NUTRITION
April 14, 2015

Holy Mackerel! (And Other Foods)

Malcolm D. Kearns, M.D. and Vin Tangpricha, M.D., Ph.D.
Fatty fish — and the sun — help us get the vitamin D we need. But too much D can be toxic.

Dr. Kearns is a Medical Resident and Dr. Tanguricha is Associate Professor, the Division of Endocrinology, Metabolism and Lipids of the Department of Medicine, Emory University School of Medicine in Atlanta, Georgia.

Vitamin D is tremendously important to health, particularly bone health. In the first segment of this two-part series, we discussed the reasons why so many people are deficient in vitamin D, as well as some of the diseases for which vitamin D appears to offer protection. In this second and final installment, we will give you an idea of ways to make sure you get enough of this essential vitamin in your diet and in your life.

Your need for vitamin D — your recommended daily allowance — changes over the life span. Here's what you need to know about getting enough D naturally and through supplements, and how to avoid over-supplementing with vitamin D. Toxicity is possible and dangerous.

How Much Vitamin D Does Your Body Need?

The Food and Nutrition Board has created the Dietary Reference Intakes (DRIs) for vitamin D and other nutrients in order assess the adequate intake of each nutrient in healthy individuals. The DRI varies by age and gender and can include: 1) the Recommended Dietary Allowance (RDA), or average daily intake needed to meet the requirements of 97-98% of healthy people; 2) Adequate Intake (AI), or the amount of a nutrient that ensures nutritional adequacy when an RDA cannot be developed; and 3) Tolerable Upper Intake Level (UL), or the maximum daily amount of a nutrient unlikely to cause adverse events .

Five to 30 minutes of sun exposure between 10 AM and 3 PM at least twice a week without sunscreen may lead to sufficient vitamin D synthesis.

The RDA for vitamin D represents the daily intake of vitamin D sufficient to maintain blood concentrations over 20 ng/ml and increases with age, from 400 IU/day for children from birth to one year, to 600 IU/day for children and adolescents from age 1 to 18 years, to 1500-2000 IU/day for everyone over 18 years old . However, higher doses of vitamin D are frequently required for vitamin D-deficient children or adults. The upper level intake increases by age as well, from 1000 IU/day for infants from birth to 1; to 2500 IU/day for children 1 to 3 years; to 3000 IU/day for kids 3 to 8 years; to 4000 IU/day for those over 9 years .

Treating and Preventing Vitamin D Deficiency
Sun exposure remains the best way to prevent vitamin D deficiency . Sun exposure can be quantified as a minimal erythemal dose (MED), the amount of time in the sun for the skin to turn pink. One MED is equivalent to ingesting 10,000-25,000 IU of vitamin D3 (3) and can last twice as long in the blood as ingested vitamin D

Cloud cover, pollution and shade can block up to 60% of UV radiation and sunscreen blocks a majority of vitamin D-producing radiation, making it difficult to create guidelines for adequate sun exposure. How far North or South one lives is also a factor, especially in relation to the season.

It is not known whether adequate sun exposure for vitamin D sufficiency can be obtained without increasing cancer risk.

To get enough sun for sufficient vitamin D synthesis, it's estimated that you need from 5 to 30 minutes of sun exposure between 10 AM and 3 PM at least twice a week without sunscreen. Commercial tanning beds can also be effective , but these are only recommended for people who are deficient, given the risk of skin cancer.

The potential carcinogenic effects of UV radiation needs to be considered — it is not yet known whether adequate sun exposure for vitamin D sufficiency can be obtained without increasing skin cancer risk.

For this reason, if you suspect you have inadequate sun exposure, the first step should be to supplement your diet with foods containing vitamin D.

Sources of Vitamin D

Vitamin D2 and D3 come from different food sources . The best natural food sources of vitamin D3 are from fatty fish (such as salmon, tuna, mackerel) and fish liver oils . Certain mushrooms provide vitamin D2 .

In the U.S., fortified foods are the best source of dietary vitamin D . Nearly all infant formulas and milk are fortified with vitamin D. This has nearly eliminated rickets since its introduction in 1930 .

A list of vitamin D-containing foods published by the U.S. Department of Agriculture’s (USDA’s) Nutrient Data Laboratory (169) is below. It is important to recognize that the amounts of vitamin D in these products can vary widely from what is advertised .

Natural and fortified sources of vitamin D. Adequate sun exposure remains the primary means of preventing vitamin D deficiency . However, those who receive inadequate sun exposure should ingest vitamin D in their diet. The best natural food sources of vitamin D are from fatty fish and fish liver oils for vitamin D3 and certain mushrooms for vitamin D2 , . In the U.S., fortified foods are the best source of dietary vitamin D , . The following list of vitamin D-containing foods is adapted from the U.S. Department of Agriculture’s (USDA’s) Nutrient Data Laboratory .

