Glucose & Whey Shakes Drop Testosterone 10%+ in Teens: Anti-Anabolic?

This discussion centers on a crossover study involving adolescent participants, focusing on acute hormonal shifts. However, this is neither the sole nor the primary factor explaining why you need not worry about the traditional bodybuilding post-workout shake. Upon closer examination of the findings

This discussion centers on a crossover study involving adolescent participants, focusing on acute hormonal shifts. However, this is neither the sole nor the primary factor explaining why you need not worry about the traditional bodybuilding post-workout shake. Upon closer examination of the findings, it appears we are dealing with what could be described as a 'protein-anabolic drop in testosterone.' This concept might sound unusual at first, but it ties directly into the function of your androgen receptors (AR) and, ultimately, your muscle-building potential, regardless of whether you are young or more mature.

In earlier analyses, I explored the potential negative impacts of extremely high protein consumption on testosterone concentrations, especially when paired with a calorie deficit that also limits glucose and fat intake. Now, recent research published in Clinical Endocrinology by Schwartz and colleagues in 2019 reveals intriguing interactions between testosterone and both protein and carbohydrates. These interactions lead to notable short-term reductions in serum testosterone among individuals whose levels should naturally be at their peak: adolescent males.

The investigation by Schwartz et al. involved twenty-three adolescent males aged 12 to 18 years. Only those exhibiting testosterone levels consistent with mid-to-late puberty were included in the detailed analysis. Researchers tracked several key biomarkers, including testosterone, luteinizing hormone (LH), active GLP-1, acylated ghrelin, glucose, insulin, and subjective feelings of appetite.

These measurements were taken at baseline (time zero) and then again at 20, 35, and 65 minutes following the intake of a test beverage. The beverages consisted of either 1 gram of glucose monohydrate per kilogram of body weight or 1 gram of plain whey protein isolate per kilogram of body weight. A non-caloric drink served as the control.

The whey protein isolate used had a protein purity of 90.4%, with minor components including 5.7% moisture, 2.2% ash, 1.18% fat, and 0.6% carbohydrates. This made the test drinks nearly isocaloric: approximately 3.74 kcal per kg of body weight for the protein version and 4 kcal per kg for the glucose version. The small caloric variance is insufficient to undermine the study's conclusions. To ensure consistency, all drinks were flavored identically using 1.5 mL of chocolate extract and diluted in 500 mL of water, mimicking prior experimental designs.

The whey protein and control drinks were sweetened with 0.2 grams of sucralose to match the sweetness of the glucose beverage. Sucralose was selected because prior evidence shows it does not influence post-meal blood glucose or insulin levels. Beverages were prepared the night before, refrigerated, and served chilled the next morning in large, covered, opaque cups via straw to maintain blinding.

Each participant underwent three separate morning sessions after a 12-hour fast. In a randomized crossover format, they consumed one of the beverages—protein, glucose, or control—within five minutes, followed by 50 mL of plain water to rinse away any lingering aftertaste.

Understanding the origins of testosterone reductions in adolescents

This study builds directly on Schwartz et al.'s earlier 2015 research, where they noted an acute decline in serum testosterone following a combined glucose and protein drink. The current work aimed to isolate the effects of glucose alone versus protein alone to determine if either replicated the previously observed drops, which reached up to -20% in mid-to-late pubertal males.

Additionally, the researchers hypothesized that these liquid meals might differently affect appetite and subsequent voluntary food consumption. To test this, after the final blood draw, participants were offered an ad libitum pizza meal. They were told to eat until comfortably full within 20 minutes, with pizza types selected based on pre-study preferences. Each 87-gram slice of pepperoni pizza provided 9 grams of protein, 6 grams of fat, and 23 grams of carbohydrates, totaling 180 kcal. Three-cheese pizza slices (81 grams) offered similar macros: 10 grams protein, 6 grams fat, 23 grams carbs, also 180 kcal.

No statistically significant differences emerged in the number of pepperoni or three-cheese pizza slices consumed, regardless of beverage type or participants' baseline body weight. Overall, the adolescents averaged around 1,300 kcal from pizza, unaffected by prior beverage consumption or weight status (F = 2.23, P = 0.14).

In stark contrast, testosterone responses varied significantly when comparing overweight and normal-weight adolescents.

