Observational study finds higher animal protein intake linked to lower testosterone levels — Evidence Review

A new study from the University of Eastern Finland found that men who eat more animal protein tend to have slightly lower testosterone, while those who consume more plant protein may have slightly higher levels. Most related studies show either modest or inconsistent effects of protein type on testosterone, supporting the new findings’ cautious interpretation (original source).

• While some controlled studies and meta-analyses report high or very high protein intake—especially from animal sources—can reduce testosterone, most show that moderate protein consumption has little effect, and the specific influence of plant versus animal protein remains uncertain 4 11 12.
• Several studies in humans and animals suggest that low total protein intake can lower testosterone, but changing the protein source (plant vs. animal) produces only small and inconsistent shifts, supporting the new study’s conclusion that the observed differences may not be clinically significant 2 4 6 9 10.
• Research on soy and plant proteins indicates they do not meaningfully reduce testosterone in men, and some evidence suggests they may support reproductive health, aligning with the new study’s findings of slightly higher testosterone among higher plant protein consumers 6 7.

Study Overview and Key Findings

Dietary choices can influence many aspects of health, but the specific relationship between protein source and male hormone levels has not been thoroughly examined. This new study is notable for its large population sample and for distinguishing between different sources of dietary protein rather than just total protein intake. By exploring nuanced links between animal, dairy, plant, and meat protein consumption and testosterone, the research adds detail to an area with mixed evidence and ongoing debate.

Property	Value
Organization	University of Eastern Finland
Journal Name	International Journal of Food Sciences and Nutrition
Authors	Miika Wynne-Ellis
Population	Men aged 42 to 60 in eastern Finland
Sample Size	n=2546
Methods	Observational Study
Outcome	Testosterone levels, SHBG concentrations
Results	Higher animal protein intake linked to 1.2 nmol/l lower testosterone

The study analyzed dietary records and blood samples from over 2,500 middle-aged men in eastern Finland, collected in the 1980s, to examine how different protein sources may relate to testosterone and sex hormone-binding globulin (SHBG) levels. Men consuming the most animal protein had average testosterone concentrations 1.2 nmol/l lower than those consuming the least, while higher plant protein intake was linked to 1.1 nmol/l higher testosterone. Dairy protein was specifically associated with lower testosterone and SHBG, whereas plant protein (especially from vegetables and non-cereal sources) correlated with higher testosterone. The differences remained statistically significant after adjusting for potential confounders such as age, BMI, and lifestyle factors. However, the observed effects were relatively small, and the study was cross-sectional, so it cannot show whether protein intake causes changes in testosterone.

Literature Review: Related Studies

To place these findings in context, we searched the Consensus database of over 200 million research papers for relevant studies. The following search queries were used:

1. animal protein testosterone levels
2. plant protein testosterone comparison
3. dietary protein testosterone effects

Below, we summarize key themes from related studies in the literature and how they connect to the new findings.

Topic	Key Findings
Does total dietary protein intake affect testosterone levels?	- Low protein diets in animals consistently reduce testosterone and reproductive organ mass, suggesting adequate protein is important for hormone health 2 5. - Very high protein intake (>3.4 g/kg/day) in men may decrease testosterone, but moderate and high (up to 3.4 g/kg/day) protein intake generally does not have a significant effect 4 12.
How do animal and plant protein sources differ in their hormonal impact?	- Human studies show that switching from animal to plant protein has limited or modest effects on testosterone, with some evidence of slightly higher levels with plant-based diets, but most differences are not clinically significant 6 9 11. - Animal studies (in lambs and birds) suggest that certain plant proteins (e.g., soybean) may support higher testosterone and reproductive health compared to some animal proteins 1 5.
What is the effect of soy and plant proteins specifically?	- Meta-analyses and systematic reviews find that soy protein and isoflavones do not lower testosterone or estrogen levels in men, regardless of dose or duration 6 7. - Short-term supplementation with soy may slightly blunt the acute testosterone response to resistance exercise, but does not lead to long-term reductions 8.
How do protein intake and diet composition interact with other factors?	- Obesity and weight loss have a much larger impact on testosterone than protein type; weight loss (from either high-protein or high-carbohydrate diets) increases testosterone in overweight/obese men 10. - The ratio of protein to carbohydrate intake may influence both testosterone and cortisol, with high carbohydrate diets associated with higher testosterone in some small studies 11 12.

Does total dietary protein intake affect testosterone levels?

