How does Vitamin D affect bone health?

Vitamin D is a fat-soluble (it needs fat to be absorbed) vitamin that humans do not make themselves and acquire instead solely from UV sunlight and diet. There are two forms of this, Ergocalciferol (D2), which is a plant-based source, and Cholecalciferol (D3), which we acquire from fish and animal sources. Vitamin D plays a pivotal role in bone health as it helps us regulate calcium and phosphorus mineral levels in our body and is involved in bone remodelling and formation. Low vitamin D levels increase an individual’s risk of osteoporosis and fractures.

Both D2 and D3 have been shown to have similar levels of bioavailability for bones. This means they affect our bodies in similar ways. Deficiency can result in rickets, osteoporosis, and fractures. For adults in the UK it is required, according to SACN (the Scientific Advisory Committee on Nutrition), that we have 10µg/day of Vitamin D. The vitamin D we take into our bodies through sun or supplements, or diet is broken down for use in 2 hydroxylation reactions, one in the liver to make 25-hydroxyvitamin D or 25[OH]D, and one in the kidney to make 1,25-dihydroxyvitamin D. We use the serum level of 25[OH]D as a biomarker for the level of vitamin D in the body.

Where do we get vitamin D from?

Most of our vitamin D comes from UV sunlight. Sunlight provides UVB radiation which induces vitamin D synthesis in our skin. Therefore, the level of vitamin D we acquire changes with seasons, with increases in deficiency in winter. Melanin in the skin sequesters (steals away) the UVB radiation and so those with darker skin naturally absorb less vitamin D and so require more from the diet, sun and supplements. The more melanin you have the less UVB there is to make vitamin D. This is a biological survival mechanism to stop those in hot climates, with more melanin to protect their skin from the suns rays, from acquiring too much vitamin D. Vitamin D production also reduces as you tan and stops when the skin burns and the use of sunscreens can block UVB and vitamin D production.

We can also get vitamin D from key dietary sources such as:

· Salmon and other oily fish

· Cod liver oil

· Dairy products

· Fortified plant-based milks

· Mushrooms (these plants are like humans and make vitamin D in their skin and so are a great source of vitamin D for vegans).

· Egg yolk

· Soy-based products such as miso, tofu, soya milk

· Almonds

· Fortified cereals

Dietary supplements can also be great sources of Vitamin D. However, be careful not to overdose as levels above 300µg can cause hypercalcaemia (too much calcium) and be toxic.

What lifestyle choices lower vitamin D?

Increased time indoors can lower vitamin D production (more screentime), as can less time in the sun, less oily fish, less dairy, wearing clothes that block the sun (such as facial coverings, long clothes and hats), smoking and weight status. Although vitamin D is a fat-soluble vitamin, being overweight lowers vitamin D levels. This is because a larger body mass dilutes the vitamin D across a larger body, so more is needed to reach sufficient biological levels. Also, there is some evidence suggesting too much body fat also stops vitamin D storage. Too low body weight, with a low body fat percentage, and a diet low in dietary fat, reduce vitamin D absorption from the gut in digestion, reducing calcium and phosphorus absorption and increasing the risk of bone fractures and diseases.

Do supplements work for vitamin D?

Vitamin D supplements are advised for people who work in office jobs or are inside a lot, or who live in areas such as Scotland in the UK with low daily levels of sunlight, especially in winter. They are also needed for those who cover their skin in public for cultural, personal or religious reasons, and for those with low dietary intake of fat or vitamin D-containing foods, such as vegetarians and vegans. Although plants are an excellent source of D2 vitamin D, many people dislike the plants that the vitamin is in, such as mushrooms, almonds and soya milk and so often unknowingly do not consume enough vitamin D. Also excess consumption of certain plant-based foods, such as spinach, potatoes, potato chips, rhubarb and beetroot, can also lead to a build-up of oxalates present in these plants, which can hinder calcium and iron absorption and result in bone weakening and anaemia (low iron levels, fatigue and cognitive decline).

The elderly and menopausal women are also at increased risk of osteoporosis, and pregnant women need more vitamin D as they need it for their growing child, as well as themselves. Therefore, it is advised by the World Health Organisation, that the elderly, those with minimal UVB sun exposure, pregnant women, menopausal women, those under and overweight and vegans and vegetarians (high-risk groups) take 10µg vitamin D to support their bones and help prevent fracture. Supplements can be prescribed by a doctor or bought over the counter and are available in the UK to children for free for less financial means families and the elderly. Supplementation can be the prevention that helps boost bone health.

Copyright Laura Campbell 01/12/2022