Vitamin D deficiency and bone density: what low levels mean for osteoporosis risk
- Jul 27
- 7 min read
Updated: Aug 8
How Vitamin D deficiency impacts bone density and osteoporosis risk, and why maintaining Vitamin D levels is crucial for bone health.
In Brief
Vitamin D and bone density are connected through calcium. Persistently low vitamin D levels reduce how much calcium the body can absorb. This article follows that pathway, from gut to parathyroid glands to bone, explains what a deficient or insufficient result means, and covers when testing has a role in osteoporosis care.
This article explains:
In clinic, I often see clients who have been told their vitamin D is deficient or insufficient, with little explanation of what that means for their bones.
When vitamin D is low for long enough, the body starts drawing calcium out of bone to keep blood calcium steady. That process is what affects bone density.
What a low vitamin D level does to calcium absorption
Vitamin D is converted in the liver into 25-hydroxyvitamin D, a storage form, then in the kidney into calcitriol, its active form. Calcitriol acts on the gut to increase how much dietary calcium is absorbed into the bloodstream. The NIH Office of Dietary Supplements describes this as one of vitamin D's core roles: promoting calcium absorption and maintaining the calcium and phosphate levels bone needs.
When vitamin D is deficient, less of the calcium reaching the gut is absorbed. Blood calcium begins to fall before bone is affected at all.
Why the body draws calcium from bone
Blood calcium is kept within a narrow range because nerves, muscles and the heart depend on it. When absorption falls and blood calcium starts to drop, the parathyroid glands respond by releasing more parathyroid hormone.
The Society for Endocrinology describes this response, known as secondary hyperparathyroidism, plainly: in vitamin D deficiency, less calcium from food is absorbed in the gut, so blood calcium levels are low, and the parathyroid glands secrete more parathyroid hormone to bring calcium levels back up to normal.
Parathyroid hormone raises blood calcium partly by acting on bone, triggering the release of stored calcium into the bloodstream. This is where breakdown outpaces rebuilding: bone is broken down faster to supply calcium the diet is not delivering, and while parathyroid hormone stays elevated, that process keeps running.
What this means for bone density and osteoporosis risk
A single instance of this response has little consequence. The concern is what happens when vitamin D deficiency, and the raised parathyroid hormone that comes with it, continues for months or years.
The NIH Office of Dietary Supplements notes that insufficient vitamin D is associated with secondary hyperparathyroidism, raised parathyroid hormone, and bone resorption, and that sustained secondary hyperparathyroidism raises the rate of bone turnover. Left untreated, secondary hyperparathyroidism can impair mineralisation and reduce bone mass. The Society for Endocrinology puts the bone-specific consequence directly: untreated secondary hyperparathyroidism can increase the risk of osteoporosis.
Vitamin D deficiency counts as one of several risk factors for osteoporosis. Bone density also depends on calcium intake, hormonal status and physical activity. A persistently low vitamin D level shifts the remodelling balance toward breakdown by keeping parathyroid hormone elevated, adding to that combined risk.
Deficient, insufficient or sufficient: what your test result means
A vitamin D blood test measures 25-hydroxyvitamin D, the storage form of the vitamin, in nanomoles per litre (nmol/L). Lab Tests Online UK, run by the Association for Laboratory Medicine, sets out the thresholds used in UK guidance from the National Osteoporosis Society, the Royal Osteoporosis Society's earlier name:
Below 25 nmol/L: deficient
25 to 50 nmol/L: may be inadequate for some people
Above 50 nmol/L: sufficient for almost the whole population
A result below 25 nmol/L is usually treated regardless of symptoms. A result in the 25 to 50 nmol/L range is more individual: treatment is more likely if there is a fragility fracture, a diagnosis of osteoporosis, treatment with an antiresorptive medicine, a raised parathyroid hormone level, or symptoms that suggest deficiency.
These are the thresholds used in UK guidance. The NIH Office of Dietary Supplements sets the equivalent US figures slightly differently: below 30 nmol/L (12 ng/mL) is classed as too low, and 50 nmol/L (20 ng/mL) or above is considered adequate for most people, a slightly different scale from the UK figures above. If your result came from outside the UK, check which threshold it is being measured against.
When vitamin D testing has a role in osteoporosis care
For the general population, without an osteoporosis or osteopenia diagnosis, routine vitamin D testing is not usually advised. The Royal Osteoporosis Society's general guidance is that anyone unsure whether they are getting enough vitamin D from sunlight should start a standard supplement as the first step.
In the UK, sunlight cannot produce vitamin D for much of the year, so this guidance is latitude and season dependent: someone in a consistently sunny, low-latitude climate may get enough from sun exposure alone, while someone at UK latitude typically needs a supplement through autumn and winter. Readers outside the UK should check their own national guidance.
Osteoporosis care is treated differently. Lab Tests Online UK lists osteoporosis and fragility fracture among the specific reasons a GP may request a vitamin D test, and the Royal Osteoporosis Society confirms testing happens automatically before two specific medicines: if you are starting zoledronate or denosumab, vitamin D is tested first, and any deficiency is treated before the medicine begins.
