Well, here's an answer from Ask Alice that sounds reasonably accurate.
Go Ask Alice!: Nutritional value in a serving of semen
A typical ejaculation fills up about one teaspoon; the actual amount is determined by a man's age (younger men usually make more semen), when he last ejaculated, and how long he's aroused before ejaculating, among other factors. Contrary to what you've heard, semen is not loaded with calories. Each teaspoon of ejaculate has about 5 - 7 calories and some 200 - 500 million sperm. Since sperm make up only about 1 percent of semen, what accounts for the other 99 percent? Well, its other ingredients include:
- Fructose sugar
- Water
- Ascorbic acid (a.k.a., vitamin C)
- Citric acid
- Enzymes
- Protein
- Phosphate and bicarbonate buffers (bases)
- Zinc
