Vitamin D and the "low energy" diagnosis

Vitamin D is the supplement most likely to be recommended by your primary care doctor and most likely to be over-claimed by your wellness influencer. Both have a point.

What vitamin D actually does

Vitamin D's primary, undisputed role is regulating calcium absorption and bone health. Severe deficiency causes rickets in children and osteomalacia in adults. That's the mechanistic core.

Beyond that, vitamin D receptors are present in nearly every tissue in the body, and observational research links low vitamin D to a long list of conditions: depression, frequent illness, fatigue, autoimmune disease. The trouble is most of those associations don't survive randomized controlled trials. Low D may be a marker of poor health rather than a cause.

Where the evidence is strongest

• Bone density and fracture risk in deficient elderly populations: supplementation helps.
• Falls in elderly: supplementation reduces fall risk in deficient people.
• Severe respiratory infections in deficient populations: supplementation modestly reduces incidence.
• Multiple sclerosis risk: strong observational link; trials suggest a real effect.

Where the evidence is weak

• Cancer prevention in non-deficient adults: most major trials (VITAL, finERR) show no effect.
• Cardiovascular events: most major trials show no effect.
• Cognitive decline: mixed.
• "Energy" in non-deficient people: trials don't show a benefit.

Are you deficient?

The blood test is 25-hydroxyvitamin D, expressed in ng/mL or nmol/L. Reference ranges:

• <20 ng/mL: deficient
• 20–30: insufficient
• 30–50: sufficient
• >50: high
• >100: potentially toxic (rare without supplementation)

If you live above 35° latitude (roughly: north of Atlanta in the US), spend most of your time indoors, have darker skin, or are over 65, you're statistically more likely to be deficient. A blood test is cheap (~$50 cash, or covered by insurance with the right ICD codes).

The dose question

If you're deficient, typical replacement protocols:

• 50,000 IU once a week for 8 weeks, then 1,000–2,000 IU/day maintenance, OR
• 5,000 IU/day for 8 weeks, then taper

If you're sufficient and not in a risk group, you don't need a supplement. A multivitamin (1,000 IU) plus 15 minutes of midday sun a few times a week is usually enough.

If you're in the gray zone (insufficient but not deficient), 1,000–2,000 IU/day is reasonable and very low-risk.

The K2 question

Some advocates argue vitamin D should always be paired with vitamin K2 (to direct calcium to bones rather than arteries). The evidence for this in humans is suggestive but not strong. If you supplement D long-term, a small amount of K2 (90–180 mcg/day) is reasonable and low-risk, with one exception: warfarin or other vitamin K antagonist users should not supplement K2 without medical supervision.

The "I take D and feel more energy" effect

This is real for deficient people. For non-deficient people, it's mostly placebo. The most honest framing: get tested, treat deficiency if present, don't expect a dramatic effect if you weren't actually low.

What it costs to ignore

Severe vitamin D deficiency is bad. It's also easy to fix — daily supplementation is inexpensive and well-tolerated. The downside of testing and treating is small. The downside of ignoring it is meaningful for genuinely deficient people.

Get tested once. Treat what you find. Don't expect miracles if you weren't low.