Figure 1 illustrates the varying impacts of treatments based on weight status (Panel A) and the broader influence of weight on plasma testosterone (Panel B). Panel 1.B initially implies that leaner individuals (average BMI 21.1 ± 0.9 kg/m²) experience more pronounced negative effects from protein or carbohydrate shakes compared to overweight or obese peers (average BMI 29.8 ± 1.2 kg/m²). Yet, a deeper look at Panel 1.A and the absence of significance markers under the normal-weight bar in 1.B reveals that the apparent large drops in the 12 normal-weight subjects were not statistically significant overall.

Thus, excess body weight appears to alter how glucose and protein beverages influence adolescent testosterone, with each additional pound potentially exerting a protective yet negative modulating effect.

Table 1 presents baseline values for appetite- and sex-related hormones. Notably, responses to protein or glucose did not differ significantly between normal-weight and overweight subjects across metabolically important markers.

Does this imply that carrying extra fat is detrimental? To explore, consider whether metabolic parameters explain the discrepancies. Based on reported changes, no clear patterns emerged. Insulin, GLP-1, and ghrelin shifted—increasing, increasing, and decreasing, respectively—similarly across all groups in response to the beverages. Crucially, responses to protein versus glucose were comparable, with no differential effects observed.

Without impacts on satiety or hunger hormones, it follows that pizza intake remained unchanged, as noted earlier. This held true even though subjective appetite ratings dropped significantly after the glucose beverage (p = 0.0198 vs. control; p = 0.0247 vs. protein), but not after protein.

Key insights from the research

Beyond boys' apparent universal love for pizza, Schwartz et al. highlight three critical findings:

• Both glucose and protein beverages cause acute reductions in testosterone levels among adolescent males.
• These effects are independent of the specific macronutrient makeup of the drinks.

Furthermore, the testosterone fluctuations did not correlate with appetite regulation or food intake changes, contrary to the researchers' initial expectations for this 2015 follow-up.

These findings align with prior research, applicable beyond adolescents

This work reinforces existing evidence rather than breaking new ground. Similar testosterone declines appear in Caronia et al. (2013), where men ingested 75 grams of glucose, and in Schwartz's 2015 study on overweight/obese adolescents with mixed beverages.

The precise mechanisms remain elusive. University of Toronto researchers propose that glucose or amino acids—especially leucine—may trigger these via mTOR signaling activation, promoting protein synthesis, and/or by suppressing AMPK activity.

Table 2 details correlations between testosterone, LH, and post-beverage deltas. Theoretically, such pathways could enhance androgen receptor (AR) mRNA expression, though Shen et al. (2014) observed AMPK upregulation reducing AR in prostate cells—a pattern likely mirrored in muscle.

The role of androgen receptors in this context

Unlike transient testosterone swings, muscle androgen receptor density has been linked to resistance training outcomes in recent studies. Elevated AR expression facilitates greater testosterone uptake into muscle cells, thereby reducing circulating levels—potentially matching the drops seen here.

This mechanism could clarify why neither glucose nor protein demonstrates anti-anabolic properties in isolation or combination. If shakes boost AR expression and sequester testosterone into tissues, it underscores their pro-anabolic endocrine influence, not the opposite.

Claims that this supports low-carb ketogenic diets over traditional high-protein, low-fat approaches for bodybuilders or muscle enthusiasts are misguided. Definitive answers require longer-term trials incorporating resistance exercise and body composition metrics. Currently, with matched protein and calories, carb variations seem less pivotal than debated, bridging nutritional divides.

References

• Caronia, Lisa M., et al. "Abrupt decrease in serum testosterone levels after an oral glucose load in men: implications for screening for hypogonadism." Clinical Endocrinology 78.2 (2013): 291-296.
• Schwartz, Alexander, et al. "Acute decrease in serum testosterone after a mixed glucose and protein beverage in obese peripubertal boys." Clinical Endocrinology 83.3 (2015): 332-338.
• Schwartz, Alexander, et al. "Acute decrease in plasma testosterone and appetite after either glucose or protein beverages in adolescent males." Clinical Endocrinology (2019).
• Shen, Min, et al. "The interplay of AMP‐activated protein kinase and androgen receptor in prostate cancer cells." Journal of cellular physiology 229.6 (2014): 688-695.