Kefir has not been shown to increase testosterone in humans. The idea gained traction after a striking mouse study found that a specific probiotic strain common in kefir, Lactobacillus reuteri, boosted testosterone and testicular size in aging mice. But when researchers tested that exact strain in a human clinical trial, it had no effect on testosterone levels at any dose over 12 weeks.
That said, the story is more nuanced than a simple no. Kefir contains nutrients and bacterial strains that interact with hormonal pathways in ways scientists are still mapping out. Here’s what the evidence actually shows.
The Mouse Study That Started the Hype
In 2014, researchers at MIT published a study in PLOS One that got enormous attention. Male mice consuming Lactobacillus reuteri in their drinking water had significantly larger testicles and higher circulating testosterone than mice that didn’t, regardless of whether they ate a normal diet or a high-fat one. The differences grew more pronounced with age. At 12 months old, probiotic-fed mice had testicular tissue that looked dramatically younger under a microscope.
The cellular details were impressive. L. reuteri increased the number of Leydig cells, the cells in the testes responsible for producing testosterone. It also prevented the shrinkage of seminiferous tubules (the structures where sperm develop) that normally happens with aging. The researchers described the testes of probiotic-fed mice as being “rescued” from typical age-related decline, with “remarkably high levels of circulating testosterone” to match.
This study is the primary reason kefir gets associated with testosterone. L. reuteri is one of many bacterial species found in kefir grains. But mouse physiology and human physiology respond very differently to the same interventions, which is why the human trial matters so much.
What Happened in the Human Trial
A 12-week, double-blind, placebo-controlled trial tested L. reuteri ATCC PTA 6475, the same strain used in the mouse study, in healthy aging men. Participants received either a high dose, a low dose, or a placebo. Researchers measured testosterone at baseline, six weeks, and 12 weeks.
The results were unambiguous. Testosterone levels didn’t budge in any group. The low-dose group started at an average of 18.74 nM and ended at 18.9 nM. The high-dose group started at 16.29 nM and ended at 15.97 nM. The placebo group stayed essentially flat as well. The study’s conclusion was direct: “The present study does not support the hypothesis that a probiotic supplementation with L. reuteri can increase testosterone levels in ageing men.”
This is the only published human trial testing this specific claim, and it found nothing.
How Gut Bacteria Influence Hormones
The lack of a direct testosterone boost doesn’t mean gut health is irrelevant to hormone production. Researchers have proposed what they call a “gut-testis axis,” a set of pathways through which intestinal bacteria can influence reproductive hormones.
Short-chain fatty acids produced by gut bacteria can act on the testes directly. Bacterial toxins like lipopolysaccharide (LPS), which increase when the gut lining is compromised, can suppress the brain’s hormonal signaling chain. Specifically, LPS interferes with the hypothalamus and pituitary gland, reducing the release of luteinizing hormone, which is the primary signal telling Leydig cells to produce testosterone. A healthier gut lining means less LPS leaking into the bloodstream, which could theoretically support more stable hormonal signaling.
Kefir, with its dense and diverse microbial community, likely supports gut barrier integrity. But supporting normal function is different from boosting testosterone above your baseline. Think of it as removing a potential drag on the system rather than adding a turbocharger.
Kefir’s Nutrients and Testosterone
Kefir does contain several nutrients involved in testosterone production, though the amounts are modest.
Vitamin K2 is one worth noting. Kefir made with traditional kefir grains contains around 4.8 micrograms of MK-7 (a form of vitamin K2) per 100 grams. Some analyses have found higher levels of MK-4, another form. In rat studies, MK-4 stimulated testosterone production in Leydig cells by activating an enzyme pathway involved in converting cholesterol into testosterone precursors. It increased the expression of a rate-limiting enzyme in that conversion process. But these were cell and animal studies using concentrated doses, not the small amounts you’d get from a cup of kefir.
Kefir also provides zinc, magnesium, calcium, and B vitamins, all of which play roles in hormonal health. Zinc in particular is essential for testosterone synthesis, and deficiency is clearly linked to low testosterone. A cup of kefir contributes to your daily intake of these minerals but isn’t an unusually rich source of any of them compared to other dairy products or whole foods.
Stress, Cortisol, and the Indirect Route
One indirect pathway worth considering is stress. Cortisol and testosterone have an inverse relationship: chronically elevated stress hormones suppress reproductive hormone production. In mouse studies, kefir consumption reversed stress-induced increases in corticosterone (the rodent equivalent of cortisol) and restored brain-derived neurotrophic factor levels that chronic stress had depleted.
If kefir helps manage the physiological effects of chronic stress in humans the way it appears to in animals, that could indirectly support healthier testosterone levels by reducing cortisol’s suppressive effect. This remains speculative for humans, but it’s a more plausible mechanism than a direct testosterone boost.
Milk Kefir vs. Water Kefir
Most research on kefir and hormones involves milk kefir, which has a richer and more diverse microbial community than water kefir. Both contain lactic acid bacteria and yeast, but the specific species differ. Milk kefir grains produce a unique polysaccharide called kefiran and harbor species like Lactobacillus kefiranofaciens that aren’t found in water kefir. Water kefir relies on different organisms for fermentation.
The vitamin K2 content, zinc, and other micronutrients discussed above come from milk kefir specifically. Water kefir, fermented with sugar water or fruit juice, has a completely different nutritional profile and lacks the dairy-derived minerals. No endocrine research has been conducted on water kefir.
The Bottom Line on Kefir and Testosterone
The honest answer is that kefir does not increase testosterone based on current human evidence. The mouse findings with L. reuteri were genuinely exciting, but the human trial using the same bacterial strain found zero effect on testosterone at any dose over three months. Kefir provides nutrients that support the biological machinery involved in hormone production, and it may help maintain a gut environment that doesn’t actively hinder hormonal signaling. But if you’re drinking kefir specifically to raise your testosterone, the evidence says you should expect your levels to stay right where they are.