At what point does a extreme diet become a eating disorder ?

It sometimes is not a clear cut threshold. A major sign is always wanting to lose more and more, even though you have reached the point of being underweight. Being unable to realise how much weight you've lost and even though you know you're not supposedly fat, still feeling that you should weight less. Another part is obsession, extremely strong feelings associated either with losing or gaining weight and restriction to the point where most food is deemed inedible. A rule of thumb is: when things become too important and when you don't see the weight lost reflected on your body.

It becomes an eating disorder when it controls your life, and you think about it all the time.

When it starts to damage your health (both mental and physical). Try to research/think about what humans are supposed to eat from a biological & evolutionary aspect when choosing a diet. good luck :)

Do you feel obsessed ? Do you feel guilty or the desire to purge if you break one of your rules? Do you exercise compulsively or go long hours without eating to compensate for the calories you ingested or as an extreme resort to lose weight? Do you have a negative body image (e.g ashamed)? If you answered yes to most/all of these questions , you probably have an eating disorder and should seek medical help promptly.

I believe that dieting is not a bad thing, as long as you love your body and accept it, and the main goal of a diet is to be healthy, then you are in the safe side ! :)

When dieting hurts, when dieting becomes an obsession, when you cannot stop. When it rules and takes over ones life.

When you stop eating or go to extreme measures of a diet such as binging and purging (Eating as much as you feel you can and forcing yourself to vomit it up) that is when it becomes a bad eating disorder. However it can be something such as cutting down your food by so much that you eat hardly any. To be honest there is no real answer but just making sure you eat well is fine.

When your skipping meals and not eating at all except a little bit of cereal or something like that