Vitamin D

Vitamin D is probably the most commonly deficient micronutrient in the modern world, and the reason is straightforward — the body produces it primarily through sun exposure, and most people in modern life don’t get enough of that. Office work, indoor living, sunscreen use, living at higher latitudes, and spending the majority of daylight hours inside all contribute to a situation where a significant portion of the population is running low on a vitamin that affects an enormous range of biological functions.

What vitamin D actually does goes well beyond the bone health association most people have for it. It functions more like a hormone than a typical vitamin — it’s involved in immune regulation, muscle function, mood, sleep quality, and the expression of hundreds of genes across multiple tissue types. The bone connection is real and important — vitamin D is essential for calcium absorption, and without it bones gradually lose density — but it’s a narrow way to think about a nutrient with this much reach.

Getting vitamin D from food alone is genuinely difficult. Very few foods contain meaningful amounts naturally — fatty fish, egg yolks, and beef liver are the main ones, and fortified foods like milk and some cereals add modest amounts. Sun exposure is the primary source for most people historically, and for those with limited sun exposure, supplementation is often the most practical path to adequate status. Vitamin D3 is the preferred supplemental form — it raises blood levels more effectively than D2.

It’s also one of the vitamins worth actually testing for. Unlike most micronutrients where dietary variety is a reasonable proxy for adequacy, vitamin D status varies enough between individuals — based on skin tone, geography, sun habits, and body composition — that knowing your actual blood level is more useful than guessing.

Reference Card

Vitamin type: Fat-soluble Pillar: Nourish

What it does for you

Essential for calcium absorption and bone density
Supports immune function and regulation
Affects mood and has a well-established association with depression when chronically low
Supports muscle function
Involved in sleep quality and hundreds of gene expression processes

Where to get it

Sun exposure — the primary source for most people historically
Food — salmon, mackerel, sardines, egg yolks, beef liver, fortified milk and cereals
Supplementation — often necessary for people with limited sun exposure; D3 is the preferred form

Considerations

Fat-soluble — stored in the body; take supplements with a meal containing fat for best absorption
Deficiency is widespread — particularly in people who spend most of their time indoors, live at higher latitudes, or have darker skin tones which require more sun exposure to produce equivalent vitamin D
Blood testing is worth doing — vitamin D status varies enough between individuals that guessing isn’t reliable
Vitamin D works closely with vitamin K2 — K2 helps direct calcium to bones rather than soft tissue, which matters when supplementing D at higher doses

Signs your intake might be low

Fatigue that doesn’t resolve with adequate sleep
Low mood, particularly in winter months
Muscle weakness or achiness
Getting sick frequently

Common myths

You can get enough vitamin D from food alone — the food supply provides very limited vitamin D naturally; sun exposure and often supplementation are necessary for most people
Sunscreen completely blocks vitamin D production — high SPF sunscreen significantly reduces vitamin D synthesis, but incidental daily sun exposure on skin still contributes meaningfully for most people
Vitamin D supplements are unnecessary if you live somewhere sunny — skin tone, time outdoors, and how much skin is exposed all affect synthesis significantly; living somewhere sunny doesn’t guarantee adequate status