According to WebMD, eating 1,200 calories a day is safe. A recommended, safe way to lose weight is to eat 1,000 to 1,200 calories a day and exercise for at least an hour a day. Eating fewer calories on a regular basis is not recommended unless under the direct supervision of a physician.
WebMD reports that a sedentary adult female needs 1,600 to 1,800 calories a day, and a sedentary adult male needs 2,000 to 2,400 calories a day. Therefore consuming 1,200 calories a day should result in weight loss but is considered to be safe.
The 1,000 to 1,200 "safe" level of calories per day is based on studies of people on very low-calorie diets. According to WebMD, eating fewer than 1,000 calories per day can result in malnutrition, gallstones, electrolyte imbalances, infertility, loss of muscle mass and other adverse events. Individuals placed on physician-supervised very low-calorie diets are given nutrient-dense meal substitutes and are not encouraged to continue on the very low-calorie diets for more than 12 weeks.
WebMD also says that it does not hurt otherwise healthy people to consume less than 1,000 calories a day occasionally. A brief period of severe calorie restriction does not result in harm. An occasional complete fast of up to 48 hours is considered safe for healthy adults.