Ray Klerck reveals the sports science breakthrough proving winter saps an athlete's power, and how elite fighters can weaponize their muscles to store sunlight itself.

Something weird happens to certain fighters’ performance in the dead of winter. Between the freezing morning workouts, the gym’s fluorescent strip lighting, and cuddling of hot coffees, there’s a quiet deletion. It’s not the motivation. Not the toughness. Or the discipline. It’s sunlight. Many Northern Hemisphere fighters look bulletproof under the July sun yet seem like sluggish shadows of themselves on a winter card. We saw flickers of this play out across some of this year’s biggest winter cards. At UFC 325, Diego Lopes, who trains out of Puebla, Mexico, spent his camp in cloud-covered Northern Hemisphere, then went down an energized Volkanovski, whose body was saturated by months of Australian summer sun. The same northern-southern-hemisphere divide played out with Max Holloway when he lost to Charles Oliveira, and with Caio Borralho and Renier de Ridder. It’s not true in all cases, but a tan seems to offer in some instances, so what happens when it’s North Vs North? Look at the Justin Gaethje Paddy Pimblett clash, where the former looked explosive and unrelenting. The difference? Gaethje spends his winters training in the high-altitude, high-UV mountain sun of Colorado, keeping his internal battery topped off. The latter spent his training camp under the UK’s chronically gray, light-starved winter skies. Historically, coaches may have blamed these performance dips on poor weight cuts or overtraining, but nutritional science is beginning to recognize that the real culprit might be far more elemental. The human body has evolved to operate under the sun, and when it disappears, the foundational chemistry of athletic power changes as part of a natural circadian rhythm. For years, we’ve known that vitamin D, your sunlight hormone, is essential for bone density and basic immune health. But a groundbreaking new study has pulled back the curtain on a much deeper, more fascinating relationship between combat athletes, their muscles, and how they store and use this vital nutrient. Here’s why it matters and what you can do to shore up your stores of this vitamin. 

YOUR MUSCLES MAY STORE SUNLIGHT

The new study, released in late February 2026, sounds like something a sadistic strength coach would cook up. Researchers recruited 22 trained MMA athletes and 12 untrained men, then put them through a brutal, repeated 30-second Wingate cycle-sprint protocol. This tests raw anaerobic capacity and power, forcing an athlete to pedal violently until their legs file a formal complaint against their owners. They measured the participants' vitamin D levels before and after the exercise stress. What they found was bizarre. After intense exercise, the trained fighters actually saw their circulating vitamin D levels rise, while untrained men saw theirs crash. Specifically, after completing a four-week vitamin D supplementation block, the fighters taking 3,500 IU experienced a sharp post-exercise spike in blood vitamin D levels of 17.8% at 30 minutes, which ballooned to a staggering 22.7% surge 24 hours later. The elite fighters taking the higher 6,000 IU dose also saw their blood levels enjoy a clean 12% increase by the 24-hour mark. Meanwhile, the untrained men who supplemented with vitamin D experienced a complete system drain, with circulating vitamin D levels decreasing by 16.8% 24 hours after getting in the saddle for the bike test. In simple terms, an elite fighter’s body appears capable of actively mobilizing and releasing stored vitamin D back into the bloodstream during supreme physical exercise. The untrained men, by contrast, looked like they were burning through their baseline reserves like a teenager discovering Redbull drinks for the first time. Researchers now suspect that intensive, long-term athletic training changes skeletal muscle tissue to act as a storage bank for vitamin D, holding onto the hormone and releasing it precisely when metabolic pathways demand it most. 

THE BIG D

The dosage findings were even more eye-opening. The untrained men taking 3,500 IU of vitamin D per day saw their resting blood levels rise from roughly 30.3 ng/mL to an optimal 42.1 ng/mL in just four weeks, comfortably crossing into what’s called the ‘sufficient’ range. But when the MMA athletes took that exact same 3,500 IU dose? They barely moved at all, stalling out from 29.6 ng/mL to just 30.6 ng/mL. Their bodies were consuming and redistributing the nutrient into active tissue just to stay afloat. It wasn't until the fighters upscaled to a heavy 6,000 IU daily dose that they finally broke through the ceiling, matching the 42.3 ng/mL threshold. This suggests that MMA fighters need drastically higher daily intakes of vitamin D just to prevent their training from draining their structural bank accounts. The ultimate proof was in the performance payload. The fighters taking 6,000 IU didn’t just fix their blood markers; they became significantly more explosive. Their peak power output jumped by 7.5%, mean power increased by 4.9%, and total work capacity rose by a clean 5.0%. In elite MMA, where world titles are won and lost by razor-thin margins, those numbers are astronomical. That is the exact physiological difference between exploding through a thunderous double-leg takedown in the championship rounds or looking like you're trying to sprint through wet concrete.

