This is because weight gain and weight loss are governed by many factors and mechanisms that are a balance between energy intake and expenditure. Even after reducing calorie intake weight may not be lost effectively if the metabolic rate of the body is lowered. Metabolic rate can be increased by exercising to burn excess calories.
The amount of calories intake on daily basis depend upon the the age and sex of the person, more younger and active person can consume more calories.
To loose weight by consuming low calorie diet can could reverse the whole dieting scene. Shortage of calorie intake could result in body storing fats, as a fear of starvation. It could also result in diet distress, just the metabolism becomes slow and body do not loose any weight, and frustration could lead you to over-eat. On the other hand, long term use of such low calories diet could result in fatigue, constipation, diarrhea etc. Gallstones are mostly observed in people using low calories diet.
