If you are a man over 40, you have probably heard that protein is your friend. Eat more of it, the advice goes, and you will protect your muscles, your metabolism, and maybe even your testosterone. Steak, eggs, cheese, and a protein shake have become the unofficial menu of masculine health.
A new study published in September 2026 in the International Journal of Food Sciences and Nutrition adds a surprising wrinkle to that story. Researchers in Finland found that men who ate the most animal protein had slightly lower testosterone levels, while men who ate the most plant protein had slightly higher levels. In other words, it may not just be how much protein you eat that matters, but where it comes from.
Before you clear the cheese out of your refrigerator, let’s walk through what this study actually found, what it can and cannot tell us, and what it might mean for you.
A look inside the study
The research team, led by Miika Wynne-Ellis and Jyrki Virtanen at the University of Eastern Finland, drew on the Kuopio Ischaemic Heart Disease Risk Factor Study. This long-running project has followed men from Eastern Finland for decades to understand heart disease and other chronic illnesses.
For this analysis, they looked at 2,546 men aged 42-60. Each man kept a detailed 4-day food diary, recording three weekdays and one weekend day. A nutritionist then sat down with each participant to review the diary and reduce reporting errors. The men also gave a fasting blood sample between 8 and 10 in the morning, when testosterone is normally highest and most reliably measured.
The researchers measured three things in the blood. The first was total testosterone, the number most men see on a standard lab report. The second was sex hormone-binding globulin (SHBG), a liver-made protein that carries testosterone through the bloodstream. The third was free testosterone, the small unbound portion that is most readily available to your tissues. In this study, researchers calculated free testosterone from the other two values rather than measuring it directly.
On average, the men ate about 93 grams of protein a day. About 70% came from animal sources (dairy, meat, fish, and eggs), and the rest from plants such as grains, vegetables, legumes, nuts, and seeds.
The researchers adjusted their results for a long list of factors known to affect testosterone, including age, body weight, alcohol, smoking, exercise, education, diabetes, heart disease, fiber, fruit and vegetable intake, and the types of fat in the diet. That careful adjustment is one of the study’s real strengths.
Animal protein and plant protein pointed in opposite directions
When the researchers divided the men into four groups based on how much animal protein they ate, a clear pattern emerged. Men in the highest group, eating a median of about 82 grams of animal protein a day, had total testosterone levels about 1.2 nmol/L lower than men in the lowest group, who ate about 49 grams a day. Their SHBG was also about 3.0 nmol/L lower.
Plant protein showed the mirror image. Men who ate the most plant protein (a median of about 28 grams a day) had total testosterone about 1.1 nmol/L higher and SHBG about 3.0 nmol/L higher than men who ate the least (about 15 grams a day).
The balance between the two mattered too. Men whose diets were most heavily tilted toward animal protein, eating nearly four grams of animal protein for every gram of plant protein, had lower total testosterone, lower SHBG, and lower calculated free testosterone than men whose ratio was closer to 1.7 to 1.
Which foods drove the pattern?
This is where the findings get interesting. When the researchers broke animal protein down by source, dairy stood out. Higher intake of dairy protein, especially fermented dairy (yogurt, kefir, sour milk, cottage cheese, and cheese), was linked to lower total testosterone and lower SHBG. Protein from meat was linked to lower SHBG but not to lower total testosterone. Protein from fish and eggs showed no meaningful link to either after the researchers accounted for other lifestyle factors.
On the plant side, protein from non-grain plant foods (vegetables, legumes, nuts, seeds, and mushrooms) was linked to higher total testosterone and was the only individual protein source associated with higher calculated free testosterone. Protein from grains was linked to higher SHBG.
The researchers also ran substitution models, which estimate what happens on paper when one food is swapped for another with the same calories. Replacing just 2% of daily calories from low-fat dairy protein with plant protein was linked to about 1.75 nmol/L higher SHBG. Other animal-to-plant swaps did not reach statistical significance, although nearly all leaned in the same direction, toward higher testosterone and SHBG when plant protein replaced animal protein.
How big are these differences?
It is important to keep these numbers in perspective. The average total testosterone in these men was around 21 nmol/L (roughly 600 ng/dL). A difference of 1.2 nmol/L is about 5-6%. That is a modest shift, not the kind of change that would, by itself, turn a man with healthy levels into a man with low testosterone. What makes the finding notable is how consistently animal and plant protein pulled in opposite directions.
The SHBG twist: why total testosterone is only part of the story
To understand this study, it helps to know how testosterone travels through your body. Most of your testosterone does not float freely in the blood. It rides on carrier proteins, mainly SHBG and albumin, which deliver it where it is needed and help regulate how much is active at any moment. Only about 2-3% circulates unbound as free testosterone. It is an elegantly designed transport system, with SHBG acting something like a reservoir.
Here is why that matters. When a lab reports your total testosterone, it counts both bound and free hormone together. So if your SHBG drops, your total testosterone number can drop along with it, even if your body is producing the same amount of hormone and your tissues are receiving the same amount of free testosterone. The authors themselves point this out, noting that lower SHBG may contribute to lower measured total testosterone without necessarily meaning lower testosterone production.
That is exactly what makes the free testosterone results worth noticing. For most individual foods, free testosterone did not change much, suggesting that much of the shift in total testosterone was driven by SHBG. Two findings did break through, though. A diet heavily tilted toward animal protein was linked to lower free testosterone (about 10 pmol/L lower), and a higher intake of protein from vegetables, legumes, nuts, seeds, and mushrooms was linked to higher free testosterone (about 8 pmol/L higher).
