If you ask people what the best exercise for weight loss is, most would answer some type of cardiovascular exercise.  This is, however, not necessarily true.  In fact, excessive amounts of cardiovascular exercise may be highly counterproductive to weight loss.

The body is a highly efficient machine.  It is excellent at conserving energy and will optimize the body for whatever circumstances that it regularly encounters.  If you were to compare the human body to a car, it would be the highest performance, most fuel efficient vehicle on the road.  The body will adapt to any situation.  If it is short on food, it will slow it's metabolism down.  If it is not being fed certain nutrients, it will trigger hunger for foods that contain the nutrients it needs.  If it is being physically challenged, it will adapt the musculoskeletal system to a more effective configuration for that particular activity.

In regards to long-duration cardiovascular exercise (exercise being performed for greater than 45 minutes at a time), the body is being given a stimulus to become more efficient at endurance activity.  The body knows that it is not efficient to carry extra weight during an endurance activity so it looks to make the body lighter.  After all, which would you rather do, run with a 50 pound backpack or without?  So where does the body look to reduce weight?  It looks to the muscle on the body.  The body only needs enough muscle to perform the endurance activity is being asked to perform.  Any extra muscle is considered excess weight.  After all, muscle is 80% water.  Water is a very heavy, dense substance.  The body considers this extra weight to be inefficient and, therefore, it causes the muscle to atrophy or shrink.

In addition to being heavy, muscle is a highly metabolically active tissue.  1 pound of muscle burns 35 calories a day, at rest.  Because the body is already burning a lot of calories in order to perform the endurance activity the body views this extra expenditure of calories (to maintain an unnecessary tissue) as inefficient.  This is another reason that the body is stimulated to reduce muscle mass on an individual that is training the body with long-duration endurance activity.

Unfortunately, the loss of muscle is bad news for weight loss.  Muscle is highly metabolically active `tissue.  Muscle speeds up your metabolism.  The more muscle you have on your body the more calories you burn – every day.  This helps you lose weight.

But don't I burn a considerable amount of calories when I perform cardiovascular or endurance activities?  The answer is yes.  During a one-hour bout of running, at a moderate pace, you'll burn approximately 600 calories per hour.  This will definitely help with losing weight.  For most of us, however, pounding out an hour on the treadmill, elliptical machine, or bicycle on a regular basis is highly unlikely to occur indefinitely.  We might be motivated to do so for a few months but beyond that the torturous, repetitive boredom will take over and most people quit their exercise program. Now we have a problem.  Because we lost muscle mass performing our long-duration endurance activities our metabolism is no longer as high as it was prior to engaging in exercise.  Plus, we're no longer burning calories with our endurance exercise.  This means we're in prime position to regain any lost weight and possibly end up in a worse predicament than when we started.

So what should I do?  You need to do 20 minutes, three days a week, of moderate intensity cardiovascular exercise in order for your heart to receive a cardiovascular training affect (for your heart to get healthy).  You should also engage in weight training three days per week in order to encourage the maintenance, or growth of muscle tissue to maximize your metabolism.  Save the long-distance endurance activities for runners and cyclists who perform these activities out of love.  Don't choose endurance activities for long-term weight loss/maintenance strategies.