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Answers from Dr. Hart

QUESTION: Are there different types of fitness? What is the difference between activity and fitness?

DR. JACQUELINE HART: The two most common forms of fitness are aerobic and musculoskeletal. Both types are important. Aerobic fitness is how well you can use oxygen during exercise. For example, are you able to run for an extended distance or are you only able to walk? If you can only walk, then your aerobic fitness is not as good as that of someone who can run far. Greater aerobic fitness improves the function of your blood vessels and may help reduce your risk of heart disease and some forms of cancer.

Musculoskeletal fitness is your ability to have the strength and flexibility to do everyday tasks and keep your balance. Maintaining musculoskeletal fitness is critical as you age. Keeping your strength up helps protect you from becoming frail, disabled, and bedridden as you age.

QUESTION: Why did I gain weight once I started exercising?

DR. JACQUELINE HART: Exercise changes the composition of your body so that you have more muscle and less fat. Muscle actually weighs more than fat. So, your weight may increase as you gain muscle and lose fat.

The important difference, however, is that you are trimmer and more lean. Ignore the number on the scale and gauge your progress by the inches you are losing from your waist and elsewhere and by how much better your clothes are fitting.

QUESTION: Is walking just as good as running?

DR. JACQUELINE HART: One key to exercise is finding what works best for you. This means an activity that allows you to work within your particular target heart range. For many people, walking at a brisk pace can bring you to that level. Others must move faster to reach their target zone.

Another key to exercise is to find something you enjoy. Some love the exhilaration of running while others find it too challenging, become too winded, or find it puts undue strain on their joints.

In terms of working your heart and preventing conditions like high blood pressure, heart disease, and osteoporosis, one is not better than the other as long as you stay within your target heart zone for at least 30 minutes 5 - 6 days per week. One difference that scientists have determined, however, is that to get equal benefit, you may have to walk longer than you would run, especially if your goals include weight loss. In other words, running for 30 minutes 5 days a week may be perfectly adequate for losing weight, but to obtain the same desired effects in your appearance from walking, you should aim for 60 minutes each workout.

With that said, consistency, safety, and sticking with your chosen exercise over the long haul is more important than pushing the intensity beyond your level of comfort or ability. In fact, many people who over exert themselves, particularly early on, wind up quitting altogether. You certainly don't want to do that. Again, the best exercise is what works for you.

It’s important to remember that almost any level of exercise is better than a sedentary lifestyle. Even 30 minutes of walking every day may improve your health. However, once you start to exercise, most doctors would encourage you to increase your intensity of your exercise, and how much you do it. Many recommend at least 60 minutes of exercise for maximum health benefit. The more intense and longer your exercise workouts are, the greater the benefit to your personal fitness and health, while also reducing your risk of premature death.

QUESTION: What kind of shoes should I wear when walking or running? Does it matter?

DR. JACQUELINE HART: Here are a few things to look for when selecting exercise shoes:

  • Basic comfort -- Always try sneakers on and walk around in them before you buy them. Make sure they fit, don't rub, and are generally comfortable. This doesn't guarantee that you won't get blisters the first few times you exercise in your new shoes, but it is a good start.
  • Your own biomechanics -- In other words, the shape of your foot and structure of your walk are very important for determining the best shoe for you. For example, people who have lower than normal arches tend to pronate (roll the foot from the outer to the inner edge with each step). Over time, this can cause shin splints, knee problems and other injuries. Some sneakers are specifically designed to control this motion. Working with someone knowledgeable and experienced can help determine which sneakers are right for you. At a shoe store that specializes in sports shoes, for example, someone there can watch you walk and recommend the best shoe based on your particular foot shape and biomechanics.
  • Different shoes for different exercises -- Walking, running, and basketball shoes are different in important ways. Think about the kind of exercise you want to do before you purchase the shoes. If you think you will do a variety of exercises, “cross-trainers” might be a good choice.

QUESTION: Is it true that exercising for short lengths of time is just as good for your heart as longer workouts? How short is too short?

DR. JACQUELINE HART: Short bouts of exercise may be as effective as longer exercise sessions, particularly when you are first starting out, as long as the total exercise time is at least 30 minutes per day. For example, if you can't find the time or it feels too strenuous to walk for 30 consecutive minutes, you could walk for 10 minutes, 3 times per day. There is no evidence that exercising for less than 10 minutes at a time is effective, however so, 6 sessions of 5 minute exercises cannot replace longer sessions.

It is important to understand that how much you exercise and how hard you exercise are important. If you exercise with greater intensity, you will need to exercise for shorter time periods to obtain the best benefit.

You can also use regular household activities to gain benefit from exercise.

If losing weight is part of your objective, evidence suggests that you should work your daily amount of physical activity up to a total of 60 minutes per day. You can break this up into three 20-minute sessions if time is a problem for you. This includes gardening, walking from your car to the office, walking upstairs instead of taking the elevator, and doing construction work. Add up all of your activities throughout the day and, again, strive for a total of at least 60 minutes.

QUESTION: I don't usually feel hungry after I exercise, even though I exercise first thing in the morning. Why is that? Should I eat before I work out?

