Often enough, when people ask about what exercise to do to lose weight the standard response is "count calories" and "CICO" and "what matters is a deficit". All true.
However, this thread is here to also investigate/discuss the positive (or negative) effects of exercise on weight loss. In general, if we agree with the adage "weight loss is 80% diet" what is going on with the "other 20%"?
First off, obviously exercise can contribute to calorie expenditure. Someone exercising very regularly can attribute 600+ daily calories to wilful exercise. This might corresponds to moving from up in one or more categories in "activity level" and be similar to a significant lifestyle change.
Beyond that, exercise actively results in lean mass retention - being active means that less muscle will be lost during weight loss which in turn corresponds to a small additional daily burn (from 7 to 21 cals per day per lb of lean mass - depending on lifestyle.)
But wait! There's more!
Exercise actually changes your metabolic function. Not only is there a significant body of research that exercise directly affects enzymes, protein production, improves risks of cardiovascular and metabolic diseases directly
, there is also clear evidence that it affects how we oxidise fat and show, surprisingly, that food has a higher thermogenic response for the active. We release (and use up) a little more energy digesting food as active individuals than as sedentary people*. In other words, research suggests that "endurance training induces metabolic changes that favour leanness".
Then there are the positive psychological effects of exercise that can counter some of the mental pressures of calorie restriction... a bit beyond what I want to focus on here but certainly worth mentioning in passing.
So when recommending that people focus on calorie deficits also remember that activity and exercise also have a multi-factorial positive effect that shouldn't be forgotten.
*see Increased thermogenic response to food and fat oxidation in female athletes: relationship with VO(2 max). López P1, Ledoux M, Garrel DR. ( http://www.ncbi.nlm.nih.gov/pubmed/10950828