In 1920, vitamin D was found. This was the end of a long search for a way to treat rickets, a painful bone disease in children. Within a decade, vitamin D was added to foods, and rickets became a rare disease in the UK. But finding a way to stop rickets was just the beginning of what scientists learned about vitamin D. The results of research suggest that vitamin D may be important for the health of people in other ways.
Getting rid of old rules
Vitamin D is one of the 13 vitamins that were found in the early 1900s by doctors who were studying diseases caused by a lack of nutrients. Since then, scientists have thought of vitamins as organic (made of carbon) chemicals that can't be made by the body's tissues and must be gotten from food. Vitamins are very important to the way our bodies work, but we only need very small amounts of them.
Vitamin D is one of the four fat-soluble vitamins, but it is not a vitamin in the strictest sense. It's true that it's important for health, but only tiny amounts are needed. But it doesn't follow the other rules for vitamins because it's made in the human body, it's not in any natural foods except for fish and egg yolks, and the body has to change it before it can help.
Science has shown that chronic, low-grade inflammation can become a silent killer that can lead to heart disease, cancer, type 2 diabetes, and other health problems.
Most of us can't rely on our bodies to make vitamin D the way they used to because our habits have changed. Instead, we are getting more and more of this important nutrient from foods and pills that have been added to them. In the modern world, this substance might end up meeting the technical definition of a vitamin.
How does vitamin D work?
Vitamin D is made up of more than one chemical. The natural type is made in the skin from 7-dehydrocholesterol, a type of cholesterol that everyone has. The key is sunlight: Its UVB energy changes the vitamin D3 precursor into vitamin D3. Most dietary supplements, on the other hand, are made by exposing a plant sterol to ultraviolet light, which makes vitamin D2. Because D2 and D3 do almost the same thing, they are both called vitamin D. However, neither will do anything until the body does its thing (see figure).
How vitamin D is made in your body
The energy from the sun changes a chemical in your skin into vitamin D3. Your liver and kidneys then change vitamin D3 into active vitamin D.
The first stop is in the liver, where extra oxygen and hydrogen molecules are added to vitamin D to make 25-hydroxyvitamin D, or 25(OH)D. This is the chemical that doctors usually test to see if there isn't enough vitamin D. But even though 25(OH)D is used to figure out what's wrong, it can't do its job until it gets to the kidney. There, it gets a final pair of oxygen and hydrogen molecules to become 1,25-dihydroxy vitamin D. Scientists call this active form of the vitamin 1,25(OH)2D or calcitriol, but most people just call it vitamin D.
How it works
Most people know that vitamin D helps keep bones healthy by making it easier for the body to absorb calcium. Without enough vitamin D, the body can only absorb 10% to 15% of the calcium it eats. But when vitamin reserves are normal, the body can absorb 30% to 40% of the calcium it eats. When kids don't get enough vitamin D, they get rickets. When adults don't get enough, they get osteomalacia. Both of these bone diseases are now very rare in the United States. However, osteoporosis, which causes broken bones and twisted spines, is on the rise.
Low levels of vitamin D cause bones to have less calcium, which makes them more likely to break. Even if vitamin D only kept bones healthy, it would still be important. But researchers have found signs that it may do a lot more than that. In fact, vitamin D receptors, which are proteins that bind to vitamin D, are found in a lot of the body's tissues. In the intestines, the receptors pick up vitamin D, which makes it easier for the body to absorb calcium. But there are also similar receptors in a lot of other organs, like the heart, blood vessels, muscles, and endocrine glands. And work that is still going on shows that when vitamin D binds to these receptors, good things happen. The most important thing is to get enough vitamin D, which a lot of Americans don't do.
