Emerging evidences suggest that vitamin D is important for the prevention of various diseases like cancer, neurological diseases, and diabetes
As the harsh winter is approaching at full pace, the sun has also started playing hide and seek. Our over clothed bodies find it hard to embrace the diminishing sunlight, if any. The downside of this inadequate sunlight availability is the reduced synthesis of the sunshine vitamin (vitamin D) by our skin cells. That is one of the main reasons for joint pain in elderly people, particularly during the winter season. Thin, brittle, and fragile bones accompanied by persisvent muscle pain in the joints of legs, knees, hips, and shoulders are some of the physical symptoms of vitamin D deficiency. In severe cases, the deficiency may also affect the immune system and mental health leading to stress, learning and memory impairment, cognitive decline and make us more susceptible to anxiety, depression and life-threatening diseases like cancer and diabetes.
What is Vitamin D?
Vitamin D is a type of steroid (secosteroid), which is essential for healthy bones and overall well-being. Our body can obtain this essential vitamin by three sources. First, it is primarily synthesized by the skin cells of our body in presence of sunlight. Second, it is naturally present in many food products and third, it can be obtained in the form of supplements.
The primary source for vitamin D synthesis is the skin where sunlight through a chemical reaction converts 7-dehydrocholesterolinto pre-Vitamin D and later into Vitamin D3 (Cholecalciferol) – biologically inactive forms. Vitamin D3 is then transported into liver cells where it is converted into 25-hydroxy vitamin D, and subsequently into the biologically active form of 1, 25-dihydroxy vitamin D in the kidneys. It is from the kidneys that the biologically active form of vitamin D is circulated into the blood and transported to most of the cells of the body including bone cells, muscle cells, and immune cells for their proper function.
Dietary vitamin D is provided by both animal and plant products. While animal products are the major source of Vitamin D3, plant products contain a similar form called vitamin D2 (ergo sterol). Both these forms follow the same liver-kidney pathway for synthesis of the biologically active form. However, the amount of vitamin D derived from plant products is inadequate and the people relying solely on a vegan diet must consume sufficient amounts of these plant products.
Physiological role: Vitamin D and Calcium
The major physiological function of vitamin D is attributed to its role in regulating the transport and absorption of calcium for bone growth and proper immune function. The dietary calcium is only utilized when there are adequate levels of vitamin D, whereas its deficiency reduces the calcium uptake by the bones. This function of vitamin D is mediated through the expression of various proteins by the action of vitamin D on its receptors. These proteins, in turn, regulate the transport of vitamin D into the cells like bone cells and immune cells. However, insufficiency of vitamin D reduces the levels of these calcium transport proteins and thus the amount of calcium in the cells. This is the process of outmost importance, which suggests that calcium cannot be used by the bones in absence of vitamin D. Thus, people who are recommended to take calcium supplements for bone health, should always take it along with the vitamin D, otherwise the effect will be undermined. Generally, it is the vitamin D insufficiency that is the culprit rather than calcium which is usually provided in sufficient quantities by our diet. Besides its role in calcium metabolism, vitamin D also increases retention of other ions like phosphate, potassium and sodium in the body. Additionally, emerging evidences suggest that vitamin D is important for the prevention of various other diseases like cancer, neurological diseases, and diabetes.
Diagnosis of vitamin D deficiency
Circulating serum concentration of vitamin D provides the information about its adequacy, inadequacy and deficiency. Tests are available for the measurement of both 25-hydroxy vitamin D and 1,25-dihydroxy vitamin D, where the former is generally considered as the better indicator. A considerable variability exists between various diagnostic tests like ELISA or liquid chromatography and between the testing laboratories. Thus, a reference range is provided and the actual concentration is measured based on the reference value. In adults, the serum concentration of vitamin D under health and deficiency condition is:
Healthy: ≥50 nmol/L or 20ng/mL
Insufficient: 30 to <50 nmol/L (12 to <20ng/mL)
Vitamin D deficiency:<30 nmol/L (<12ng/mL)
Insufficient refers to the inadequate growth of bones and other health complications, whereas deficiency leads to the disease conditions like osteomalacia in adults and rickets in infants. Osteoporosis is also associated with vitamin D deficiency – a condition that causes bones to become weak and more likely to fracture.
People who are at risk of vitamin D deficiency
Normally, the diet and adequate exposure to sunlight maintains the sufficient levels of vitamin D in the body. However, some health conditions can interfere with maintaining the normal levels. One of the major reasons for vitamin D deficiency is insufficient exposure to sunlight, which may be due climatic conditions and modern work culture. Apart from the insufficient exposure to sunlight, elderly people are more prone to vitamin D deficiency as their aged skin cells have compromised synthesis machinery. Diseases of liver and kidney also interfere with synthesis of the biologically active form of this vitamin. Similarly, genetic defects in the vitamin D receptors have been identified to lead to the associated disease conditions. Although people with defects in vitamin D receptor may obtain the optimal quantities via diet or by exposure to sunlight, the calcium absorption by their cells is hampered, which leads to a similar condition called vitamin D resistance.
People with dark skin are more prone to vitamin D deficiency due to the synthesis of high levels of pigment melanin which interferes in vitamin D synthesis pathway by the sunlight. Other conditions which lead to vitamin D deficiency include inflammatory bowel disease, obesity and gastric bypass surgery.
Signs and Symptoms
Getting sick or infected often; fatigue and tiredness; Bone and back pain; impaired wound healing; bone loss; hair loss and depression. Some of the major signs listed here may be overlapping with other disease conditions other than vitamin D deficiency but if all of them are present together, then the chances of being diagnosed with vitamin D deficiency are higher.
Prevention and Treatment
Adequate Sunlight: Some exposure to sunlight is the most appropriate way for obtaining adequate amounts of vitamin D. Though it is not clear how much time one needs to spend in the sunlight to get adequate vitamin D, but an optimal range is 15-30 minutes exposure each day during the mid-day hours. Factors that may reduce the amount of vitamin D synthesized by sun exposure include ageing, application of sunscreen creams, covering of legs, arms and face, and pigmented skin.
Diet: Dietary intake of vitamin D is provided by some of the animal products and can be obtained from a very few plant sources. Dairy sources include milk, egg yolk, fish oil, fatty fish, cheese, meat, and chicken and plant sources include mushrooms, fortified soy milk and almond milk.
Supplements: To prevent vitamin D deficiency in persons with vegan diet and inadequate exposure to sunlight, vitamin D supplements are recommended. Earlier, the recommended daily intake dose was 200 IU for children and adults below 50 years, and 400 IU for adults above 50 years. Recently, the daily intake for children and adolescents has been revised by the American Academy of Pediatrics. Children and adolescents who do not ingest one litre of vitamin D-fortified milk per day or do not get regular exposure to sunlight; the recommended dose is 400 IU. The supplements should be taken only after medical consultation. The amount of the supplement intake is based on the age of the person, which can be recommended by a physician after examination and proper diagnosis.
(Author is Assistant Professor, Jamia Millia Islamia New Delhi)