Dear HEALTHbeat subscriber,

How you cook your vegetables may make a big difference in how much nutrition you get from them. And you might be surprised to learn that using a microwave may help retain the vitamins, minerals, and other compounds that make vegetables such a health boon to begin with. This issue of HEALTHbeat explains how microwave cooking affects the nutrient content of food and also gives tips for healthier microwave cooking. Also in this issue, Dr. Harvey Simon, editor in chief of the Harvard Men’s Health Watch, talks about getting hydrated properly during summer exercise.

Wishing you good health,


Nancy Ferrari
Managing Editor
Harvard Health Publications
HEALTHbeat@hms.harvard.edu

Boiled, steamed or fried

Cooking vegetables in water causes some of the nutrients to leach out into that same water. For example, boiled broccoli loses glucosinolate, the compound that may be responsible for its cancer-fighting properties. However, you can incorporate the nutrient-rich water from boiled vegetables into sauces or soups. Steaming vegetables may help retain more nutrients. Steamed broccoli holds on to more glucosinolate than boiled or fried broccoli.

No matter how you slice it, vegetables are good for you pretty much any way you prepare them, and most of us don’t eat enough of them. And the microwave oven? A marvel of engineering, a miracle of convenience — and sometimes nutritionally advantageous to boot.

Healthy microwave-cooking tips

Here are some things to keep in mind when using the microwave:

• Most takeout containers, water bottles, and plastic tubs or jars made to hold margarine, yogurt, whipped topping, and foods such as cream cheese, mayonnaise, and mustard are not microwave-safe.
• Microwavable takeout dinner trays are formulated for one-time use only and will say so on the package.
• Don’t microwave plastic storage bags or plastic bags from the grocery store.
• A recycle symbol does not mean a container is safe to use or reuse in the microwave oven. Only a microwave-safe icon or wording to that effect does.
• Before microwaving food, be sure to vent the container: leave the lid ajar, or lift the edge of the cover.
• Don’t allow plastic wrap to touch food during microwaving because it may melt. Wax paper, kitchen parchment paper, or white paper towels are alternatives.
• If you’re concerned about plastic wraps or containers in the microwave, transfer food to glass or ceramic containers labeled for microwave oven use.

For more information on food and your health, order our Special Health Report, Healthy Eating: A guide to the new nutrition, at www.health.harvard.edu/HE.

Q: I’ve always been told that it’s important to drink plenty of fluids during exercise, and it seems to work for me. But now I’ve read that too much water can be very dangerous. Which is right?

A. Both are right: good hydration is important, but overhydration can be hazardous, even lethal. Fortunately, common sense and moderation will protect you from both extremes.

Until the late 1970s, most athletes were advised not to drink before or during exercise. The idea was to avoid bloating and to improve performance. Some coaches may have also believed that withholding fluids would “build character.” It didn’t work. In fact, dehydration increases the risk of muscle cramps, heat exhaustion, and heat stroke during exercise in warm weather. And even in mild weather, dehydration can leave exercisers grumpy or groggy for hours afterward.

When the hazards of dehydration became apparent, experts began to encourage fluids during exercise. A common recommendation was to drink 20 to 40 ounces per hour of exercise. But these guidelines were formulated for elite male athletes whose high-intensity exercise produced lots of fluid loss in sweat. And they were intended to apply to exercise lasting two hours or less.

As a result of these guidelines, athletes began to drink. Spurred on, in part, by manufacturers of sports drinks and bottled water, they drank more and more. And some drank too much, producing water intoxication and hyponatremia (low blood sodium levels). Hundreds of cases and a number of deaths have been recorded in medical journals.

The tragic death of a female runner in the 2002 Boston Marathon called attention to the problem and has stimulated advice against drinking too much during exercise. The caution is justified and the advice is important, but it’s not easy to drink enough to get into trouble. The typical victim of water intoxication is a runner who is out on a marathon course for over four hours and who consumes enough fluids — often well over 3 quarts (96 ounces) — to gain weight during the race. That is way, way too much.

How much should you drink? In hot, humid weather, you can lose over a quart of sweat in an hour of moderate to intense exercise; in cool weather, much less. Plan to drink two to three cups of water an hour, but boost the amount if you are sweating heavily. If you drink a quart, or even two, during a long summer tennis game or hard workout, you won’t get into trouble. Extra salt is not required to prevent hyponatremia, and unless you get way behind in your fluid replacement, sports drinks won’t be any better than water. The trick is simply to avoid drinking too much fluid.

Best of all, evaluate your own needs. Drink when you feel dry or thirsty, but don’t arbitrarily force down huge amounts. To learn precisely what you need to stay in balance, weigh yourself before and after exercise, stripped down to avoid the effects of sweaty clothing. For each pound you lose, you’ll need a pint of fluids. You can also keep an eye on your urine output. If your urine is scant and concentrated, you’re dry, but if it’s clear and copious, you are fully hydrated. Remember, though, that if you gain weight, feel bloated, or experience nausea and vomiting, you’re on your way to hyponatremia and big trouble.

Medicine is a science, but its teachings often seem to swing like a pendulum. If you stay centered and use common sense in evaluating advice, you won’t be left high and dry — or, for that matter, all wet.

This Question and Answer first appeared in the July 2008 Harvard Men's Health Watch, available at www.health.harvard.edu/men.

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