Vitamin D deficiencies
When most men rolled up their sleeves and went to work in sunny fields, vitamin D deficiency was rare. But that changed as people moved from farms to offices. Because pigmentation can stop the skin from making over 90% of the vitamin D it needs, non-white people are especially at risk. Patients with intestinal diseases that make it hard for them to absorb fat and people with kidney or liver diseases that make it harder for vitamin D to be turned into its active form, calcitriol (1,25(OH)2D), are also likely to be deficient. Also, some medications make vitamin D less available or less effective. Even in healthy people, getting older makes them more likely to not get enough vitamin D.
Even though standards vary, most experts agree that levels of 25(OH)D below 20 ng/ml (nanograms per millilitre) show a clear-cut lack of vitamin D, while levels between 20 and 30 ng/ml are borderline.
Several things can play a role. Not getting enough sun is at the top of the list. People who live above 37 degrees north or below 37 degrees south of the equator don't get enough UVB energy from the sun to make all the vitamin D they need, except during the short summer months. The same is true for people who spend most of their time indoors and for those of us who try to stay out of the sun and use sunscreen to protect our skin from the harmful effects of ultraviolet radiation (see box below). It's an example of an unintended result of being smart. Taking vitamin supplements can also help you protect yourself from the sun and give you strong bones.
Like politicians, doctors often have to find middle ground. When it comes to the sun, most politicians promise blue skies, but most doctors end up being the shady guys, or at least the ones who push for sunscreen.
There are two kinds of energy in sunlight: ultraviolet A (UVA) and ultraviolet B (UVB) (UVB). UVB gives your skin the energy it needs to make vitamin D, but it can also burn the skin and damage cells more, which can lead to cancer. UVA also causes skin damage and ageing before it should.
To stay safe, stay out of the sun during the summer, especially between 10 a.m. and 2 p.m. Wear a wide-brimmed hat, a dark, tightly woven long-sleeved shirt and long trousers whenever you can when you go out in the sun.
But summer clothes are often thin and show a lot of skin. So, a sunscreen comes in handy. Look for something that has an SPF of 30 or more. Look for a sunscreen with a "broad spectrum" that will protect you from both UVA and UVB. Use sunscreen early, often, and in large amounts.
Because of all of these things, the UK has a shockingly high number of people who don't get enough vitamin D. Even though standards vary, most experts agree that levels of 25(OH)D below 20 ng/ml (nanograms per millilitre) show a clear-cut lack of vitamin D, while levels between 20 and 30 ng/ml are borderline. Using the same criteria, researchers have found deficiencies in 42% of non-white women aged 15 to 49, 41% of non-hospitalized patients aged 49 to 83, and up to 57% of hospitalised patients. Even healthy young adults often have low levels of vitamin D. In one study, more than a third of people between the ages of 18 and 29 were deficient.
Numbers can never tell the whole story, but in this case, "D-ficiencies" add up to a wide range of health problems.
Broken bones and osteoporosis
It's a paradox that the best-known benefit of vitamin D is healthy bones, but it's also the most controversial. Doctors agree that not getting enough vitamin D makes you more likely to get osteoporosis and break bones, but they disagree about the benefits and best dose of supplements.
If you don't get enough vitamin D, your intestines can't absorb calcium well. But because calcium in the blood is important for nerve and muscle function and for the heart, the body doesn't let levels drop. Instead, it pours out a hormone called parathyroid hormone, which gets calcium out of the bones. The amount of calcium in your blood stays normal, so your heart and nerves continue to work well. But your bones take the most damage: As the amount of calcium in bones decreases, bones become weak and more likely to break.
Most studies show that a lack of vitamin D increases the risk of osteoporosis and the likelihood of hip and other non-spinal fractures. But there is considerable disagreement about how much supplements reduce the risk of fractures. Some studies only include women, while others include both men and women; some only use frail, elderly, or institutionalised people as subjects, while others use people who are physically active; some use vitamin D alone, while others use vitamin D along with different amounts of calcium; and some give 400 international units (IU) of vitamin D a day, while others give up to 800 IU a day.
