Is a high protein diet bad for testosterone levels?

(Originally appeared on Medium)

People who train care about their testosterone levels. And every healthy male should.

There is an idea floating around that states that too much protein can be detrimental for testosterone levels, and by extension, to overall health and muscle building.

What does the data really show?

I have repeatedly come across this source as containing the reasoning behind the “high protein-lower testosterone” connection.

The guy from the article states:

Many men are unknowingly ingesting way too high amounts of protein which negatively impacts their hormonal environment and hinders muscular gains and overall men’s health.

He uses mainly two studies to support his assertion that a high protein diet is detrimental for testosterone levels: Volek et al., 1997 (1) and Anderson et al., 1987 (2).

For (1), he says (my highlights):

In terms of the hormonal effects of protein, perhaps the most interesting study comes from Volek et al. In the study, the researchers made their resistance trained subjects consume different diets with different macronutrient compositions, and examined their hormonal changes during those (low-fat, high carb, high protein, etc).

First, in the Volek study, they did not manipulate the diet at all. They analyzed food records from the subjects that were recruited. There were no “low fat, high carb, high protein” diets. They basically recruited a bunch of subjects who trained and asked them to record their diet for 17 days. All subjects appeared to follow a mixed diet judging by means presented in Table 2 (20% protein, 56% carbohydrates, 23% fat). So his description of the study is flat out wrong.

From the methods:

(…) subjects recorded 17 consecutive days of food records.(…) Food record forms were analyzed for total food energy, carbohydrate, protein, fat, saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), cholesterol, and dietary fiber by using Nutritionist IV, version 4, nutrient-analysis software (N-Squared Computing, First Databank Division, Hearst, San Bruno, CA).

He continues:

What they found was that the higher the subjects went in total protein intake, the lower their serum testosterone levels fell. Same was true for the protein/carbohydrate ratio; more protein and less carbs, resulted in significantly suppressed testosterone synthesis.

This is based on the following results (from Figure 2 of the study):

Based on the previous misrepresentation of the study, the wording implying causality is incorrect. Besides that, some things to consider from this data (which remember, is only correlational).

There is a clear outlier (red circle in the images) which has a very low level of free testosterone (~4 nmol/L). Normal values of free testosterone are at least higher than ~10 nmol/L, if you are being generous. So, remove the probably hypogonadal subject and your correlation is gone:

The same trend is observed for the protein/carbohydrate correlation, which is again driven by the diet consumed by the subject with very low testosterone levels.

Additionally, there are only 12 data points which are all over the place (specially if you remove the outlier). With such data, it is better not to attempt to fit any regression line.

Moreover, looking at the data points themselves, one can see that a subject consuming almost the same relative amount of protein (28%) as the subject with the very low testosterone levels (33%) has higher or equal testosterone levels than 4 subjects consuming < 20% of calories as protein.

Most importantly, this study did not measure free testosterone levels, which ultimately reflects the bio available hormone concentration (3, 4).

In summary, the negative correlations between total testosterone and protein/protein:carbohydrate ratio are driven by an outlier with deficient testosterone levels. Thus, the data does not allow concluding that a high protein intake reduces testosterone levels, let alone imply causality or a negative dose-response relationship between dietary protein and testosterone.

What about (2)? He says (again my emphasis):

Anderson et al. conducted a similar study where the researchers compared the hormonal effects of high protein and high carb diets on non-trained men, total caloric intake and fat intake was kept identical on both of the diets, so the only thing that changed was the ratio of protein to carbohydrate.

Findings? Similar to as what was seen in the study by Volek et al. testosterone (and DHT) levels were significantly suppressed on a high-protein diet when compared to high-carb diet. In addition they also saw that higher protein intake resulted in lower SHBG and higher cortisol levels. Their data indicates that the only “negative” effect of a low-protein diet, would be slightly higher production of SHBG (…)

Now in this study we have more data as they also measured SHBG. A lower protein-high carbohydrate diet raised both total testosterone and SHBG, and vice versa. However, with these values, we can then calculate the concentration of “free” testosterone under both dietary conditions:

In relative (% of testosterone available) and absolute terms, the bio available testosterone in both groups was basically the same (the difference between the two diets is 20 ng/d;, 211 ng/dL in the high P/C and 231 ng/dL in the low P/C). So it appears that protein intake, under energy balance, does not influence significantly the amount of bio available testosterone.

Conclusion

From the available data, it is impossible to conclude that a higher protein diet (and higher protein-to-carb ratio) reduces bio available testosterone levels. In addition, it is simply ludicrous to suggest that a higher protein intake will hinder muscle gains based on both human and mechanistic evidence.

Besides promoting an increase in lean mass under a calorie deficit (combined with resistance exercise) (5), a high protein intake promotes lean mass gains even without resistance exercise under overfeeding conditions, compared to lower protein intakes (6). Interestingly, in this last study, the protein:carbohydrate ratio of the high protein diet was 0.6, which is the same ratio as the diet consumed by the subject with very low testosterone levels in (1).