Simply put, calories are energy. Consume more energy than your body needs and excess calories are stored as fat. Fat storage was an important part of human survival--allowing people to survive when food was scarce. Most modern Americans don't need to worry about famine, but your body still stores fat as if there wasn't a 24-hour convenience mart selling thousands of empty calories right down the street. To lose weight, you'll need to burn more calories than you eat through a combination of diet, exercise and lifestyle changes.
USDA Estimated Calorie Requirments
The U.S. Department of Agriculture publishes a very simple calorie requirement guideline based on age, gender and activity level. As a general rule, men need more calories than women, adults need more calories than children--at least until age 52 when caloric needs decline, and active people need more calories than sedentary people. The USDA believes that women ages 19 to 30 years old need 2,000 calories if they're not active, 2,200 calories if they're moderately active and 2,400 calories if they're very active. From ages 31 to 50 women need 200 calories less at each activity level and a further 200 calorie reduction after age 52. Sedentary men ages 19 to 30 need 2,400 calories, moderately active men need between 2,600 and 2,800 calories and active men need 3,000 calories daily. Caloric needs decrease by 200 calories as men age, just like women.
Individual Caloric Needs
The USDA guidelines are very general and don't take size into account. Although the USDA can give you a rough estimate of how many calories you should eat, the University of Maryland has a more individualized suggestion. To maintain your current body weight, multiply your weight, in lbs., by 12 calories if you're sedentary and 14 if your active. For example, a 24-year-old woman who is 5 feet 2 inches and weighs 110 lbs. would only need between 1,320 and 1,540 calories to maintain her current weight--far less than the USDA recommended 2,000 to 2,400 calorie range. Very athletic people may need more than 14 calories per lb.
Cutting Calories for Weight Loss
To lose weight, you need to burn more calories, creating a calorie deficit--it takes a 3,500 calorie deficit to lose 1 lb. By cutting 500 to 1,000 calories from your daily diet, you should see a weight loss of 1 to 2 lbs. per week. The Cleveland Clinic notes that a loss of 1 to 2 lbs. per week is safest and gives you the best chance of long-term success. Extreme diets that promise a loss of several pounds per week often result in loss of water weight or loss of lean muscle mass--when what you want is fat loss. It's also important not to eat too few calories, which can slow your metabolism and stall weight loss. The American College of Sports Medicine suggests women eat at least 1,200 calories and men eat at least 1,800 calories daily to keep their metabolism functioning.
Other Weight Loss Tips
Don't try to lose weight by making changes to your diet alone--burn calories by increasing your activity level. Even cleaning your house or walking the dog more often can help boost weight loss. Eat a variety of nutrient-dense fruits and vegetables, lean proteins--especially fish high in omega-3s, and unsaturated fats. Eat smaller meals more often rather than sitting down to three large meals. Watch your portion size and don't let yourself get too hungry, which can result in overeating. Keep track of what you eat--writing everything down in a journal will help you understand how many calories you're actually consuming.