In this issue of HEALTHbeat:
  • The benefits of cooking veggies in the microwave
  • Hydration for summer exercise

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Harvard Health Publications -- Harvard Medical School HEALTHbeat
July 8, 2008

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

In This Issue
1 The benefits of cooking veggies in the microwave
2 Notable from Harvard Medical School:
* Healthy Eating
* Exercise: A program you
   can live with

3 Hydration for summer exercise

From Harvard Medical School
Healthy Eating: A guide to the new nutrition
Some foods are good for you, some are bad. But which are which? While some age-old advice like “eat your vegetables” still holds true, many early assumptions have turned out to be wrong. The Healthy Eating report discusses new research on nutrition, describes the food-health connection, and takes on controversial topics like food additives, cooking methods, the role of carbohydrates, and more.
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1\ The benefits of cooking veggies in the microwave

The microwave oven is standard issue in today’s kitchen, from the home, to the workplace, to the dorm room. Their speed and convenience feel like nearly a necessity for today’s busy lifestyles. But does this method of cooking and heating meals and snacks compromise the nutritional value of foods we eat?

Microwave ovens use waves of oscillating electromagnetic energy to heat food. These waves are similar to radio waves but move back and forth at a much faster rate. They are also remarkably selective, primarily affecting polar molecules—molecules with a negatively charged end and a positively charged end. Water is a polar molecule, so a microwave oven cooks by energizing (heating) water molecules, and the water energizes its molecular neighbors. The reason glass, ceramics, and many types of plastic don’t get hot in a microwave oven is because they’re made up of nonpolar molecules.
In addition to being more selective, microwave-oven energy is also more penetrating than heat that emanates from an oven or stovetop. It immediately reaches molecules about an inch or so below the surface. In contrast, regular cooking heat goes through food rather slowly, moving from the outside in.

Some nutrients break down when they’re exposed to heat, whether that heat comes from a microwave or a regular oven. Vitamin C is perhaps the clearest example. So, cooking with a microwave probably does a better job of preserving the nutrient content of foods because the cooking times are shorter.

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

For more information on food and your health, order our Special Health Report, Healthy Eating: A guide to the new nutrition.
    [Back to top]

2\ Notable from Harvard Medical School
** Healthy Eating: A guide to the new nutrition
Some foods are good for you, some are bad. But which are which? While some age-old advice like “eat your vegetables” still holds true, many early assumptions have turned out to be wrong. The Healthy Eating report discusses new research on nutrition, describes the food-health connection, and takes on controversial topics like food additives, cooking methods, the role of carbohydrates, and more.

** Exercise: A program you can live with
Exercise: A program you can live with describes specific types of exercise and explains the complementary roles of structured exercise and daily activity. It will also help guide you through starting and maintaining an exercise program that suits your abilities and lifestyle. You’ll find advice on being a savvy consumer when it comes to fitness products and equipment, as well as useful tools and tips designed to help make exercise work for you.

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3\ Hydration for summer exercise

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.

— Harvey B. Simon, M.D.
Editor, Harvard Men’s Health Watch

This Question and Answer first appeared in the July 2008 Harvard Men's Health Watch, available at

For more information exercising safely, order our Special Health Report, Exercise: A program you can live with.



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Harvard Medical School publishes authoritative Special Health Reports on a wide range of topics. Each report delivers practical information on diagnosis, treatment, and prevention of major health concerns in clear, easy-to-understand language. For more information on a specific topic, click the appropriate link below:

Alzheimer’s, Arthritis, Bladder, Cholesterol, Depression, Diabetes, Digestion, Energy, Exercise, Eye Disease, Headache, Heart Disease, High Blood Pressure, Memory, Menopause, Prostate, Sexuality, Sleep, Stroke, Vitamins

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Harvard Medical School offers special reports on over 50 health topics. Visit our Web site at to find reports of interest to you and your family.

Copyright 2008 by Harvard University.
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