Why vitamin D matters
Vitamin D helps your body absorb calcium and phosphate, minerals essential for keeping bones, teeth and muscles healthy. Without enough vitamin D, bones can become soft and weak — leading to rickets in children and a condition called osteomalacia in adults. Vitamin D also plays a role in muscle function and the immune system.
Unlike most vitamins, the main source of vitamin D is not food but sunlight: your skin makes it when exposed to sunshine. This is why deficiency is so common in places with limited sun, during winter, and among people who spend most of their time indoors.
Symptoms of deficiency
Many people with low vitamin D have no obvious symptoms, especially when the deficiency is mild. When symptoms do occur, they can be vague and easily attributed to other things. They may include:
- •Tiredness and fatigue
- •Bone pain or aches
- •Muscle weakness, aches or cramps
- •Low mood
- •Frequent infections
- •In children, delayed growth or bone deformities (rickets)
Who is at risk?
Some people are much more likely to become deficient. Risk factors include:
- •Living in regions with little sunlight, or spending most time indoors
- •Having darker skin, which makes less vitamin D from the same sun exposure
- •Always covering the skin or using sunscreen consistently
- •Older age, as skin makes vitamin D less efficiently
- •Being overweight or having obesity
- •Certain medical conditions affecting fat absorption
- •Exclusively breastfed infants, who may need supplements
Treatment and prevention
Vitamin D deficiency is diagnosed with a simple blood test and is usually straightforward to treat. Treatment typically involves vitamin D supplements, sometimes at higher doses initially to restore levels, followed by a maintenance dose. Your doctor can advise on the right amount based on your blood test.
To maintain healthy levels, safe sun exposure, vitamin D-rich foods (such as oily fish, egg yolks and fortified foods) and, where needed, a daily supplement all help. In many countries, health authorities recommend that most people consider a vitamin D supplement, particularly in autumn and winter. Because excessive supplementation can be harmful, follow recommended doses or your doctor's advice rather than taking very high amounts.
Sunlight, supplements and safe levels
Because sunlight is the body's main natural source of vitamin D, understanding how to balance safe sun exposure with skin protection is useful. When skin is exposed to sunlight, it produces vitamin D, but the amount made depends on factors like the time of year, time of day, latitude, skin tone and how much skin is exposed. In many regions, the sun is not strong enough in autumn and winter for the skin to make adequate vitamin D, which is a major reason deficiency is so common.
This creates a balance, because while some sun exposure supports vitamin D, too much increases the risk of skin damage and skin cancer. Short periods of sun exposure are generally enough for vitamin D, and there is no need to risk burning. For many people, particularly in low-sun seasons or for those with limited sun exposure, a supplement is a reliable way to maintain healthy levels without relying on the sun.
When taking supplements, it is important to stick to recommended doses. While deficiency is common, taking very high doses over long periods can cause harm, including a build-up of calcium that can affect the heart and kidneys. Standard preventive doses are safe for most people, but if in doubt — especially before taking high doses — it is best to seek advice from a doctor or pharmacist.
Vitamin D, bones and overall health
Vitamin D's best-established role is in bone health, where it works together with calcium. It helps the body absorb calcium from food and maintain the right levels in the blood, which is essential for building and maintaining strong bones. Severe, prolonged deficiency causes bones to become soft and weak — leading to rickets in children, where bones can become deformed, and osteomalacia in adults, causing bone pain and muscle weakness.
Beyond bones, vitamin D supports muscle function, which is why deficiency can contribute to muscle weakness and, in older adults, an increased risk of falls. It also plays a role in the immune system. Research has explored possible links between vitamin D and a range of other health outcomes, though the evidence in many of these areas is still developing and less certain than its established role in bone and muscle health.
The practical takeaway is that maintaining adequate vitamin D is a simple, worthwhile part of looking after your health, particularly for bones and muscles. For most people this means a combination of sensible sun exposure, vitamin D-rich foods, and a supplement where needed, especially during the darker months or for those at higher risk of deficiency.