So, I joined MFP, put my stats in and went for a reasonable 1.5lb weight loss a week, understanding that 2lb is considered a reasonable aim.

Then I join the forums and read a lot of disparaging remarks (to put it nicely) about people who eat at 1200 calories. Can anyone shed any light on the contradiction here?

(For the record I'm struggling on 1200 so doing exercise to be able to afford a little chocolate each day).

I think it's because 1200 a day is under the daily amount your body NEEDS just to function, so eating less than this is dangerous for your body as it won't have the fuel necessary to function properly. I'm no expert though

Broadly there is a lower threshold that your system needs to sustain normal function, while that varies according to age, gender, general build etc the recommended minimum by various national health agencies is somewhere between 1200 and 1400 calories for women, and generally between 1400 and 1600 for men. That's seen as the level at which the body can function.

As an individual tries to lose weight by restricting calorie intake the loss will come from a combination of fat, muscle and organ tissue. The body uses the calories eaten to function on a day to day basis, and to regenerate tissue. If the intake is too low then the tissue isn't regenerated. Further to that the body will consume energy in the form that it most easily has access to, so muscle tissue and organ tissue are preferred sources of fuel, with fat actually being harder for the body to consume as fuel.

Clearly that's not going to happen overnight, but an intake lower than the level required to sustain function is going to cause damage in the long term.

That's one of the reasons for some form of resistance training, and some form of cardiovascular training in your programme. The resistance training causes the system to retain lean muscle mass, and the CV training improves your bodies ability to convert fuel. Both together encourage the weight loss to be fat, rather than muscle and organ tissue.

My experience, about this time last year, was that I set my goal at 1600 per day, and I really struggled with it. I found that I was so underfuelled I was struggling at work, had no energy and my training wasn't particularly high quality. Given the problems I was having my loss plateaued, I was so tired I'd dropped from being lightly active to being almost completely sedentary I reset my goals to give me 1900 per day and it helped me a lot. clearly an N=1 sample size doesn't tell you much, but by being more realistic about my loss goals I was more successful. Mind you, at the same time I stopped using the scale as a measure of success, I now track resting heart rate, running performance and bodyfat percentage as metrics, noting that my bf% has reduced significantly.