The short answer is when it starts to interfere with your life. When you spend most of your time thinking about your body, your weight, and/or food. When you plan your entire life around those things. When you start avoiding social events because you're afraid of the food, or eating in front of people. Basically, when it goes from dieting to obsession. I knew I had a problem as soon as I realized I couldn't even eat my favorite meal even when I wanted to. Everyone is different, but the short answer really is, when it starts to destroy your quality of life. That aside, it is normal for healthy dieters to obsess over food. There's a major difference, though, between a healthy dieter obsessing over food, and the way someone with an eating disorder obsesses over food and/or body image.