Vitamin D: Why a Billion People Are Deficient
Vitamin D is one of the most misunderstood nutrients in modern health. Despite its name, it's not really a vitamin — it's a hormone precursor that your body synthesizes from sunlight. And the vast majority of people aren't getting enough.
The scale of the problem
An estimated 1 billion people worldwide have insufficient vitamin D levels. In northern latitudes, deficiency rates can exceed 40% even in otherwise healthy adults. If you work indoors, wear sunscreen, or live above the 37th parallel, there's a good chance you're affected.
What vitamin D actually does
Most people associate vitamin D with bone health, and that's correct — it regulates calcium absorption. But its effects are far broader:
- Immune function — Vitamin D modulates both innate and adaptive immune responses
- Mood and cognition — Low levels are associated with depression and cognitive decline
- Muscle function — Deficiency causes weakness and increases fall risk
- Cardiovascular health — Emerging evidence links adequate D levels to lower cardiovascular risk
- Inflammation — Vitamin D has anti-inflammatory properties that affect CRP and other markers
What your blood test tells you
Vitamin D is measured as 25-hydroxyvitamin D (25(OH)D) in your blood:
- Below 20 ng/mL — Deficient
- 20-30 ng/mL — Insufficient
- 30-50 ng/mL — Adequate
- 50-80 ng/mL — Optimal for most people
- Above 100 ng/mL — Potentially toxic
Many functional medicine practitioners aim for 40-60 ng/mL as the sweet spot. Standard lab ranges often mark 30 ng/mL as "normal," but emerging research suggests this is the floor, not the target.
How Merios handles vitamin D
In the Merios Nutrient Status system, vitamin D is one of the most impactful markers. Because it interacts with so many other systems — immune, bone, cardiovascular, hormonal — a deficiency ripples across your entire health score.
Merios doesn't just flag a low value. It contextualizes your vitamin D against your other markers: if your calcium is also low, or your CRP is elevated, the significance changes. That's the power of analyzing 130+ biomarkers together rather than in isolation.
Practical steps
If your levels are low, here's what the evidence supports:
- Supplementation — Vitamin D3 (cholecalciferol) at 2,000-4,000 IU daily is a common starting point for adults with insufficient levels. Always pair with vitamin K2.
- Sun exposure — 15-20 minutes of midday sun on arms and legs, without sunscreen, 3-4 times per week
- Food sources — Fatty fish (salmon, mackerel), egg yolks, and fortified foods contribute but rarely move the needle alone
- Retest — Check levels again after 3 months of supplementation to adjust dosing
The bottom line
Vitamin D is cheap to test, cheap to fix, and has an outsized impact on your overall health. If you haven't checked your levels recently, add 25(OH)D to your next blood panel. It takes 30 seconds to test and could change your health trajectory.
Merios tracks your vitamin D alongside calcium, magnesium, B12, iron, and dozens of other nutrient markers to give you a complete picture of your nutritional status. Join the waitlist to get started.