Some men mistakenly think that osteoporosis is only a problem for women, but none of them don't understand how important prostate cancer is.
Vitamin D is an important part of how cells grow and divide. Lab tests suggest that it helps stop the uncontrolled growth of cells that is a sign of cancer by slowing cell division, cutting off the blood supply to tumours (angiogenesis), making more cancer cells die (apoptosis), and stopping cancer cells from spreading (metastasis). Like many human tissues, the prostate has an abundant supply of vitamin D receptors. It also has enzymes, like some other tissues, that change the biologically inactive form of vitamin D, 25(OH)D, into the active form, 1,25(OH)2D. Normal prostate cells have a lot more of these enzymes than cancerous prostate cells do.
Do the results of these experiments have effects that are important in the real world? Possibly.
In 1998, Harvard's Health Professionals Follow-up Study of 47,781 men found that taking a lot of calcium supplements was linked to a higher risk of advanced prostate cancer. Men who got more than 2,000 mg of calcium a day from food and supplements were at the highest risk. Since then, other studies have shown that very high levels of calcium intake are linked to a higher risk, but they have cleared calcium from foods. Scientists at Harvard think that the problem is not calcium itself, but rather that there is not enough active vitamin D.
Other types of cancer
People who live far from the equator seem to be more likely to get colon cancer, breast cancer, and other cancers. Sun exposure and levels of vitamin D could be a part of the answer. In a recent clinical trial, taking 1,000 IU of vitamin D every day was not linked to a lower risk of cancer, but it was linked to a lower risk of dying from cancer.
"D" just enough
The most common recommended daily allowance (RDA) for vitamin D is 600 IU for adults up to age 69 and 800 IU for adults over age 70.
Is more better? We don't know yet, but too much of a good thing is for sure possible. Vitamin D is stored in the body's fat tissue, just like the other fat-soluble vitamins. That means your body can use its own stores if you miss a day or two of getting enough vitamin D, but it also means that too much vitamin D can build up to dangerous levels. At these levels, vitamin D can raise the amount of calcium in the blood to a point where it can cause grogginess, constipation, and even death. But toxicity only happens when too much is taken.
How to get vitamin D
You can get vitamin D the old-fashioned way by letting UVB rays from the sun hit your skin. It doesn't take much, but people who live north of 37 degrees latitude, which is roughly the imaginary line between Philadelphia and San Francisco, can't get enough UVB in the winter to do the trick. And many others will find it too easy to get too much UVB, which will increase their risk of malignant melanomas and other types of skin cancer, as well as wrinkles and skin that ages too quickly. In general, most doctors say to stay out of the sun (see box) and take vitamin D by mouth.
Diet can help, but it's hard to reach the new goals just through food. Fish and shellfish naturally contain vitamin D (oily fish have the most), but to get 400 IU, you'd have to eat about 5 ounces of salmon, 7 ounces of halibut, 30 ounces of cod, or almost two 8-ounce cans of tuna. An egg yolk has about 20 IU of vitamin D, but it also has almost a day's worth of cholesterol, so you can't really use eggs to fill up on vitamin D. Other foods have even less vitamin D, which is why milk, some yoghurt, some orange juice, and a lot of cereals are fortified with it. In general, one serving has about 100 IU. To get 400 IU, you would have to drink a quart of fortified milk.
Most people have to take vitamin D supplements to get what they need. This is the main reason to take a multivitamin every day; most of them have 400 IU. Make sure to carefully read the labels so you don't get too little or too much. Cod liver oil is full of vitamin D, but it has too much vitamin A to be used every day.
A new look at vitamin D
It used to be easy: just get a "healthy" tan and your body will make all the vitamin D it needs. All of that has changed because of desk jobs and sunscreen. At the same time, research is showing how important vitamin D is and how it might help prevent many health problems. This makes vitamin D a modern problem with a modern solution: eat fish and drink low-fat fortified milk, as well as take vitamin D supplements in the right amounts.