What is Cortisol?
Cortisol is your body’s main stress hormone
- Cortisol is produced by your adrenal glands, small glands that sit right on top of your kidneys.
- It’s part of your body’s “fight or flight” system, released in response to stress, low blood sugar, inflammation, or even just waking up in the morning.
- Its main purpose is to help you survive by making energy available when you need it most.
What cortisol does
Cortisol affects nearly every tissue in your body. Its main jobs include:
- Raising blood sugar when energy is needed (by stimulating the liver to make glucose).
- Mobilizing fuel – breaking down fat and protein for energy.
- Reducing inflammation (in short bursts).
- Regulating blood pressure and fluid balance.
- Supporting alertness and wakefulness (especially in the morning).
Think of cortisol as your body’s “built-in stimulant and energy manager.”
Cortisol follows a daily rhythm
In a healthy person, cortisol is highest in the morning (giving you energy to wake up and get moving) and gradually drops throughout the day, reaching its lowest point at night when it’s time to rest.
If you live under constant stress, don’t sleep well, over train, or under eat, that rhythm can get disrupted keeping cortisol elevated at the wrong times.
Why chronically high cortisol is a problem
Short-term cortisol spikes are normal and helpful. But when stress becomes chronic, cortisol stays elevated and that can cause:
- Higher blood sugar and HbA1c
- Insulin resistance
- Muscle loss and fat gain (especially belly fat)
- Poor sleep and fatigue
- Suppressed immunity and slow recovery
Essentially, cortisol is beneficial in short bursts but damaging when it never turns off.
Cortisol and ketogenic diets
When you first switch to a zero carb carnivore or ketogenic diet, your body may release a bit more cortisol temporarily to help maintain blood sugar while you adapt to fat metabolism.
That’s normal and short-lived.
Over time, most people on a well-formulated carnivore diet actually see lower baseline cortisol because inflammation drops, blood sugar stabilizes, and the body becomes metabolically calm.
Summary
- Cortisol is your body’s master stress hormone, essential for energy, alertness, and balance.
- Short-term cortisol keeps you alive. Chronic cortisol keeps you tired, inflamed and metabolically stuck.
Learning how to manage stress, rest deeply, and fuel properly is as important for your health as eating right because even a perfect diet can’t offset a body that never feels safe enough to relax.
How does cortisol raise HbA1c?
Cortisol’s job is to raise blood sugar
Cortisol is your body’s main stress hormone, released by the adrenal glands when you’re under physical, emotional, or metabolic stress.
Its purpose is to increase energy so you can respond to that stress. (run from a tiger)
It does this by:
- Stimulating gluconeogenesis, your liver making glucose from protein and fat.
- Reducing glucose uptake by muscles and other tissues (to keep more glucose available for the brain).
- Breaking down muscle into amino acids that can be turned into glucose.
So cortisol’s overall effect = higher blood glucose (sugar).
Chronic cortisol = chronically elevated glucose
When cortisol stays high for too long, from ongoing stress, lack of sleep, illness, overtraining, or even under eating, blood sugar remains slightly elevated much of the time.
Even if you don’t notice it on a single fasting glucose test, your average glucose over weeks and months rises.
HbA1c reflects long-term glucose exposure
Hemoglobin A1c measures how much glucose has attached (or glycated) to your red blood cells over their lifespan (about 120 days for carb eaters).
The higher your average blood sugar, the more glycation occurs and the higher your A1c.
So when cortisol keeps glucose circulating longer or higher than normal, more of it sticks to hemoglobin, pushing your A1c up.
Stress and insulin resistance
Chronically elevated cortisol also makes your cells less sensitive to insulin, especially muscle and fat cells.
That means glucose stays in the bloodstream longer, again contributing to a higher average and a higher A1c.
This effect can happen even in people who eat zero-carb or carnivore if they’re under constant stress, not sleeping or eating enough.
Summary
Cortisol raises HbA1c because it increases blood sugar production, decreases glucose uptake, and promotes mild insulin resistance.
Even if you’re eating perfectly, chronic stress can keep your blood sugar slightly higher on average enough to affect your A1c reading.
That’s why stress management, adequate sleep, and restful recovery are just as important as diet for optimal metabolic health.
You can be eating zero carb and still see elevated A1c if your body thinks it’s in a constant fight-or-flight mode.
How chronically high cortisol can cause weight gain or prevent weight loss even on a zero carb carnivore diet.
Cortisol’s Role in Energy Regulation
Cortisol is your body’s primary stress hormone, produced by the adrenal glands. Its job is to:
- Raise blood glucose when needed (for example, during fasting, low carb intake, or stress)
- Mobilize fat and protein for fuel
- Help manage inflammation
Short-term, this is adaptive and even beneficial. But when cortisol stays chronically elevated, the system shifts in ways that promote fat storage instead of fat burning.
