When you have one of the following ailments, there are certain foods and/or activities that can make you feel better or worse. What are they, and why? I tell you here.

HEADACHE

What is it? Headaches come in many forms, but all cause pain in the area of the upper head and skull. The pain may be dull and steady or throbbing, on one side or both. Visual changes, nausea and vomiting may also occur, says Dr. Holly L. Phillips, medical correspondent for CBS 2 News in New York City. According to the National Institutes of Health, there are four types of headaches: "vascular, muscle contraction (tension), traction and inflammatory. The most common type of vascular headache is migraine."

Does food cause it? Many foods are thought to trigger headaches. Some of the biggest offenders: aged foods (in particular cheese, red wine, sparkling wine and beer — particularly draft) and tyramine (which is a breakdown product of amino acids, basically a metabolic waste product) found in foods like canned, cured or processed meats, pickles, canned soups and nuts. Foods with preservatives and additives (sausage, hot dogs, luncheon meats, MSG in Chinese food), can also cause headaches, says Phillips.

Cold foods, liquids, caffeine withdrawal and missing meals may also be triggers.

Some foods that contain niacin, (even some specialty waters, such as Propel) can cause headaches because niacin is a vasodilator, says osteopath George R. Nissan, of the Diamond Headache Clinic in Chicago and spokesman for the National Headache Foundation. Some controversy also surrounds the use of artificial sweeteners, in particular Aspartame; however, according to Nissan, there is not enough supportive evidence, and it is still anecdotal.

What should I eat? If dehydration (e.g., after drinking alcohol) is contributing to the headache, drink electrolyte-rich fluids such as Gatorade or clear fruit juices. Ginger ale also is a stomach-soothing hangover option. You should also have more water, adds Nissan.

What should I not eat? Avoid any known triggers, as above.

Should I avoid exercise? Avoid exercise if you're dehydrated, says Phillips. However, regular exercise, such as swimming or vigorous walking, can also reduce the frequency and severity of migraines.

NAUSEA AND VOMITING

What is it? Nausea, according to the National Institutes of Health, is "an unpleasant, wavelike sensation experienced in the back of the throat and/or the epigastrium (the upper part of the abdomen) that may or may not culminate in vomiting." Vomiting is the forceful spitting up of food associated with stomach contractions and nausea, says Phillips.

Does food cause it? According to Dr. Lisa Feinberg, a pediatric gastroenterologist at the Cleveland Clinic, "Vomiting can be prevented, once nausea sets in, by consuming small amounts of clear, sugary liquids such as soda, fruit juices and Popsicles."

To prevent nausea, eat small meals throughout the day, eat slowly, avoid hard-to-digest foods and those that have smells (you might have to eat cold foods, because heating foods up creates odor). If you wake up nauseated, eat crackers or have a low-fat, highprotein snack before bed, says Feinberg.

What should I eat? For vomiting, Phillips recommends starting with 4-ounce portions of clear liquids (chicken broth, apple juice, ginger ale), then work up to soft foods like gelatin dessert, bananas, rice, applesauce and oatmeal. It's most important to avoid dehydration, she adds. "Consume your foods and drinks slowly and eat smaller, more frequent meals. Do not mix hot and cold foods. Avoid brushing your teeth after eating. Choose foods from all the food groups as you can tolerate them to get adequate nutrition," adds Feinberg.

What should I not eat? Avoid milk and dairy products as they can contribute to nausea, says Phillips. And according to Feinberg, for nausea you "should avoid fried, greasy or sweet foods."

Should I avoid exercise? Avoid exercise to prevent further dehydration. Plus, activity might make nausea worse and could lead to vomiting.

DIARRHEA

What it is it? Frequent, loose, watery stools.

Does food cause it? According to Phillips, food poisoning is a common cause of diarrhea. It may be viral or bacterial and can be spread by contaminated food products. Diarrhea can be caused by an acute problem, such as an infection (usually viral) or food intolerance (such as lactose), says Dr. James Tebbe, an internal medicine specialist at the Ochsner Medical Center in New Orleans.

What should I eat? Drink plenty of clear liquids (water, juices, bouillon/broth) to help prevent dehydration. Sugared liquids are preferred because they provide calories as well. "I often recommend half-strength sports drinks (such as Gatorade), to add some electrolytes (like potassium and sodium). I usually don't recommend OTC (over-the-counter) diarrhea medications. Some intestinal infections can be made worse by these medications," says Tebbe. Some encourage the BRAT diet: bananas (great for potassium), rice, applesauce and toast. If the diarrhea is accompanied by nausea, ice chips may help.

What should I not eat? Avoid fatty, greasy foods, alcohol and spicy foods, which can also worsen diarrhea. And, according to Tebbe, once symptoms are controlled for 24 hours, the diet can be advanced slowly. Start with starches (breads, pastas, etc.), then eggs and lean meats. Avoid dairy products, spicy foods and fried foods for about one week.

Should I avoid exercise? You can resume physical activity in a few days, once your fluid losses have been replaced. "If your urine is clear, you're probably well-hydrated. If your urine is dark, you probably still need to replace some of the fluid loss. Also remember that your stamina will not be exactly what it was before your illness," says Tebbe.

Charles Stuart Platkin is a nutrition and public-health advocate, and author of "Breaking the FAT Pattern" (Plume, 2006). Sign up for the free Diet Detective newsletter at www.dietdetective.com.