The real truth is that the nutritional composition of semen is extremely subjective and varies depending on a man's age and diet. The numbers that follow are rough averages based on a study of the physical and chemical properties of semen—published in a 2005 issue of the Journal of Andrology—that looked at a half-century's worth of research and semen samples. (Additionally, we've referenced material published in The RE/Search Guide to Bodily Fluids.)
There's a lot of misinformation out there about the composition of ejaculate, particularly regarding how many calories are in a typical serving of man milk—which just so happens to be one of the more common inquiries about spunk. A Google search of "semen calories" returns over 1.2 million results. Answers vary widely, but most sources peg it somewhere between 5 - 25 calories per serving.
However, a typical ejaculation—with an average volume of 3.4 ml—contains less than 1 calorie. Just like Diet Coke! This calorie count of semen was calculated using the "4-4-9" formula of macronutrients (in which there's 4 calories per gram of protein, 4 calories per gram of carbs, and 9 calories per gram of fat).
And while human semen has a great protein/carb/fat ratio that would allow it to fit into many diets like Atkins, Paleo, or Elimination, it's not exactly as protein packed as some would have you think. AskMen (of course) claims that one load of jizz has about as much protein as an egg white. But you'd actually have to drink nearly 4 oz (a half cup) of semen to match the protein of one egg white. That's a lot of blow jobs. Do you really hate cooking that much?
Another rumor is that there is as much vitamin C in human semen as in an orange. That is the kind of bullshit that somebody comes up with when he's talking out of his ass and thinking with his dick. It is completely untrue.
Semen does have an awful lot of zinc in it (3% of U.S. RDA), which is believed to be an antioxidant and slows down the aging process of skin and muscles. So there's that. It's something that's probably already in your multivitamin, though.