Author: Kate Scott, DipNut, mBANT, rCNHC, mIFM
DNApal co-founder | Nutrition & genetics specialist | BANT registered nutritionist

Vitamin D is known as the “sunshine vitamin” because it is mainly produced by our skin when exposed to direct sunlight. It is one of the fat-soluble vitamins (along with A, E, and K) but differs from other vitamins because it is actually a hormone. The active form is called cholecalciferol (vitamin D3). Vitamin D3 is produced by the liver and kidneys and helps to increase calcium absorption in the gut from food. 

One of the main roles of vitamin D is to work with calcium and phosphorus for healthy bones, muscles and teeth. As a result, vitamin D helps prevent conditions such as osteomalacia (softening of the bones), rickets and osteoporosis (brittle bones). Your muscles need vitamin D to move, and your nerves need it to carry messages between your brain and body. Vitamin D is also vital for supporting the immune system (fighting off invading bacteria and viruses), boosting your mood and reducing the risk of certain types of cancer. (a)

The recommended daily allowance (RDA) of vitamin D for adults in the UK is 10 micrograms (about 400 IU) per day, an absolute minimum amount to prevent insufficiency. According to the British Medical Journal (BMJ), this should be increased to between 25 micrograms (1000 IU) – 125 micrograms (5000 IU) for most people, especially the elderly and those with darker skin. (b)   

It is difficult to get enough vitamin D from diet alone as it is found in very few foods such as oily fish (salmon, mackerel, and sardines), eggs, mushrooms and some meat. 

The best way to maintain vitamin D levels is through adequate sunlight exposure. The amount of vitamin D you make depends on how strong the sunlight is. During the warmer months, exposing your skin to the sunlight for at least 20 minutes in the morning or afternoon is recommended. It is important to protect the skin and not engage in overexposure during the hottest times of the day, as excessive sun exposure increases the risk of skin cancer. In the UK, the ultraviolet light is only strong enough to make vitamin D on exposed skin from April – September. During autumn and winter, we can get vitamin D from our body’s stores (if we have made enough during the spring and summer), small amounts from food, and from supplementation. 

Absorptions factors to consider with Vitamin D supplements:

You may want to read our ‘How to get the most out of your Daily Nutrition Supplements’ to understand better bioavailability, vitamin agonists (friends), and antagonists (foes).

Bioavailability:

Bioavailability refers to the proportion of a nutrient absorbed by the body and made available for use. Vitamin D is a fat-soluble vitamin obtained through dietary sources, supplementation, or skin synthesis upon sunlight exposure. However, the bioavailability of vitamin D can vary depending on several factors, such as age, body weight, gastrointestinal health, and the form of vitamin D consumed. For example, vitamin D3 is considered more bioavailable than vitamin D2. In addition, certain dietary factors, such as fat and the type of food consumed, can affect vitamin D absorption. Adequate vitamin D intake is essential for maintaining bone health and preventing chronic diseases, and understanding its bioavailability is crucial for ensuring proper nutrient status.

Agonists (friends):

As mentioned earlier in the blog, Vitamin D helps with the absorption of calcium and phosphorus in the body. However, there are some vitamins and minerals that can help increase the absorption of vitamin D as well. These include:

Calcium: Calcium is essential for bone health and is also needed for vitamin D absorption. Vitamin D helps the body absorb calcium, while calcium helps the body absorb vitamin D. Eating calcium-rich foods such as dairy products, leafy greens, and fortified foods can help increase calcium and vitamin D absorption.

Magnesium: Magnesium is also needed for vitamin D absorption. It helps convert vitamin D into its active form, which the body can use. Good sources of magnesium include nuts, seeds, whole grains, and leafy green vegetables.

Vitamin K: Vitamin K is essential for bone health and may also affect vitamin D metabolism. Studies have shown that vitamin K helps activate proteins that transport calcium and may help prevent calcium from building up in the arteries. Good sources of vitamin K include leafy green vegetables, cruciferous vegetables, and fermented foods.

Zinc: Zinc is essential for immune function and may affect vitamin D absorption. Good sources of zinc include oysters, beef, chicken, nuts, and beans.

It’s important to note that while these nutrients can help increase vitamin D absorption, taking too much of any one nutrient can be harmful. It’s best to get these nutrients from a balanced and varied diet rather than relying solely on supplements.

Antagonists (foes):

Funnily enough, some of the vitamins and minerals listed as agonists, i.e. help with the absorption, can also, when taken in higher doses, become antagonists to the optimal absorption of vitamin D. Here are some to watch out for:

Vitamin A: High doses of vitamin A can reduce the effectiveness of vitamin D in the body. Conversely, vitamin D can also reduce the toxicity of vitamin A, so maintaining a proper balance between these two vitamins is important. (You can read more about vitamin A here)

Calcium: Calcium and vitamin D work together to support bone health, but high doses of calcium supplements can reduce the absorption of vitamin D. This is because calcium and vitamin D use the same transport system in the intestines.

Magnesium: Magnesium is an important mineral needed for many bodily functions, including the activation of vitamin D. However, high magnesium supplements can interfere with vitamin D metabolism and lead to a deficiency.

While these vitamins and minerals can potentially act as antagonists to vitamin D, they are also essential nutrients needed for overall health. Therefore, it’s important to maintain a balance and not overdo any one nutrient.

Genes:

Our genes can affect our ability to absorb and utilise vitamin D in several ways. One of the primary ways is through the production of a protein called vitamin D-binding protein (DBP). DBP is responsible for transporting vitamin D from the liver to other parts of the body, where it can be used to support bone health, immune function, and other critical processes.

Research has shown that certain genetic variations can impact the amount of DBP that a person produces, which in turn affects their ability to absorb and utilise vitamin D. For example, people with certain variations in the GC gene, which provides instructions for making DBP, may have lower levels of vitamin D in their blood. The VDR gene also controls the receptivity of target cells to vitamin D

Other genetic factors may also play a role in vitamin D absorption, including variations in genes that regulate the enzymes responsible for converting vitamin D into its active form. Additionally, certain conditions, such as celiac disease, can interfere with vitamin D absorption and utilisation and may be influenced by genetic factors. 

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People at risk of low vitamin D:

• Babies and young children who don’t spend time playing outdoors
• Pregnant and breastfeeding mothers 
• People over 65 years old as the skin becomes less efficient at making vitamin D
• People with darker skin tones living in colder climates 
• People who seldom expose their skin to direct sunlight 
• People who live in higher northern hemisphere areas
• Anyone who doesn’t spend time outdoors during the day and summer months (e.g. shift workers, people who are immobile, office workers) ©

Feel free to leave us a comment if you have any further questions.

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