Vitamin D: Test First, Then Dose

Vitamin D may be the most common supplement people take without knowing whether they need it or how much. Two people reach for the same 2,000 IU bottle. One started at a level of 18 and will likely still fall short of where we want it. The other started at 75 and did not need it. We test first, because the dose should come from the number.

The test and the target

The blood test is 25 hydroxy vitamin D. It is the storage form, and it reflects your sun exposure and intake over the previous several weeks. Most labs call anything above 30 ng/mL normal. Our target for the people we treat is 60 to 80 ng/mL.

Vitamin D behaves more like a hormone than a vitamin. Its receptors sit in bone, muscle, and immune cells. At low levels the gut absorbs less calcium, so the body borrows calcium from bone to keep the blood level steady. Low levels are also linked to muscle weakness and more frequent respiratory infections, though those are associations rather than proof of cause.

San Antonio has plenty of sun, and people here still test low all the time. Most of us work indoors, and skin makes less vitamin D with age and with darker pigment.

Dosing by level

A rough rule for an average adult is that each 1,000 IU a day of vitamin D3 raises the blood level by roughly 5 to 10 ng/mL once it settles, less the higher you start. So a person starting at 25 who wants to reach 65 may need 4,000 IU a day or more. A person starting at 50 may need 1,000 to 2,000. Body fat changes the math, because vitamin D is stored in fat tissue. People with higher body fat usually need more to reach the same level.

A few details matter:

• Take it with a meal that contains some fat. Absorption is noticeably better than on an empty stomach.
• D3 raises the blood level more reliably than D2 at the same dose.
• Give it time. The level plateaus after roughly 2 to 3 months on a steady dose, which is why we retest at 3 months and adjust, rather than guessing twice.

Toxicity

Vitamin D toxicity is real and rare. It shows up as high blood calcium, with nausea, thirst, and kidney stress, and it essentially only happens at very high doses, tens of thousands of IU a day, taken for months. Blood levels above 100 to 150 ng/mL are where concern starts. That is very unlikely on a sensible dose with a retest on the calendar. The retest is how we keep the level inside the range we want.

That's why we check it before anyone buys a bottle. When a supplement is the right call, we send people to our online dispensary for professional grade products, and clinic members get 35% off every order.

This article is educational and is not medical advice.

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