How Long Does It Take To Restore Vitamin D Levels

Understanding vitamin D levels

Vitamin D is a fat-soluble vitamin that regulates calcium homeostasis and bone metabolism. These nutrients are needed to keep bones, teeth and muscles healthy. The majority of people should be able to produce all of their own vitamin D from the sunshine on their skin during the summer. 10 micrograms of vitamin D per day are required for adults and children starting at age one. This includes those who are at risk for vitamin D insufficiency as well as pregnant and nursing mothers. 8.5 to 10 micrograms of vitamin D per day are necessary for infants up to 1-year-old.

Sources of vitamin D

Vitamin D2 and D3 are the main forms of vitamin D. Vitamin D2 is present in plants and yeast and is synthesised by the skin, while D3 comes from animal sources. Sources of dietary vitamin D include

  • oily fish – such as salmon, sardines, herring and mackerel, red meat, liver, and egg yolks.
  • fortified foods – such as some fat spreads and breakfast cereals. 

Another source of vitamin D is dietary supplements. Both vitamin D2 and D3 are converted to an intermediate form of the vitamin in the liver and are then converted to the most active form of vitamin D in the kidney. 

Role of vitamin D in our body

The active form of vitamin D aids increased absorption of calcium from the intestine and decreases the excretion of phosphate and calcium. Vitamin K aids in calcium integration into bones. The majority of vitamin D about 50- 90% of vitamin D is absorbed from the skin via the sunlight while the rest is absorbed from the diet. Both phosphate and calcium are important not only for building bone but also for maintaining neuromuscular function. Furthermore, vitamin D is important for the immune system’s function to fight infection and prevent autoimmunity (when the immune system attacks the body’s own tissue e.g. alopecia areata). 

What is vitamin D deficiency?

There are several causes of vitamin D deficiency. One of the causes is decreased absorption and/or consumption of dietary vitamin D. This could stem from certain diseases and/or syndromes such as 

  • cystic fibrosis
  • chronic pancreatic insufficiency
  • inflammatory bowel disease
  • gastric bypass
  • short bowel syndrome 
  • celiac disease that causes malabsorption of vitamin D

Another cause of vitamin D deficiency is decreased sun exposure. 

Decreased endogenous synthesis of the active form of vitamin D which could be due to chronic liver diseases such as cirrhosis, renal failure, or hyperparathyroidism. Hyperparathyroidism is abnormally high levels of parathyroid hormone in the blood and this hormone and vitamin D work in a feedback cycle. Some medications such as rifampin, phenobarbital, nifedipine, clotrimazole, spironolactone, and dexamethasone can activate the degradation of vitamin D.  

Many vitamin D-deficient patients have no symptoms. However, even a minor chronic vitamin D deficiency can result in hyperparathyroidism and chronic hypocalcemia, which increase the risk of osteoporosis, fractures, and falls, particularly in the older population.

Chronic and/or severe vitamin D deficiency causes a decrease in intestinal calcium and phosphorus absorption, resulting in increased calcium and secondary hyperparathyroidism. This secondary hyperparathyroidism causes phosphaturia and hastens bone demineralization. This can lead to osteomalacia (problem with bones not hardening) and osteoporosis (weakening of the bone) in adults, and rickets (same as osteomalacia in adults just a different name) in children.

How does it feel when you have a low vitamin D level?

Vitamin D deficiency is tested by measuring the amount of intermediate vitamin d (25-hydroxyvitamin D) levels in the blood. Vitamin deficiency is split into mild (25-hydroxyvitamin D less than 20 ng/mL), moderate (25-hydroxyvitamin D less than 10 ng/mL) and severe (25-hydroxyvitamin D less than 5 ng/mL).

The vast majority of vitamin D deficiency patients are asymptomatic. However, even mild chronic vitamin D deficiency can cause chronic hypocalcemia and hyperparathyroidism, increasing the risk of osteoporosis, falls, and fractures, particularly in the elderly. Patients with severe vitamin D deficiency may experience secondary hyperparathyroidism symptoms such as bone pain, arthralgias, myalgias, fatigue, muscle twitching (fasciculations), and weakness. Fragility fractures can occur as a result of chronic vitamin D deficiency, which can lead to osteoporosis. Symptoms of vitamin D deficiency in children include irritability, lethargy, developmental delay, bone changes, and fractures.

How long does it take to restore vitamin D levels?

During the summers people can generally get all the vitamin D required through sunlight on the skin and by eating a balanced healthy diet. However, during autumn and winter, the sunlight might not be strong enough for the skin to synthesise vitamin D. The amount of vitamin D absorbed through supplements would depend on the individual, the condition, weight and the amount of vitamin D taken in addition to what the individual baseline is. However, serum vitamin D levels are usually checked again after 6 – 12 weeks on the supplement. 

How to increase your vitamin D level?

To avoid vitamin D deficiency, spend at least 20 minutes per day in direct sunlight with at least 40% of your skin exposed. Vitamin D synthesis in the skin decreases with age. Dark-skinned people produce less cutaneous vitamin D. Therefore, it is difficult to maintain the required amount of vitamin D through diet alone and everyone should consider taking a daily supplement of 10 micrograms of vitamin D supplement during autumn and specifically winter. Certain populations and groups of people are recommended to take vitamin D supplements all year round due to the increased risk of vitamin deficiency. This includes breastfed babies, all children aged 1 to 4 years, and people who are not often exposed to the sun i.e. frail and people in institutions. 

Risk factors

Major risk factors for vitamin D deficiency include

  • insufficient sunlight exposure 
  • insufficient dietary intake of vitamin D-containing foods
  • malabsorption syndromes like Crohn's disease and celiac disease. 

Although clinically obvious manifestations of vitamin D deficiency, such as rickets in children or bone pain in adults, are uncommon in most developed countries. Recent research suggests that subclinical, asymptomatic vitamin D deficiency continues to play a significant role in several leading causes of death, including cardiovascular disease, cancer, and diabetes. Furthermore dark skin for people from African, South Asian or African Caribbean backgrounds may not make enough vitamin D in their bodies. 

When to see a doctor

Anyone experiencing vitamin D deficiency symptoms should consult a doctor. Many people have no symptoms, but a simple blood test can determine whether they have adequate levels. In both outpatient and inpatient settings, vitamin D deficiency is frequently overlooked. Symptoms of vitamin D deficiency are generally nonspecific and may include: Back or joint pain or muscle weakness, hair loss, headache, fatigue, low mood, depression, and sleep disturbances.


Vitamin D is important for maintaining bone strength, absorbing calcium, nerve function, anti-inflammation, and helping fight infection. A balanced lifestyle with adequate sun exposure and a healthy diet would prevent vitamin D deficiency in most people. However, people living in colder regions, or with limited exposure would benefit from vitamin D supplements. 


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  3. Aranow C. Vitamin D and the immune system. J Investig Med. 2011 Aug;59(6):881-6. doi: 10.2310/JIM.0b013e31821b8755. PMID: 21527855; PMCID: PMC3166406.
  4. Khan QJ, Fabian CJ. How I treat vitamin d deficiency. J Oncol Pract. 2010 Mar;6(2):97-101. doi: 10.1200/JOP.091087. PMID: 20592785; PMCID: PMC2835491.
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Misha Siddiqui

Master's degree, Integrated immunology, University of Oxford, England

2nd year PhD candidate at institute of cancer research and AstraZeneca applying deep learning to understanding immunometabolism using multi-omics. I have a masters in integrated immunology from the university of oxford and undergraduate in applied medical sciences from UCL.

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