UNLOCKING ATOMIC POWER

To understand why a lack of sunlight might quietly sabotage performance, you first need to understand what vitamin D actually is. Despite the boring name, it’s not really a vitamin in the traditional sense. It struts around your body, behaving more like a powerful steroid hormone that regulates massive parts of your body’s performance machinery. When your levels dip, you don’t just feel a bit flat. Your recovery slows, your explosive power dulls, your immune system weakens, and your body starts operating like an iPhone stuck on 9% battery. That matters when you look back at fighters like Lopes spending camp under dark Northern Hemisphere skies, or Paddy Pimblett grinding through another miserable British winter while Justin Gaethje topped himself up in Colorado’s high-altitude sunshine. The body runs on biology, and there’s an important distinction here because not all vitamin D is equal. The study specifically focused on vitamin D3, also known as cholecalciferol, which is the exact form your skin naturally produces when ultraviolet B sunlight hits it. Vitamin D2 comes mostly from plants and fortified foods, but D3 is significantly better at raising and maintaining active vitamin D levels in the bloodstream. That’s why virtually every serious sports performance discussion centers around D3 rather than the bargain-bin versions floating around supermarket shelves. 

PICKING THE RIGHT KIND

For everyday people, maintaining healthy vitamin D3 levels is already important. It helps regulate calcium absorption, supports immune health, protects bone density, and keeps your body functioning properly through winter. The scary part is that most people are already running low before they even decide to throw a punch at the problem. As many as 42% of Americans are vitamin D deficient, with the highest rates seen in African Americans at 82%. That means huge chunks of the population are walking around with dimmed headlights and wondering why they feel exhausted all the time. For fighters, though, the stakes become much higher. Vitamin D3 directly influences muscle contraction, nervous system efficiency, fast-twitch fiber recruitment, inflammation control, bone resilience, testosterone support, and recovery from repeated high-impact trauma. Every explosive movement, whether it’s a lightning-fast level change or an overhand right, relies on rapid calcium signaling inside muscle tissue. Vitamin D receptors help regulate that process. If levels are low, the entire engine starts misfiring. That’s why deficiency symptoms in fighters often look deceptively familiar. Slower reactions. Flat performances. Lingering soreness. Poor recovery. Increased stress fracture risk. Endurance that mysteriously disappears halfway through camp. And as the new 2026 MMA study revealed, elite MMA fighters not only use vitamin D differently than non-fighters do, but their bodies may also learn to store and recycle it thanks to years of brutal, high-intensity training.

CHARGE YOUR INTERNAL SUNLIGHT BATTERY

Fighters can top up vitamin D3 naturally, although modern life makes it weirdly difficult unless you’re a construction worker, a Bondi lifeguard, or Alex Pereira wandering barefoot through the rainforests. The best source is still free sunlight. Around 15 to 30 minutes of midday sun exposure on bare skin can dramatically boost natural vitamin D3 production, depending on your skin tone, where you live, and whether your country’s weather forecast looks like the opening scene of a Viking funeral. The problem is that most fighters now train indoors, but even someone like Merab Dvalishvili constantly trains outdoors and stays physically active outside the gym. Food does help, but nowhere near as much as people think. Fatty fish like salmon, sardines, mackerel, tuna, and herring are loaded with vitamin D3. Egg yolks, liver, cod liver oil, and fortified dairy products also contribute. The catch is that you’d need to eat enough oily fish to start smelling like a trawler captain, so that’s why supplementation can be valuable, especially through long winters. That doesn’t mean fighters should immediately start swallowing vitamin D3 capsules like Tic Tacs. Blood testing matters because excessive supplementation can cause problems, too. The bigger lesson here is that many people obsess over little things like cold plunges, electrolyte timing, and sauna protocols, but often ignore the giant glowing nuclear reactor floating above Earth that’s completely free of charge. And if this study is right, the most dangerous fighters on the planet may not just be training their muscles anymore. They might literally be learning how to store sunlight itself so should that beach holiday be take before or after a fight?