Vitamin D levels during weight loss: what fat tissue does to the number

What the lab measures is 25-hydroxyvitamin D, 25-OH-D for short, in nmol/l. That is the storage form, and the number describes your reserve over recent months rather than what you took in yesterday. Its half-life is two to three weeks, so a few weeks of extra sun or a new supplement only shows up with a delay.

The active form, 1,25-dihydroxyvitamin D, is deliberately not measured here. It clears from your blood within hours and is tightly regulated by parathyroid hormone, the hormone from your parathyroid glands that keeps your calcium steady. When the reserve is empty your body steps up the conversion, so a normal or high active value does not rule out a shortage. When a shortage is suspected, that is the wrong test.

Ask for total 25-OH-D, meaning D2 and D3 together. If you take a plant-based D2 product, mention it, because some immunoassays only partly recover D2.

Where vitamin D sits decides what you measure. It is fat-soluble and distributes into your fat tissue as well, so with a higher body fat percentage less remains in your blood at the same production and the result reads lower. That is no measurement error, but a distribution.

The lab prints a reference range of 50 to 150 nmol/l on your report. The Dutch Health Council uses no range but a floor you must meet all year round: 30 nmol/l until you turn seventy and 50 nmol/l after that. If you sit between 30 and 50, the lab flags you as low while the Health Council calls you sufficient under seventy. Which yardstick counts for you is a conversation with your doctor or coach.

During a weight-loss programme your vitamin D level tells you two things at once, and you need to keep them apart: how much sun your skin caught in recent months, and how much of your reserve sits in your fat tissue rather than in your blood.

The first is the larger effect. Between October and March the sun sits too low over the Netherlands for skin production, so a March result should read lower than a September one, even if nothing else changed. The second effect is smaller and slower. Vitamin D accumulates in fat tissue, and in studies the blood value reads lower on average at a higher body fat percentage. As your fat mass falls, the value can rise modestly at the same production. Do not count on it as a result, though: the season easily drowns that effect out, so a lower number in March says nothing about your programme.

Why the number matters is calcium and bone. Vitamin D lets you absorb calcium and phosphate from your food. When your reserve runs down, you absorb less calcium, parathyroid hormone rises and your body pulls calcium out of your bones. If you structurally eat less during a programme, you also take in less calcium and vitamin D from food; food supplies only a small share of your vitamin D, but with an empty summer reserve every share counts.

The Dutch Health Council uses a floor rather than a laboratory reference: until you turn seventy at least 30 nmol/l, all year round, and from seventy at least 50 nmol/l. Below it the term is deficiency; from seventy the Farmacotherapeutisch Kompas calls anything below 35 nmol/l a severe deficiency. The Health Council names no target above that floor, so higher is no goal to work towards. The lab also prints 50 to 150 nmol/l as its own range, and below 50 the two yardsticks diverge. Read your number against both, and alongside calcium and parathyroid hormone: a low 25-OH-D with a raised PTH weighs more than the vitamin D figure alone.

Measure a baseline before your programme starts, and repeat in the same month of the following year if you want to know what genuinely changed. Two results from different seasons mostly show you the season.

Test too if you have complaints that fit a deficiency, such as heavy thighs, difficulty standing up or bone pain, and if you get outdoors little, have darker skin or wear covering clothing. If you have had a gastric bypass or another bariatric procedure, testing is sensible in any case, because fat absorption from the gut is reduced and with it the absorption of vitamin D; your treatment team decides how often.

You do not need to fast and the time of day does not matter. The result is back within about 72 hours. If you take high-dose biotin, say so at the draw, because it can falsely raise some 25-OH-D assays.