What's more important, diet or exercise?
The answer, of course, is yes!
The most basic equation for weight loss is to eat less and exercise more. There is an intrinsic co-dependency on these two sides of the weight-loss coin. You really can't separate the two. Why?
Some people want it to be all about food. And if you reduce your calories, either by the recommend moderation, or going extreme and only eating 500 calories/day*, you will lose weight. But odds are good that you will lose muscle and protein about equally. Some of that weight loss is just water.
Aside: I cannot stress enough that I do not recommend extreme diets. More on that in another post.
It's important to note what is really lost. It's more than just a number on the scale going down. You want the loss to be fat, right? When you lose muscle during weight loss, you also lose some of your metabolism. Muscle burns calories even when you're not doing anything. Fat? Not so much.
This is why many people who get caught in the cycle of yo-yo dieting are very overweight. In the process of losing weight (muscle and fat) and then regaining weight (as fat), they continue to change their body composition to be more and more fat, and less and less muscle. Which makes it even harder to lose weight next time.
What about an exercise only approach?
You can out-eat any exercise regimen. Easily.
Aside: There are many added benefits of exercise that go beyond burning calories. Improved heart health, muscle tone, energy, better sleep, emotional health to name a few. And I believe that all of these things will ultimately help with weight loss.
Back to out-eating your exercise...
Think about the person who goes out for a 4-mile run. That's a decent workout that burns somewhere in the range of 400 calories. They justify an extra slice of pizza and beer because they worked out that day. Their added calories? Probably around 500. Which is 100 more calories than they burned. Which means they will be actively gaining weight.
Think of it another way. To lose 1 pound per week, you must account for about 500 calories per day. If you want to do this all through exercise you have your work cut out for you. Any of these will burn about 500 calories depending on your size and exertion: a five-mile run, an hour of Zumba, running stairs for 45 minutes, 50-60 minutes of beach volleyball.
Every. Day. Without eating any more than you were before.
Which brings us back to the answer of "YES" -- diet and exercise each play a non-negotiable role in weight loss or weight maintenance.