The Endocrine Society's 2024 guideline against routine population testing draws the same line: it is written for people who are otherwise healthy, and its lead author has confirmed it does not extend to people with an established osteoporosis diagnosis.
Clinical guidance also supports testing where there is a raised parathyroid hormone result, a fragility fracture, or symptoms such as widespread bone or muscle pain, whatever the diagnosis.
As a nutritional therapist, I take a testing-led approach with every client who has an osteoporosis or osteopenia diagnosis. I check vitamin D levels as part of a nutritional review. A confirmed result lets dosing be set to the individual, and separates a straightforward case of low sunlight exposure from other causes worth investigating, such as malabsorption or a medicine interaction.
Where to start if your result is low
Treatment for a confirmed deficiency is GP-directed: usually a higher-dose course of vitamin D3, followed by a standard maintenance dose once levels recover.
Retesting: a follow-up test after treatment confirms the level has moved into the sufficient range. As part of a nutritional review, I recommend retesting on a set schedule that matches your treatment.
Calcium alongside vitamin D: calcium intake needs its own check alongside vitamin D. Our article on vitamin D and calcium for osteoporosis covers why the two are assessed together.
Underlying causes: a low result that keeps recurring after treatment can point to a malabsorption issue or a medicine that interferes with vitamin D. Raise this with your GP if it happens more than once.
Working out what is right for you
If you have an osteoporosis or osteopenia diagnosis, I recommend testing vitamin D as part of a nutritional review. A review looks at your vitamin D and calcium status together with your diet, medication and other risk factors, and sets out what your own results mean specifically. The figures in this article are general population guidance.
Frequently asked questions
Does vitamin D deficiency cause osteoporosis?
Vitamin D deficiency is one of several risk factors for osteoporosis. Sustained deficiency raises parathyroid hormone, which speeds up how quickly bone is broken down to release calcium. That adds to osteoporosis risk alongside other factors such as calcium intake, hormonal status and physical activity.
What is secondary hyperparathyroidism?
It is the parathyroid glands releasing more parathyroid hormone in response to low blood calcium, most commonly caused by vitamin D deficiency. The extra hormone raises blood calcium partly by drawing calcium out of bone, which is why sustained vitamin D deficiency affects bone density.
What vitamin D level counts as deficient?
Under UK guidance from the National Osteoporosis Society, a result below 25 nmol/L is deficient, 25 to 50 nmol/L may be inadequate for some people, and above 50 nmol/L is sufficient for almost the whole population.
Does a DEXA scan show my vitamin D level?
No. A DEXA scan measures bone mineral density. The two are connected through the mechanism this article describes, but checking your vitamin D status needs its own blood test.
Do I need a vitamin D blood test if I have osteoporosis?
As a nutritional therapist, I recommend testing vitamin D as part of a nutritional review for anyone with an osteoporosis or osteopenia diagnosis. Testing also has a defined clinical role beyond that: before starting zoledronate or denosumab, and where there is a fragility fracture, a raised parathyroid hormone result, or symptoms suggesting deficiency.
How is vitamin D deficiency treated?
Once a blood test confirms deficiency, treatment usually starts with a higher-dose course of vitamin D3 prescribed by a GP, followed by a standard maintenance dose. A follow-up test confirms the level has recovered.
Structured guidance for bone health
If you’re looking to build a clearer understanding of how to support your bone health, the Nutrition for Bone Health Guide explains it in a structured and practical way.
If you would prefer to explore how this applies to your own situation, one-to-one support with Laura provides personalised guidance alongside your medical care.
Disclaimer
The information in this article is for general educational purposes. It is not intended to diagnose, treat, or replace medical advice. Bone health is influenced by many factors, and individual circumstances vary.
If you have been diagnosed with osteopenia or osteoporosis, or are taking medication that affects bone health, continue to work with your GP, consultant, or specialist team. Nutritional therapy is intended to support, not replace, medical care.
For personalised guidance, consult a registered nutritional therapist or other qualified health professional who can assess your full clinical picture.
References
National Institutes of Health, Office of Dietary Supplements. Vitamin D: fact sheet for health professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
National Institutes of Health, Office of Dietary Supplements. Vitamin D: fact sheet for consumers. Updated November 2022. https://ods.od.nih.gov/factsheets/VitaminD-Consumer/
Society for Endocrinology. Secondary hyperparathyroidism. You and Your Hormones. Reviewed April 2023. https://www.yourhormones.info/endocrine-conditions/secondary-hyperparathyroidism/
Association for Laboratory Medicine. Vitamin D. Lab Tests Online UK. https://labtestsonline.org.uk/tests/vitamin-d
Royal Osteoporosis Society. Vitamin D. Reviewed July 2025. https://theros.org.uk/information-and-support/food-and-supplements/vitamin-d/
NHS. Vitamin D. Vitamins and minerals. Reviewed August 2020. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
Pittas AG, quoted in: Pocius DM. Endocrine Society releases new guidelines advising physicians to not screen for vitamin D. Dark Daily, 24 June 2024. https://www.darkdaily.com/2024/06/24/endocrine-society-releases-new-guidelines-advising-physicians-to-not-screen-for-vitamin-d-which-could-affect-test-referrals-to-clinical-laboratories/
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