DR. JACQUELINE HART: If you eat immediately before a workout you may feel cramping and nauseous. This is because exercise causes the blood vessels in your digestive system to narrow, while blood vessels to your muscles become wider. This allows the blood to flow to the muscles you are working, supplying them with more oxygen and glucose (sugar) for work and energy, rather than to the digestive tract. You should preferably have a light meal rich in carbohydrates about 2 hours before exercising. After vigorous exercise, you are less likely to be hungry because your stomach has a smaller blood supply and is less ready to handle an influx of food.

QUESTION: How much fluid do I need when I exercise?

DR. JACQUELINE HART: Our bodies are approximately 60% water, so even at rest, we need plenty of fluid. When you exercise, you lose water through sweat. To keep up with these losses and avoid dehydration, drink 2 - 3 cups of fluid within the hour before you exercise, a half to a full cup every 10 - 15 minutes during the workout, and at least 1 - 2 cups within 30 minutes afterwards. This is as necessary in cold weather as it is in warm weather.

Here are a couple of ways to tell if you need to drink more fluid:

  • Weigh yourself before exercise and immediately after. Drink 2 cups of fluid for every pound you lost during the workout.
  • Check the color of your urine. It should be pale yellow or clear. If it is dark or bright yellow, you are dehydrated and should drink.

QUESTION: Are sports drinks and energy bars good for you?

DR. JACQUELINE HART: Sports drinks are a good source of both fluid and electrolytes (salts), both of which your body loses during exercise. Truthfully, though, water is generally the best choice for replenishing fluids, especially since many of the sports drinks contain sugar -- a particular problem if you have diabetes.

People eat energy bars for different reasons, and there are many bars on the market. If you eat them to feel more energetic during a workout, look for one that is high in carbohydrates, and eat it about an hour before you exercise. If you eat an energy bar as a snack or for breakfast, look for ones that are higher in protein, to help keep you satisfied during the day. Watch out for bars that are high in fat. And remember that just because they are labeled as "sports bars," does not mean that they are healthy or low in calories. Again, beware of hidden fat and calories, particularly if you are trying to lose weight or you have heart disease.

QUESTION: Why do I feel sore the day after I lift weights?

DR. JACQUELINE HART: Soreness comes from tiny tears in the muscle that occur during strength training. It takes your body about 48 hours to repair these tears, and this process of tearing and rebuilding is what adds mass (and therefore strength) to your muscles.

It is normal to feel a little sore for 1 - 3 days after you lift weights, especially if you used muscles you don't normally use, or you used them more intensely than normal. If you experience pain in your joints following exercise or the soreness is more than mild, however, see your doctor.

QUESTION: Is it true that exercise helps you sleep better?

DR. JACQUELINE HART: Studies suggest that exercise can help people with insomnia (trouble sleeping) fall asleep faster and stay asleep longer, but it does not appear to improve sleep for people who do not have insomnia. Also, if you have insomnia, avoid exercising after 6 p.m. because exercising later in the day may make you feel too energetic to go to sleep at bedtime.

If you have insomnia, see your doctor to find out what is causing it. Exercise may be all you need to sleep better, but consult your doctor to find out for sure.

QUESTION: Can I exercise too hard?

DR. JACQUELINE HART: Yes. While you should always strive to exert yourself, know your limits so that you do not push yourself too hard. Overexertion can lead to injury, heat exhaustion, and heat stroke.

Use the talk test to find out if you are working too hard -- if you cannot comfortably have a conversation in short sentences while exercising, slow down. Build up your intensity gradually to reach your goals, and rest when you need to.

QUESTION: I have diabetes. Besides seeing my doctor for a medical evaluation before I begin, is there anything regarding exercise that I should be cautious about?

DR. JACQUELINE HART: Following blood sugars closely is extremely important for anyone with diabetes who exercises, particularly when you are just starting out or have recently changed your routine. Check blood sugar levels before, during, and after exercise. As a general rule for anyone with diabetes, you should not exercise if your blood sugar is over 300 or below 100 mg/dL.

If you use insulin, avoid injections near muscles that you use while working out. Over time, you will likely be able to adjust your insulin or medication dosage. You need to work closely with your doctor to make these adjustments.

Also, for anyone with diabetes, it is particularly important to drink plenty of fluid. And, if you have a diabetes-related eye disorder called retinopathy or you have decreased circulation in your legs, you should avoid strength training and high-impact exercises.

Exercise has many benefits for people with diabetes. Exercise improves glucose control and reduces risk of death from complications. People with diabetes who exercise may substantially improve their quality of life and add years to their lifespan.

Dr. Hart is Board Certified in Internal Medicine and teaches lifestyle modification programs for people with heart disease. She is currently affiliated with the Department of Internal Medicine, Newton-Wellesley Hospital, Harvard University. She holds an MD from the George Washington School of Medicine and an AB in psychology from Harvard-Radcliffe University. Dr. Hart completed her residency at Brown University in Primary Care Internal Medicine.


Warburton DE, Nicol CW, Bredin SS. Prescribing exercise as preventive therapy. Cmaj. Mar 28 2006;174(7):961-974.

Warburton DE, Nicol CW, Bredin SS. Health benefits of physical activity: the evidence. Cmaj. Mar 14 2006;174(6):801-809.

Review Date: 6/28/2011
Reviewed By: Jeffrey Heit, MD, Internist with special emphasis on preventive health, fitness and nutrition, Philadelphia VA Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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