Vitamin D — sun first, then supplementation, and never without K2.

Vitamin D isn’t really a vitamin. It’s a steroid hormone the body manufactures from cholesterol when UVB radiation hits skin. It has receptors in nearly every tissue in the body — immune cells, muscle, brain, heart, pancreas, breast, prostate, bone. It modulates roughly 2,000 genes. When the body has enough of it, things work. When it doesn’t, the cracks show up everywhere.

Dr. Eric Berg has been arguing for two decades that the mainstream framing of vitamin D is wrong on three counts: the recommended intake is too low, the “normal” lab range is set far below the optimal range, and the supplementation protocol almost never accounts for the cofactors that determine whether the D you take actually does its job or causes a different problem.

Get the framing right and vitamin D is one of the highest- leverage interventions in nutritional medicine. Get it wrong and you can supplement faithfully for years without results — or worse, drive calcium into the wrong places while telling yourself you’re building bone.

Sun first

The body was designed to make vitamin D from sunlight. UVB radiation hits 7-dehydrocholesterol in the skin and converts it to pre-vitamin D3, which the liver and kidneys then activate into the hormone form (calcitriol). This pathway is the primary intended source. Supplementation is the second-best option, designed to compensate for the modern indoor lifestyle and the geographic latitudes where sun-driven synthesis isn’t available year-round.

The practical sun protocol:

• Midday sun, when UVB is at its strongest. Roughly 10 AM to 3 PM, depending on season and latitude. Early morning and late afternoon sun is mostly UVA — tans the skin but doesn’t make D.
• Arms, legs, and torso exposed. The more skin, the more synthesis. A 15-minute walk with arms covered makes very little D.
• No sunscreen during the synthesis window. SPF 15 blocks roughly 99% of UVB. The mainstream "sunscreen always" rule is one of the major drivers of modern D deficiency.
• 10–30 minutes for fair skin. Darker skin needs more — melanin is a natural UV filter, which is protective in equatorial climates but a deficiency driver at higher latitudes.
• Above roughly 35° latitude (most of the US north of Atlanta), sun-driven synthesis is meaningful only from roughly April through October. Below 35° latitude, year-round if you make the time.
• Don’t burn. Build gradually. A tan is the skin’s natural sun-protective response.

Berg’s position, and the position of the alt-health literature broadly, is that thirty minutes of midday sun three or four times a week, when geography allows, is more useful than any supplement bottle. The supplement is for the rest of the year and for people who can’t get the sun.

What vitamin D actually does

• Immune modulation. Vitamin D regulates both arms of the immune system — it activates antimicrobial peptides (cathelicidins) that kill pathogens directly, and it modulates T-cell behavior to prevent autoimmune flare. Low D status is consistently associated with worse outcomes in respiratory infections, autoimmune disease, and cancer.
• Calcium absorption. Without adequate D, only about 10–15% of dietary calcium gets absorbed. With adequate D, that climbs to 30–40%. The bone-density story isn’t really about taking calcium; it’s about having enough D to absorb what you eat.
• Mood and brain. Vitamin D receptors are dense in the hypothalamus, hippocampus, and prefrontal cortex. Low D correlates with depression, seasonal affective disorder, and cognitive decline. Repletion often resolves mood symptoms that didn’t respond to other interventions.
• Hormone production. Vitamin D is itself made from cholesterol, which is also the precursor for testosterone, estrogen, progesterone, and cortisol. Adequate D status is one of the consistent correlates of healthy testosterone in men.
• Insulin sensitivity. D modulates insulin secretion and peripheral sensitivity. Low D is a frequent component of the metabolic-syndrome cluster.
• Cancer protection. The epidemiological link between low vitamin D and several cancers (breast, prostate, colon) is strong enough that the Grassroots Health consortium of researchers recommends 50–80 ng/mL as a cancer-prevention target.
• Heart and blood pressure. D modulates the renin-angiotensin system that controls blood pressure, and low D correlates with cardiovascular disease independent of other risk factors.

The “normal” lab range problem

Standard labs report vitamin D status as 25-hydroxyvitamin D (25(OH)D) in nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L). Mainstream reference ranges flag anything above 30 ng/mL as “sufficient” and anything above 20 as “not deficient.”

Those numbers were set decades ago to prevent rickets — the most extreme visible manifestation of severe deficiency. They have nothing to do with what level optimizes immune function, hormone production, or cancer protection.

Dr. Michael Holick — the world’s most-cited vitamin D researcher, formerly at Boston University Medical Center — has argued for decades that the optimal range is 50–80 ng/mL. Below 50, the body is running on insufficient D for full immune and hormonal function. Above 100, you may be heading into excess territory (rare but possible). The Grassroots Health scientists' panel has converged on a similar range as their cancer-prevention target.

Berg’s position, on his channel and in his clinical writing: aim for 60–100 ng/mL. The “normal” range is the floor below which obvious disease shows up. The optimal range is where the body actually works the way it was designed to.

Get your 25(OH)D tested. Any major lab runs it. If your number is below 50, you have room to improve.

The K2 cofactor — the safety-critical part

This is the single most important thing to get right when supplementing vitamin D, and it’s the thing mainstream medicine almost never mentions.

Vitamin D drives calcium absorption from the gut. Vitamin K2 directs that calcium into bone, where it belongs, and away from soft tissue (arteries, kidneys, joints), where it doesn’t. Without enough K2, high-dose D can actually drive arterial calcification — the opposite of what you wanted.

This isn’t a hypothetical concern. The Rotterdam Study (2004) and several subsequent studies have documented that high vitamin D status with low K2 status correlates with vascular calcification. The fix is straightforward: take them together.

