How much cardio should I do for weight reduction / maintenance?

By Dr. Atsushi Matsubara

The short answer is: Not too much and not too hard.

Exercise is the most effective, and I dare say the only sustainable way of controlling weight. But often it is not done quite right, frequently too hard, too much, and too often, as well as too little.

The bottom line of weight gain / loss is a simple thing: If you burn more than you take in, you lose weight. If you take in more than you burn, you gain weight. As simple as that. What makes the issue complicated is how to actually shift that balance. Exercising too much or too hard does not necessarily increase the amount of calories you burn overall. Going on a strict diet does not necessarily shift the balance favorably either.

The general recommendation is 20-30min of MODERATE CARDIOVASCULAR exercise 3-4 times a week. The key points are: “moderate”, “cardiovascular”, and “routinely”. And here are the reasons:

Why “moderate cardiovascular”? The answer has got to do with which kind of energy sources your body will tap into depending on the type of exercise you do. You want to exercise in such way that your body will preferentially burn fat calories.

If you exercise too leisurely, such as slow strolling, you really don’t use many calories at all. And obviously this won’t help too much. Walking is such an efficient means of transportation you burn only about 100kcal per mile or so (not even a half a can of soda).

On the other hand, if done strenuously, even the types of exercises categorized as “cardiovascular“ will force your body to use a quick energy source, sugar stored in the liver and muscles which will be depleted within a matter of a few minutes. Once the sugar storage is used up your blood sugar will drop, which will make you feel fatigued and hungry. You may burn a rather large number of calories over a short period of time but that’s where the effect of exercise ends without burning many calories overall. You may also end up eating much more than you have burned because of the hunger you work up during this type of exercise.

When you do moderate cardiovascular exercise, such as brisk walking, light jogging, or relaxed swimming, because of the relatively slow rate of energy consumption, your body will have a chance to tap into fat calories. In order to use fat as a source of energy, your body has to convert fat into sugar , which takes extra steps and oxygen. Therefore this cannot be accomplished during anaerobic exercise or strenuous aerobic exercise.

Now, those of you who have been exercising on a high-tech treadmill may remember feeling discouraged to see how little calories you log on the meter with brisk walking or light jogging. My advise to you is: Don’t look at the meter. The goal of moderate aerobic exercise is NOT to burn calories during the exercise, but to raise your metabolism. During moderate cardiovascular ex, as I mentioned above, your body will preferentially burn fat calories by turning fat into sugar. When fat cells convert fat into sugar and release it into the bloodstream, fat cells also produce some chemicals that raise the metabolism; it is not yet known exactly how these chemicals work. It is this raising of the metabolic rate that leads to weight reduction. Although you don’t burn much during the exercise, as your metabolism goes up the effect of exercise will keep taking place in the body long after the exercise, and overall you burn a lot more calories.

As far as figuring out the appropriate intensity of exercise, there are fancy ways to do it by going to a special exercise lab, etc. But the appropriate intensity of exercise will change depending on the level of conditioning , body weight, etc., and the high-tech approach can get quite expensive. Age-based heart rate is not always a reliable indicator either. One simple clue to figuring out how hard you should be exercising is the way you feel after the exercise: you should feel energized, not tired and hungry. As long as you stay in the “fat burning range“ during the cardiovascular exercise, your blood sugar will not significantly drop, because of the abundance of calories stored in the form of fat. Because the blood sugar level is maintained during moderate aerobic exercise, you should not feel tired or hungry. If you feel hungry and/or tired after exercise, that would be an indication that you have done it too hard and gone over the “fat burning range”. The next time you might want to slow down a little bit.

To discuss the rationale for the recommended duration and frequency of exercise: 20-30min of exercise is what it takes to trigger the significant chemical / metabolic changes as described above. 3-4 times a week is advised because the chemical / metabolic changed triggered by exercise seem to persist for about 2 days after each session. But in reality, you may want to shoot for doing it daily and you will probably end up at 3-4 times a week.

Most likely as you stay in the habit of exercising routinely, you will find yourself wanting to exercise harder, longer and more frequently, because of the addictive property of aerobic exercise. But again the word of caution against too much. If you keep burning too much more than you are taking in, your body will compensate for the caloric deficit by lowering your metabolism. This would be counterproductive. So it is crucial that you don’t exercise excessively.

Some quick advice on diet: Dietary moderation is the way to go. Eating 3 moderate, well-balanced meals a day is crucial in maintaining the metabolism. Crash / drastic diets are not usually sustainable. Even if sustained, they will most likely lead to a decreased metabolic rate, or possibly other health problems. Some short-term weight loss may be achieved by such diets, but in most cases weight loss will hit a plateau and then the weight will come back up as the metabolism lowers. Personally, I would recommend against snacking. Frequent small meals are sometimes recommended, but this does not make much physiologic sense. Some authorities cite low blood sugar during fasting as the cause of overeating as well as of decreased metabolism. However, in the overwhelming majority of cases, low blood sugar is due to excessive sugar / starch intake and the resultant desperate attempt by the body to keep the blood sugar down by secreting insulin. Large amounts of insulin remain in the system too long, driving the blood sugar too low. This in turn leads to hunger and overeating. So, in order to prevent low blood sugar, you should avoid excessive intake of starch and sugar in the first place.