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All You Need to Know About Vitamin D

Vitamin D is a fat-soluble vitamin, meaning that it is absorbed along with fats in the diet and is stored in the body’s fatty tissues. (1)

Vitamin D is an essential vitamin for a wide range of body functions. Unlike other vitamins, vitamin D functions as a hormone, meaning that it serves as a messenger, controlling and coordinating activities throughout the body. Tissues in the body, as well as all the major organs, have a receptor for it. (2)

And unlike most other vitamins which must be obtained from the diet, we can make vitamin D ourselves. The process by which the body makes vitamin D is complex, but it all starts when the skin absorbs the invisible UVB rays from the sun. (3)

For this reason, vitamin D is sometimes called the ‘sunshine vitamin’.

What are the benefits of vitamin D?

Vitamin D is a nutrient that is needed by the body for health and to maintain strong bones and teeth. It does so by helping the body absorb calcium from food and supplements. (4)

Vitamin D has other functions in the body too! Muscles need it to move, nerves need it to carry messages between the brain and every part of the body, and the immune system needs vitamin D to help fight off bacteria and viruses. (5) Some research also indicates that vitamin D may help to lower the risk of diseases such as multiple sclerosis and some cancers. (6)

What are the food sources of vitamin D?

Very few foods naturally contain vitamin D. Fatty fish, such as salmon, tuna, and mackerel, and fish liver oils are some of the best sources. Small amounts of vitamin D are also found in beef liver, cheese, and egg yolks. Some mushrooms, if exposed to ultraviolet light, can also provide some vitamin D. (7)

Fortified foods provide most of the vitamin D in the American diet. Almost all of the U.S. milk supply is voluntarily fortified with 100 IU per cup. Other dairy products made from milk, such as cheese and ice cream are not usually fortified. Plant milk alternatives (almonds, cashews, and soy) are often fortified with vitamin D as well, to the amount found in fortified cow’s milk. (8)

Ready-to-eat breakfast cereals, some brands of orange juice, yogurt, margarine, and other food products may also contain added vitamin D. (8)

variety of raw foods that contain high amounts of vitamin d

Fatty fish, beef liver, egg yolks, and mushrooms naturally contain vitamin D.

How much vitamin D should I take?

The Food and Nutrition Board (FNB) sets intake reference values for vitamin D and other nutrients. The established recommended dietary allowance (RDA) represents a daily intake that is sufficient to maintain bone health and normal calcium metabolism in healthy people. See below, a list in both International Units (IU) and micrograms (mcg); the biological activity of 40IU is equal to 1 mcg. (9)

The amount of vitamin D that you need depends on your age. (9)

  • 400 IU (10 mcg) for infants 0-12 months
  • 600 IU (15 mcg) for children 1-13 years
  • 600 IU (15mcg) for adolescents 14-18 years
  • 600 IU (15 mcg) for adults 19-70 years
  • 800 IU (20 mcg) for adults 71 years and older

Am I at risk of being deficient in vitamin D?

Some people may not get enough vitamin D from the foods in their diet. People who are lactose intolerant or choose to avoid dairy products, many not get enough vitamin D from their diets. (10)

Others may be at risk of deficiency because their sun exposure is limited. People who primarily stay or work indoors or live in the more northerly regions of the US, may not get enough sunlight to produce vitamin D, especially in the winter months. (2)

People at risk include:

  • Infants who are completely or partially breastfed. While breast milk is a very nutritious choice for babies, it only has small amounts of vitamin D (about 4-40 IU per liter). For this reason, babies who are breastfed should receive a daily supplement of 400 IU from birth until they get enough from their diet. (11)
  • Adults over the age of 50 years. Older adults are at an increased risk of vitamin D deficiency in part because as they age, skin cannot make vitamin D as efficiently, but they are also likely to spend more time indoors and may not be getting enough of the vitamin from their diets. (7)
  • People with dark skin. Greater amounts of the pigment melanin result in darker skin color and decrease the skin’s ability to produce vitamin D from sunlight. (7)
  • People with medical conditions that cause fat malabsorption. Since vitamin D is a fat-soluble vitamin, its absorption depends on the guts’ ability to absorb fat. Fat malabsorption is associated with medical conditions including celiac disease, Crohn’s disease, cystic fibrosis, and some forms of liver disease. These individuals might require vitamin D supplementation. (12)

Symptoms of vitamin D deficiency

A vitamin D deficiency can occur when usual intake is lower than recommended over time, exposure to light is limited, absorption from the digestive tract is altered, or the body cannot efficiently convert the sun’s UV rays into vitamin D. (9)

In children, vitamin D deficiency can result in rickets, a condition in which the bones become soft and bend. Bowed legs is a common symptom of rickets. (13) In adults, vitamin D deficiency leads to osteomalacia, causing muscle weakness and bone pain. (14)

Do I need a vitamin D supplement?

People who do not get enough vitamin D from their diets or have low blood levels of vitamin D may need a supplement. Adults with osteopenia or osteoporosis may also need to supplement with vitamin D. To find out your vitamin D levels, visit your healthcare practitioner. Your clinician can help you decide if you should take a supplement and how much you should take. Vitamin D supplements are available over-the-counter as pills, chewable tablets, and drops.

vitamin d softgel supplement on a wooden spoon

Vitamin D supplements should only be taken on the advice of a healthcare practitioner.

Can I take too much vitamin D?

Too much vitamin D can be harmful to your health. Because vitamin D is stored in fat cells, excess amounts can build up to dangerous levels, causing high blood calcium and damage to the heart, blood vessels, and kidneys. (9)

Total vitamin D intake should remain below the level of the Tolerable Upper Intake Level (UL) to avoid possible side effects. The UL is determined by the FNB as the maximum daily intake unlikely to cause adverse health effects. Long-term intake above the UL increases the risk of adverse health effects. (7)

The total daily intake from food and supplements combined should not exceed: (7)

  • 1000 IU for infants 0-6 months
  • 1500 IU for infants 7-12 months
  • 2500 IU for children 1-3 years
  • 3000 IU for children 4-8 years
  • 4000 IU for children over 9 years of age and adults (including pregnant or lactating women)

The bottom line

You should aim to get the majority of your vitamin D from foods, fortified foods, and beverages. Foods contain other vitamins, minerals, and dietary fiber that contribute to good health. In some cases, dietary supplements can provide vitamin D that would otherwise be consumed in less-than-recommended amounts.

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