Semen, a biological fluid, contains various components, including proteins. These proteins are part of a mixture designed to support sperm and facilitate reproduction, rather than serving as a nutritional source.
Composition of Semen
Semen is primarily composed of water, making up 90-95% of its volume. The remaining 5-10% consists of various organic and inorganic substances, including sugars, enzymes, and minerals. Secretions from the seminal vesicles contribute 46-80% of the fluid and provide fructose, the main energy source for sperm. The prostate gland adds 13-33% of the volume, contributing proteolytic enzymes, citric acid, and zinc.
Minor contributions come from the testicles and epididymis, providing sperm, which account for about 5% of the total volume. Bulbourethral and urethral glands also add 2-5%, consisting of clear mucus that acts as a lubricant. The overall mixture also includes amino acids, potassium, sodium, magnesium, and phosphorus. These components create an environment conducive to sperm survival and function. The normal pH range for human semen is between 7.2 and 7.8, which helps counteract the acidic environment of the vagina.
The Role of Proteins in Semen
Proteins within semen perform functions aimed at protecting sperm and enabling their journey towards fertilization. Semenogelins (SEMGs), secreted by the seminal vesicles, form a gel-like coagulum immediately after ejaculation. This temporary coagulation traps sperm, preventing premature motility and protecting them within the female reproductive tract.
Following coagulation, prostate-specific antigen (PSA), also known as kallikrein-related peptidase 3 (KLK3), initiates the liquefaction process. PSA, an enzyme from the prostate gland, hydrolyzes semenogelins and fibronectin, causing the gel to dissolve into a more watery consistency within 15-20 minutes after ejaculation. This liquefaction is a necessary step, allowing sperm to become progressively motile and move freely towards the egg.
Other kallikrein enzymes, such as KLK2, KLK5, and KLK14, also contribute to this proteolytic cascade, further breaking down the coagulum. Seminal plasma proteins also help protect sperm from the acidic environment of the vagina, with basic polyamines like spermine and spermidine buffering the acidity. Zinc, present in high concentrations from prostatic fluid, initially inhibits kallikrein activity, but its binding by semenogelins after ejaculation allows the enzymes to become active, regulating the liquefaction process.
Nutritional Value of Semen
While semen contains proteins and other organic compounds, its nutritional contribution to the human diet is minimal. The average volume of an ejaculation ranges from 1.5 to 7.6 milliliters, with 3.4 to 5 milliliters being common. Within this small volume, the average protein concentration is 5,040 milligrams per 100 milliliters.
This translates to 250-252 milligrams, or 0.25 grams, of protein per ejaculation. To put this into perspective, this amount of protein is less than what is found in a single small nut or a sip of milk. For example, a single blueberry contains more sugar than the total fructose and glucose found in an average ejaculate. Therefore, despite containing various components, semen does not provide a significant source of dietary protein or other nutrients for the body.