Andrew Fields:
Would you not say that it's important to still exercise regularly (this time, cardio and strength) for the purposes of maintaining physical fitness? Maybe you don't do it to build muscle or lose weight, but just to keep the body you have.
My other question (and I want you to know that I really am looking for your opinion, not just sniping you for an argument) is this: what do you do when diet stops being enough? Every diet is slammed with the dreaded plateau of weight loss, and this is where a lot of people lose hope and give up. I am actually coming up on my plateau now. But I am determined to get past it. Is it healthy/safe to lower your calorie intake at this point? Do you load up on a certain vitamin/mineral to power through it?
Both diet and fitness are the cornerstones of any attempt at health and fitness. There should be no confusion about that. We can all argue about the nature of those two elements but I doubt anyone will seriously contest that both are necessary. If anyone does want to argue about that, I'm going to have to get very patronizing and self-righteous indeed.
I think many people get confused as to why they should do cardio, or any kind of training. Yes, the calories burned will offset calories consumed (although even that is nowhere near as linear or predictable as many would like to believe). No, that's not why you should do it. The easiest and best way to manage calories is to control your intake, in terms of macronutrient ratios, quality of food, and volume.
Why is it not good to consume a large pizza, say, then 'burn it off'? Michael Phelps does this, and he won all those Gold medals!, you might say. Well, Michael Phelps is literally AGING himself prematurely by taking in over 10,000 calories and burning them off. He is offsetting that to a considerable extent by maintaining phenomenal physical condition, but he will pay the price in later years. So while you want to exercise to be healthy, you don't want to exercise more than you need. Doing cardio to work off your calories is a bad cycle you don't want to get into.
Forget about weight loss and calories. Focus on lean body mass. You may think this is the same thing, but many people miss the point. Most everyone (i.e. everyone below 'athlete') needs more muscle, and less fat to be healthy. So you need a two-pronged approach of using both diet and exercise to build muscle and reduce fat. This may result in an increase in weight. So what. After a while, you won't need a scale to tell you whether you are healthy and fit and strong. A mirror and a stopwatch will be all you need.
So use diet to build muscle, use diet to reduce body fat (which emphatically does NOT mean reducing dietary fat), exercise to build muscle, exercise to reduce fat. Approx. 80% of body composition changes will occur from diet, but that 20% may be where you are lacking.
I have no idea what you are doing right now so I have no idea how to advise you on a plateau. In general, though, I advise setting performance goals, not weight goals. If you get to the point where you can, say, deadlift 400lbs AND run a 400m in 60 seconds or less, your weight will be close to ideal. Aim for that. Even if you fall short, you'll probably still look and feel pretty awesome. (I'm shy of both myself...these are fairly lofty goals for a middle-aged man who sat on his ass most of his life).