What is vitamin D?
Vitamin D is technically not a vitamin — it's a hormone that your body makes when sunlight hits your skin. It plays a role in hundreds of processes, from bone health to immune function to mood regulation.
The blood test measures 25-hydroxyvitamin D (sometimes written as 25(OH)D), which is the form your body stores and the best indicator of your overall vitamin D status.
Why it matters for longevity
Vitamin D deficiency is one of the most common nutrient deficiencies in the world — and it's linked to a surprisingly wide range of health problems:
- Bone health: Vitamin D is essential for calcium absorption. Without enough of it, bones weaken over time, increasing the risk of osteoporosis — a major concern for women as they age.
- Immune function: Vitamin D helps your immune system fight off infections and may reduce the risk of autoimmune diseases.
- Mood and energy: Low vitamin D is associated with depression, fatigue, and seasonal mood changes.
- Muscle strength: Deficiency is linked to muscle weakness and increased fall risk, especially in older adults.
- Cardiovascular health: Some research links low vitamin D to higher heart disease risk, though this relationship is still being studied.
Studies consistently show that people with adequate vitamin D levels have lower rates of all-cause mortality.
What the numbers mean
The conventional ranges for 25(OH)D are:
- Deficient: below 20 ng/mL
- Insufficient: 20–29 ng/mL
- Sufficient: 30–100 ng/mL
However, many longevity and integrative medicine practitioners recommend an optimal range of 40–60 ng/mL. At this level, research suggests the best outcomes for bone density, immune function, and overall health. Going above 80 ng/mL is generally unnecessary and could carry risks.
What affects your vitamin D
- Sun exposure: The biggest factor. Your skin makes vitamin D when exposed to UVB rays, but this depends on latitude, season, time of day, skin tone, and sunscreen use. Most people don't get enough from sun alone.
- Skin tone: Darker skin has more melanin, which reduces vitamin D production from sunlight. People with darker skin tones are at higher risk of deficiency.
- Where you live: If you live above about 35° latitude (roughly north of Atlanta or south of Buenos Aires), you get little to no UVB during winter months.
- Body fat: Vitamin D is fat-soluble, meaning it gets stored in fat tissue. Higher body fat can "trap" vitamin D and make less available to your bloodstream.
- Age: Your skin becomes less efficient at making vitamin D as you get older.
- Diet: Very few foods contain meaningful amounts. Fatty fish, egg yolks, and fortified foods provide some, but supplementation is usually necessary to reach optimal levels.
How to get tested
A 25-hydroxyvitamin D blood test is the standard test. It does not require fasting. You can get it through your doctor or order it directly through online lab services. Testing once or twice a year (ideally at the end of winter and the end of summer) gives you a clear picture of your highs and lows.
How to improve it
- Get some sun. Aim for 10–20 minutes of midday sun exposure on bare skin (arms and legs) several times a week during warmer months. This is the most natural way to boost vitamin D.
- Supplement wisely. Vitamin D3 (cholecalciferol) is the preferred form. Most people need 1,000–4,000 IU daily to maintain optimal levels, but your ideal dose depends on your current level, body weight, and sun exposure. Take it with a meal that contains fat for better absorption.
- Pair with vitamin K2. Vitamin K2 helps direct calcium to your bones (where you want it) rather than your arteries (where you don't). Many vitamin D supplements now include K2 for this reason.
- Eat vitamin D-rich foods. Wild salmon, sardines, mackerel, egg yolks, and mushrooms exposed to UV light all provide some vitamin D. It's hard to get enough from food alone, but every bit helps.
- Retest after 3 months. If you start supplementing, check your level again after about 3 months to make sure you're in the optimal range and adjust your dose if needed.
Since vitamin D affects so many systems in your body, optimizing it is one of the simplest and most impactful steps you can take. Check your Biological Age to see how your overall health picture compares to your actual age.