Vitamin D: Why It Matters, How to Get It, and How Much You Need
Vitamin D is one of the most essential nutrients for overall health and longevity. But where does it come from, why is it so crucial, and how do you know if you’re getting enough?
What Is Vitamin D?
Vitamin D functions as both a nutrient and a hormone your body produces. Research shows it:
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Supports immune function
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Reduces cancer cell growth
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Improves cognitive performance
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Strengthens bones
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Reduces inflammation
Scientists continue to investigate additional roles vitamin D may play in long-term health.
What the Research Says
Bone Fractures
Vitamin D is crucial for maintaining strong bones. Low levels are associated with a higher fracture risk in older adults.
A meta-analysis of 12 randomized controlled trials (over 42,000 adults ages 65+) found:
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500–800 IU/day reduced hip and non-spine fractures by ~20%
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≤400 IU/day showed no significant benefit
Multiple Sclerosis (MS)
Genetics and environmental factors—especially low vitamin D—play a role in MS risk.
Studies show:
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Higher vitamin D levels are linked to less disease activity and slower progression in people with MS.
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MS rates are much higher far north or south of the equator, where sunlight exposure is lower.
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Healthy individuals with high vitamin D levels showed dramatically lower MS risk:
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U.S. young adults: 62% lower risk
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Sweden: 61% lower risk
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Finland: Low vitamin D linked to 43% higher risk
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While trials are underway for treatment, prevention trials are unlikely due to MS being relatively rare. However, current evidence suggests up to 40% of cases might be preventable by correcting vitamin D insufficiency.
Flu, Colds & COVID-19
Flu tends to peak in winter partly because vitamin D levels drop.
Research shows:
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Children with low vitamin D are more prone to respiratory infections.
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Adults with low levels report more colds and coughs.
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Low vitamin D levels significantly increase COVID-19 risk (according to a large meta-analysis).
A randomized trial in Japanese schoolchildren found:
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1,200 IU/day vitamin D reduced Type A flu by ~40% compared with placebo.
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No significant difference was observed in Type B flu.
Autoimmune Conditions
(rheumatoid arthritis, psoriasis, Hashimoto’s, Graves’, and more)
An extensive 5-year randomized, double-blind, placebo-controlled trial of 25,000 adults (50+) found:
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2,000 IU/day of vitamin D reduced the incidence of autoimmune disease by ~22%.
Conditions included rheumatoid arthritis, psoriasis, polymyalgia rheumatica, and autoimmune thyroid diseases.
Cognitive Decline
In an analysis of over 427,000 White European participants:
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Low vitamin D levels (<25 nmol/L) were linked to a 54% higher risk of dementia compared with adequate levels (50 nmol/L).
Vitamin D From Food
Most people consume only ~140 IU/day, and even less if they’re vegan or have digestive conditions (IBD, Crohn’s, ulcerative colitis) or a history of gastric bypass surgery.
Foods naturally high in vitamin D3 include:
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Wild salmon
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Sardines
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Mackerel
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Tilapia
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Tuna
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Swordfish
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Trout (with skin)
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Cod liver oil
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Beef liver
Smaller amounts are found in egg yolks and in fortified foods such as milk, cereal, and orange juice.
Mushrooms
Some UV-exposed mushrooms contain higher levels of D2. You can boost the vitamin D in your mushrooms by placing them in the sun for 20–30 minutes.
Vitamin D From Sunlight
Most vitamin D comes from sun exposure—but absorption varies.
Your body stores vitamin D from summer sun, but levels decline over winter, and many people become deficient by late winter.
Factors that reduce vitamin D absorption:
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Sunscreen use (reduces absorption by 90%+)
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Full-coverage clothing
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Staying indoors (sunlight through windows doesn’t count)
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Darker skin (more melanin = reduced D synthesis)
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Obesity (vitamin D gets trapped in fat tissue)
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Aging (reduced ability to synthesize vitamin D)
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Living in northern climates
Deficiency & Toxicity
An estimated 1 billion people worldwide have inadequate vitamin D, including ~42% of Americans.
Symptoms of Vitamin D Deficiency
Many people have no symptoms. Others may experience:
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Muscle pain or weakness
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Fatigue
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Hair loss
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Mood changes (depression, irritability, anxiety)
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Frequent illness
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Tingling in hands/feet
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Bone pain or fractures
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Muscle twitches or spasms
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Rickets (in children)
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Osteomalacia (in adults—reversible)
Vitamin D Toxicity
Extremely rare and caused only by excess supplements, not sun or food.
Symptoms include stomach pain, constipation, nausea, or dizziness.
Should You Supplement?
Recommended Dietary Allowance (RDA):
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Adults 19–70: 600 IU/day
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Adults 70+: 800 IU/day
Functional Medicine Perspective
Many functional practitioners recommend:
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5,000+ IU/day for adults
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500–1,000 IU/day for children
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Or 35 IU per pound of body weight
Example:
120 lbs × 35 IU = 4,200 IU/day
The FDA is transitioning from IU to mcg for standardization:
1,000 IU = 25 mcg
If you want a supplement recommendation, here is the one I take: Vitamin D
Vitamin D2 vs. D3: What’s the Difference?
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Vitamin D2 (ergocalciferol): from plants; common in fortified foods
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Vitamin D3 (cholecalciferol): produced by the body; found in animal foods
Most experts prefer D3, as it raises blood levels more effectively.
Tip:
Take vitamin D early in the day with a high-fat meal, or with vitamin K, for optimal absorption.
How to Check Your Vitamin D Levels
Western medicine considers 30–100 ng/mL normal.
Functional practitioners aim for 50–80 ng/mL, arguing the standard range is too broad.
The USPSTF says routine testing isn’t necessary for everyone, so you often need to request that vitamin D be added to lab work specifically.
Ideally, test twice per year—once in summer and once in winter.
If you want a vitamin D supplement recommendation, this is the one I use: VITAMIN D
If you have questions or want to book me for an event, email aftannfit@gmail.com or follow me on Instagram or Facebook.
