Q: Why do my doctors tell me not to drink green tea 24 hours before a blood test or three days before surgery? How is it harmful? — Vera, North Bay, Ontario, Canada

A: Some of the very properties that make green tea so great for you have the potential to mess with your blood tests and operations. Green tea can prevent blood clotting, much like aspirin does (which is why you need to tell your doctor if you drink it).

The real reason not to drink it before blood tests is the same reason docs don't want you to eat anything before blood tests: Like food, the green brew can alter your blood sugar levels in ways that could throw off the results of a diabetes test. If you drink green tea with sugar in it, that also could elevate your triglyceride levels. Finally, your docs may consider green tea an herbal supplement, and many physicians want you to stop all herbs and supplements before tests or surgery, since they may contain contaminants that could interfere with testing or medications. (Technically, green tea is an herb, but so are regular tea and coffee, since they're also plant-based.)

Q: My fiance has polycystic kidney disease, and his kidneys function at only 23 percent. I've heard there is a diet that will help him, but I haven't been able to find any information on it. Do you know of such a thing? — Darlene, Beaumont, Texas

A: There is some evidence that a low-protein diet may slow the progression of kidney disease by taking some of the workload off these organs.

When your body processes a steak, for example, it produces a waste product called urea, which healthy kidneys filter out of the body. Kidneys that aren't working very well have to work hard to get rid of it, and they don't always do such a great job: Urea often builds up in the bloodstream and causes fatigue and loss of appetite. It also can build up in the kidney itself, speeding up the decline in kidney function.

For people with kidney disease, the standard recommendation is small amounts of protein, plus grains and vegetables, and in some cases, healthy fats. But for someone close to kidney failure, an extra-low-protein diet often is better.

In addition, these kidney-sparing diets also limit sodium, potassium and phosphorous, and can require supplements of essential amino acids that normally would come from protein-containing foods.

This very restrictive diet may even halt or reverse kidney failure. But it's not easy to do on your own. Your boyfriend should work with a kidney specialist and a nutritionist in order to stick with it safely.

Q: My 17-year-old daughter refuses to eat meat and fish, avoids eggs and now won't eat anything with gelatin. She doesn't like many vegetables, and survives mainly on mac and cheese, peanut butter and French fries. Is this a phase? Is there a magic spell to get her to eat healthier foods? — Leslie, Jefferson City, Mo.

A: She's already eating a vegetarian diet, and her refusal to eat gelatin (which is made from animal bones) suggests that she may be on her way to becoming a vegan — someone who doesn't eat or use any animal products at all.

There's a healthy upside here: Vegetarians who make healthy food choices are less likely than omnivores to be overweight, have heart disease, get cancer or die young. However, unless the cheese in her macaroni is low-fat or soy-based and her French fries are oven-baked, your daughter won't get those benefits, because her diet is loaded with saturated fat. But it doesn't take a magic spell, just some Nutrition 101 smarts, to turn her half-healthy diet into a good one. Her protein can come from beans, lentils and chickpeas; eggs (if she still eats them); nuts and nut butters (which are full of heart-healthy fats); and low-fat or soy cheeses. She should toss any vegetables she does like into her mac and cheese ... and into chili made with either beans or textured soy protein (trust us, the family won't know the difference). Since she's not eating meat — the prime source of iron — get her to eat foods high in vitamin C (tomatoes, citrus fruits, broccoli) at every meal, because C helps wring iron out of plant-based sources such as broccoli, raisins, watermelon and spinach. She might be missing out on vitamin B-12, but a bowl of fortified cereal or a vitamin tablet every day will take care of that. It wouldn't hurt for the rest of the family to join her meat-free madness a few times a week. You'll all benefit.

The YOU Docs, Mehmet Oz and Mike Roizen, are authors of "YOU: On a Diet." Want more? See "The Dr. Oz Show" weekdays at 9 a.m. on Fox/Ch. 13. To submit questions, go to www.RealAge.com.

© Michael Roizen, M.D. and Mehmet Oz, M.D.

Distributed by King Features Syndicate Inc.