Sorry to be a pain in the butt on this, but the thing is-you see a lot of these 80%/20% etc percentages thrown around. None of them have any grounding whatsoever in science. Many studies have shown diet intervention to be more effective in the short term for weight loss, but exercise is needed for long term weight loss. A few papers have even found the opposite, which makes sense in terms of calorie balance, but is generally too difficult for the majority of people to adhere to. It has also been shown that diet alone, especially if calories are restricted, can have long lasting detrimental impact on metabolic rate, which is part of the reason that exercise needs to be used in conjunction with weight loss.
I could ramble for days about this-but that is WEIGHT LOSS-it's easy to get it confused with HEALTH, but they are very different. Exercise is important for so many other reasons down to a biochemical level, most related to disease risk. I was asked to give a seminar on this last year with part of my message being that I feel it is better to think of weight loss, or "getting in shape" as a by-product of using exercise to avoid being dead-which is a bit of a harsh statement, but we need to think about exercise as an important part of our make up and not something we do until we lose x amount of kilos or can see our biceps or abs grow. Diet and exercise are both important, and sometimes for different reasons, but both effecting the whole person, which is difficult to study empirically.
I realise I'm putting more than my 2 cents worth in here-I reckon I'm up to a dollar or so, but I am passionate about this stuff and get annoyed with so much media led rubbish about diet and exercise. I hope my rants come across in the positive manner I want them too :)
Here's a couple of articles for some light reading-just to balance the scales a little
journals.lww.com/acsm-essr/Citation/2000/28040/Exercise_Alone_Is_an_Effective_Strategy_for.5.aspxAn easier way to look at the above review
www.unm.edu/~lkravitz/Article%20folder/exandwtloss.htmlThis is the study I was looking for but can't find it-article instead
www.newsmax.com/Health/Diet-And-Fitness/exercise-diet-weight-loss/2015/04/03/id/636327/