ok, fat has a lot more calories then carbohydrates do, meat has more nutrition for the weight the vegetables do, though again to much of any thing including water is a bad thing too.... the biggest problem for people into the early 20th century was to NOT starve to death or suffer from malnutrition. we, humans are Omnivores. its simple, we need a balanced diet of animal and/or fish and/or dairy product proteins and fruits and vegetables to get all the nutrition and calories to survive. you can in an equatorial or similar climate survive on only vegetables,and fruits, but only if you are not doing a lot of hard labor or other wise stressed badly. the technology we have now reduces the calories needed by heating the spaces we live in, and the amount of hard physical labor we need to do. so yes you can be a vegan and survive, but if you are say doing hard manual labor.. then its unlikely you can do so even short term if there is cold or any other stresses on you and get enough calories from vegetables and fruits and grains. That is why combat rations are not vegetarian with out some kind of fats added. my great uncle was a dairy farmer till he died. even with central heating he burned over 7,000 calories every day!!! a combat soldier in combat is expected even in warm weather to burn at least 5000 calores a day... usually more. in a cold climate like mountain warfare or arctic warfare they figure a lot closer to 12,000 calories a day is needed to sustain them....