The relationship between the amount of dietary protein and testosterone has been studied primarily in animal models and controlled feeding studies in humans. Most evidence indicates that both very low and very high protein intakes can reduce testosterone, while moderate consumption supports normal hormone levels. The new study found only small differences in testosterone across typical dietary patterns, aligning with the broader evidence.

• Animal studies show that inadequate dietary protein reduces testosterone and reproductive organ size, indicating protein sufficiency is important for endocrine health 2 5.
• Meta-analyses in humans indicate that only extremely high protein intake (>3.4 g/kg/day) lowers testosterone, while more typical intake levels (1.25–3.4 g/kg/day) do not have significant effects 4 12.
• The new study’s modest differences in testosterone across protein intake levels are consistent with these findings, suggesting that protein source may be less important than overall adequacy or excess.
• Other health factors, such as body weight and metabolic health, may play a bigger role in testosterone variation than dietary protein amount alone 10.

How do animal and plant protein sources differ in their hormonal impact?

Direct comparisons of animal versus plant protein and their effects on testosterone are limited, but available evidence suggests differences are generally small, and sometimes plant proteins are associated with slightly higher testosterone.

• Cross-sectional studies in humans, including US and Finnish populations, find no significant association between plant-based diet indices and testosterone, though some studies observe a small positive trend with higher plant protein 9 11.
• In animal studies, diets rich in certain plant proteins (like soybean) support healthy testosterone and reproductive tissue development, sometimes outperforming some animal protein sources 1 5.
• The new study’s observation of slightly higher testosterone with more plant protein and lower with animal protein fits with these small, source-dependent differences seen in prior research.
• Overall, the effect of switching from animal to plant protein on testosterone is much smaller than the impact of total protein adequacy or overall diet quality 2 9.

What is the effect of soy and plant proteins specifically?

Soy protein and isoflavones have been a particular focus of research due to concerns about potential estrogenic effects, but the majority of clinical evidence shows no negative impact on male hormones.

• Large meta-analyses of randomized trials find that neither soy protein nor isoflavones lower total or free testosterone, estradiol, or SHBG in men, regardless of dose or duration 6.
• Some studies of athletes report that soy protein may blunt the immediate testosterone response to heavy exercise, but this does not translate into lower baseline testosterone in longer-term studies 7 8.
• The new study’s finding that plant (including soy) protein is not associated with decreased testosterone, and may even relate to slightly higher levels, is consistent with these meta-analyses.
• This body of evidence suggests that moderate intake of soy and other plant proteins is safe for male reproductive health 6 7.

How do protein intake and diet composition interact with other factors?

Diet composition—especially the balance of protein to carbohydrate and the presence of obesity—can influence testosterone, sometimes more than protein type alone.

• Randomized trials show that weight loss, regardless of whether it comes from a high-protein or high-carbohydrate diet, raises testosterone and improves sexual function in overweight and obese men 10.
• Experimental studies find that high-carbohydrate diets raise testosterone and SHBG compared to high-protein diets, while high-protein, low-carb diets may reduce testosterone, particularly at very high protein intakes 11 12.
• These interactions suggest that overall diet quality, energy balance, and macronutrient ratios are important mediators of hormonal effects, beyond the specific protein source 10 11 12.
• The modest associations observed in the new study may reflect these broader dietary patterns rather than protein source alone.

Future Research Questions

While the new study provides valuable insights, further research is needed to clarify the causal relationships between protein source, dietary patterns, and male hormone health. Important areas for future investigation include longitudinal impacts, underlying mechanisms, and potential health outcomes of small hormonal changes.

Research Question	Relevance
Does changing from animal to plant protein cause meaningful changes in testosterone over time?	Longitudinal intervention studies are needed to determine whether switching protein sources has sustained effects on testosterone beyond what is observed in cross-sectional designs 9 11.
What are the mechanisms linking protein source to testosterone and SHBG regulation?	Understanding the biological pathways could clarify why modest hormonal differences are observed with different protein sources and whether they have functional significance 11 12.
How do protein intake and diet composition interact to influence male hormones?	Since macronutrient ratios and energy balance affect testosterone, research should explore how protein type interacts with carbohydrate and fat intake in different populations 10 11 12.
Are the modest testosterone differences from dietary protein sources clinically relevant?	The new study finds statistically significant but small hormonal differences; further research should determine if these translate to measurable health outcomes in men 4 6.
Does protein source affect testosterone in different age groups or populations?	Most research focuses on middle-aged or older men; studies in younger adults, women, and diverse ethnic groups could reveal population-specific effects 5 9.

Future studies designed to address these questions—particularly randomized interventions in broader populations—will help clarify the practical significance of protein source choices for hormonal and overall health.