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Nutrition and Metabolism

Vitamin D

What is vitamin D?

Vitamin D is the sunshine vitamin. Human body can synthesize sufficient vitamin D from cholesterol when the skin is exposed to adequate amount of sunlight. Vitamin D deficiency can lead to rickets, a disorder featuring softening and weakening of the bones. Sufficient vitamin D in human body helps prevent many chronic diseases such as osteoporosis, rheumatoid arthritis, high blood pressure, depression, and certain cancers.

How is vitamin D used in human body?

Vitamin D helps maintain normal levels of calcium and phosphorus in blood circulation by controlling their absorption from diet and their release from bone deposit.

What is the normal vitamin D level in human body?

Vitamin D level is measured by serum 25-hydroxy vitamin D (25-OH vitamin D or calcidiol) concentration, which is considered adequate between 30 and 74 ng/ml.  Concentrations between 20 and 29 ng/ml are regarded as insufficient and that less than 20 ng/ml indicates deficiency. An excess of vitamin D (75 ng/ml or more) may cause high blood calcium, which can lead to overcalcification of bones and soft tissues, kidney stones and hypertension. You need to seek advice from your health providers if your serum 25-hydroxy vitamin D is too high.

How much vitamin D do I need in my diet?

If you have adequate sunlight exposure, equivalent to 10-15 minutes of sun exposure twice a week on the face, arms, hands, or back without sunscreen during summer, you body will synthesize enough vitamin D from cholesterol and you will not need it from diet.  Otherwise, you need to get vitamin D from diets or supplements. The Institute of Medicine of the National Academies recommends AI (adequate intake) and UL (Upper Intake Levels) for different human population based on age and gender (Table 1). 

Table 1. AI and UL of Vitamin D recommended by the Institute of Medicine of the National Academy of Sciences. 1 μg is equivalent to 40 IU (International Unit).

Age (years)  AI (μg/day) UL (μg/day)
1–70 15 100
> 70 20 100

Why vitamin D is more important for people carrying certain gene variants?

Genetic variations in two genes, GC and CYP27B1, are responsible for lower serum 25-hydroxy vitamin D concentrations. GC gene encodes the vitamin D binding protein, the main transporter of vitamin D in circulation. CYP27B1 encodes an enzyme that activates vitamin D. People carrying one variant of GC, about 22% in general population, and one variant of the CYP27B1, about 65% in general population, need to pay more attention to increase sunlight exposure or to increase their vitamin D intake to avoid deficiency.

Which foods contain high level of vitamin D?

Not that many natural foods contain vitamin D. The richest natural vitamin D source is cod liver oil. Some fish (mackerel, catfish, tuna, sardine and herring) and certain mushrooms (Maitake, also known as hen-of-the-wood, chanterelle, morel) are also good sources of natural vitamin D rich foods. Eggs also contain significant amount of vitamin D, mainly in egg yolk. Table 2 represents the top 10 vitamin D rich foods. Many fortified foods in grocery stores, such as milk and ready-to-eat cereals, also have the amount of added vitamin D listed in the Nutrition Facts on the label.

Table 2. Top vitamin D-rich foods.

Fish Oil
Fish oil
Per 100 g (μg) 250
Per Serving (μg) 11
Serving Size 1 teaspoon (4.5 g)
Fish
Fish, mackerel
Per 100 g (μg) 16.1
Per Serving (μg) 14
Serving Size 3 oz (85 g)
 
Fish, Catfish
Per 100 g (μg) 12.5
Per Serving (μg) 10.6
Serving Size 3 oz (85 g)
 
Fish, tuna
Per 100 g (μg) 5.7
Per Serving (μg) 4.8
Serving Size 3 oz (85 g)
 
Fish, sardine
Per 100 g (μg) 4.8
Per Serving (μg) 4
Serving Size 1 cup (89 g)
 
Fish, herring
Per 100 g (μg) 4.2
Per Serving (μg) 3.6
Serving Size 3 oz (85 g)
Mushrooms
Mushroom Maitake
Per 100 g (μg) 28.1
Per Serving (μg) 19.7
Serving Size 1 cup (70 g)
 
Mushroom, Chanterelle
Per 100 g (μg) 5.3
Per Serving (μg) 2.9
Serving Size 1 cup (54 g)
 
Mushroom, Morel
Per 100 g (μg) 5.1
Per Serving (μg) 3.4
Serving Size 1 cup (66 g)
Fortified Cereals
Cereal, GENERAL MILLS, TOTAL
Per 100 g (μg) 8.3
Per Serving (μg) 2.5
Serving Size 1 NLEA serving (31 g)
Eggs
Eggs
Per 100 g (μg) 2
Per Serving (μg) 0.9
Serving Size 1 medium (44 g)
Dairy Products
Milk
Per 100 g (μg) 1
Per Serving (μg) 2.4
Serving Size 1 cup (245 g)

How to get sufficient vitamin D?

The best way to get sufficient vitamin D is to have adequate amount of sunlight exposure.  This way you do not need dietary vitamin D.

If you cannot get enough sunlight exposure, you are recommended to use the GB Food Calorie and Nutrition Calculator to get the most accurate estimate of your current vitamin D intake.

To get dietary vitamin D, some fish and certain mushrooms listed in Table 2 are good natural food sources. Most of these foods are healthy in general.

Vitamin D is one of the nutrients that are required to be listed on the food labels. If the foods listed in Table 2 are not part of your life style, you have to then relay on fortified foods such as milk, cereal, and many other read-to-eat foods in grocery stores. You need to pay attention to the Nutrition Facts in the label to understand the vitamin D content per serving.

Vitamin D supplement is the last line to ensure enough intakes. In this case, an upper intake level of 100 μg/day need to be observed. For pregnant women or people with certain conditions such as parathyroid disorders, advice from health providers are recommended before taking any supplement.

 

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