Vitamin D and its role in health; A review of mechanisms, risk factors, sources, and health benefits

Vitamin D is crucial for maintaining optimal health. This review study evaluates health problems related to deficiency and ways to maintain sufficient levels. We conducted a review of the association between vitamin D inadequacy and various diseases, explored the various sources of vitamin D and the risk factors associated with vitamin D insufficiency, examined the consequences of insufficient vitamin D levels


Introduction
Vitamin D is a fat-soluble substance that helps maintain bones by helping the body absorb calcium, magnesium, and phosphate (1).It also contributes to bodily processes like supporting our immune system, improving cell growth, and aiding in neuromuscular function (2).This vitamin is naturally occurring in certain foods, added to others, and exists in supplement form (3). When sunlight touches the skin, it triggers a process that produces vitamin D. Vitamin D is synthesized in response to exposure to ultraviolet (UV) rays (2,3).We can obtain vitamin D from food sources like fish and fortified foods or produce it naturally when exposed to sunlight (2).However, most people worldwide don't get vitamin D, which causes the incidence of vitamin D insufficiency and is correlated with disorders like infections, type 2 diabetes, and COVID-19 (4).

Objective
We investigate how vitamin D is correlated to diseases, what are its different sources and risk factors of its inadequacy, the implications of lacking sufficient vitamin D, and the main advantages of taking vitamin D supplements for health.

Mechanisms of vitamin D action
Vitamin D mechanisms are quite complex as they involve a series of steps, including synthesis, metabolism, and activation.Vitamin D can be obtained from the food we eat or synthesized by our bodies when exposed to sunlight.Once we acquire vitamin D, it goes through two hydroxylation processes, including in the liver and kidneys, causing the production of a form called 1,25 dihydroxy-vitamin D (1,25(OH)2D).This active form bonds with receptors in tissues, such as the intestine, bones, and immune cells.Its role is regulation of calcium, phosphate balance, bone health control, and metabolism as a support immune functions (5).In addition to its classical functions, vitamin D is known for regulating cell proliferation, differentiation, and apoptosis (5).

Sources of vitamin D
Vitamin D can be produced by the body through sunlight or taken from supplements and food sources.The two common types of vitamin D, including cholecalciferol (D3) and ergocalciferol (D2), are available in both food

Implication for health policy/practice/research/ medical education
Vitamin D insufficiency is common with various health problems, such as osteoporosis, autoimmune diseases, infection, cardiovascular disease, diabetes, and depression.Therefore, maintaining a sufficient vitamin D level with a balanced diet and regular exposure to sunlight can prevent this common disease and improve health and quality of life.
sources and supplements.The best food sources include fatty fish, eggs, sun-exposed mushrooms, liver, and other organ meats (6,7).Other sources are yogurt, eggs, cheese, fortified milk, beef, liver, dark chocolate, fat spreads, breakfast grains, orange juice, and plant-based beverages (8).There are limited food sources that provide a significant amount of vitamin D. However, there are a few examples including fish liver oils, Reindeer lichen, and mushrooms (7).The main dietary sources of vitamin D vary by country and depend on factors like sex, age, body composition, vitamin D levels, and income.Ensuring a sufficient intake of vitamin D is crucial for maintaining good health, and it is recommended by the European Food Safety Authority (EFSA ) to consume 15 μg/d on a daily basis (7).

Deficiency of vitamin D
Many individuals struggle with vitamin D insufficiency worldwide due to severe health issues caused by it (1,9).The most important cause of vitamin D deficiency is insufficient sun exposure, which is the primary source.There are not many foods that are naturally rich in vitamin D, and the ones that are fortified with it often don't meet human requirements (9).Both developed and developing countries have a worldwide prevalence of up to 1 billion individuals with subclinical vitamin D deficiency (1).An analyzed data from the National Health and Nutrition Examination Survey (2005 to 2006) regarding vitamin D levels in adults showed that the prevalence of vitamin D deficiency is 41.6%, and the highest rates are 82.1% and 69.2% in black and Hispanic races respectively (10).

Prevalence of vitamin D insufficiency
The following studies show the prevalence rates of worldwide vitamin D insufficiency:

Symptoms of vitamin D deficiency
It is crucial to be aware that a lack of vitamin D in the body can lead to various symptoms, which may not necessarily be specific to vitamin D deficiency and could be caused by other medical conditions.It is crucial to be aware that a lack of vitamin D in the body can lead to various symptoms, which may not necessarily be specific to vitamin D deficiency and could be caused by other medical conditions.Some of the most common related symptoms to vitamin D deficiency are muscle pain and weakness, reduced performance speed, bone pain and deformities, increasing risk of falling, hypocalcemia, and Dental abnormalities (4,(15)(16)(17)(18).

Risk factors for vitamin D deficiency
Maintaining good bone health and overall health is essential, and vitamin D is an important nutrient that helps in achieving this.Unfortunately, many people suffer from vitamin D deficiency, which can lead to various health issues.According to previous studies, the following are some risk factors for vitamin D deficiency: • Insufficient sunlight exposure: When the skin is exposed to sunlight, it produces vitamin D. This means that individuals who spend the majority of their time indoors, wear clothing that conceals most of their skin, or reside in regions with limited sunlight are at a greater risk of developing (4,17).• Inadequate dietary intake: Some foods contain vitamin D, including egg yolks, fortified foods, and fatty fish.If someone does not eat enough of these foods or follows a vegetarian diet, they may be at risk of developing this disease (4, 12).(19,20).It is worth noting that certain individuals may have a greater likelihood of experiencing a shortage of vitamin D compared to others.These may include seniors, those with darker skin tones, and individuals who are overweight (17).As a result, it's advisable to undergo testing for vitamin D deficiency and consider taking supplements if needed.

Prevention of vitamin D deficiency
Some behavior and lifestyle changes can reduce the risk of vitamin D deficiency, including being exposed to sunlight (17,21), consuming rich vitamin D foods and supplements (20,22), being physically active (22), avoiding smoking and opium, maintaining a healthy weight, and Improving the economic status, educational level, and environmental conditions (20).

Health-related problems of vitamin D deficiency and health benefits of its supplication
Vitamin D inadequacy can cause softening of bones (Osteomalacia) in both children and adults and rickets in children (1,9), and also is correlated with osteoporosis, risk of falling, and older age fractures.Recent studies show an association between vitamin D deficiency and cardiovascular and cancer disease, autoimmune diseases, diabetes mellitus, and depression (1).On the other hand, vitamin D deficiency, due to its role in immune function, can lead to various diseases such as infections, type 2 diabetes, and COVID-19 (2), and its supplementation can improve bone health and reduce falls in seniors (6).

Conclusion
Vitamin D deficiency is a prevalent condition associated with various health issues.It is crucial to maintain a sufficient level of vitamin D through a balanced diet and regular exposure to sunlight or supplements if needed.