Well, a couple of times I said it was probably a good target, and the reason I said that was because the people asking the question were women and shorter than me. When I was focused on weight loss, my target was 1500, though I did have some fast days and other days where the number was lower.

You want to create a decent calorie deficit while maintaining good nutrition. If you can figure out what your maintenance requirements are and then subtract 500, then you'll have a 3500 calorie deficit per week. This should translate into losing about a pound a week.

The main reason not too go to low is nutrition. You've got to get the nutrition in- enough protein to maintain your lean body mass, for instance. Good fats are high in calories, but still need to be part of a diet, because we always need them, and because- for the SAD people- the fat ratios within their bodies are all screwed up and it takes at least two years to fix that.

I always heard the 1200 being the minimal caloric intake because it is thought that this is the minimal amount of calories one can take in while still getting the bare minimum RDA of vitamins and minerals. This was at least the justification I've heard anytime watching a special on TV with some doctors debunking super low cal diets, like Kimkins, Hcg, etc.

People just like nice round numbers.

Never trust nice round numbers.

It is a made up number that continues to be repeated without any backup.

Alternatevely, if you include the "minimum recommended macronutrients" from the Food and Nutrition Information Center, you get approximately 1100 calories (including 130 grams of carbs, 20 grams of fat (none saturated), 10 grams of Omega 6, 1.6g of Omega 3, 56 grams of Protein. (http://www.iom.edu/Global/News%20Announcements/~/media/C5CD2DD7840544979A549EC47E56A02B.ashx )

However, in 1943, the suggested calorie intake for a ONE TO THREE YEAR OLD CHILD is 1200 calories. Recomendations for Women over 21 were 2500, and 3000 for 21+ men. http://ajph.aphapublications.org/doi/abs/10.2105/AJPH.33.1.58

So, the 1200 came to be the minimum for anyone of any age. Personally, I like to eat more than a 3 year old.

To Compare, the "Minnesota Starvation Study" was 1560 calories, usually with starchy tubers such as potatoes, rutabagas, turnips, with some bread products. (http://en.wikipedia.org/wiki/Minnesota_Starvation_Experiment#Goals_and_methods )

Supposedly below this number and your body downshifts into starvation mode. That is, it decreases your homeostatic metabolic rate in anticipation of famine. Your body no longer optimizes for performance, but to try survive the coming hard times.

This is bad because (1) daily performance is affected; (2) caloric burn rate drops; (3) later on, your metabolism may not magically upshift when you want it to. People seem to mess themselves up, and have difficulty find their original equilibrium.

Because you don't want your body to starve and start eating muscles/organs..

In the reading I have done, 1200 calories is not a goal for everyone. You should not go below your BMR (Basal Metabolic Rate), which is the number of calories you burn doing nothing but breathing and beating your heart all day. Below that number, you will sometimes go into "starvation mode" as another poster noted. You may also have trouble getting enough vitamins and nutrients into your body. Most of what I've read recommends that you find the BMR for someone with your height and weight (and preferably body fat percentage) and not go significantly below that for long periods of time.