Bryan Johnson Is Running Cold — And That's a Problem

Your 98.6°F isn't arbitrary. It's millions of years of evolutionary negotiation with the fungal kingdom — and deliberately lowering it may be one of the riskiest longevity experiments anyone could run.

Brehan Crawford, MAcOM, LAc — on why running cold may be opening the door to fungi

The longevity world is obsessing over a number that evolution already optimized for us.

Bryan Johnson has been publicly celebrating a resting body temperature around 35°C — roughly 95.1°F — as a biomarker of slowed aging and efficient metabolism. He's not entirely wrong. There's legitimate science connecting lower metabolic rate with reduced oxidative stress. A cool body, like a piece of fruit in the refrigerator, does slow certain processes down.

But that fruit in the fridge will still grow mold eventually. And that's the part of the equation Bryan isn't talking about.

I don't bring this up to dunk on the biohacking world. I bring it up because I see the downstream consequences of disrupted thermal terrain every week in clinical practice — in people with fibromyalgia, MCAS, IBS, long COVID, and chronic Lyme. And a lot of them are already running cold. Voluntarily making that worse is, from my vantage point, a serious oversight.

Your warmth is your immune system's first line of defense.

Most fungi in our environment are what mycologists call mesophilic — they thrive at moderate, ambient temperatures and cannot replicate efficiently at normal human body heat. This is not an accident. The landmark work by Casadevall and Bergman at Einstein College of Medicine modeled the exact trade-off: higher body temperatures confer protection against fungal species but require more metabolic energy to maintain. Their model found the optimal temperature — maximizing fungal exclusion while minimizing caloric burden — lands right at 36.7°C. That is essentially 98.1°F. Essentially, us.

Look at amphibians and reptiles. Chytrid fungus has driven multiple frog species to extinction. Bat white-nose syndrome has killed tens of millions of animals. These are cold-blooded creatures with no thermal exclusion zone, and the fungal kingdom has been devastating them for it. We mammals escaped that fate precisely because we run hot.

At Bryan's reported baseline of 35°C, his body can now support roughly 10–12% more fungal species than someone running at normal temperature. That's not a longevity hack. By the logic of mammalian evolutionary biology, that's a regression toward the thermal vulnerability of a cold-blooded animal.

For people with chronic illness, candida isn't theoretical — it's already in the room.

All of us carry some Candida in our gut, our mouth, and on our skin. At normal temperatures, a healthy immune system and our thermal barrier keep it in check. Candida is essentially a mesophilic organism. It's comfortable at ambient room temperatures and begins to thrive as the body drops below the normal thermal exclusion zone. At 35°C, Candida is squarely within its comfort range.

This matters enormously for the people I work with. Research has identified that fibromyalgia patients show elevated rates of fungal overgrowth and candidal infections compared to healthy controls — and studies modulating the gut mycobiome in fibromyalgia patients found that suppressing fungal overgrowth tracked with meaningful clinical improvement. IBS patients consistently show altered gut mycobiome profiles, with Candida interactions playing a central dysregulatory role. And for anyone dealing with MCAS, Candida is one of the documented triggers that can send mast cells into a histamine-releasing spiral.

The piece Bryan is really missing: the ability to mount a fever.

When your immune system encounters a pathogen, one of its most powerful responses is to raise your core temperature. Researchers describe this as creating a thermal exclusion zone — a targeted, active defense that shuts fungal and bacterial replication down in real time. Your fever isn't a malfunction. It is the immune system doing precisely what it evolved to do.

If your baseline is already 35°C, you've dramatically narrowed the headroom for that fever response. Clinically, patients who become hypothermic rather than febrile during infection consistently have worse outcomes. The fever isn't the problem — it's the defense.

This is exactly what concerns me about the low-body-temp longevity trend. You're not just opening the door to more fungal species. You're also blunting your primary adaptive immune tool for fighting them once they arrive. And if you're already dealing with a chronic condition — Lyme, fibromyalgia, long COVID — your immune system doesn't have the redundancy to absorb that tradeoff.

Gut Harmony — Botanical Support for a Balanced Microbiome

Maintaining a healthy fungal-bacterial balance in the gut is one of the most foundational things you can do for terrain health. Gut Harmony brings together a clinically informed botanical formula — rooted in TCM and reviewed against modern pharmacological research — to support digestive ecology, not just suppress symptoms.

Classical medicine was already describing this — just in a different language.

In Traditional Chinese Medicine, cold and damp are the two environmental pathogens most consistently associated with what we'd now call fungal and mycotoxin-driven disease. The clinical pattern of "cold-damp invading the middle jiao" maps remarkably well onto what we see in candida-driven IBS, fibromyalgia with gut involvement, and the kind of low-grade chronic fungal dysregulation that never quite triggers a conventional diagnosis.

When I work with patients in this pattern, a major part of the protocol is warming the digestive center — not with artificial heat, but with the right botanicals, movement practices, and dietary shifts that restore metabolic vitality. We are trying to reestablish the internal environment that the immune system needs to do its job.

What Bryan's experiment is doing — from a TCM terrain perspective — is deliberately inducing cold-damp. He's taking a system that evolved to run warm and cooling it down in the name of efficiency. But the fungi don't care about his efficiency metrics. They just know the door is open.

I'm not suggesting you try to run a fever all the time. I'm suggesting you don't deliberately sabotage the thermal firewall that 200 million years of mammalian evolution spent building. Sometimes the old wisdom holds. There's a reason we evolved to run hot.

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References

1. Casadevall A, Bergman A. Mammalian endothermy optimally restricts fungi and metabolic costs. mBio. 2010;1(5). PMC: https://pmc.ncbi.nlm.nih.gov/articles/PMC2975364/
2. Casadevall A, et al. Immunity to fungi. Annual Review of Immunology. 2022;40. DOI: 10.1146/annurev-immunol-101220-034306
3. Jacobs C, et al. Fungal feelings in the irritable bowel syndrome: the intestinal mycobiome and abdominal pain. Gut Microbiome. 2023. PMC: https://pmc.ncbi.nlm.nih.gov/articles/PMC9897793/
4. Corrias MV, et al. Modulation of gut bacterial and fungal microbiota in fibromyalgia patients following a carb-free oloproteic diet. PMC. 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC12472566/
5. Casadevall A, Pirofski LA. The damage-response framework of microbial pathogenesis. Nat Rev Microbiol. 2003.