Glucose and Insulin Effects
Even on a zero-carb diet, cortisol can raise blood glucose by stimulating gluconeogenesis (the creation of glucose from protein in the liver).
If this happens frequently:
- Blood glucose stays mildly elevated.
- The pancreas releases small but consistent amounts of insulin to control it.
Over time, this high-cortisol/high-insulin environment makes it harder to burn fat, because insulin suppresses lipolysis (fat breakdown).
So even if you eat a zero carb carnivore diet, stress can mimic the metabolic effects of eating carbs (cake).
Muscle Breakdown and Slower Metabolism
Cortisol also promotes the breakdown of muscle tissue to supply amino acids for gluconeogenesis (turning protein into glucose).
Less muscle mass = lower resting metabolic rate, meaning:
- You burn fewer calories at rest.
- The same food intake can now lead to weight gain or plateau.
But I eat plenty of protein, how could this be happening!?
Cortisol’s Role Is Catabolic, Not Just Compensatory
Cortisol doesn’t just mobilize nutrients when there’s not enough dietary intake it’s a catabolic hormone, meaning its default action is to break down tissue (especially muscle) to ensure the body has raw material for glucose production and stress response.
So even if you’re eating plenty of protein, chronically high cortisol still signals the body to break down muscle tissue for amino acids. The reason is priority: under stress, your body values maintaining blood glucose above preserving muscle.
Why Dietary Protein Doesn’t Always Protect Muscle
If cortisol is persistently elevated:
- Muscle protein breakdown increases (via the ubiquitin-proteasome pathway).
- Protein synthesis decreases (since cortisol inhibits mTOR — the growth pathway).
- Gluconeogenesis ramps up — the liver converts amino acids (from both diet and muscle) into glucose.
Even if you’re consuming a high-protein diet, those amino acids can be:
- Pulled preferentially into gluconeogenesis instead of muscle repair.
- Combined with suppressed anabolic signaling (from mTOR inhibition and lowered testosterone/IGF-1).
The net effect can still be muscle loss, or at least reduced lean mass over time.
Note:
Gluconeogenesis is demand-driven it only ramps up when your body actually needs glucose.
The myth? Cutting protein prevents it from “turning into sugar.”
Reality: Eat less protein, and your body simply breaks down your own muscle to meet the demand driving catabolism and further tanking your metabolism.
It’s About Hormonal Context, Not Just Nutrient Availability
Think of it this way:
| Condition | Cortisol | Insulin | mTOR (Muscle Building) | Result |
| Calm, well-fed, low stress | Normal | Normal/Low | Active | Muscle maintenance or growth |
| Chronic stress, poor sleep, overtraining | High | Often elevated intermittently | Suppressed | Muscle breakdown, slower repair |
So even with adequate protein, the hormonal environment determines what happens to that protein whether it rebuilds tissue or gets burned for energy.
- In a high cortisol environment adequate protein is necessary but not sufficient for muscle maintenance.
- Chronic cortisol:
- Inhibits protein synthesis (via mTOR suppression)
- Stimulates muscle breakdown (via proteolytic enzymes)
- Promotes amino acid diversion to glucose production
- Lowering cortisol (via rest, recovery, stress control) can restore protein balance even if diet stays the same.
Fat Redistribution
Cortisol changes where fat is stored:
- It promotes visceral fat (around the belly and organs), not subcutaneous fat.
- Visceral fat cells have more cortisol receptors and are more insulin-sensitive.
- This type of fat is metabolically active and can itself increase cortisol production a vicious cycle.
Sleep, Hunger, and Appetite Hormones
Chronic cortisol dysregulation also:
- Disrupts sleep → raises ghrelin (hunger hormone) and lowers leptin (satiety hormone)
- Increases cravings for carby foods (even if you resist them)
- Raises overall stress eating drive and lowers metabolic flexibility
- Can lead to over eating
Summary
| Mechanism | Effect |
| ↑ Cortisol → ↑ Gluconeogenesis | Raises blood sugar → small insulin response |
| ↑ Insulin + Cortisol | Fat storage, reduced fat oxidation |
| ↑ Muscle Breakdown | Slower metabolism |
| ↑ Visceral Fat | More cortisol activity → more stress signaling |
| ↓ Sleep & Hormonal Balance | More hunger, less satiety |
What Helps
- Adequate calories: Chronic under-eating keeps cortisol high.
- Proper sleep: 7–9 hours consistently.
- Stress management: Walking, sunlight, deep breathing, self care, etc.
- Avoid excessive fasting or over training: Both are stressors that can keep cortisol elevated long-term.
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Related post: Why High Fasting Blood Glucose Is Normal On A Carnivore Diet
