Animal Protein Linked to Lower Testosterone in 2,546 Middle-Aged Men

Men who ate relatively more animal protein than plant protein had lower testosterone levels, according to a new study of 2,546 men aged 42 to 60. The finding runs against the popular claim that a meat-heavy diet boosts the hormone, ScienceAlert reported.

The researchers stress that the differences were small, that the work is observational, and that it cannot show animal protein is the cause. Swapping animal protein for plant protein in the researchers' models did not clearly change testosterone either way.

What the Finnish researchers measured

The work comes from a team at the University of Eastern Finland and was published in the International Journal of Food Sciences and Nutrition. Participants reported what they ate over four days. The researchers then compared diets with hormone levels in the blood.

Men with the highest ratio of animal to plant protein had lower total testosterone. They also had the lowest levels of free testosterone, which is the portion not bound to a protein in the bloodstream and therefore more readily available to the body. Men who ate the most plant protein from non-grain sources, such as vegetables, legumes or nuts, had higher free testosterone.

Why the result comes with big caveats

Nutrition researcher Miika Wynne-Ellis said the differences were statistically significant but relatively small. In the words of Wynne-Ellis, “the study cannot establish whether these differences have implications for health.”

The group of men was predominantly Caucasian, so the results may not apply to other populations, according to ScienceAlert. Because participants simply reported their eating habits and were not assigned diets, the study can show an association, not a cause.

The team also ran statistical models of what would happen if a middle-aged man replaced animal protein with plant protein. In those models, the swap was not significantly linked to total or free testosterone. That suggests a simple substitution may not raise or lower the hormone, ScienceAlert said.

Why animal protein might matter, and what is still unknown

Scientists cannot yet say whether animal protein lowers testosterone, or why. ScienceAlert noted two possibilities that remain speculation. Animal protein could trigger inflammatory markers that suppress testosterone secretion. Or animal and plant proteins could lead to different metabolic effects, such as different insulin responses.

The authors also pointed out that plant protein sources usually bring more fiber, carbohydrates and unsaturated fatty acids, while animal sources may bring more saturated fat and cholesterol. Those other nutrients could be part of the picture, though the study does not settle it.

Why it matters for the carnivore-diet debate

In recent years, some social media influencers have promoted eating mostly meat as a remedy for falling testosterone in later life. ScienceAlert said there is no robust evidence for that claim, and the new study adds to other recent research suggesting high-protein diets are not necessarily tied to testosterone in the way many assumed.

ScienceAlert also reported that physician Paul Saladino, once known as the “Carnivore MD,” had to cut back on meat after his testosterone reportedly dropped and brought sleep problems, heart palpitations and muscle cramps. That is a single reported case and not evidence about the wider population.

For middle-aged men, the practical takeaway is limited. The study does not show that eating animal protein harms health or that eating plant protein improves it. Higher testosterone is also not automatically better, since the hormone needs to stay in balance, and research is still working out how diet affects that balance.

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Around Prague, based on reporting by ScienceAlert