Here is the most important thing that you need to know about exercise – and it applies to either weight loss or weight gain.  Exercise matters because it has an impact on body weight; specifically, body weight ratio.  Most basically, our body weight is composed of fat mass and fat free mass (sometimes referred to as lean body mass).  Fat free mass involves several components, including, bones, muscles and water.  When you change your body weight, it is essential for good health and calorie expenditure that you increase your fat free mass and/or preserve your fat free mass.            
   Exercise helps you to do so because it builds muscle.  Muscle where most energy is burned.   If you lose lean body mass you may weigh less but have a higher ratio of body fat and this is not good.
   A person can be overweight and not have metabolic disturbance if they exercise and keep up their fat free mass (they may still have joint problems though).  A person can be thin and have metabolic dysfunction because they have a higher proportion of fat mass.
   Cardio exercise can certainly keep you toned, but to build muscle I recommend that you consider strength training (resistance or weight lifting) at least twice a week (as the PGA suggest.)
   I also want to share with you this sentence from the recently released report U.S. Health in International Perspective: Shorter Lives, Poorer Health from the Institute of Medicine and the National Research Council (2012).

[D]ecades of research suggest that avoiding sedentary behavior and engaging in regular physical activity (independent of body weight, body mass index, and dietary habits) exerts its own protective effect on the risk of heart disease and stroke and possibly other conditions, including cancer, depression, and dementia (Lee et al., 2012; U.S. Department of Health and Human Services, 2008a).

   As the sometimes controversial Robert Lustig, MD says, “exercise is the anecdote for anything that is wrong.”  I agree – and you know that I do.  He also thinks fructose is the root of all evil and that foods that cause the release of insulin are the culprit for our obesity epidemic or the diseases that accompany it.  I am not sure – but he has a new book out that you might like to search for if you are interested.  I agree with him 85% of the time, somewhat less than I agree with Walter Willett (90+%) and a lot more than I agree with that Dr. Oz fellow.  

Interested in the full report regarding the poor health of Americans compared to similar countries... click here.