I’m not a doctor. I want to be clear about that upfront.
What I’m about to share is my personal experience and some research I’ve found compelling enough to act on. You should talk to your healthcare provider about your own situation. Get your bloodwork done. Know your numbers.
That said — this might be the most important health article I’ve ever written.
Most Men Are Walking Around Deficient
Here’s something that doesn’t get nearly enough attention: the majority of American adults have insufficient vitamin D levels. Some estimates put it as high as 42% of the U.S. population being clinically deficient — and that number is higher for men over 40, men with darker skin, men who work indoors, and men who live above the 37th parallel (basically anywhere north of a line from San Francisco to Richmond, Virginia).
You’re probably one of them. I was.
Vitamin D isn’t really a vitamin — it’s a hormone precursor. Your body makes it when sunlight hits your skin. The problem is that modern life has made that almost impossible for most men: you wake up before sunrise, commute in a car, work indoors under fluorescent lights, drive home, and sit on the couch. Your skin barely sees the sun.
And here’s what most people don’t know: vitamin D is involved in far more than bone health. It plays a role in immune function, testosterone production, mood regulation, inflammation control, and metabolic health. Every one of those matters if you’re a man over 35.
What the Research Shows
A few years back, I came across some research that changed how I think about vitamin D.
During a period when respiratory illness was a major global concern, researchers started looking at vitamin D status as a potential factor in outcomes. What they found was striking:
A 2020 study published in PLOS ONE by Maghbooli et al. analyzed hospitalized patients and found that those with vitamin D levels above 30 ng/mL had significantly better outcomes — including a 51.5% lower risk of dying. Patients who were vitamin D sufficient were also less likely to become unconscious or experience dangerously low oxygen levels.
But here’s the finding that really got my attention: multiple studies suggested that patients with vitamin D levels above 50 ng/mL had dramatically better outcomes. A 2022 study by Borsche, Glauner, and von Mendel analyzed data from multiple countries and found a strong inverse correlation between vitamin D levels and severity of respiratory illness. Their analysis pointed to an “optimal” level of approximately 50 ng/mL or above for immune protection.
An Israeli study by Dror et al. (2022), published in PLOS ONE, analyzed pre-infection vitamin D levels in nearly 1,000 hospitalized patients and found that those who were vitamin D deficient before becoming ill had a 14 times higher likelihood of severe illness compared to those with adequate levels.
The numbers kept pointing in the same direction: higher vitamin D levels — particularly above 50 ng/mL — correlated with dramatically better outcomes during respiratory illness.
What I Did About It
Again — I’m not a doctor. This is what I did for myself.
After reading this research, I got my vitamin D levels tested. I was in the mid-20s ng/mL — technically “insufficient” by conventional standards, but the conventional target is only 30 ng/mL. Functional medicine practitioners — including many at organizations like Forum Health — often target significantly higher levels, in the 50-80 ng/mL range.
I started supplementing with vitamin D3 daily. I take it with a meal containing fat (vitamin D is fat-soluble — it needs dietary fat to absorb properly). I also make sure I’m getting adequate vitamin K2 and magnesium, which work together with vitamin D for proper calcium metabolism.
I retested after about 3 months and my levels were above 60 ng/mL.
Here’s my personal observation — and I want to be careful to frame this as exactly that, a personal observation: since getting my vitamin D levels up, I rarely get sick. When the people around me are catching colds and respiratory bugs, I seem to dodge them. Is that because of vitamin D alone? I can’t prove that. Too many variables. But the research gave me enough confidence to make the change, and my experience has been consistent with what the studies suggest.
What Most Doctors Won’t Tell You
The “recommended daily allowance” for vitamin D is 600-800 IU per day. Most conventional doctors will tell you that’s sufficient.
Here’s the problem: that amount was established to prevent rickets — a bone disease. It was never designed to optimize immune function, testosterone production, or metabolic health.
A 2021 review published in PMC argued that the upper safety limit for vitamin D supplementation should be revisited, noting that doses up to 10,000 IU daily have been used safely in clinical settings for extended periods, provided levels are monitored through regular blood testing.
