In the 19th century, an epidemic raged in Europe and the United States. Children didn’t grow. Their bones were soft and as they got older, their legs would gradually became deformed and they suffered from soreness and pain in their bones. The disease became known as rickets or “the English disease”. 

Three children with rickets. Friends' Relief Mission, Vienna XII, n.d.
Three children with rickets. Friends’ Relief Mission, Vienna XII, n.d.

The cause? Lack of vitamin D. You may know it as the vitamin that your body can make from sunshine. But to get enough vitamin D, you also need to have it in your diet.

Rickets is thought of as an old disease. You might be surprised to know that rickets continues to be a common disease today in children globally (Elder and Bishop 2014). But it can start as early as in the womb. 

There is still much we don’t know about the metabolic changes that occur in pregnancy, when the mother’s body must cover all of the nutritional needs of the fetus as well as her own. During pregnancy, the mother transfers on average 30 grams of calcium to the fetus to build and mineralize the fetus’ bones (Gustafsson 2018; Kovacs 2016; Moon et al. 2015). (That is roughly equivalent to the amount of ground coffee beans needed to make two cups of coffee.)

To be able to do this without using the calcium from her own bones, the mother’s body becomes much more efficient at absorbing calcium from her intestines. Vitamin D is essential for this process. 

Why vitamin D is important for your bones

Calcium is involved in many processes inside of your body. It helps your cells to communicate with one another, it helps your proteins to function and it preserves your bone health.

Your bones act as a storage compartment for calcium. Much like a bank, your body stores its calcium in your bones. If you withdraw too much of your calcium savings, you’ll be in trouble. But at the same time, you need to keep some cash on hand – or in this case, some calcium in your blood. The level of calcium that is in your blood and available for your cells is carefully controlled by what is put into or withdrawn from your bone bank, intestines and kidneys. If the level of calcium in your blood is too high or too low, you can get severely sick and your bones can suffer.

Specialized cells sense the level of calcium in your blood and control several chemical messengers known as hormones. Together these hormones ensure that the level of calcium in your body, and the balance of calcium across your bones and your blood, is kept just right. One of these hormones is vitamin D.

Both a pregnant mother and her fetus are dependent on her own vitamin D levels to build and maintain a strong skeleton. When the infant is born and begins nursing, the mother’s vitamin D levels also determine the amount available in her breast milk. 

Given the importance of vitamin D to our health from the start of our development, it is curious that there is still so much we don’t understand about this vitamin.

“Over the last ten years, around 30,000 papers have been published on vitamin D and yet we still don’t fully understand how it works,” said Associate Professor Miriam K. Gustafsson. She has studied the alterations of vitamin D during pregnancy and how it relates to the health of women and their newborns.

What is we know is that vitamin D is important for many bodily processes, the function of our cells and our immune system. Insufficient vitamin D levels are associated with what researchers refer to as total mortality. That means that on average, people with low levels of vitamin D have higher rates of mortality (death) than those with adequate levels. This could be due to an elevated cancer risk, a less effective immune system, autoimmune disorders, type-2 diabetes and even depression (Nair and Maseeh 2012). All of these health issues might be affected by vitamin D levels.

Vitamin D is important in bone development as well as cell communication and brain health.
Vitamin D is important in bone development as well as cell communication and brain health.

How much vitamin D do you need?

You can get vitamin D from your diet or your body can actually create it in your skin when your skin is exposed to the sun’s ultraviolet B rays. Vitamin D is carried from your skin or your gut to your liver and then to your kidneys. In your kidneys, it is converted to an active form, a hormone. The active vitamin D hormone is called 1,25-dihydroxyvitamin D and its production is tightly regulated. For simplicity, let’s just call it vitamin D. 

In your body, vitamin D is mainly bound to vitamin D-binding protein. Only the free, unbound vitamin D is available to do work in your body. Active vitamin D signals to cells through a receptor on cells’ surfaces. This receptor is a protein that acts like a telephone to the inside of the cell and tells it what genes to turn on and off. In this way, vitamin D can turn on or off hundreds of genes in many different cell types. In addition to controlling calcium levels in your bones and blood, vitamin D is involved in controlling cell division, producing other hormones and regulating your immune system.

Because vitamin D is involved in so many important processes, your body puts tight guardrails on the amount of active vitamin D available to your cells.

There is disagreement about what the optimal level of vitamin D is, including for pregnant women. The recommendations differs between health authorities and experts, but it is generally agreed that less than 50 nmol/L is an insufficient vitamin D status and less than 30 nmol/L is a deficiency. 50 nmol/L of vitamin D corresponds roughly to 20 nanograms vitamin D per milliliter (one nanogram is a billionth of a gram) (Gil et al. 2018).

