Losing weight involves cutting calories, but you should be careful about how much you're slashing from your diet. You still need enough calories to ensure your metabolism stays revved and you have enough energy for your workout plan. "According to the National Institutes of Health, an active 30-year-old female should eat 2,400 calories daily. So, if you take that average and deduct 300 calories you will lose one pound in 10 days," Juliet explains. "I would look to increase her expenditure of calories through working out as well as lowering her actual caloric intake to 1,200 to 1,800, depending on the person and their level of fitness." Your caloric intake will vary depending on your weight, activity level, age, goals, and more, but make sure you don't dip below 1,200 calories a day.