The standard alt-health ratio is roughly 100–180 mcg of K2 (as MK-7, the long-acting form) per 5,000 IU of vitamin D3. Many quality products combine both. If yours doesn’t, take a separate K2 supplement alongside the D.

Two forms of K2 exist: MK-4 (short-acting, four-hour half-life) and MK-7 (long-acting, three-day half-life). For most people, MK-7 is the practical choice — once-daily dosing covers the work. MK-4 requires multiple daily doses to maintain levels.

Magnesium gates the whole system

People who supplement vitamin D faithfully and don’t see their levels rise are usually magnesium-deficient. The enzymes that activate vitamin D in the liver and kidneys are magnesium-dependent. Without enough magnesium, the D you take stays in its inactive storage form and never becomes the hormone the body actually uses.

See the magnesium article in this section for the full treatment. The short version: 400–600 mg of elemental magnesium per day, glycinate form or transdermal, alongside any vitamin D supplementation. They’re a single protocol, not two separate ones.

What deficiency looks like

• Frequent colds, flu, and respiratory infections
• Fatigue, particularly in winter months
• Low mood, seasonal depression
• Bone and joint pain, especially in the spine and hips
• Muscle weakness, especially proximal (thighs, upper arms)
• Slow wound healing
• Hair loss
• Autoimmune flare — thyroid, joints, gut
• Low testosterone in men, irregular cycles in women
• Brain fog and cognitive slowdown
• Insulin resistance and stubborn weight gain

These symptoms overlap with magnesium deficiency, B12 deficiency, and several other common shortfalls. Test the level rather than guess.

Dosing

• Maintenance: 5,000 IU vitamin D3 per day for most adults. Adjust up or down based on lab values and sun exposure.
• Repletion (from a deficient starting level): 10,000 IU per day for 8–12 weeks, then retest. Some clinicians use 50,000 IU once weekly for the repletion phase, which delivers similar total dosing with less daily friction.
• Acute illness: Some protocols call for 50,000 IU for several days at the first sign of respiratory infection. The clinical backing is real, especially with adequate K2 and magnesium support.
• Pair with K2: roughly 100–180 mcg MK-7 per 5,000 IU of D.
• Pair with magnesium: 400–600 mg elemental magnesium per day.
• Take with fat. D is fat-soluble; absorption is much better with a meal that contains fat.

Test your 25(OH)D level at baseline, then again three months in. Aim for 60–100 ng/mL. Adjust dose based on what your blood shows, not what the bottle says.

Forms — D3, not D2

Two forms of vitamin D exist in supplements:

• Vitamin D3 (cholecalciferol) — the form the body makes from sunlight. The form to supplement with. Animal-sourced (typically from lanolin or fish oil), well-absorbed, raises serum levels effectively.
• Vitamin D2 (ergocalciferol) — the plant-derived form, often prescribed by mainstream doctors at 50,000 IU weekly. Less well-absorbed, less effective at raising serum levels, shorter half-life. Skip it.

When you see a product label, make sure it says D3 or cholecalciferol. If it says D2 or ergocalciferol, put it back.

The toxicity question

Mainstream guidance often warns against doses above 4,000 IU per day. That number is the Tolerable Upper Intake Level set by the Institute of Medicine in 2010 — a number set very conservatively to ensure a wide safety margin for the general population, including pregnant women and people with conditions that make D regulation unusual.

Actual vitamin D toxicity (hypercalcemia from excess D) requires sustained intake of 40,000–100,000+ IU per day for months, or serum levels above roughly 150 ng/mL. The doses described in this article (5,000–10,000 IU daily) don’t come close to toxic territory in healthy adults with adequate K2 and magnesium support.

Holick’s clinical data from thousands of patients on 5,000–10,000 IU per day over years is the most comprehensive safety record we have on the higher-dose protocol. Toxicity at those doses is essentially unheard of.

Where I buy vitamin D

• Sports Research Vitamin D3 with K2 — 5,000 IU D3 + 100 mcg K2 (MK-7) per softgel, coconut oil base. My default everyday product. Reasonable price, clean formulation.
• Thorne Vitamin D/K2 Liquid Drops — 500 IU D3 + 200 mcg K2 per drop. Flexible dosing, easy to titrate based on lab values. Higher price per IU, but the most customizable.
• Pure Encapsulations D3 + K2 — 1,000 IU D3 + 50 mcg K2 per capsule. Useful for people who want a lower per-dose option to titrate up.
• NOW Foods Vitamin D-3 5,000 IU — the budget option, D3 only. Take separately with a K2 product if you go this route.

Where to start

From April through October, the sun does most of the work: 20–30 minutes of midday sun on most days, arms and legs exposed, no sunscreen during the synthesis window. A supplement of 2,000–5,000 IU is enough through those months.

From November through March, the sun isn’t strong enough at most temperate latitudes. A common winter dose is 10,000 IU of D3 with K2 daily, taken with the fattiest meal of the day, alongside magnesium glycinate at night.

Testing 25(OH)D twice a year — once in early spring after the low-sun season, once in early fall after the high-sun season — keeps the dose honest. A blood level in the range of 65 to 90 ng/mL is the common target.

Closing

Vitamin D is the rare nutritional intervention where the mainstream is still substantially wrong — on the recommended intake, on the optimal blood range, on the cofactors required to make it work. Berg, Holick, the Vitamin D Council, and the Grassroots Health researchers have been making the same case for decades, and the clinical data keeps catching up to them.

Get sun when you can. Supplement when you can’t, and always with K2 and magnesium. Test your level. Aim for 60–100 ng/mL. The investment is a few dollars a month and a few minutes outside. The return is one of the most-studied hormones in human physiology running at the level it was designed to.