Many experts recommend an intake of 10 micrograms of vitamin D per day. That equals about one teaspoon of fish oil (a great source of vitamin D), 100 grams of salmon or 5 to 15 minutes of sunlight on the face and arms 2-3 times per week in the summer (in the northern hemisphere) (Public Health England 2016; Public Health England 2016). For people above 75 years of age, Norwegian authorities recommend 20 micrograms of vitamin D daily.

“Vitamin D is essential for bone, muscle and other aspects of health, and therefore an important aim is to restore and maintain vitamin D levels in the normal range,” Miriam said. “Most experts will agree that we don’t need routine vitamin D screening of everyone. However, among people at high risk, including ethnic minority groups with dark skin and those with malabsorption syndromes, measuring vitamin D levels in blood should be considered. Maternal vitamin D status affects fetal skeletal growth in the womb. So we need to make sure that the mother gets enough vitamin D.”

But how much vitamin D is needed during pregnancy? Again, we don’t know for sure.

“During pregnancy, in regards to supplementation, there are large differences in official recommendations. The World Health Organization does not recommend any extra supplements. In Norway, we recommend a daily intake of 10 micrograms of vitamin D supplements per day, while the Nordic council doesn’t. In England, the recommendation is also 10 micrograms per day with at least 35 in the dark period of the year,” Miriam said. 

However, Miriam has found that fewer than one fourth of pregnant women adhere to these guidelines.

If not through supplements, how can you make sure that you have enough vitamin D, especially if you are a woman who is pregnant or trying to get pregnant?

A mother supplies her fetus and breastfeeding infant child with vitamin D from her own diet. But a growing child and moth can both get vitamin D from their diet and sunlight. Photo by Xavier Mouton Photographie on Unsplash.
A mother supplies her fetus and breastfeeding infant child with vitamin D from her own diet. But a growing child and mom can both get vitamin D from their diet and sunlight. Photo by Xavier Mouton Photographie on Unsplash.

Vitamin D from sunlight

For most of us, the most important source of vitamin D is UV-B radiation in sunshine. 

“Many factors can affect Vitamin D status, including season, air pollution, skin pigmentation, aging, sunscreen, clothing habits and obesity,” Miriam said. 

Most people living in the Nordic countries can actually get enough vitamin D by wearing short sleeves and being outside in the sun for short periods (5-10 minutes) a few times weekly during the summer. But you should be careful not to burn your skin. 

Note that sunscreen protects the skin from UV-B radiation, which is what triggers the production of vitamin D in the skin. Nevertheless, if you are planning to be out for long, stay safe in the sun and use sun protection. Getting enough vitamin D and protecting yourself from the sun aren’t mutually exclusive options. Spending around 5-10 minutes in the sun a few times per week isn’t harmful for most adults, and wearing sunscreen at the beach isn’t going to make you vitamin D deficient.

Taking advantage of the sun might be difficult, however, for people who work inside. Even in low-latitude countries where sunshine is readily available, insufficient vitamin D status is still very common.

“Many of us have jobs that involve sitting inside, so even if there is sunlight we might not be able to take advantage of it,” Miriam said. “Clothing and coverage for cultural reasons and behavioral attitudes can also affect vitamin D status. Higher latitudes and longer distances from the equator reduce the amount of sunlight that is available to us.”

When sunlight isn’t an option, vitamin D must come from our food or from supplements.

“We have a six month vitamin D winter,” Miriam said. “From October to April, the sun is too low in the sky for the skin to synthesize vitamin D.” 

Food sources of vitamin D.
Food sources of vitamin D.

Vitamin D in your food

As mentioned above, sunshine and fish oil are excellent sources of vitamin D. Outside of sunlight, the best sources of vitamin D are fatty fish, fish liver oils, egg yolks, certain wild mushrooms and dark chocolate!

In Norway and across the globe, cod fish oil, made by extracting the oil from fish livers, has been a popular dietary supplement ever since it was discovered that the oil could prevent rickets in children. Cod fish oil is rich in vitamin D. But people today aren’t eating fish or fish oil as often as they used to.

“Unfortunately, fish oil is getting out of fashion, also in Norway,” Miriam said. 

Fish oil is recommended for infants in Norway and although many will happily eat it, my daughter did not. Baby puke is never nice, but after having my apartment smell like fish oil-puke for three days in a row, I gave up. I must confess that I have never personally taken fish oil either. Thankfully there are other options both for babies and adults. 

