I'm Jackie Arnett, Registered Dietitian, and this is how to gain weight and still stay healthy. Fewer than 2 percent of Americans are underweight and have to think about gaining weight but it can be very frustrating and occasionally a medical concern to gain weight. Usually being underweight is due to a medical condition like hyperactive thyroid or a malabsorption issue like Celiac's Disease but just like with weight loss weight gain is also something that can be done in a healthy way or in an unhealthy way. To do it the healthy way the first thing to do is to avoid the big trap of eating everything and anything just for the sake of calories. What you want to do is pick nutrient dense calories. So you can eat things like healthy oils, avocados, and nuts. Peanut butter, cheese, and other items that are both nutrient dense and calorie rich so that you're getting health and wellness along with the calories. You'll also not want to eat too much at once. What you should do is eat frequently. So eat every 3 to 4 hours that way you're not causing a shock to your system and you're not letting glycogen stores get used up. Another thing you can do to gain weight healthily is to try and drink calories. It's fun and easy really. All you can have is great smooth and smoothie shakes and you can use protein powder and other smoothie shakes that include a lot of nutrients and calories. In general if you want to gain weight you want make sure you want to do it in a slow and steady way and make sure your getting a lot of nutrients along with the calories.