Vitamin D is a fat-soluble vitamin (along with A, E & K), meaning it dissolves in fat/oil and can be stored in the body. The two majors forms are D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D helps to regulate calcium and phosphorus in the body, which are needed to keep bones, teeth and muscles healthy. Other important roles include cell growth, immune function and the reduction of inflammation.
For most people the main source of vitamin D is through exposure to sunlight containing UVB radiation, but it can also be obtained from food or dietary supplements. When exposure to sunlight is limited, dietary sources of vitamin D are essential in order to maintain adequate levels and avoid health problems linked to deficiencies. The main dietary sources of vitamin D in the UK are foods of animal origin, fortified foods and supplements. Good sources of vitamin D3 include egg yolk, oily fish (herring, salmon, mackerel, sardines) and red meat. Wild mushrooms are a rich natural source of vitamin D2. Foods fortified with vitamin D2 and D3 in the UK include dairy products, margarine, fat spreads and breakfast cereals. After vitamin D is synthesized in the skin following exposure to sunlight containing UVB radiation or you eat food or supplements containing vitamin D, it is converted to its active state in two stages in the body. This happens firstly in the liver, where it is converted to calcidiol, and secondly in the kidney, where the calcidiol is converted to calcitriol. This is the active, steroid hormone form of vitamin D, which travels round the body, entering the cells and binding to the vitamin D receptors (VDR), influencing the expression of over 200 genes. If your liver or kidneys are not functioning properly, or you are not absorbing the vitamin D in the small intestine, it can all impact the conversion process, so good liver, kidney and gut health is important too.
The importance of vitamin D in connection with musculoskeletal health is well known. Infants need vitamin D, along with calcium and phosphorus, to meet the demands of rapid growth for healthy skeletal development. Prolonged deficiency of vitamin D can lead to rickets and osteomalacia (softening of the bone). In children/adolescents, vitamin D is important for bone formation, particularly during growth spurts. Whilst rickets is more common in infancy, it can occur during adolescence. Deficiency during this period can also affect bone mineral density. In adulthood, peak bone mass is reached in the early 20’s and vitamin D is required to maintain healthy bone. Deficiency can lead to osteomalacia and in later life, osteoporosis. Osteoporosis is a progressive disorder usually associated with ageing. The risk of bone fractures increases as the condition progresses, with women at greater risk than men, mainly due to the decrease in oestrogen production after the menopause, which directly impacts bone loss. This is why it's particularly important for women undergoing the menopause to check their vitamin D levels and maintain adequate levels.
The UK daily-recommended values (DRV) for vitamin D were set in 1991, based on the protection of musculoskeletal health. At that time a dietary intake of vitamin D wasn’t considered necessary for people between 4y - 64y with adequate exposure to sunlight. However, the results of recent studies suggest that in addition to musculoskeletal health, there are benefits for a much wider range of conditions including cancer, pre-eclampsia, CVD, depression, diabetes, autoimmune diseases, autism and helping you live longer. These findings have led to a thorough review of the DRV’s by the government's Scientific Advisory Committee on Nutrition (SACN).
Following this review by SACN, Public Health England (PHE) issued new advice on vitamin D levels and supplementation in July 2016. It states that children and adults over the age of 1y should consider taking a daily supplement of 10 micrograms (MCG) of vitamin D, particularly during autumn and winter, in order to guard against deficiencies and support health. People who have a higher risk of vitamin D deficiency are being advised to take a supplement all year round, (at-risk groups include people whose skin has little or no exposure to the sun, like those in care homes, or people who cover their skin when they are outside). People with dark skin, from African, African-Caribbean and South Asian backgrounds, may also not get enough vitamin D from sunlight in the summer. They should consider taking a supplement all year round as well. When choosing a supplement I go for vitamin D3 with added vitamin K2, as studies show this enhances calcium absorption.
*The above supplement recommendations are based on the average population. However if you take certain medication, or have certain health conditions it may necessitate different requirements and in these cases, I would suggest that individual advice be sought from your GP or registered NT. Your GP or NT can check your vitamin D levels with a blood test.