What Causes Vitamin D Deficiency?

Vitamin D stands out from other vitamins because the body can produce sufficient amounts through sunlight exposure. It is synthesized in the skin from a compound derived from cholesterol, meaning dietary intake is not always essential if sun exposure is adequate. [1] Vitamin D is considered a hormone and plays numerous vital roles in the body. This article will explore the key aspects of vitamin D, focusing specifically on its deficiency.

Functions of Vitamin D

Bone Growth and Maintenance: Vitamin D is part of a complex system responsible for bone formation and health, which includes vitamins A, C, K, parathyroid hormones, calcitonin, collagen protein, calcium, phosphorus, magnesium, and fluorine. It regulates calcium and phosphorus levels in the blood, enabling bones to absorb and store these minerals effectively. Vitamin D enhances their absorption in the digestive tract and reabsorption in the kidneys, working alongside parathyroid hormone to mobilize minerals from bones when levels are low. Adequate intake of calcium and vitamin D helps keep parathyroid hormone low, preserving bone density. [1][2]

Recent research has identified vitamin D receptors in various body cells—including immune cells, brain, nervous system, pancreas, skin, muscles, cartilage, and reproductive organs—highlighting its role in cell differentiation and growth. It also acts in breast and colon cells, preventing abnormal growth and reducing cancer risk. [1][2][3]

Studies suggest vitamin D may lower the risk of multiple sclerosis and autoimmune-related rheumatoid arthritis by regulating immune function, which is often impaired in autoimmune diseases like type 1 diabetes and inflammatory bowel diseases. [1][2][4]

Some research also links vitamin D to insulin sensitivity, indicating a protective role against type 2 diabetes. [5] It is essential for muscle contraction and strength, including the heart muscle. [2]

Daily Vitamin D Requirements

Consequences of Vitamin D Deficiency

When vitamin D is deficient, the production of calcium-binding proteins that facilitate calcium absorption in the intestines drops. This causes calcium deficiency even if dietary intake is adequate. In adolescents, this can prevent achieving peak bone mass. In children, deficiency causes rickets, while in adults, it leads to osteomalacia and osteoporosis. [1]

Rickets

Rickets remains a common childhood disease worldwide, caused by insufficient calcium supply to bones, leading to delayed growth and bone deformities. Weakened bones cannot support body weight, causing bowed legs once children start walking—a hallmark sign of rickets. Rib bones may become bead-like due to weakened cartilage connections, causing chest protrusion. Additional symptoms include an enlarged forehead, wrist and ankle swelling, bone pain, muscle weakness, and spasms. Delayed and malformed teeth are also common. [7]

Dark-skinned children, those breastfed exclusively without vitamin D supplements, children with chronic fat absorption disorders, and those on anticonvulsant therapy are at higher risk. Historically, rickets was more prevalent among poor children in developed cities with limited sun exposure. [7]

Osteomalacia

Osteomalacia is the adult form of rickets, often affecting women with insufficient calcium intake and inadequate sun exposure, especially those with frequent pregnancies and breastfeeding. This causes bone thinning, reduced density, bowed legs, and spinal curvature. It also results in microfractures, muscle weakness, and an increased risk of fractures, particularly in the spine, femur, and arms. [1][7]

Osteoporosis

Osteoporosis is characterized by decreased bone mass and fragility, most common in postmenopausal women but also affecting older men. Lack of adequate vitamin D causes calcium loss from bones, increasing fracture risk. Studies have found that half of hospitalized women with osteoporosis and hip fractures had undiagnosed vitamin D deficiency. [1][7]

Other Effects of Vitamin D Deficiency

• Depression: A correlation exists between vitamin D deficiency and depression. Supplementation has helped improve depressive symptoms in deficient individuals. [8]
• Obesity: Some studies link low vitamin D levels with increased fat accumulation and obesity. [9]
• Infections and Asthma: Deficiency raises susceptibility to viral and bacterial respiratory infections and is linked to asthma risk and severity in children. [3][10]
• Cardiovascular Disease: Low vitamin D levels associate with higher mortality from heart and vascular diseases. [10]
• Cognitive Decline: Deficiency correlates with cognitive impairments in the elderly. [10]
• Cancer: Associations have been found between low vitamin D and certain cancers. [10]

Vitamin D Deficiency in Older Adults

Vitamin D deficiency is especially common in older adults due to decreased skin, liver, and kidney function that reduces conversion to the active form. Elderly people often spend most of their time indoors with limited sun exposure and consume less fortified milk, raising their risk of bone loss and fractures. [1]

Sources of Vitamin D

Diet alone usually cannot meet the body’s vitamin D needs; adequate sun exposure is essential. About 10–15 minutes of sunlight exposure on sunny days, two to three times per week, is generally sufficient for most people, although those with darker skin require longer exposure. [1][7]

Natural dietary sources are mainly animal-based: fish liver oils are the richest, followed by butter, cream, egg yolks, and liver. [7] Vitamin D supplements and fortified foods such as juices, cereals, milk, and margarine are common additional sources. Breast milk and regular cow’s milk are poor sources, so breastfed infants require vitamin D supplements prescribed by a doctor, while commercial infant formulas are typically fortified. [7]