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Q: Are potato skins nutritious? What if they have green spots?

A: Ounce for ounce, the skin is the most nutritious part of the potato, with lots of fiber (2 grams per ounce), plus iron, potassium, calcium, vitamin C, B vitamins, and other nutrients and phytochemicals. You should wash the potato well and pare away any green areas, gouge out any sprouts, and trim visible blemishes. If the potato is green below the skin, has gone soft, or is excessively cracked, bruised, or sprouted, throw it out.

Why not "go green?" The green tinge is chlorophyll, which forms when the potatoes are exposed to light. Chlorophyll is harmless but indicates that solanine and another glycoalkaloid called chaconine may be present in increased amounts, mostly in the skin. Potatoes develop these toxins as a defense against fungi, insects, and animals. Potatoes that are bruised or otherwise damaged or rotten may also have elevated solanine and chaconine, even if they are not green. Potato sprouts contain the most of these chemicals.

In large amounts glycoalkaloids can cause stomach pain, vomiting, headaches, fever, rapid heart rate, confusion—even coma and death on rare occasions. But because a potato with high levels of these toxins would taste so bitter, such poisoning is uncommon. If a potato does taste bitter, do not eat it.

Q: Is it okay to use metal bottles for water in order to avoid chemicals in reusable plastic bottles? I noticed an odd build-up in my metal bottle. Could that be harmful?

A: Metal bottles (aluminum or stainless steel) are okay as reusable water bottles. Most plastic bottles are fine, too—though we recommend against reusing the disposable kinds over and over, since they were not designed for this and may become contaminated with bacteria. Manufacturers are phasing out those hard polycarbonate bottles (for hikers and cyclists) that contain bisphenol-A, or BPA. It’s unclear what risks BPA poses in consumer products (see Wellness Letter, February 2009). Still, most people don’t want to be exposed to BPA, or to expose their children, who may be more vulnerable to any possible adverse effects. In particular, heating polycarbonate bottles, as in a dishwasher, can damage the material and cause lots more BPA to leach out. Makers of metal bottles have made much of the dangers of BPA, but most plastics today do not contain it.

Remember, though, that your metal bottles may be lined (to prevent a metallic taste), and the lining may contain plastic. Manufacturers usually claim the plastic does not contain BPA or does not leach BPA, but you can’t be sure. Metals from unlined bottles—minuscule amounts of nickel, aluminum, or iron—may leach into the drinking water. This happens with cookware, too, and should not be a concern. The link between aluminum and Alzheimer’s has never been proved—though marketers of stainless-steel bottles sometimes imply otherwise.

A build-up in a metal bottle could simply come from hard water or from the contents of the bottle. Try washing it with a brush and soapy water. Or let it soak in a vinegar solution overnight and then rinse with warm water and baking soda. Wide-mouthed bottles are easier to clean, and most can go in the dishwasher. Follow the manufacturer’s instructions.

Q: What’s better—a snack food with less saturated fat but some trans fat, or a food with more saturated fat but no trans fat? Is there a ratio of saturated to trans fats I can use to rate a food’s healthfulness?

A: There’s no official ratio of fats to aim for in a specific food, but the American Heart Association (AHA) offers some perspective of how much saturated versus trans fat you should get in a day. Both types of fat have undesirable effects on blood cholesterol and raise heart disease risk. Trans fats are even worse because they not only raise LDL ("bad") cholesterol (like saturated fats do), but also lower HDL ("good") cholesterol.

The AHA recommends limiting saturated fats to less than 7% of your total calories (that’s about 15 grams if you eat 2,000 calories a day), and trans fats to less than 1% of your total calories (about 2 grams for a 2,000-calorie diet). So you can consume a lot more saturated fat (seven times as much) relative to trans fat, and still have a healthy diet. Another way of looking at it is that trans fats are so bad, it doesn’t take much to reach your daily limit.

Keep in mind, this 7-to-1 ratio of saturated to trans fats applies to your whole diet, not to specific foods. It’s what you eat over the entire day (or days) that makes your diet healthy or not—and the bottom line is that you should limit both types of fat. Best yet, look for foods that are free of trans fat altogether. Many companies have reduced or removed trans fat from their products. But even if a label says "0" trans fat, if partially hydrogenated oil is listed as an ingredient, that means some trans fat is present.

Q: Should I stop exercising when I have a cold?

A: It’s a myth that you should stop. Studies have found that it doesn’t matter whether people with colds exercise or rest—the duration and severity of the colds are the same.

Colds vary in severity, of course, and people react differently to them. A lot depends on how bad you feel and how much you exercise. For instance, long, strenuous exercise, such as running a marathon, can briefly depress the immune system and may actually make you more susceptible to colds and flu. Your expectations may also play a role: if you expect that your cold will interfere with your usual workout and make you feel worse, it’s more likely it will.

If you have a cold or feel one coming on, it won’t hurt to exercise. It’s best to start slowly and reduce the intensity of your workout, and see how you feel. If you feel worse, stop. If you feel okay, work up to your normal routine. If you have signs of a more serious infection (fever, swollen glands, fatigue, diarrhea, or vomiting), discontinue your workouts until you have fully recovered.

UC Berkeley Wellness Letter, March 2010