Because of the possible double-edged sword of sun exposure for synthesizing vitamin D3, consuming vitamin D from the diet or supplements is the alternative.
However, there are a limited number of food naturally rich in vitamin D. Good sources of vitamin D are fatty fish (salmon, tuna, etc.) and their oils (such as cod liver oil). The amount in fatty fish varies greatly with wild-caught salmon being the highest. One study showed that farmed salmon contained almost 75% less vitamin D than wild-caught salmon1. It is not known whether this disparity exists between other types of farmed and wild-caught fish varieties.
Table 12.121 Vitamin D content of fish1
|Fish||Vitamin D (IU/oz)|
|Blue Fish||280 ± 68|
|Cod||104 ± 24|
|Grey Sole||56 ± 36|
|Farmed Salmon||240 ± 108|
|Wild Salmon||988 ± 524|
|Farmed Trout||388 ± 212|
|Tuna||404 ± 440|
Like vitamin C, E, and A, international units (IU) are also used for vitamin D.
For vitamin D the conversions are:
1 ug of D3 = 40 IU
1 ug of 25-OH-D = 200 IU2
Thus, since not many foods contain vitamin D, many brands of milk have been fortified with vitamin D2 or D3 (100 IU/8 oz) since the 1930s3. However, the actual measured amount of vitamin D in many brands of milk is far less than stated on their labels4,5. Part of this problem stems from a lack of a standardized method for measuring vitamin D in the past. Without standardized analysis, there inevitably was a wide range of variation from lab to lab in the reported amount of vitamin D.
Another issue with relying on dairy products to provide vitamin D is the common problem of lactose intolerance. Lactose intolerant individuals don’t have lactase, the enzyme needed to break down lactose. Common symptoms of this condition include:
Lactose intolerance is a fairly common problem worldwide, as shown in the map below.
The following table shows the percent of people who are lactose intolerant by race:
Table 12.122 Lactose intolerance rates8
|Race or ethnicity||% Lactose Intolerant|
|Mexicans (rural communities)||74%|
|North American Jews||69%|
|Mexican American Males||55%|
|Descendants of N. Europeans||5%|
Thus, you can see that many people are lactose intolerant. Coincidentally, many of these people have darker pigmented skin, meaning that they have an increased risk of vitamin D deficiency/insufficiency because they require greater sun exposure to synthesize adequate amounts of vitamin D3.
Other foods that are sometimes fortified are breakfast cereals and orange juice. Despite the fact that orange juice doesn’t contain fat, and vitamin D is fat-soluble, vitamin D is quite bioavailable in orange juice10.
Vitamin D in supplements is found as vitamin D2 or D3. However, based on the recent evidence suggesting that D2 isn’t as beneficial as D3, many are being reformulated to contain D311.
References & Links
1. Lu Z, Chen TC, Zhang A, Persons KS, Kohn N, et al. (2007) An evaluation of the vitamin D3 content in fish: Is the vitamin D content adequate to satisfy the dietary requirement for vitamin D? J Steroid Biochem Mol Biol 103(3-5): 642.
2. Anonymous. (1997) Dietary reference intakes for calcium, phosphorus, magnesium, vitamin D, and fluoride. Washington, D.C.: National Academies Press.
3. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
4. Holick MF, Shao Q, Liu WW, Chen TC. (1992) The vitamin D content of fortified milk and infant formula. New England Journal of Medicine, the 326(18): 1178.
5. Faulkner H, Hussein A, Foran M, Szijarto L. (2000) A survey of vitamin A and D contents of fortified fluid milk in ontario. J Dairy Sci 83(6): 1210.
9. McBean LD, Miller GD. (1998) Allaying fears and fallacies about lactose intolerance. J Am Diet Assoc 98(6): 671.
10. Tangpricha V, Koutkia P, Rieke S, Chen T, Perez A, et al. (2003) Fortification of orange juice with vitamin D: A novel approach for enhancing vitamin D nutritional health. Am J Clin Nutr 77(6): 1478.