Table of Contents
What Are Vitamins?
Vitamins are a group of organic substances that the body needs for normal cell function, growth, development, and many other biological processes. There are 13 essential vitamins:
- Vitamin A
- Vitamin C
- Vitamin D
- Vitamin E
- Vitamin K
- Vitamin B1 (thiamine)
- Vitamin B2 (riboflavin)
- Vitamin B3 (niacin)
- Vitamin B5 (pantothenic acid)
- Vitamin B6 (pyridoxine)
- Vitamin B7 (biotin)
- Vitamin B9 (folate or folic acid)
- Vitamin B12 (cobalamin)
Each vitamin has different functions in the body. A vitamin deficiency can develop when a person does not obtain enough of a particular vitamin over time or cannot properly absorb or use it.
A varied diet containing vegetables, fruits, whole grains, legumes, protein foods, healthy fats, and other nutrient-rich foods can help provide many of the vitamins the body needs. You can learn more about maintaining a balanced diet in our guide to nutrients in your diet.
Vitamin D
Vitamin D is one of the essential vitamins the body needs to maintain healthy bones and support normal calcium and phosphorus metabolism. It helps the intestines absorb calcium from food, which is important for maintaining bone strength.
Severe vitamin D deficiency can cause rickets in children and osteomalacia in adults, conditions in which bones become softer and weaker.
Vitamin D also has roles in muscle function and immune system regulation. However, although vitamin D is important for overall health, research has not established that taking extra vitamin D prevents or treats every condition associated with low vitamin D levels.
Some Vitamin D Roles in the Body Include:
- Supporting strong bones: Vitamin D helps the body absorb calcium and maintain normal bone mineralization.
- Supporting muscle function: Adequate vitamin D is important for normal muscle function.
- Supporting immune function: Vitamin D contributes to normal immune system activity.
- Supporting calcium and phosphorus balance: These minerals are essential for healthy bones and teeth.
- Supporting overall health: Vitamin D participates in many biological processes throughout the body.
Research has investigated possible links between vitamin D and conditions such as type 1 and type 2 diabetes, hypertension, glucose intolerance, and multiple sclerosis. However, an association between vitamin D levels and a disease does not necessarily mean that vitamin D supplements can prevent or treat that disease.
For information about vitamin D more generally, see our detailed guide to Vitamin D.
Reasons for Vitamin D Deficiency
Vitamin D occurs naturally in relatively few foods, although some foods are fortified with it. The body can also produce vitamin D when exposed to sunlight. Several factors can increase the risk of deficiency.
1. Inadequate Dietary Intake
A person may have a higher risk of deficiency if they do not consume enough vitamin D over time. Natural food sources include fatty fish, egg yolks, and some animal products, while fortified milk and plant-based alternatives can also provide vitamin D.
People following strict vegan diets may need to pay particular attention to fortified foods or supplements because naturally occurring vitamin D is limited in plant foods.
2. Limited Sun Exposure
The body can produce vitamin D when the skin is exposed to ultraviolet B (UVB) radiation from sunlight.
People who spend very little time outdoors may have a higher risk of deficiency. Factors such as season, geographic location, clothing, sunscreen use, skin pigmentation, and lifestyle can affect vitamin D production.
Sun exposure should not be used as a reason to avoid appropriate protection against excessive ultraviolet radiation.
3. Darker Skin
Melanin can reduce the skin’s production of vitamin D in response to UVB exposure. As a result, people with darker skin may require greater sun exposure to produce the same amount of vitamin D as people with lighter skin.
However, vitamin D status depends on multiple factors, and individual blood levels can vary considerably.
4. Reduced Kidney Function
The kidneys play an important role in converting vitamin D into its biologically active form. Certain kidney conditions can therefore affect vitamin D metabolism and increase the risk of deficiency.
5. Digestive Problems
Some medical conditions can interfere with the absorption of vitamin D from food. These include conditions affecting fat absorption, such as Crohn’s disease, celiac disease, and cystic fibrosis.
6. Obesity
People with obesity are more likely to have lower blood concentrations of vitamin D. Vitamin D can become distributed into body fat, which may reduce its concentration in the bloodstream.
A low vitamin D level does not necessarily mean that obesity is caused by vitamin D deficiency. The relationship between vitamin D status and body weight is complex.
