Vitamin D: The Blood Test Most Executives Fail Without Knowing It

Of all the blood markers I review with new clients, vitamin D is the one that comes back low most often — and it is not close. Executives who live indoors, travel through airports instead of walking outdoors, and work through daylight hours are functionally sun-deprived for most of the year, and their lab work shows it. The good news is that vitamin D is also one of the cheapest, simplest, and most reliably correctable deficiencies in all of health. The bad news is that most people have never had it tested, and many who have were told a technically "normal" result that is nowhere near optimal.

I'm Dr. Jason Rannfeldt, a health performance coach who works with founders, executives, and high performers on building energy and resilience that holds up under real schedules. In this article I'll cover what vitamin D actually does, why the standard reference range misleads people, what level to target, and the exact testing and supplementation protocol I use with clients.

What vitamin D actually does

Vitamin D is misnamed — it functions less like a vitamin and more like a hormone, with receptors in nearly every tissue in the body. It regulates calcium absorption and bone metabolism, which is the role everyone knows, but its reach is far broader. It modulates immune function, which is why low levels are consistently associated with higher susceptibility to respiratory infections — the resilience theme I covered in Immune Resilience for Executives. It influences muscle function and strength, which matters for the training adaptations in Strength Training After 40. It plays a role in mood regulation, and low levels are reliably associated with higher rates of low mood and seasonal energy decline. And it supports the skeletal system I covered in Bone Density and Skeletal Longevity for Executives — vitamin D deficiency is one of the few ways you can undermine a good strength program from the inside.

Why executives are almost universally low

Vitamin D is synthesized in the skin in response to UVB sunlight, and the modern executive lifestyle is engineered to prevent exactly that. Office work keeps you indoors during peak UV hours. Air travel happens in metal tubes at altitude. Northern latitudes — anywhere above roughly the 37th parallel, which includes most of the northern United States and all of Canada and northern Europe — produce essentially no usable UVB from late fall through early spring, a gap I discussed in Morning Light and Circadian Performance. Sunscreen, glass windows, and darker skin tones all reduce synthesis further. The result: studies of indoor-working populations routinely find deficiency or insufficiency rates of 40 to 70 percent, and in my client population the number is at the high end of that range. If you work indoors and live north of Atlanta, assume you are low until a blood test proves otherwise.

The reference range problem

Here is where most people get misled. The standard lab reference range for 25-hydroxyvitamin D — the marker you test — typically starts at 20 or 30 ng/mL, and anything above that floor gets reported as "normal." But that floor was set to prevent rickets and frank bone disease, not to support optimal immune function, mood, and performance. Most researchers and clinicians who work in this space target a range of roughly 40 to 60 ng/mL, and many of the observed associations with better immune and cardiometabolic outcomes sit in that band. A client who comes to me with a level of 24 ng/mL has been told they are fine; functionally, they are running a deficit that shows up as winter fatigue, frequent colds, and flat training recovery. This is the same pattern I described in Executive Bloodwork and Biomarkers — reference ranges describe populations, not performance.

The symptoms nobody connects

Vitamin D insufficiency rarely announces itself clearly. It shows up as a cluster of low-grade complaints that are easy to attribute to stress or age: persistent low energy that sleep does not fully fix, catching every cold that moves through the office, low mood that worsens in winter, muscle aches and slower recovery from training, and disrupted sleep quality. Individually, each of these has a dozen possible causes — the fatigue differential I walked through in The Afternoon Energy Crash applies here too. But when several cluster together in someone who lives indoors, vitamin D belongs on the shortlist, and a $50 blood test settles the question definitively. This is why I never guess at supplementation: test first, dose to the result, retest to confirm.

The protocol: test, dose, retest

The protocol I use with clients has three steps. Step one: test. Ask your physician for a 25-hydroxyvitamin D test, or order one through a direct-access lab — it is inexpensive and widely available. Test in late winter if you can, when levels are at their annual low; that is your true baseline. Step two: dose to the gap. A common starting point for adults is 2,000 to 5,000 IU of vitamin D3 daily, taken with a meal that contains fat, since vitamin D is fat-soluble and absorbs poorly on an empty stomach. The right dose depends on your starting level, body weight, and sun exposure — larger gaps and larger bodies need more. I typically pair it with vitamin K2 and the magnesium strategy from Magnesium for Executives, because magnesium is a required cofactor for vitamin D metabolism and most people are short on both. Step three: retest in 8 to 12 weeks. Vitamin D levels move slowly; retesting confirms your dose is actually landing you in the 40 to 60 ng/mL target range rather than guessing. Adjust, then recheck annually. This test-dose-retest loop is the same discipline I apply to the supplement framework in Supplements for Executives — no supplement earns a permanent place in the cabinet without a measurement behind it.

Food, sun, and the honest limits

Can you fix this with food and sunlight alone? In theory, partly. Fatty fish — salmon, sardines, mackerel — are the best dietary sources, and a serving of wild salmon provides several hundred IU, which fits naturally into the protein framework in Omega-3s for Executives. Fortified foods add a little more. But diet alone rarely closes a significant gap. Sunlight is more powerful — 15 to 20 minutes of midday summer sun on arms and legs can produce several thousand IU — but it is seasonal, latitude-dependent, and incompatible with most work calendars for half the year. For the indoor executive, supplementation is not a shortcut; it is simply the reliable delivery mechanism. I wrote a shorter companion piece on foundational supplements over at jasonleerannfeldt.me for readers who want the quick version.

What vitamin D will not do

A word of honesty, because the supplement industry oversells this one. Correcting a deficiency is high-leverage; megadosing beyond sufficiency is not. Once you are in the target range, more vitamin D does not produce more energy, more immunity, or more performance — and very high levels carry real risks around calcium metabolism. Vitamin D also does not compensate for the fundamentals: it will not rescue the sleep debt from Sleep Optimization for Executives, the metabolic dysfunction in Metabolic Health, or a missing training base. It is a foundation stone, not the building. The executives who get the most from correcting it are the ones who treat it as one calibrated input in a measured system — the same philosophy behind Recovery Metrics: HRV, RHR, and Sleep Score.

The bottom line

Vitamin D is the rare health intervention that is cheap, safe when dosed to testing, and almost universally needed by people who work indoors. Get the blood test, dose to the gap, retest in three months, and stop guessing. If you want your labs, supplements, training, sleep, and nutrition sequenced into one system built around your actual calendar, that is the work inside Crush 90 and Thrive 90, alongside the broader framework in Longevity Strategies for High Performers. Start with a short conversation on the contact page, or read more about the approach on the about page.