With the media attention paid to paleo, keto, and carnivore diets, along with a surge of enthusiasm for strength training and protein intake, the high-protein approach seems like it's everywhere. "I’ve been eating low-carb, high-protein to lean out."
As a physician and a lifelong trainer, I care about what these strategies actually do to your body—especially your hormones, and in this article, I will focus specifically on what these strategies are doing to men. Testosterone and cortisol aren’t just lab values; they drive how you feel, recover, build muscle, handle stress, and protect long-term health.
A 2022 paper, “Low-carbohydrate diets and men’s cortisol and testosterone: Systematic review and meta-analysis” by Whittaker and Harris, pulled together the best available intervention studies to answer a simple question:
How do low-carbohydrate diets affect men’s testosterone and cortisol compared with higher-carb diets?
Whittaker and Harris reviewed 27 intervention studies including 309 healthy, physically active men. All were assigned to diets that differed mainly in carbohydrate content:
Low-carb (LC): ≤35% of total energy from carbohydrates
High-carb (HC): >35% of total energy from carbohydrates
They looked at:
Resting cortisol
Post-exercise cortisol (0, 1, and 2 hours after exercise)
Resting total testosterone
Post-exercise testosterone
They also paid close attention to:
Diet duration: short-term (<3 weeks) vs. long-term (≥3 weeks)
Protein intake: moderate protein (<35% of calories) vs. high protein (≥35%)
Exercise duration: short (<20 minutes) vs. long (≥20 minutes)
Short-term low-carb (<3 weeks):
Resting cortisol went up moderately compared with higher-carb diets.
Long-term low-carb (≥3 weeks):
Resting cortisol returned to baseline—no consistent difference vs. high carb.
Translation:
When you first switch to low-carb, your stress hormone rises as your body adapts. After a few weeks, that baseline cortisol rise seems to settle down.
When men exercised for 20 minutes or longer on a low-carb diet:
Post-exercise cortisol was much higher at 0, 1, and 2 hours compared with higher-carb diets.
For short, intense bouts (<20 minutes):
Post-exercise cortisol sometimes looked lower on low-carb, but the data were limited and less certain.
Translation:
If you’re doing longer endurance or conditioning sessions on low-carb, your body leans harder on cortisol to manage fuel demands. That stress signal stays elevated well into recovery.
Overall, low-carb diets appeared to lower resting testosterone, but the story changes once you separate protein intake:
Moderate-protein low-carb (<35% protein):
No consistent effect on resting testosterone.
High-protein low-carb (≥35% protein):
Resting testosterone dropped substantially—about 5.2 nmol/L, roughly a 37% decrease from typical levels in young men.
Translation:
It’s not “low-carb alone” that looks problematic for testosterone—it’s low-carb plus very high protein. That extreme combination may stress the body’s ability to process nitrogen (via the urea cycle) and your hormonal system compensates by pushing testosterone down and cortisol up.
Long-term, moderate-protein low-carb:
Post-exercise testosterone tended to be higher, suggesting a potentially stronger anabolic response—but the sample sizes were small, so this is more hypothesis-generating than definitive.
Short-term, high-protein low-carb:
Post-exercise testosterone was lower, mirroring the drop seen at rest.
Translation:
Reasonable-protein, long-term low-carb might support a good testosterone response to training. Extreme high-protein, low-carb may do the opposite.
Not necessarily—but you should be intentional.
Here’s what this meta-analysis suggests for healthy, active men:
Short-term low-carb is a stressor.
Expect higher cortisol for the first few weeks as your body adapts to using more fat and ketones for fuel.
Long-term low-carb is not automatically harmful for cortisol.
Resting levels drift back toward baseline, but exercise still provokes a bigger cortisol response when carbs are low—especially for longer training sessions.
Very high-protein, low-carb is where red flags appear.
When protein climbs to ≥35% of calories on a low-carb base, testosterone can drop significantly. That’s not what most lifters, athletes, or everyday gym-goers are aiming for.
Most everyday diets don’t naturally hit that extreme.
Population data suggest most people hover around 15–17% of calories from protein. The problematic patterns are the ultra-high-protein, ultra-low-carb “bro science” diets followed very strictly.
Hormones are just one piece. Sleep, total calories, training load, alcohol, medications, and underlying health all matter, too. But if you care about strength, recovery, libido, and long-term health, it’s worth avoiding unnecessary hormonal stress from extreme macros.
Here’s how I would translate Whittaker & Harris into practical guidance on the gym floor:
Carbs are a tool, not the enemy. Keeping some carbohydrate—especially around harder training sessions—can help moderate cortisol surges and support performance.
Protein should be adequate, not excessive. Your daily protein needs are largely a function of how much energy you expend — the more you train, move, lift, and recover, the more protein your body requires to repair tissue and support muscle. But there’s a balance: you want enough protein to support training and recovery, not so much that it dominates your total calorie intake or displaces other essential nutrients.
Be skeptical of extreme “shred” protocols. If a plan sounds like “all meat, no carbs, all the time,” and you’re training hard, there may be a hormonal cost—especially over weeks to months.
Match your diet to your training and goals. Strength, hypertrophy, endurance, body composition, and health all place different demands on recovery and hormones. There’s no single macro pattern that solves everything.
At FITNESS SF, this is where coaching matters. A trainer who understands both physiology and real life can help you:
Choose a carb level that supports your training and energy
Set protein targets that build muscle without pushing you into “testosterone penalty” territory
Adjust your plan if you’re noticing red flags: chronic fatigue, poor recovery, stalled progress, low libido, or mood changes
You bring your goals and your body. We bring the science, a barbell, and a plan grounded in both medicine and movement. Your hormones are listening to the choices you make every day. Let’s make sure we’re sending the right signals.
*The content on this blog is provided for general informational purposes only. It does not constitute medical advice. No responsibility or liability is assumed for any actions taken based on the information provided.