Low-Carb in Summer? How Strong Sunlight Increases Glucose and Cortisol Demand
The Overlooked Metabolic Cost of High UV Exposure
Kendall Toerner
Published: February 9, 2026
Low-carb diets are often framed as universally anti-aging. Lower insulin. Less blood sugar damage. Better fat burning.
But nature never designed one fuel strategy for all environments.
The body does not run on food alone.
It runs on light, electrons, and timing — with food adapting to the season.
Once you understand when glucose is made and why, the confusion around carbs, protein, cortisol, and seasons disappears.
Glucose is not “bad”, it’s contextual
Glucose has one main job:
to stabilize energy flow when demand is high or variable.
Your brain, red blood cells, and nervous system still require glucose — even when you’re fat-adapted. The question is not whether glucose is needed, but how it’s supplied.
There are two ways glucose enters circulation:
- You eat it (carbs)
- You make it (gluconeogenesis)
Which path dominates depends on environment, not ideology.
What gluconeogenesis really means (simply)
Gluconeogenesis means:
Making glucose when you don’t eat carbs
Your liver can convert:
- Protein
- Some fat-derived substrates
into glucose to protect the brain.
This process is normal and essential.
But it is not free.
Gluconeogenesis is hormonally supervised — mainly by cortisol.
Cortisol’s real role
Cortisol is not a villain. It is an energy translator.
It helps:
- Convert stored material into glucose
- Keep blood sugar stable
- Bridge gaps between energy demand and fuel availability
Cortisol rises when the body senses:
- Low glycogen reserves
- Low insulin tone
- High or unpredictable energy demand
Short pulses are adaptive.
Chronic elevation is aging.
Sunlight changes everything
Sunlight is not just a signal — it is fuel.
When sunlight hits your skin, eyes, and blood:
- Electrons are excited
- Mitochondrial efficiency increases
- Energy throughput rises
In strong sunlight:
- Dopamine rises
- Activity rises
- Metabolic speed rises
- Repair and growth are allowed
But here’s the key point:
Sunlight increases energy demand before it increases chemical fuel.
If biochemical fuel doesn’t rise to match that demand, cortisol fills the gap.
Why strong sunlight needs some carbs
In summer or high-UV environments:
- Electron flow is high
- Mitochondria run faster
- The nervous system expects abundance
Glucose becomes important not as primary fuel, but as a buffer.
That buffer is liver glycogen.
When liver glycogen is sufficient:
- Cortisol can drop
- Sleep deepens
- Thyroid signaling improves
- Repair dominates over survival
When carbs are extremely low:
- Liver glycogen stays depleted
- The body runs on anticipatory stress
- Cortisol stays slightly elevated “just in case”
This is why:
- Low-carb can feel fine at first
- Then slowly feel wired, flat, or fragile
- And why carbs suddenly feel restorative
They shut cortisol off by refilling the buffer.
Where protein fits — and why it matters more in summer
Protein raises glucagon, not insulin.
Glucagon tells the liver:
“Be ready to make glucose.”
In strong sunlight, with:
- Low carbs
- Low insulin
- High activity
High protein signals:
“We may need glucose now.”
Cortisol steps in to manage the conversion.
This is why:
- High-protein + low-carb is most cortisol-elevating
- Especially in summer or bright environments
- Especially with activity
Protein isn’t the problem.
Protein without buffering is.
Winter flips the rules
Winter is the opposite environment.
In winter:
- Sunlight is weak
- Electron flow is lower
- Activity drops
- Metabolic demand falls
Now glucose buffering is less important.
The body doesn’t need speed — it needs stability.
Nature’s winter fuel is fat.
Fat:
- Supplies steady energy
- Does not require cortisol to regulate
- Preserves protein
- Supports long sleep and repair
In true winter conditions:
- Low carbs make sense
- Gluconeogenesis slows
- Cortisol can stay low if calories are sufficient
This only works when:
- Total calories are high enough
- Protein is moderate
- Activity is reduced
- Light cycles are respected
The modern mismatch
Problems arise when we mix seasons incorrectly.
Summer mismatch:
- Strong sunlight
- High activity
- Very low carbs
- High protein
→ Cortisol-dependent metabolism
Winter mismatch:
- Weak sunlight
- Artificial light at night
- Low carbs
- Low calories
- High mental stress
→ Chronic stress without recovery
Nature never intended either.
The core rule nature follows
When light is strong, glucose buffers stress. When light is weak, fat sustains life.
Carbs are not “always needed.”
Fat is not “always better.”
Fuel follows environmental energy.
Anti-aging is seasonal, not rigid
Anti-aging biology looks like:
- Low cortisol at rest
- Strong circadian rhythm
- Flexible fuel use
- Fast recovery
A body that can relax in abundance and conserve in scarcity ages slower than one locked into permanent restriction.
A real-world example: when permanent summer breaks a winter fuel strategy
A useful public example of seasonal fuel mismatch is animal-based diet creator and influencer Paul Saladino.
Importantly, he did not experience major problems during the early and middle years of carnivore.
For a long time, the approach worked.
That’s not accidental.
