To determine how many calories you should eat per day, first calculate the number of calories needed to maintain your current weight--roughly 15 calories per pound of body weight in a moderately active person (someone who gets at least 30 minutes of moderate to intense physical activity every day). A completely sedentary person may require just 12 calories per pound to maintain weight.
A pound of body fat contains 3,500 calories. To lose 1 to 2 pounds per week--a gradual and safe rate of weight loss--you must eat 500 to 1,000 fewer calories per day than what is needed to maintain your weight. (The calorie cutback need not be so severe if you also begin to exercise regularly.) Calorie intake should not drop below 1,200 per day in women or 1,500 per day in men (unless the diet is medically supervised and you are taking a vitamin/mineral supplement), since it would be difficult to get all the nutrients you need.
While reducing calorie intake is essential for losing weight, focusing on calories per se may leave people feeling hungry and frustrated unless the overall composition of the diet is also considered. Replacing dietary fat with complex carbohydrates automatically lowers calorie intake, while allowing a satisfying volume of food. There are other reasons to reduce the fat content of your diet. Some evidence suggests that a prolonged high-fat diet may trigger an upward adjustment in the body's set point. In addition, fewer calories are burned when dietary fat is converted into body fat than when carbohydrates or protein are converted into fat. Moreover, a low-fat diet can help to lower blood cholesterol levels and may reduce the risk of colon and prostate cancer.
Once you decide on an appropriate calorie intake, you need to determine the amount of total fat you should eat. Most experts now recommend that a diet should derive no more than 35 percent of its calories from fat (even when substituting monounsaturated for saturated fats). Most people should not reduce fat intake to less than 20 percent of calories, and the American Heart Association cautions against cutting fat below 15 percent for certain groups of people (older adults, for example), owing to concerns over malnutrition and a possible negative effect on blood lipids.
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