May 28, 2017

View as List Vitamin D: How Much Do You Know?

  • Vitamin D: How Much Do You Know?

    In recent years, countless headlines have proclaimed vitamin D a nutritional superstar because of its purported health benefits, making it one of the top-selling dietary supplements. How much do you know about the “sunshine vitamin"? Take our quiz and find out.

  • 1

    How many people have blood levels of vitamin D that are either deficient of insufficient?

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    (a)  5 percent

    (b)  15 percent

    (c)   25 percent

    (d)  At least 50 percent

    (e)  90 percent

  • 2

    Answer: (d) At least 50 percent.

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    By many estimates, at least half of people in the US and many other countries have low blood levels.

  • 3

    Why do so many people fall short on vitamin D?

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    (a)  They spend less time in the sun

    (b)  They wear sunscreen

    (c)  They live in northern areas where sunlight is inadequate most of the year

    (d)  They are obese

    (e)  Not many foods supply vitamin D.

    Hint: There is more than one correct answer.

  • 4

    Answer: All are correct.

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    The skin synthesizes vitamin D when it is exposed to ultraviolet (UV) rays of the sun, but many people limit or avoid sun exposure because of the risk of skin cancer, or else they use sunscreen to block UV. Obesity is linked with lower blood levels of D because the vitamin is trapped in fat tissue. Surprisingly few foods supply vitamin D.

  • 5

    True of false:

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    Vitamin D is a hormone.

  • 6

    Answer: True.

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    But only after the kidneys convert it into its biologically active form, called calcitriol. Vitamin D is actually a group of related compounds with a mind-numbing array of names and a complex metabolism. The vitamin D made in the skin and found in most foods and capsules is called vitamin D3, which is first converted to a compound known as calcidiol—the form usually measured in blood tests. These forms of vitamin D are considered pre-hormones.

  • 7

    True or false:

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    Younger people need to consume more vitamin D than older folks.

  • 8

    Answer: False.

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    Older people need to consume more because the skin’s ability to produce vitamin D from the sun’s ultraviolet rays declines with age. A 70-year-old makes only about one-quarter as much vitamin D, on average, as a 20-year-old from the same sun exposure. What’s more, in older people the liver and kidneys are not as efficient in converting vitamin D to its active form, and vitamin D is not absorbed as well from the intestine. 

  • 9

    The daily Recommended Dietary Allowance (RDA) for vitamin D for most adults is:

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    (a)  400 IU

    (b)  600 IU

    (c)  800 IU

    (d)  1000 IU

  • 10

    Answer: (b) 600 IU.

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    In 2010 the Institute of Medicine (IOM) raised the RDA for most adults from 400 IU up to 600 IU. Plus, it raised the RDA for people age 70 and older to 800 IU. The IOM said that most Americans don’t need to take supplements to achieve adequate blood levels of vitamin D. However, many experts disagree and recommend supplements of 800 to 2,000 IU a day for people likely to have low blood levels.

  • 11

    The best evidence of benefit from vitamin D is for:

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    (a)  Preventing cancer and heart disease

    (b)  Treating arthritis

    (c)  Treating depression and cognitive decline

    (d)  Maintaining healthy bones

    (e)  Maintaining muscle function and balance

  • 12

    Answer: (d) and (e)

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    Observational studies have linked low vitamin D levels to everything from arthritis, depression, cancer, and heart disease to diabetes, infections, MS, and Parkinson’s disease. But such studies don’t prove causation. Most well-designed clinical trials support vitamin D’s role in keeping bones strong, and some have found muscle benefits. Several large clinical trials on vitamin D are underway, but results are not expected until 2017 to 2020.

  • 13

    The main natural dietary source of vitamin D in the US diet is:

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    (a)  Milk

    (b)  Certain fish

    (c)  Mushrooms

    (d)  Eggs

    (e)  Breakfast cereal

  • 14

    Answer: (b) Certain fish.

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    Fish such as trout, salmon, and tuna are naturally rich in D. They provide 230 to 645 IU per 3-ounce serving. But milk is by far the main dietary source of vitamin D because it is fortified (100 IU per cup) and we drink so much of it. Some breakfast cereals, orange juice, and other foods are now also fortified with vitamin D. Mushrooms produce vitamin D only when exposed to ultraviolet light from the sun or UV lamps. 

  • 15

    True or false:

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    Cod liver oil is a good source of vitamin D.

  • 16

    Answer: True, but we don’t recommend it.

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    Cod liver oil is naturally rich in vitamin D, which is concentrated in the liver. But because the oil is also very high in vitamin A, it is not a good option. Though research is inconsistent, some studies have linked high doses of vitamin A to impaired bone health. Don't take cod liver oil, unless the label says that the vitamin A content has been reduced.

  • 17

    How much time does the average person need to spend in the sun to make adequate vitamin D?

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    (a)  5 minutes a week

    (b) 15 minutes twice a week

    (c) At least 15 minutes nearly every day

  • 18

    Answer: (b)...

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    ...though some experts would say nearly every day. The standard recommendation has been to get about 10 to 20 minutes of sun exposure between 10 a.m. and 3 p.m. twice a week to the face, arms, and legs without sunscreen to produce adequate vitamin D. However, the skin’s synthesis of vitamin D varies from person to person, depending on factors such as age, skin color, geographic location, and genetics. 

  • 19

    True or false:

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    Your skin can make vitamin D when exposed to sunlight through a car window.

  • 20

    Answer: False.

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    Glass filters out most of the ultraviolet-B rays that trigger vitamin D production in the skin. Ultraviolet-A rays, which don’t increase vitamin D production but do cause skin aging and contribute to skin cancer, do get through glass. 

  • 21

    For more on vitamin D

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    Read our article Vitamin D: What's the Latest?

    Or order our 48-page special report on vitamin D.