Table 1. Natural and Fortified Sources of Vitamin D
Source Amount of Vitamin D
Sun exposure 10,000 IU-25,000 IU of vitamin D3 per minimal erythemal dose
Vitamin D-containing foods (IU/100 g)
Fish
Atlantic mackerel, raw 643 IU
Atlantic cod, raw 36 IU
Bluefin tuna, raw 227 IU
Canned salmon 547 IU
Canned sardines 193 IU
Cod liver oil 10,000 IU
Pickled herring 130 IU
Meat
Beef liver, raw 49 IU
Dairy/Eggs
Butter, salted 60 IU
Cheddar cheese 24 IU
Egg, scrambled 72 IU
Swiss cheese 20 IU
Fungi/Mushrooms
Portabella mushrooms, raw 10 IU
Shitake mushrooms, raw 18 IU
Vitamin D-fortified foods (per 100 g)
Fortified 2% milk 40 IU/ 98 IU per cup
Fortified American cheese 274 IU/ 52 IU per slice
Fortified yogurt 52 IU/ 127 IU per cup

If You Are Deficient

Treatment typically calls for daily or weekly high-dose vitamin D followed by a maintenance dose once serum vitamin D concentrations are over 30 ng/mL (28). Treatment varies by age:

  • For children under a year treatment for vitamin D deficiency involves six weeks of vitamin D at either 2000 IU/day or 50,000 IU once a week, followed by a 400-1000 IU/day maintenance dose .

  • Children ages 1-18 years generally receive the same six weeks of high-dose treatment followed by a maintenance dose of 600-1000 IU/day . Vitamin D deficient adults should receive 8-12 weeks of treatment with vitamin D at either 50,000 IU/week or 6000 IU/day followed by a maintenance dose of 1500-2000 IU/day .

While these recommendations do not specify whether vitamin D2 or D3 should be used , there is some evidence to suggest that vitamin D2 is less potent than vitamin D3 at high doses. It is also possible that single, large doses of vitamin D may be more effective than daily doses and offer better adherence compared to daily and monthly dosing .

Factors That Can Influence Your Response to Vitamin D Supplementation
Vitamin D interacts with certain medications that can increase its breakdown (such as corticosteroids and several anti-convulsants like phenobarbital) or reduce its absorption (such as the cholesterol-lowering medication cholestyramine and the weight-loss drug Orlistat .

The type of vitamin D you take (vitamin D2 vs. D3) may also impact your response to vitamin D supplementation. Both vitamin D2 and D3 have been used for treating vitamin D deficiency, some studies suggest that vitamin D3 may be more effective than vitamin D2 for normalizing vitamin D status ,.

Toxic Levels of Vitamin D
As mentioned in the first part of this series, it is possible to have too much vitamin D. The risk for vitamin D toxicity is thought to begin at concentrations greater than 150-200 ng/mL .

Hypercalcemia — too much calcium in your blood — is responsible for producing most of the symptoms of vitamin D toxicity, which can be non-specific (such as nausea, vomiting, headache, anorexia, metallic taste, constipation, frequent urination) . Over a longer period of time, elevated calcium leads to vascular and tissue calcification (40) and pancreatitis .

Sun exposure, even excessive sun exposure, will not cause vitamin D intoxication.

Taking high doses of vitamin D for prolonged periods of time is usually the cause of toxicity. There is no consensus as to what oral dose leads to intoxication, though ongoing consumption of approximately 40,000 IU/day is necessary to cause toxicity . Lethal doses of vitamin D have been reported as high as 840,000 IU/kg . Sun exposure, even excessive sun exposure, will not cause vitamin D intoxication, since heat from the sun breaks down previtamin D and vitamin D3 as it is formed in the skin .

Conclusion

Vitamin D deficiency is too common and affects both those who are healthy and those with certain diseases. If your doctor diagnoses you as deficient, he or she will suggest ways you can regain vitamin D sufficiency, such as spending adequate time outdoors, eating vitamin D fortified foods and taking daily supplements.

Though more studies are needed both to determine what concentration of serum vitamin D is sufficient, evidence suggests that serum concentrations of 25-hydroxyvitamin D (the blood test for vitamin D status) should be above 30 ng/mL.

Vitamin D sufficiency appears to reduce mortality and is well-established in bone health, but further randomized controlled trials are necessary to validate other physiological functions of vitamin D, such as in heart health and cancer prevention, in the body.

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