Symptoms of Vitamin D Deficiency
Many people with vitamin D deficiency do not experience obvious symptoms, particularly when the deficiency is mild.
When deficiency becomes more significant, possible symptoms and effects may include:
Frequent Illnesses
Vitamin D plays a role in immune function. However, frequent infections are not specific to vitamin D deficiency and can have many other causes.
Fatigue and Muscle Weakness
Low vitamin D levels can be associated with muscle weakness and may contribute to fatigue in people with significant deficiency. However, fatigue has many possible causes, so it should not automatically be attributed to vitamin D deficiency.
Bone and Joint Pain
Severe vitamin D deficiency can contribute to bone pain and muscle weakness. Joint pain, however, can have many causes, including arthritis and other musculoskeletal conditions.
Osteoporosis
Vitamin D supports calcium absorption and normal bone metabolism. Inadequate vitamin D can contribute to reduced bone mineralization and weaker bones.
Adequate vitamin D, calcium, protein, and regular weight-bearing physical activity are all important components of maintaining bone health.
Depression and Low Mood
Research has found associations between low vitamin D levels and depression in some populations. However, evidence does not establish that vitamin D supplementation alone prevents or treats depression in everyone.
People experiencing persistent depression or significant changes in mood should seek appropriate medical or mental health support.
Slow Wound Healing
Vitamin D participates in processes involved in immune function and tissue repair. However, slow wound healing can have many causes, including diabetes, poor circulation, infection, nutritional deficiencies, and certain medications.
Anyone with a wound that is slow to heal should seek medical evaluation rather than assuming vitamin D deficiency is responsible.
Vitamin D Deficiency in Children
Children can also develop vitamin D deficiency. Severe deficiency can affect normal bone development.
Possible signs of significant deficiency in children can include:
- Irritability
- Lethargy
- Bone pain
- Muscle weakness
- Delayed growth
- Bone deformities
- Delayed motor development
Infants and children have specific vitamin D requirements, and parents should discuss supplementation with a pediatrician or other qualified healthcare professional when appropriate.
Complications
Rickets
Rickets is a condition in children in which bones become soft and weak because of impaired mineralization. Severe vitamin D deficiency can cause rickets, although other nutritional and medical factors can also contribute.
Signs may include bone deformities, delayed growth, muscle weakness, and abnormalities affecting the legs, wrists, and other bones.
Treatment depends on the underlying cause and may include vitamin D and calcium under medical supervision.
Cardiovascular Disease
Research has identified associations between low vitamin D levels and cardiovascular disease in some populations. However, clinical trials have not consistently shown that taking vitamin D supplements prevents cardiovascular disease in people who are not deficient.
Heart health is influenced by many factors, including blood pressure, cholesterol, smoking, physical activity, diet, body weight, and genetics.
Autoimmune Disease
Vitamin D is involved in immune system regulation, and researchers have investigated its relationship with autoimmune diseases.
Low vitamin D levels have been associated with conditions such as:
- Type 1 diabetes
- Multiple sclerosis
- Rheumatoid arthritis
- Crohn’s disease
However, these associations do not prove that vitamin D deficiency directly causes autoimmune disease or that supplementation can prevent or cure these conditions.
How Can You Support Vitamin D Intake?
Vitamin D can come from several sources, including:
- Fatty fish such as salmon, sardines, and mackerel
- Egg yolks
- Fortified milk
- Fortified plant-based milk alternatives
- Some fortified cereals
- Vitamin D supplements when recommended
A balanced diet is important for obtaining vitamin D along with other essential nutrients. For additional nutrition guidance, see our article on nutritional tips.
Conclusion
Vitamin D is an essential nutrient that plays an important role in bone health, calcium absorption, muscle function, and immune regulation. Vitamin D deficiency can occur for several reasons, including limited sun exposure, inadequate dietary intake, certain medical conditions, reduced kidney function, and obesity.
Because many people with mild deficiency have no obvious symptoms, testing may be appropriate for people at higher risk. Vitamin D supplements can be useful when deficiency is confirmed or supplementation is recommended, but taking unnecessarily high doses can cause harm.
If you are concerned about vitamin D deficiency or are considering supplements, consult a healthcare professional. They can determine whether testing or supplementation is appropriate based on your individual health needs.
