1000 Calories per day is only suitable for a small framed woman who engages in minimal amounts of exercise.

If you intend to follow such a restricted diet it is very important to ensure nutritional balance is maintained. 

Ensure you drink plenty of water and do not follow this amount of calories for sustained period of time. Daily calories should vary (zigzag) in order to maintain metabolic rate. That said, managed properly, such a diet can aid with weight loss goals.

A 1200 calorie diet plan would be appropriate for an adult woman who gets little to no activity and who desires weight loss. A 1200 calorie plan may also be appropriate for small-to-medium framed women over the age of 50 who are only lightly active. This calorie level is probably too low for most men.

These meal plans are around 1400-1450 calories, and are higher in protein. This calorie level is appropriate for a woman engaged in physical activity (such as 3-4 times per week), and who wants to lose weight. Ensure you drink plenty of water - aim for 8-10 glasses per day. This calorie level is not appropriate for men.

1800 calories per day is a generalized recommendation for men trying to lose weight. This exact amount has been trialed and has shown to lead to weight loss. Low-carb and Mediterranean-type diets were more successful than low fat. This is a balanced meal of about 50% carbs, 30% proteins, and 20% fats. Ensure you drink plenty of water - aim for 8-10 glasses per day. Only other drinks allowed are herbal teas, green tea, and the occasional black coffee.