by Sue Bedford, nutritional therapist
Vitamin D is a fat soluble vitamin which is only found naturally in small amounts in a few foods. Therefore, to make vitamin D more available to us, it is added to dairy products, juices, and cereals that are then said to be ‘fortified with vitamin D’.
Most vitamin D – 80 per cent to 90 per cent of what the body gets – is obtained through exposure to sunlight.
It is often referred to as the sunshine vitamin, as sunlight is necessary for the synthesis of this vitamin (which is produced underneath the skin following exposure to sunlight). It occurs in two forms: vitamin D2, which is present in a small number of foods, and vitamin D3, which is formed in the skin when exposed to sunlight. Both D2 and D3 are converted into a form that the body can use in the liver and the kidneys. People need varying degrees of vitamin D depending on where they live and their diets.
In the UK (and people living in the northern hemisphere) we don’t get enough of the kind of sunlight that causes our bodies to manufacture vitamin D under the skin. Only one kind of solar radiation does this: UVB sunlight. Vitamin D is synthesised only when we’re exposed to UVB rays – and unless UVB rays are present it doesn’t matter how warm it is, or in fact how brightly the sun is shining: your skin cannot produce vitamin D. In the UK we are exposed to UVB in reality from April to October.
It is the opinion of Harvard Medical School that ‘except during the summer months, the skin makes little if any vitamin D from the sun at latitudes above 37 degrees north or below 37 degrees south of the equator. People who live in these areas are at relatively greater risk for vitamin D deficiency’.
Why is vitamin D important in the body?
- for the normal absorption/utilisation of calcium and phosphorus
- it contributes to normal blood calcium levels
- for the maintenance of normal bones and teeth
- for the maintenance of normal muscle function
- it contributes to the normal function of the immune system
- it plays a role in the process of cell division
- it is needed for normal growth and development of bone in children
In addition, Vitamin D is used in the treatment of conditions of the heart and blood vessels, including high blood pressure and high cholesterol. It is also used for diabetes, obesity, muscle weakness, multiple sclerosis, rheumatoid arthritis, chronic obstructive pulmonary disease (COPD), asthma, bronchitis, premenstrual syndrome (PMS), and tooth and gum disease. Some people use vitamin D for skin conditions including psoriasis, actinic keratosis, and lupus vulgaris. More recently the importance of vitamin D has been recognised as a significant factor in relation to fertility, although there is still a lot more research to be done.
Although the recommended daily dose of vitamin D is 400 international units (IU), new research suggests taking at least 1,700 IU of vitamin D daily to get its health benefits. And while sunshine is the best source of vitamin D, you can still load up on the vitamin by adding vitamin D-rich foods to your diet.
Vitamin D, fertility and IVF
In the world of fertility, the importance of vitamin D has been studied in experiments involving mice. These studies have shown us that mice that are vitamin D deficient or lack a vitamin D receptor have an underdeveloped uterus or an inability to form mature eggs. If a pregnancy did occur in these mice the foetus produced often showed impaired growth. With vitamin D supplementation reproduction is returned to normal in mice but not by giving calcium alone – suggesting that vitamin D’s role in female reproduction is not related to the ability to absorb calcium.
In humans, the vitamin D receptor is present in many female organs including the uterus, ovary and placenta.
The active form of vitamin D (D3) has various important roles in human reproduction. It is thought to help control the genes involved in making oestrogen. It also controls several genes involved in the implantation of the embryo. Once a woman is pregnant vitamin D3 is involved in the organisation of the immune cells in the uterus. During pregnancy, if a woman is deficient in vitamin D it has been linked to some complications such as diabetes and hypertension.
In men, vitamin D status has been associated with semen quality and sperm count, motility and morphology. There is evidence to suggest that if a man is not deficient in vitamin D then there is a positive effect to be seen on semen quality, testosterone concentrations and fertility outcomes. Further studies are required in this area.
IVF helps in the study of the role of vitamin D during preconception from egg development to the implantation of the embryo. In a recent study it was discovered that women with higher vitamin D levels were significantly more likely to achieve a pregnancy from IVF compared to women with lower levels of vitamin D.
The study was repeated in a different IVF unit and it was found that there was a fourfold difference in pregnancy achievement between these with sufficient vitamin D levels in comparison those women who were deficient. Further research is needed into this emerging evidence that vitamin D levels may be linked to IVF success.
Which foods are good sources of vitamin D?
- Egg yolk
- Cod and halibut liver oils
So, the question we all need to ask ourselves; are we getting enough vitamin D in our diet?