Vitamin D is a fat-soluble vitamin that is present in two forms – D2 (ergocalciferol) and D3 (cholecalciferol). It is also casually referred to as the ‘sunshine vitamin’ as both forms are naturally produced in organisms in the presence of the sun’s ultraviolet-B (UV-B) rays. Vitamin D2 is produced in plants and fungi, while D3 is produced in animals. Hence, when we talk about vitamin D supplements for humans, we usually refer to D3.

Natural Synthesis of Vitamin D

The steroid 7-dehydrocholesterol is naturally found in our skin. Upon exposure to UV-B rays from the sun, this steroid is broken down to release vitamin D3, which then enters circulation in the body. Generally, exposure of around 15-20 minutes per day, three days a week is believed to synthesise a sufficient amount of vitamin D in young, healthy individuals. Sun rays are most powerful between 10:00 am and 3:00 pm, hence optimal UV-B absorption would ideally occur within this time frame.

Alternative Sources of Vitamin D

Vitamin D can be obtained through foods such as:

• fatty fish (e.g. salmon, swordfish, mackerel, tuna, sardine)
• cod liver oil
• fortified milk and other dairy products such as cheese, yoghurt
• egg yolks
• mushrooms
• beef liver
• fortified orange juice
• fortified cereal

Apart from this, vitamin D may also be taken as a supplement (usually in the form of capsules). Doctors recommend an average of between 1000 – 2000 IU (international units) a day. However, this would vary with factors such as age, underlying health conditions, ongoing medication, and in the case of pregnant / lactating women and breastfed infants (human milk is a poor source of vitamin D).

Role of Vitamin D in Humans

Many organs and tissues in the human body have been found to contain vitamin D receptors, suggesting that the chemical plays a significant role in the normal functioning of our body. As such, among other roles, the vitamin is known to:

• optimise absorption of calcium as well as phosphorus from the diet, both of which are important for bone formation and maintenance of bone strength
• play a role in the nervous and muscular systems, particularly to improve muscle strength
• improve cardiovascular health by maintaining the flexibility of arteries and regulating inflammatory cells that are involved in cardiovascular disease
• improve general immunity (hence, doctors have been recommending a daily intake of vitamin D supplements since the inception of the Covid-19 pandemic)

Research also suggests that vitamin D may play a role in:

• reducing the risk of cancers such as that of the colon, prostate and pancreas
• reducing the incidence of Type 2 diabetes mellitus
• improving lifespan by lowering the risk of premature death

Risk Factors for Vitamin D Deficiency

There are several risk factors that could contribute towards the incidence of vitamin D deficiency. These include:

• limited exposure to direct natural sunlight – exposure through windows does not count as UV-B gets cut out by glass
• geographical location and season – latitudes further away from the equator receive weaker UV-B, as do regions experiencing winter
• skin tone – having more melanin in one’s skin (i.e. darker complexion) acts as a sort of natural sunscreen, reducing UV-B absorption
• using sunscreen or wearing clothes that cover the skin to a large extent, when outdoors
• age – 7-dehydrocholesterol levels in the skin reduce with age, particularly in those over the age of 50 years
• obesity – body fat may bind to the vitamin, preventing it from entering circulation
• inflammatory bowel diseases – such as Crohn’s Disease, celiac disease and ulcerative colitis that affect the digestion of fat, the absorption of which is required for vitamin D absorption
• other medical conditions – such as hyperparathyroidism, granulomatous diseases, lymphomas, osteoporosis, chronic liver / kidney disease
• medication – such as glucocorticoids, laxatives, antifungal drugs, anti-seizure drugs, drugs for HIV/AIDS, drugs for cholesterol
• gastric bypass surgery – this often removes the upper part of the small intestine where most vitamin D absorption occurs

Symptoms and Complications of Vitamin D Deficiency

Symptoms of vitamin D deficiency may include:

• fatigue
• aching bones
• feeling weakness in or aching / cramping muscles
• mood changes (e.g. depression)

Severe cases of vitamin D deficiency could lead to complications such as:

• osteomalacia – softening and weakening of bones in adults, often causing bone pain and muscle weakness; can be reversed with vitamin D supplementation
• osteoporosis – loss of bone density in older adults, making bones porous and increasing the risk of fracture; usually irreversible
• rickets – failure of bone tissue to harden in children, causing soft and deformed bones (most common in children of African ethnicity)

Vitamin D Toxicity

An excess of vitamin D, too, could be harmful. This usually happens if too much vitamin D is consumed in the form of supplements, and is unlikely to occur due to excessive exposure to sunlight. Symptoms and complications of vitamin D toxicity may include:

• nausea and/or vomiting
• loss of appetite
• weight loss
• anorexia
• weakness
• confusion
• constipation
• irregular heart beat
• hypercalcaemia (elevated levels of blood calcium), potentially causing damage to the heart and kidneys as blood vessels and tissues may harden
• ataxia (nervous disorder causing unsteadiness and slurred speech)

Hence, vitamin D supplements should ideally be taken in consultation with your doctor who will decide on the best dosage for you based on age, medical history and any other supplements you might be taking.

Levels of blood vitamin D and subsequent detection of deficiency or toxicity may be determined using a simple blood test. No fasting is required.

Cover illustration adapted from iStock