The Truth About Vitamin D: Why 25 ng/mL is Too Low
When most people hear “vitamin D,” they think bone health or maybe “get some sunshine.” But the truth is, vitamin D is far more powerful than we give it credit for—and most of us are walking around with levels that are far too low.
Vitamin D deficiency is one of the most common nutrient deficiencies worldwide. An estimated 1 billion people globally have insufficient levels. In the U.S. alone, about 40% of adults are considered deficient, with even higher rates in women, people with darker skin, and those who live in northern climates or spend little time outdoors.
If your recent lab work showed a vitamin D level of 20-30 ng/mL, you might have been told, “That’s fine.” But in reality? That number is borderline deficient. So, what does “optimal” actually mean? Why does vitamin D matter so much beyond bone health? And how can you safely bring your levels up? Let’s dig in.
What Is the Optimal Range?
Laboratory reference ranges for vitamin D (measured as serum 25-hydroxyvitamin D) typically list 20–100 ng/mL as “normal.” Laboratory “reference ranges” are often misleading. Labs understate the problem. They’re based on averages of the population, not on what’s actually healthy. If the average population is deficient, the “normal” range is skewed low. So there’s an important distinction between what’s “statistically common” in the population and what’s biologically optimal. This is why functional medicine practitioners push for higher targets supported by clinical research.
Deficiency: <20 ng/mL
Insufficiency: 20–29 ng/mL
Optimal range: 40–60 ng/mL (some sources extend to 80 ng/mL for therapeutic benefit)
Toxicity: >100–150 ng/mL (very rare, usually from excessive supplementation)
In other words: 25 ng/mL may not raise alarms on your lab report because many labs set “normal” anywhere from 20–100 ng/mL., but it leaves you vulnerable to subtle deficiencies that impact energy, immunity, mood, and metabolic health.
Functional medicine and nutrition experts typically recommend 40–60 ng/mL as the sweet spot for optimal health.
Levels below 30 ng/mL are consistently associated with increased risks for immune dysfunction, bone weakness, mood disorders, and chronic disease.
So, while 25 ng/mL may not get flagged as “deficient” in your lab portal, it’s a far cry from optimal.
Why Is Vitamin D So Important?
Vitamin D is more than just a vitamin—it’s technically a hormone precursor, or prohormone (rather than a vitamin) that influences the expression of over 200 genes in your body once activated. Think of it as a master switch that supports nearly every system. Its effects extend far beyond bone health.
Immune system regulation – Vitamin D helps your body respond effectively to infections without tipping into autoimmune overdrive.
Adequate vitamin D supports production of antimicrobial peptides that help fight viruses and bacteria.
Deficiency has been linked with increased risk of respiratory infections, autoimmune disorders, and poor vaccine response.
Mood and brain health – Low vitamin D has been linked to depression, seasonal affective disorder, brain fog, and cognitive decline.
One meta-analysis found that people with depression were significantly more likely to have low vitamin D levels.
Hormone & Metabolic health – Vitamin D plays a role in insulin sensitivity.
Low levels are associated with insulin resistance, higher rates of metabolic syndrome and obesity, and increased risk of type 2 diabetes.
It also influences reproductive health, including ovulation and fertility in women.
Bone health – Vitamin D regulates calcium and phosphorus absorption, protecting against osteoporosis.
Vitamin D is essential for calcium absorption and bone remodeling.
Deficiency increases risk of fractures, osteoporosis, and muscle weakness.
Longevity and disease prevention – optimal levels may reduce risk of cardiovascular disease, certain cancers, and inflammatory conditions.
This includes hypertension and strokes.
Deficiency is associated with higher all-cause mortality.
Simply put: vitamin D isn’t optional. It’s foundational.
How to Increase Your Levels
If your vitamin D levels are low or hovering around 20-30 ng/mL, diet and lifestyle adjustments alone may not be enough—you’ll likely need targeted supplementation. Still, a multi-pronged approach works best. Here are the most effective strategies to bring them up:
Sunlight
Midday sun exposure (10 a.m.–2 p.m.) on bare arms and legs for 15–30 minutes, a few times per week, can raise levels.
Effectiveness depends on skin tone, latitude, season, and whether you use sunscreen.
In northern latitudes or during winter, sun exposure is insufficient
Food sources – Food alone typically isn’t enough to correct a deficiency, but can support maintenance.
Fatty fish: salmon, sardines, mackerel, tuna
Cod liver oil
Pastured egg yolks
Mushrooms (especially UV-exposed)
Fortified foods (dairy, plant milks, cereals)
Supplements
Vitamin D3 (cholecalciferol) is the preferred form and superior to D2 for raising serum levels.
Pairing with vitamin K2 helps for better absorption and calcium regulation, directing calcium into bones rather than soft tissues.
General maintenance dose: 1,000–2,000 IU/day, but those with low levels may need more.
Deficiency correction: often 4,000–5,000 IU/day, short-term, under professional guidance
Always retest after 3 months to assess effectiveness and safety.
Important: Supplementation isn’t one-size-fits-all. Genetics, body weight, absorption, and co-nutrient status (like magnesium) affect your response. This is why professional guidance matters.
What Happens at Optimal Vitamin D Levels?
This is where the transformation happens—and it’s often subtle but powerful. My clients who raise their vitamin D into the 40–60 ng/mL range reported benefits such as:
More stable moods – less winter/seasonal depression, fewer “blah” days, and less irritability.
Better energy – waking up with improved morning alertness, less fatigue, and fewer mid-day crashes.
Stronger immunity – fewer colds, milder seasonal illnesses, faster healing and recovery times.
Sharper thinking – brain fog clears, focus sharpens, and cognition improves.
Physical resilience – improved muscle strength and fewer bone aches.
Stronger bones and muscles – particularly noticeable in women approaching menopause or with a family history of osteoporosis.
Research backs these observations. Studies link optimal vitamin D to improved cardiovascular outcomes, reduced risk of autoimmune flare-ups, and better mental health markers.
It’s not a “miracle pill,” but when your vitamin D is dialed in, the difference is noticeable.
Bringing It Back to You
If your bloodwork shows vitamin D sitting at 25 ng/mL, don’t settle. Don’t accept “normal” as good enough. Ask yourself:
Do I feel my best right now?
Am I experiencing low energy, poor mood, or frequent colds?
Could my mood, energy, or immune health improve if I optimized this one marker?
Have I had labs reviewed by someone who looks beyond reference ranges?
Do I have the right guidance on how much vitamin D I should personally take?
Do I have a plan to safely and effectively optimize my vitamin D?
As a dietitian specializing in women’s hormone and longevity health, I see low vitamin D pop up on labs all the time. And correcting it is often one of the simplest, most empowering first steps toward better health.
Final Thoughts
Vitamin D is not just about bones—it’s about your immune system, mood, hormones, and long-term vitality. And a level of 25 ng/mL, while technically “normal,” is a signal that your body needs more support.
With the right guidance, optimizing vitamin D is achievable and life-changing.
Next Step For You
Curious where your vitamin D stands—or how it ties into your hormone balance, energy, and long-term wellness?
Book a nutrition consultation with me. Together, we’ll review your labs, uncover hidden deficiencies, build a sustainable plan that supports your whole-body health, and make sure you’re not just “normal,” but optimal.