisine for thousands of years. Commonly available dried and ground into a powder, turmeric is inexpensive and readily available in most grocery-store spice sections.

You may know that turkey, especially white meat, is a better choice for people whose primary concern is cutting saturated fat and cholesterol. But if your doctor suggests you get more iron in your diet to counteract the fatigue and weakness associated with iron deficiency, “beefing up” your diet may be a smart choice.

Creatine is a dietary supplement promoted for enhancing athletic performance. It's commonly available in the form of creatine monohydrate. During high-intensity, short-duration exercise, creatine -- a protein building block -- is converted to a major source of fuel for the body.

B-complex vitamins are found in a variety of fruits and vegetables, as well as poultry, eggs, fish and meat. Following the recommended dietary guidelines set by the USDA can ensure you are getting enough of these vitamins daily.