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It will be useful to track your total calorie intake using a food journal, food journaling app or website.  This will help you manipulate your predetermined caloric goal to result in any desired weight changes.  It can also help keep you accountable to your diet plan.  Food journals are also a great way to get insight into what you currently eat and how that compared to your determined caloric goal. Food journals will be able to give you an idea of where you're eating most of calories during the day. Lastly, journaling will be able to help you track and successful gain, lose or maintain your weight. If you're trying to lose weight, you'll want to make sure that you consistently have a net negative calorie intake each day. You can do this by reducing the number of calories you eat, burning more calories through exercise, or both.  In general, losing about 3500 calories per week equates to losing one pound of body weight.  Cutting out 500 calories daily will help you reach this weekly deficit.  Don't lose weight too quickly or cut out too many calories.  Most reputable sources recommend losing no more than about 1-2 pounds per week.  This can be dangerous and may leave you feeling weak, tired and low in essential nutrients.  Keep in mind that as you shed pounds you'll gradually have to work harder to maintain your weight. Having a low weight decreases your BMR and the number of calories you burn from exercise, meaning you'll have to further lower the number of calories you eat per day or exercise harder to continue to lose weight. Take in more calories than you burn through daily activities and exercise to help you gain weight.  You can do this by increasing the number of calories you eat or decreasing the amount of calories you spend through exercise or with a combination of both. Regardless of your reason to gain weight, choose healthy, yet high calorie foods to help you meet a higher calorie goal.  Choosing fried, processed or other unhealthy foods isn't ideal. Note that some exercise is necessary to maintain good health.  Don't discontinue exercise unless instructed by your doctor. Though everyone's physical activity needs are different, most medical sources recommend about two and a half hours of moderate aerobic exercise with strength training on two days out of the week (or one and a half hours of intense aerobic exercise).
Start a food journal. Cut calories to lose weight. Increase calories to gain weight.