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Flu shot lowers testosterone level

If your immune system has to learn a new trick, your testosterone level goes down. This is also what happens if you get vaccinated against a virus, researchers at the University of California in Santa Barbara discovered. Students who were given a flu jab had slightly reduced testosterone levels for about two weeks after having the shot.

Testosterone [see its structure right] is an anabolic hormone. The higher the testosterone level, the more the body is geared up for reproduction and muscle growth, and the less it is focused on the immune system.

But the immune system needs energy as well. The learning part of the immune system is continuously scanning for new intruders, and when it finds them it activates a category of cells that can fight the intruders. That costs energy: a high androgen level is bad for the immune system because androgens rob the immune system of energy.

This is why we only make limited amounts of androgen naturally. Evolution has not been kind to individuals that make excessively high amounts of androgens.

Thatís the theory at least. But does it hold water?

The Americans tested the hypothesis by doing an experiment with a group of about forty students. The students were given a flu shot containing a vaccine against an influenza virus. A vaccine contains an inactivated pathogen, which does not make the body really ill, but stimulates the immune system to start producing a new type of immune cells. This enables the person to fight the disease if they are exposed to the real virus. Vaccinations give the immune system a crash course, which costs the body less energy than fighting an infection from the original pathogen would do.

The researchers measured the testosterone levels of twenty students before they were given a flu shot, and again two weeks afterwards. They did the same with a control group who received no vaccination.

Flu shot lowers testosterone level

The Americans assume that a real infection would have a greater effect on the testosterone level, but the results of the experiment have at least confirmed their theory: "The present results suggest that pathogen exposure can cause a decline in testosterone, presumably as a means of prioritizing energy investment away from mating effort and into immune responses", they write.

Am J Hum Biol. 2009 Jan-Feb;21(1):133-5.