Fat, fat and fat.
If you are hardcore, you can get products like: http://innordicclub.com/produkt/kompeet/kompeet_3_kg/#.VOrq9vmG8wBNaturlig energi til heste This one is a danishDanish product though, but it is 99,5% pure vegetabe fat in powder, odor-/tasteless. Pretty cool to add to your food, you get the highest density of calories possible and it just melts in your mount, without a taste.