Many people who spend time outdoors or take vitamin D supplements still find their blood tests show low levels. One hidden reason can be body composition: vitamin D is fat soluble, which means it dissolves in and is stored by body fat rather than staying freely circulating in the blood. That storage behaviour can make vitamin D less available to the organs that need it.
The fat-soluble nature of vitamin D
When we say vitamin D is fat soluble, we mean it mixes with fat and is kept in fatty tissues and the liver rather than being flushed out quickly in urine. This contrasts with water soluble vitamins, like vitamin C, which the body removes more readily. A brief note on terminology: adipose tissue simply means body fat, and BMI (Body Mass Index) is a common way to describe weight relative to height.
Skin exposed to ultraviolet B (UVB) radiation from the sun makes vitamin D in the form of vitamin D3. Some vitamin D also comes from food and supplements. Regardless of the source, a proportion of that vitamin D moves into fat cells. For people with more adipose tissue, that storage capacity is larger, so a greater share of vitamin D can be held in fat and appear lower on routine blood tests even though the vitamin exists in the body.
Why higher body fat can lower blood vitamin D
Research consistently shows an inverse relationship between BMI and circulating vitamin D concentrations: as BMI increases, measured blood levels often fall. This is not necessarily because someone with a higher BMI makes less vitamin D in the skin, nor because they eat less, but because distribution changes — more vitamin D is sequestered, or effectively hidden, in fat.
That sequestration means the vitamin is less available for tasks such as supporting bone health, which is one reason clinicians monitor blood concentrations. It also helps explain why two people with similar sun exposure and supplement habits can have different blood results. The amount of vitamin D you need to reach a particular blood level depends on location, season, skin tone, body composition and other individual factors.
Why supplements and sun aren’t a simple fix
Because adipose tissue draws vitamin D out of circulation, standard supplement doses that work for someone with lower BMI may be less effective for a person with higher BMI. Oral vitamin D is absorbed through the gut into the bloodstream and then can be taken up by fat cells in the same way as vitamin D made in the skin.
That said, increasing supplement doses is not a decision to take lightly. Unlike vitamin D made in the skin — where production tails off naturally when enough has been produced — supplements can raise blood levels beyond safe limits if taken inappropriately. Vitamin D toxicity (too much) is uncommon but can cause symptoms and needs medical oversight. For a deeper look at balancing sunlight and supplements, see our article on Sunlight vs Supplements, and for guidance on target ranges, read about optimal vitamin D levels.
Short, regular sun exposures are still a practical way to help maintain levels, but sunscreen and sun protection remain essential. UV cannot be seen or felt and is not the same as heat, so you shouldn’t judge UV risk by temperature. In Australia, combined sun protection is recommended when the UV Index is 3 or above: protective clothing, a broad-brimmed or legionnaire-style hat, Australian-standard sunglasses, shade and a broad-spectrum water-resistant SPF50+ sunscreen. Apply sunscreen generously about 20 minutes before going outdoors and reapply every two hours or sooner after swimming, sweating or towel-drying.
Practical steps to manage your vitamin D and skin safety
Start with a measured approach. A simple blood test ordered by your doctor will show your circulating vitamin D concentration and provide a baseline for safe action. If levels are low, discuss tailored strategies with your clinician. They can recommend an evidence-based supplement dose that accounts for your body weight and monitor levels over time rather than suggesting high doses without follow-up.
If you are working on weight management, know that losing fat can gradually release stored vitamin D back into the circulation, and many people show rises in blood levels after sustained weight loss. This is one potential benefit of a broader healthy-lifestyle approach, but individual responses vary and change occurs over weeks to months.
Be careful about at-home treatments for suspicious skin spots. Any new, changing, bleeding or non-healing lesion needs professional assessment; a biopsy or clinical evaluation should come before destructive cosmetic procedures on an uncertain pigmented lesion. Regular professional skin checks are especially important for Australians over 40, because early detection is central to good outcomes. For practical sun-safe ways to gain vitamin D from brief exposure in our climate, read about the six-minute summer walk approach and remember that individual needs differ by season and skin type.
Take Action for Your Skin Health
If you’re concerned about low vitamin D or sun-damaged skin, take a sensible, evidence-based next step: book a professional skin assessment to check any worrying spots and discuss a personalised plan for testing and supplementation. Our clinic offers comprehensive skin cancer checks and evidence-based treatments for sun damage such as photodynamic therapy. If you’d like to arrange a consultation to review your vitamin D results, discuss safe sun strategies and have your skin examined, you can book an appointment.