Why carnivore worked for so long
Before relocating permanently to a high-sun environment, his lifestyle still contained seasonal signals, even if unintentionally:
- Periods of lower sunlight
- Travel through cooler climates
- Less constant heat and sweating
- Natural reductions in activity at times
- Environmental cues that resembled “winter”
In those conditions:
- Metabolic demand was lower
- Fat-based fuel worked well
- Gluconeogenesis stayed episodic
- Cortisol spikes could fully resolve
This is why zero-carb can feel excellent for years when seasons are still being cycled.
The body was receiving winter-appropriate signals.
What changed in a permanent summer environment
The shift happened after moving to a near-permanent summer climate (Costa Rica):
- Strong, consistent sunlight year-round
- High UV exposure
- High daily activity
- Frequent sweating
- Little true “winter” signaling
At the same time:
- Carbohydrate intake remained near zero
- Protein intake stayed high
- Training volume remained elevated
This created a new physiological equation.
Why symptoms appeared only then
Strong sunlight increases:
- Electron flow
- Dopamine
- Metabolic speed
- Glucose buffering requirements
But without carbohydrates:
- Liver glycogen stayed depleted
- Cortisol had to stay elevated to maintain glucose
- Kidneys lost sodium more easily
- Plasma volume dropped with sweating
Over time, this presented as:
- Cold hands and feet
- Heart palpitations
- Poor sleep and early waking
- Reduced exercise tolerance
- A “wired but fragile” feeling
These are not signs of failure.
They are signs of cortisol-dependent adaptation in a high-energy environment.
Why reintroducing carbs helped
When carbohydrates (fruit, honey) were added:
- Liver glycogen refilled
- Cortisol demand dropped
- Insulin allowed sodium retention
- Fluid and electrolyte balance improved
- Stress symptoms resolved
The fix wasn’t ideological — it was environmental alignment.
The key lesson from this example
This case does not show that:
- Carnivore is wrong
- Low-carb is broken
- Gluconeogenesis failed
It shows this:
A winter fuel strategy placed into a permanent summer environment will eventually rely on stress hormones.
Seasonal cycling had been silently protecting the system before.
When the seasons stopped changing, the mismatch surfaced.
Why this matters beyond one person
Modern life allows us to:
- Live in endless summer
- Train year-round
- Remove carbs permanently
- Ignore environmental feedback
Nature never designed metabolism for that.
Fuel is meant to follow light.
Carbs as a signal, not a staple
The takeaway is not “eat lots of carbs in summer.”
Carbohydrates are meant to act as a seasonal signal, not a constant fuel source.
In nature, carbs:
- Appear periodically
- Come with light, activity, and growth
- Are limited by availability, effort, and season
They are pulses, not a baseline.
Why carb quantity and frequency still matter
Even in strong sunlight, excessive or frequent carbs:
- Raise insulin too often
- Increase glycation pressure
- Reduce mitochondrial efficiency
- Push metabolism toward storage instead of signaling
That does accelerate aging.
The benefit of carbs in high-light environments comes from:
- Small amounts
- Strategic timing
- Clear purpose (buffering cortisol, restoring glycogen, stabilizing fluids)
Not from chronic intake.
The correct role of carbs in summer
Carbs work best when they:
- Refill liver glycogen, not muscle saturation
- Suppress nighttime cortisol
- Support electrolyte and fluid balance
- Allow stress hormones to turn off
This usually requires far less carbohydrate than modern diets provide.
Often:
- Small fruit portions
- Honey as a signal, not a meal
- Post-activity or sunset timing
Once the buffer is restored, more carbs add cost without benefit.
Why this still fits anti-aging biology
Aging accelerates when:
- Insulin is high too often
- Cortisol never shuts off
- Fuel signals are constant instead of rhythmic
Longevity comes from contrast:
- Low baseline insulin
- Low resting cortisol
- Occasional, meaningful signals
- Long periods of metabolic quiet
Carbs belong to the signal category — like sunlight, heat, or movement.
Too little, and stress hormones compensate.
Too much, and insulin never rests.
Nature’s balance point
Strong sunlight raises demand. Small carbohydrate pulses buffer it. Fat remains the foundation.
Winter removes the need for buffering.
Summer introduces it — briefly.
The mistake is turning a seasonal cue into a permanent input.
Bottom line
- Sunlight raises energy demand; glucose acts as a buffer
- Without glucose buffers, protein pushes cortisol higher
- Strong light favors small, strategic carbs; winter favors fat
- Carbs only exist in meaningful amounts in summer
- Longevity comes from seasonal alignment, not constant carbs or constant restriction
- Carbs help by lowering stress hormones—but not when eaten frequently or in excess
Carbs are a message.
Fat is the fuel.
Light decides when the message is needed.
Apply the Framework
Structured seasonal protocols and implementation systems are published inside the private Substack.
The Foundation
The Sunlight Cure
by Kendall Toerner
Preventing Aging and Reversing Disease Through the Epigenetic Signals of Nature
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- Jiang G, Zhang BB.
- Fromentin C, et al.
- Plano SA, et al.
- Poggiogalle E, et al.
- Kuzmenko NV, et al.
- Manninen AH.
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