If you have a fast metabolism or are underweight, you may need to work on gaining weight. Your metabolism is mostly affected by your body size and composition, your sex and your age. Though you can change your body composition by gaining muscle or fat and losing muscle or fat, you cannot affect the other determinants of your metabolism. It is, however, possible to slow down your metabolism and gain weight without taking any drugs. You simply need to change your eating and exercise patterns.
Start eating more calories than what you are used to eating. UCLA's Student Nutrition Awareness Campaign says it takes an extra 3,500 calories to gain a pound. So if you eat 500 extra calories every day, you will gain one pound per week. To do this, you need to eat larger portions at all of your meals and more calorie-dense foods, which include dried fruit, nuts, meats, full-fat dairy products, granola and trail mix.
Start adding extras to your meals. For instance, drizzle 2 tbsp. of olive oil onto your spaghetti, then add the sauce, instead of only adding the sauce. You can add mayonnaise to sandwiches, peanut butter to crackers, honey to your tea and butter to your toast. These extras will help you gain weight.
Space your meals out more to slow down your metabolism. According to Cooking Light magazine, the more frequently you eat, the faster your metabolism runs and the more calories your body burns. Therefore, if you eat only three times per day instead of snacking often and having five or six meals per day, you will slow down your metabolism and gain weight.
Exercise at lower intensities and for shorter periods of time. Vigorous exercise such as running will make you lose weight, so instead, stick to moderate walking for your cardiovascular exercise. Add resistance weight training to your exercise routine so that you can gain some muscle instead of losing weight. You can replace your typical 30 minutes of running three times a week with 30 minutes of strength training three times a week and 30 minutes of walking twice a week. This will help slow your metabolism so you gain weight.