One of our Peak Testosterone Forum members posted a link to an incredible study from 2014 that somehow escaped the attention of the health world showing a strong link between probiotics and testosterone. Although the study was on mice, it probably has far-reaching implications for us human males and verifies some past research that I will discuss below.
The study tied into the fact that "Inflammation is the Root of All Evil." (There is a research review entitled this, and it is VERY true.) Inflammation is a root cause for cancer, heart disease, autoimmune disease and now it looks like for andropause. Yes, inflammation may actually be the main reason that you lose your testosterone as you age!
To show this, researchers gave mice a well-known and well-studied strain of probiotics called L. Reuteri. This is not one of the common strains in yogurt, which are usually acidophilis, bulgaricus and casei. Nevertheless, L. reuteri is in many probiotic supplements already and researchers love it as a strain, because it is known for lowering inflammation. For example, L. reuteri inhibits TNF alpha, the inflammatory messenger "bad boy" that has been tied into so many different aspects of chronic disease from Alzheimers to arterial plaque.  And it is so powerful that it is currently being studied in the treatment of colitis, an inflammatory-based gut disorder.
So what happened the researchers have L. reuteri to these mice? The results were nothing short of astonishing: both Leydig cell counts and sperm cell-producing tissue mass were increased when compared to controls.  Essentially, L. reuteri slowed down testicular aging. The researchers stated that "this showed that criteria of gonadal aging were reduced after routinely consuming a purified microbe such as L. reuteri." The authors then tested this by giving the mice an anti-inflammatory therapy and found that this was indeed the source of L. reuteri's anti-aging magic.
Now does this mean that probiotics can increase your Leydig cell count and restore youthful testosterone levels? (The Leydig cells produce about 95% of our T as men.) There is no indication of that in the study - this isn't magic after all. However, what it does show is that you can likely slow down the age-related decline in testosterone by focusing on gut health and proper probiotic strains. And it may even mean that andropause is not inevitable!
Do you think that's crazy talk? Well, let me give you two lines of evidence that it may not be as crazy as you think. Young men in particular should check this out:
1. The "Healthy Man" Study. There was a recent study that showed that a significant percentage of men never experienced andropause.  In fact, what researchers found was that senior men who felt good were associated with youthful levels of testosterone. These men were NOT rare oddities either:
"'A decline in testosterone levels as men grow older is likely the result -- not the cause -- of deteriorating general health', say Australian scientists, whose new study finds that age, in itself, has no effect on testosterone level in healthy older men. Handelsman and his team, however, found that serum (blood) testosterone levels did not decline with increasing age in older men who reported being in excellent health with no symptoms to complain of. 'We had originally expected age to have an effect on serum testosterone, so the findings were a bit of a surprise,' Handelsman said."
Thus losing your testosterone does not appear to be inevitable and may easily be controllable and perhaps inflammation levels is simply the major lifestyle factor involved.
2. The Okinawans. More evidence of this comes from the Okinawans, who experienced the highest known senior age testosterone levels of any recorded culture. Essentially, Okinawan men lost their testosterone levels much more slowly than the typical Westerner, something I discuss in my page on How to Avoid Andropause.
What was the Okinawans secret? Well, they had many positive aspects to their lifestyle, including relatively low calories levels. However, it should be noted that they ate a largely plant-based, whole foods lower fat diet. (They were not super low fat, but somewhere between what we term low fat and Mediterranean.) This kind of diet is very good for gut health. They were also known for eating "probiotic foods." One of them was called Gobo or Burdock root and scientists noted that "Burdock root is low in caloric density and is high in two kinds of fiber: inulin and a spongy fiber called mucilage, which is a thick, glutinous substance related to the natural gums and used in medicine as an emollient. Inulin extracted from edible burdock has also shown probiotic properties that could promote health by increasing beneficial bacteria in the gut." 
CONCLUSION: One of the huge questions that plague any health conscious male is "where has all the testosterone gone?" First of all, there is the age-related decline in testosterone that we all know about, which is often termed "andropause." Secondly, recent studies show DRAMATIC reductions in testosterone levels and fertility parameters among even young men. I was not able to locate the study, but a few years ago researcher found that young men had average testosterone in the upper 300's (ng/dl)! This is very low testosterone compared to historical levels, something I discuss in my page on Normal Male Testosterone Levels by Age. What is causing this issue and how can it be reversed? One of the issues may be gut health and inflammation, and, perhaps, falling testosterone levels can be slowed down by focusing on certain probiotic strains and gut-healthy lifestyles.
1) BMC Microbiology, "Probiotic Lactobacillus reuteri biofilms produce antimicrobial and anti-inflammatory factors"
2) Infect Immun, 2004 Sep, 72(9): 5308 5314, "Live Lactobacillus reuteri Is Essential for the Inhibitory Effect on Tumor Necrosis Factor Alpha-Induced Interleukin-8 Expression"
3) PLoS One, 2014 Jan 2, 9(1):e84877, "Probiotic microbes sustain youthful serum testosterone levels and testicular size in aging mice"