The Endocrine Society has recommended that adults may need 1,500-2,000 IU daily just to maintain a level above 30 ng/mL. To get above 50 ng/mL — the level where the immune research gets interesting — many people need significantly more.
This is why functional medicine practitioners tend to approach vitamin D differently from conventional medicine. They’re not treating for deficiency — they’re optimizing for health. It’s the difference between “not sick” and “actually thriving.”
What You Should Do
Get tested. A simple 25-hydroxyvitamin D blood test tells you exactly where you are. It’s inexpensive and widely available. Many men’s health providers include it in standard panels.
Know your number. Here’s how to think about it:
| Level (ng/mL) | What It Means |
|---|---|
| Below 20 | Deficient — address immediately |
| 20-30 | Insufficient — most conventional doctors start here |
| 30-50 | “Adequate” by conventional standards |
| 50-80 | Optimal range — where functional medicine practitioners aim |
| Above 100 | Potentially toxic — avoid without medical supervision |
Supplement intelligently. If you’re deficient, you’ll likely need more than the standard 600-800 IU to get your levels up. Work with your provider to find the right dose. Take D3 (not D2) with a meal containing fat. Consider adding vitamin K2 and magnesium for proper absorption and calcium metabolism.
Retest every 3-6 months. Your levels change with the seasons (lower in winter, higher in summer if you’re getting sun exposure). Track your number like you’d track your weight or your cholesterol.
Don’t just supplement — get outside. 15-20 minutes of midday sun on exposed skin (arms, face) can produce significant vitamin D. It’s free, and there’s no pill that fully replicates what natural sunlight does for your body.
Why This Matters for Men Over 35
Vitamin D isn’t just an immune story. For men specifically:
Testosterone. Multiple studies have found a positive correlation between vitamin D levels and testosterone. Men with sufficient vitamin D tend to have higher testosterone levels than men who are deficient. If you’re doing everything right with intermittent fasting, resistance training, and sleep — but your vitamin D is in the basement — you’re leaving results on the table.
Belly fat. Vitamin D deficiency is associated with increased visceral fat — the deep belly fat that drives metabolic disease. Some research suggests that optimizing vitamin D may help with fat distribution, particularly in the midsection.
Mood and energy. Vitamin D receptors are throughout your brain. Low levels are consistently associated with fatigue, low mood, and reduced motivation. If you’ve ever noticed you feel better in summer than winter, vitamin D is part of that equation.
Inflammation. Chronic low-grade inflammation is behind most metabolic disease. Vitamin D has well-documented anti-inflammatory effects. For men doing intermittent fasting — which also reduces inflammation — optimizing vitamin D may amplify the benefits.
The Bottom Line
Most men over 35 are vitamin D deficient and don’t know it. The standard recommendation of 600-800 IU was designed to prevent rickets, not optimize health. The research on immune function, testosterone, and metabolic health suggests that levels above 50 ng/mL may be significantly protective — and functional medicine providers have been saying this for years.
I got my levels tested, started supplementing based on my results, and I can tell you from personal experience that I feel markedly better and rarely get sick. That’s my story. Yours may be different.
The first step is dead simple: get tested. Know your number. Then decide what to do about it with your healthcare provider.
You can do this. It’s one blood test.
Stay strong, keep busy and active.
To Your Success,
Bob
References:
- Maghbooli, Z., et al. (2020). Vitamin D sufficiency, a serum 25-hydroxyvitamin D at least 30 ng/mL reduced risk for adverse clinical outcomes in patients with COVID-19 infection. PLOS ONE.
- Borsche, L., Glauner, B., & von Mendel, J. (2022). COVID-19 Mortality Risk Correlates Inversely with Vitamin D3 Status, and a Mortality Rate Close to Zero Could Theoretically Be Achieved at 50 ng/mL. Nutrients.
- Dror, A.A., et al. (2022). Pre-infection 25-hydroxyvitamin D3 levels and association with severity of COVID-19 illness. PLOS ONE.
- Vitamin D supplementation: upper limit for safety revisited? (2021). PMC.
- Pilz, S., et al. (2011). Effect of vitamin D supplementation on testosterone levels in men. Hormone and Metabolic Research.