Is there any single element (e.g. possibly carbon) that can be eaten, which has nutritional value in that it provides calories? I found on Wikipedia that http://en.wikipedia.org/wiki/Dietary_element that there are certain elements that our bodies require. I suppose oxygen could count as such a nutritional element, although we breathe vs. eat it.
Interesting question! You may get a better answer, but I believe the answer is no. Calories are units of heat, and represent energy available to the body. The form of fuels that can be (biochemically) "burned" by the body are hydrocarbons; i.e. molecules of carbon and water, like sugars. Carbon alone I do not believe can work. The only other single element fuel is hydrogen, and again I do not think this is biologically available to the body. By the way, oxygen is required as the oxidizer in any combustion reaction, and we definitely need oxygen (from breathing) to convert the food we eat to energy. But oxygen is not a fuel: it is the oxidizer that is reduced during combustion.
Caloric value is basically stored energy. Plant based foods create this stored energy through the absorption of sunlight, and in some extreme situations I imagine there may be plants that generate energy from thermal vents and such. They then store that energy in a molecular combination that you can think of as compressing a spring, giving it potential energy.
The same goes for animals, whereas rather than sunlight that energy is created from eating either plant or other animal life.
Mass inherently has energy only at the mass-energy equivalent level, and biologically we are not capable of accessing that.
From conservation of energy, if the element you are putting in your body is not part of a molecular "energy spring" as per the analogy above, it will not spontaneously posses energy just because you are ingesting it.
As far as Carbon, this is one of the great misunderstandings about fuel. We burn Hydrocarbons as a fuel source. Wood originally, then coal, and now we have jet fuel. They are basically all the same thing, with decreasing Carbon to hydrogen ratios. The energy stored is actually being released from the hydrogen bonds. The ideal goal is for us to be using carbon-less hydrocarbons as fuel sources as the only burned waste is water as it combusts with the air, no CO2 providing the highest weight to fuel ratio (no carbon along for the ride reduces the weight) and doesn't pollute.