Nutrition Hotline
by Reed Mangels, Ph.D., R.D.
Question: How much sunlight do I need to get adequate vitamin D? D.O., NY
Answer: Vitamin D is also known as the sunshine vitamin. It can be made by our bodies following exposure to sunlight. The amount of sunlight which is needed to promote production of adequate vitamin D varies depending on the season of the year, the time of day, where you live, how fair skinned you are, and whether or not you use sunscreen.
In the midwest United States, infants and toddlers need two hours a week of sun exposure if only their faces are exposed, or 30 minutes per week wearing only a diaper. Adults have been estimated to need about 10 to 15 minutes of sun on hands, face, and arms 2 to 3 times a week, although this depends on location and the season of the year. Those with dark skin have been shown to need as much as six times more sunlight than those with light skin to reach the same blood level of vitamin D.
Sunscreens block vitamin D production, so should be used after you've been in the sun long enough to promote vitamin D production. Because so many factors affect how much vitamin D our bodies make, unless you are regularly in the sun, another source of vitamin D is recommended.
Dietary sources for vegetarians include fortified cereals, fortified plant milks, and fortified cow's milk.
Question: Can I get vitamin B-12 from eating organic produce? C.C., NV
Answer: The simple answer to this question is possibly "yes." Plants can apparently absorb vitamin B-12 by their roots from soil which contains vitamin B-12. Vitamin B-12 could be found in soil because of the use of cow manure or sewage sludge. In reality, however, while some vitamin B-12 appears to be found in organically grown plants, the amount may not be enough to meet requirements.
For example, one study reported on the amount of vitamin B-12 in organically grown spinach. It appears that more than 23 cups of spinach would have to be eaten every day in order to meet the adult RDA for vitamin B-12. Produce cannot be depended on as a reliable vitamin B-12 source because the level of vitamin B-12 in plants varies widely depending on the type of plant and the soil in which it is grown. Also, vitamin B-12 analogues (look alikes) may be found in soil and absorbed by plants. If these analogues are present, they could either interfere with the plant's uptake of vitamin B-12 or with the usefulness of the plant's vitamin B-12 for humans.
Mozafar, A. Is there vitamin B-12 in plants or not? A plant nutritionist's view. Vegetarian Nutrition: An International Journal 1/2: 50-52, 1997.
Mozafar, A. Enrichment of some B-vitamins in plants with application of organic fertilizers. Plant and Soil 167: 305-311, 1994.
