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Q: Is it safe to re-use a disposable plastic water bottle?

A: Yes, but only up to a point. The bottles, designed for one-time use, are usually made of polyethylene, a flexible plastic that can be damaged with repeated use and washing. Some preliminary research suggests that this can release potentially harmful chemicals. But the studies have had inconsistent results, and researchers disagree about whether these compounds pose a significant risk.

A better reason not to re-use the bottles over and over is that they can become contaminated with bacteria from your hands or mouth. Bacteria don’t grow easily in water, but can thrive when saliva and food particles are present, especially if the bottle is left at room temperature

If you do refill your water bottle, do so only a few times. Before refilling, wash it in hot, soapy water, as you would anything you drink from—but not in the dishwasher. Wash the cap, too, and inside the narrow neck. Keep a re-used bottle out of heat or sunlight, which may speed degradation and increase bacterial growth.

Q: What are natural flavorings, and how are they different from artificial flavorings?

A: Natural flavorings come from natural sources—such as cinnamon bark, coffee, citrus, blueberries, and almonds—that are used in small amounts to flavor foods. They may also come from meat, seafood, poultry, eggs, dairy, and fermented foods. Artificial flavorings, on the other hand, are combinations of synthetic chemicals that often match flavors found in nature. Flavorings have no calories or nutrients.

The term "natural" is somewhat misleading because natural flavorings aren’t just squeezed out of the plant or animal material; they undergo lab processing to extract the flavor chemicals. Moreover, there’s often little or no difference between natural and artificial flavorings, since they may contain the same key ingredients. For example, the chemical in bananas that provides their recognizable taste, amyl acetate, can be either distilled from bananas (called "banana flavor" or "natural banana flavor") or manufactured using vinegar, alcohol, and an acid ("artificial banana flavor"). Either way, it’s the same chemical.

So why go artificial? For one, it’s easier and cheaper to make something in the lab. It also saves plants—sometimes scarce plants—from destruction. Artificial flavors are also more stable and less likely to spoil, so they have longer shelf life. And contrary to popular belief, artificial flavorings are no less safe than natural flavorings; some people may be less likely to be allergic to them.

The FDA does not require foods to list the source of flavoring, only whether it is natural or artificial. But starting in 2006 the source of natural flavorings must be disclosed on the label if it is a major allergen, such as peanuts, shellfish, or milk.

Q: What is miso, and how nutritious is it?

A: Miso (pronounced MEE-so) is a fermented soybean paste used in Asian cuisine. It’s traditionally made by combining soybeans and a grain, usually barley or rice, with salt and a special mold (koji), and then left to ferment in wooden vats for up to three years. There are many types, such as hatcho, kome, and mugi miso, depending on what grains and other ingredients are used, how much salt is added, and how long it’s aged. Each has its own flavor (variations of earthiness and salty-sweetness), texture (chunky or smooth), and color (from white to red to deep brown). Darker misos have been aged longer, have stronger flavor, and are saltier. Miso is dissolved in water to make soup stock. Or it’s used to make spreads, sauces, marinades, and dressings.

On average, a tablespoon of miso (enough to make a cup of broth) contains 35 calories, 2 grams of protein, 5 grams of carbohydrate, 1 gram of fat, 1 gram of fiber, and very small amounts of zinc, manganese, iron, and B vitamins. It also has small amounts of isoflavones (phytochemicals found in soy).

The problem with miso is its high sodium content—470 to 850 milligrams per tablespoon, as much as one-third to one-half the daily limit. Thus, it should be used sparingly—for instance, an occasional cup of miso soup before a meal.

Q: I’m a mostly sedentary woman of 60. My doctor tells me I need exercise. I now have access to a gym, but weight machines and treadmills look pretty scary. Is a personal trainer worth the money?

A: A recent survey in the prestigious Cochrane Database of Systematic Reviews found that exercise "interventions"—that is, people to exercise more effectively. The studies available for review, however, were not all top-notch. Still, a qualified trainer can often help.

A trainer can be expensive, with a 60-minute session ranging from $20 to $150. An average is $1 a minute. But you don’t need a trainer the rest of your life. Most gyms offer packages, such as sessions twice a week for four weeks. Some gyms offer two-for-one training—you go with a friend and split the fee. If the trainer helps you design and get started on a safe, regular exercise program, that could be worth a lot to you.

How to find a good trainer? Talk with the manager of your gym. Try to find someone who understands your goals. Check references, if possible. The National Board of Fitness Examiners is devising a standardized national exam this year, but it won’t be obligatory for hiring in gyms.

UC Berkeley Wellness Letter, May 2005