This fits with older research comparing vegan and meat-eating men. Those studies found that vegans tended to have higher SHBG and higher total testosterone, with little or no difference in free testosterone. In other words, eating more plants does not appear to lower a man’s active testosterone, a worry many men still carry.
One more reason to pay attention to SHBG itself. Low SHBG is not just a lab curiosity. It is one of the classic companions of insulin resistance, and in large studies, low SHBG has been tied to a higher risk of type 2 diabetes in men. A diet pattern that keeps SHBG healthier may be telling us something about the metabolic health of the whole person, not just his hormones.
Why might the source of protein matter?
This study was not designed to prove cause and effect, so the authors are careful about explanations. Still, they point to a few plausible pathways, and each of them lines up with what we see every day in metabolic medicine.
Inflammation. Chronic, low-grade inflammation strains the hormone-producing system. Men with higher levels of C-reactive protein (CRP), a common blood marker of inflammation, tend to have lower total testosterone and SHBG. In a controlled experiment, a brief inflammatory challenge in healthy, lean men was followed by a drop in their testosterone. Meanwhile, a UK Biobank analysis of more than 128,000 adults aged 60 and older found that those who ate more plant protein had lower CRP levels.
Insulin. Animal and plant proteins are digested and absorbed differently. A review of controlled feeding trials found that animal proteins tend to provoke a larger insulin surge after a meal than plant proteins, although the results were not entirely consistent. Because the liver makes SHBG and it responds to insulin, a steady diet that keeps insulin elevated is a reasonable explanation for lower SHBG.
The whole plate. Protein never arrives alone. Beans, lentils, nuts, seeds, and vegetables bring fiber, minerals, and a wide range of protective plant compounds along with their protein. Cheese and processed meats tend to bring saturated fat and sodium. The researchers did adjust for fiber, produce, and dietary fats, which makes it less likely those factors alone explain the results, but the overall eating pattern that comes with each type of protein may still play a role.
Two worries this study helps put to rest
Many men avoid soy because they have heard it lowers testosterone. A meta-analysis of clinical studies found no effect of soy protein or soy isoflavones on testosterone, free testosterone, or SHBG in men. Plant protein, including soy, is not the threat it is often made out to be.
Others worry that any amount of protein above the minimum is hard on hormones. Earlier evidence suggests that only very high protein intakes, above about 3.4 grams per kilogram of body weight per day, consistently lower testosterone. That’s more than 238 grams a day for a 155-pound man. Even the highest-protein men in the Finnish study averaged about 109 grams a day, far below that range. The message here is not “eat less protein.” It is about the balance of sources.
What this study cannot tell us
Every study has limits, and an honest reading of this one requires naming them.
First, it was a snapshot. Diet and blood were measured at the same point in time, so the study shows an association, not proof that animal protein lowers testosterone. It is possible that men with certain hormone profiles simply tend to eat differently.
Second, the researchers measured each man’s hormones only once, and testosterone naturally varies from day to day. The authors calculated free testosterone rather than measuring it directly, which they say should be interpreted cautiously.
Third, the researchers tested many different protein sources, which raises the chance that one or two results were due to chance. For that reason, the authors rightly emphasize the overall pattern rather than any single food.
Fourth, the data were collected from eastern Finnish men between 1984 and 1989. Diets, food processing, and body weights have changed since then, and the findings may not apply equally to women, younger men, or other populations. A 4-day food diary may also miss foods eaten only occasionally, such as fish.
Finally, the study measured blood levels, not symptoms. It did not assess energy, libido, muscle mass, mood, or other outcomes men actually feel. The authors call for longer studies and clinical trials to confirm whether changing protein sources truly changes hormone health.
What this means for you
This single study is not a reason to panic over a slice of cheese. But it adds to a growing body of evidence that the source of your protein matters, and it gently challenges the idea that more animal protein is always better for a man’s hormones.
If you want to act on it, the most sensible step isn’t to eat less protein, but to rebalance its sources. In this study, the healthiest pattern looked like a lower ratio of animal to plant protein, with more of that plant protein coming from beans, lentils, peas, nuts, seeds, vegetables, and mushrooms. Soy foods such as tofu, tempeh, and edamame are a perfectly good part of that mix. Fish and eggs showed no harmful link, while dairy, especially fermented dairy and cheese, was the animal source most associated with lower testosterone and SHBG. A practical first step might be swapping a daily cheese-heavy snack or a dairy-based protein shake for a handful of nuts, a bowl of lentil soup, or a pea or soy protein alternative.
These choices support healthy blood sugar, lower inflammation, a healthy gut, and a healthy heart. Our bodies were designed to thrive on whole foods from the garden, and caring for them well is good stewardship. It is encouraging when hormone research points in the same direction as everything else we know about eating well.
Know your numbers
If you are concerned about your testosterone, do not rely on total testosterone alone. Ask your doctor to check total testosterone and SHBG together, along with free testosterone, from a morning fasting blood sample. Because low SHBG so often travels with insulin resistance, a fasting insulin level is a valuable addition. Seeing these numbers side by side gives a clearer picture than any single value, and retesting a few months after changing your diet is the best way to see how your body responds.
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