A person needs X number of calories to keep their body warm, they need Y amount of protein to keep their muscles intact, they need Z amount of Iodine and various other micro-nutrients, and they should have a certain amount of fiber as roughage, though I believe quite a number of inorganic items would suffice. And they need oxygen and water of course. How you get these is irrelevant to your body unless the configuration results in dangerous excesses of one of the others or your intake method isn't resulting proper absorption. I don't believe any of the food groups can't be done without it might just require either taking vitamin supplements or carefully tailoring your picks from the others. Peppers and brussel sprouts and several other veggies for instance are equal or better sources of Vitamin C then oranges. Meat's a much better source of protein but there are wads of veggies with sufficient quantities in them that you needn't go heavy on the calories to hit your protein intake goal, and so on. The whole food pyramid concept and its parallels are just loose rules of thumb, especially if vitamin supplements are allowed in the game. And very loose too because 'fruits', 'vegetables', and 'grains' are very vague and arbitrary concepts compared to 'vegetation' or 'berries' or legumes etc. and I think one of the reasons we have so many hokum health fads is what we do teach people is essentially pseudo-scientific rules of thumb.