Fatty fish, egg yolks, certain wild mushrooms and dark chocolate are excellent sources of vitamin D. Many countries also fortify milk products or bread to contain vitamin D. If you don’t eat fish, it might be difficult to cover you daily vitamin D intake solely from food. While you need as little as 100 grams of salmon (about one portion) to get 10 micrograms of vitamin D, you need about 2 liters of milk. 

Notice that fish liver is not generally recommended for pregnant women because of environmental toxins. This means that getting enough vitamin D solely from food is often difficult for pregnant women. Vegetarians and vegans will also find it difficult to cover their vitamin D intake solely from food. Supplements or vitamin D fortified foods are therefore useful for many of us.

Is there any other time in life when what you eat gets more attention than during pregnancy?

Probably not. There is so much you should or shouldn’t eat that it can get very confusing. Add cravings or morning sickness to your day and you might end up living on watermelon and yoghurt like I did when I was pregnant – although I would in no way recommend that.  

If you are pregnant and find that you simply can’t eat any foods rich in vitamin D, or you are avoiding them out of safety concerns, you should talk to your doctor about taking a supplement. For example, if you would prefer not to eat fish, or if you’re unsure about the safety of fish and fish oil during pregnancy, synthetic vitamin D pills could be an excellent alternative. 

Here are some recipe ideas for how you can get more vitamin D into your diet!

Salmon is a great source of vitamin D. Photo by PTMP on Unsplash.
Salmon is a great source of vitamin D. Photo by PTMP on Unsplash.

Vitamin D and nursing

It is generally recommended that fully breastfed infants are given 8.5-10 micrograms of vitamin D per day. Most mothers don’t have enough vitamin D to fully cover their baby’s and their own needs – which is perfectly okay. With an infant to care for, you have more than enough to think about. You would have to increase your daily intake to around 100 micrograms to fully cover your baby’s and your own needs. The exact amount depends on your initial vitamin D levels, your diet, daily sun exposure and pigmentation. Vitamin D is therefore recommended for infants as a precaution.

Generally speaking, the dangers of vitamin D deficiency in infancy outweigh the risk of getting too much. The safety range of vitamin D is very broad and there is generally no risk of vitamin D overdose by supplementing breastfed infants with up to 10 micrograms per day. Babies who get formula will typically not need any additional supplements, because formula already contains vitamin D (and other vitamins and minerals). 

Even though infants (and their parents) can take 10 micrograms of vitamin D per day without any problems, there is no additional health benefit of taking more. Unless you are diagnosed with vitamin D deficiency and told by your physician to take additional supplements, you should not take more than the daily recommendation. Too much vitamin D can actually be toxic.

Avoid too much vitamin D

If you combine several different forms of supplements and exceed the daily safety range from 10 – 250 micrograms of vitamin D, you might overdose and eventually get toxic levels of vitamin D. When you talk to your physician, be sure to mention your diet and any other supplements that you are taking that might contain vitamin D.

Because vitamin D is stored in fat, it is released when you lose weight. The vitamin D levels in your blood can therefore increase as your weight decreases. If you are in the process of losing weight, be sure to mention this to your physician as well. 

It is practically impossible to overdose on vitamin D from the sun or your diet alone. But if you eat a lot of vitamin D-enriched foods and/or combine several different supplements, you might overdose on vitamin D. Too much vitamin D can cause the calcium levels in your blood to rise to problematic levels, a condition known as hypercalcemia. Hypercalcemia is a serious condition that affects multiple organs and can cause kidney-stones, heart problems, confusion, fatigue and depression. 

Taking vitamin D supplements in the range of 1000-25000 micrograms daily for several months is not safe and will eventually lead to health problems because of vitamin D overdose.

With vitamin D, as with other vitamins and minerals, it’s important to remain within the healthy range. You want to have enough, but not too much. Science still has a ways to go before we know exactly how much vitamin D each of us should have and how much we should get from our food (or supplements) every day. In the meantime, we know that vitamin D deficiency is widespread. We also know that groups at risk for deficiency such as pregnant women, people with darker skin or people who are inside for most of the day, can benefit from increasing their vitamin D intake. 

Whether you go for sunshine or fish oil, that is up to you!


Signe Elisabeth Åsberg, PhD

I'm interested in everything biology, but mainly antibiotics. Scientist by day, science blogger by night. My research is focused on the interactions between the immune system, intracellular bacteria and antibiotics. I blog about everything immune related.

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