Obese individuals have vitamin D levels 3-8 ng/mL lower than non-obese

Claude AI - Oct 2026

Typically about 15–30% lower, which works out to roughly 3–8 ng/mL (8–20 nmol/L) in most adult populations. Vitamin D deficiency is about a third more common in the obese. The gap gets larger when people take supplements or get sun, and smaller when everyone is low.

Source	Comparison	Difference	≈ ng/mL	Ref
Ekwaru 2014, 17,614 adults, many supplementing	Obese vs normal BMI	19.8 nmol/L lower	≈ 7.9	Ekwaru
Ekwaru 2014	Overweight vs normal	8.0 nmol/L lower	≈ 3.2	Ekwaru
NHANES, users of >800 IU/day	Obese vs normal	15.5 nmol/L lower	≈ 6.2	NHANES supp
Camargo Cohort, Spain	Obese 20.9 vs normal 24.4 ng/mL	~14% lower	≈ 3.5	Camargo
Walsh 2016, UK, fall/spring	Overweight/obese ~41–45 vs normal 59 nmol/L	~23–30% lower; none in winter	≈ 5–7	Walsh
NHANES, fully adjusted	Obese vs normal	3.4 nmol/L lower	≈ 1.4	NHANES 2013-14
Vimaleswaran 2013, 42,024 adults	Per +1 BMI unit	1.15% lower	BMI 35 vs 22 ≈ 14% lower	Vimaleswaran

What changes the size of the gap

Deficiency prevalence: Deficiency was 35% more prevalent in obese people than in normal-weight people, and 24% more prevalent than in the overweight. The association held regardless of age, latitude, or the cutoff used. [Pereira-Santos 2015]
Severe obesity (BMI >40): Across 96 studies the mean was 18.65 ng/mL. About 59% were below 20 ng/mL. [Severe obesity meta]
More input widens the gap: After the same whole-body UVB exposure, the rise in blood vitamin D3 was 57% smaller in obese subjects. Their skin's capacity to make it was normal. [Wortsman 2000] This explains why the supplement-user cohorts show the largest differences. Conversely, the UK study found no difference in winter, when nobody was making much.
Direction of causation: The Mendelian randomization analysis indicates that higher BMI lowers 25(OH)D, while low vitamin D has little effect on BMI. [Vimaleswaran 2013]
Mechanism: A hyperbolic model based on total body weight removed all of the obesity-related variation in 25(OH)D. This supports dilution into a larger body volume rather than sequestration in fat. [Drincic 2012]
Dosing implication: Ekwaru recommends 2–3× the dose for obese people and 1.5× for overweight people. [Ekwaru 2014] In obese adults, about 2.5 IU/kg/day raised 25(OH)D by 1 ng/mL. [Drincic 2013]
Weight loss helps only modestly: In RCTs, losing weight raised 25(OH)D by only about 3.1 nmol/L (≈1.2 ng/mL) more than maintaining weight. [Mallard 2016]
Caveat: The UK group also found lower free 25(OH)D and 1,25(OH)₂D in the obese. They suggest a larger distribution pool is the cause, and that low 25(OH)D may not mean worse bone health in the obese. [Walsh 2016]

Rule of thumb: expect about 1.15% lower 25(OH)D per BMI unit. That is about 15% lower at BMI 35 and about 20% lower at BMI 40, compared with BMI 22, at the same intake. With supplementation, the absolute gap can roughly double.

References

Obese individuals have vitamin D levels 3-8 ng/mL lower than non-obese

Claude AI - Oct 2026

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