Let me start off by saying that I have never and will never have this problem lol. However, if I have a random low calorie day, I just take it for what it is. I don't force myself to eat extra if I'm not hungry. However, if you continuously have this problem, I'd say nuts are a good way to go. Good for you and very calorically dense. Also, make sure you've had all your servings of fruits and veggies. If not, I'd blend them up into a smoothie w/ some protein powder. You can easily make that into 300 cals w/o even trying.