Vitamins and minerals are a key part of nutrition, and I realized that I haven’t been addressing them a lot here. I lean towards a “whole foods” approach of nutrition. For the average person, it is incredibly time-consuming and therefore, in my opinion, unreasonable to calculate the exact amounts of particular nutrients. It is better to keep the big picture in mind, and “eat food, not too much, mostly vegetables” (to borrow from the tagline of Michael Pollan’s book “In Defense of Food”).
However, vitamins and minerals are key components of healthy nutrition, and it is my job as a dietitian to educate consumers about them.
So, welcome to my version of Vitamin D 101.
Vitamin D is a fat soluble vitamin, and is sort of an outlaw among the vitamins. As most people know, the body can synthesize vitamin D from the sun. And, even though it is called a vitamin, it is technically a hormone because it is a chemical messenger that causes a response in the intestines, kidneys, and bones.
Vitamin D regulates calcium and phosphorus absorption, and is therefore very important for bone mineralization and strength. In addition, research suggests that vitamin D helps to maintain muscle strength, promote a healthy immune system, and help regulate cell growth and differentiation.
Here’s a list of the different diseases and conditions for which vitamin D is perhaps involved in preventing or treating. You will notice that I use the word “perhaps”. Because scientific investigations are constantly expanding and evolving, it is generally not a good idea to use definite words such as “proven” or “will prevent” or “will cure”. It is difficult to nail down 100% definitive answers. But anyways, here’s the list (1):
- Cancer—best evidence for colon and colorectal cancers
- Chronic fatigue
- Chronic pain
- Congestive heart failure
- Diabetes—both type 1 and 2
- Falling and balance problems
- Gum disease
- Heart disease
- Lupus/systemic lupus erythematosus
- Multiple sclerosis
- Muscle loss and weakness
- Preeclampsia in pregnancy
- Rickets resurgence
- Rheumatoid arthritis
- Risk of death
- Stress fractures
You can get vitamin D from food or from the sun. When UV light from the sun hits the skin, it reacts with 7-dehydrocholesterol (a vitamin D precursor made from cholesterol in the liver) to create cholecalciferol (vitamin D3).
This cholecalciferol, and vitamin D from foods, are converted in the liver to 25-hydroxy vitamin D, then sent to the kidney to be converted into 1,25-dihydroxy vitamin D3, the biologically active form.
Several factors that contribute to vitamin D deficiency include: lack of vitamin D in the diet, dark skin, lack of exposure to sunlight, and liver or kidney disease. It is estimated that more than 50% of men and women ages 65 and older in North America are vitamin D deficient (2).
There are not many foods that naturally contain vitamin D. Therefore, in the United States, most dietary vitamin D comes from fortified foods such as breakfast cereals, milk, soy milk, and margarine. Regulations state that all commercially available milk must be fortified with 400 IU/quart (3).
Food sources of Vitamin D include:
- Salmon (3.5 oz) 360 IU
- Mackerel (3.5 oz) 345 IU
- Canned Tuna (3.5 oz) 200 IU
- Fortified Orange Juice (8 oz) 100 IU
- Fortified milk (8 oz) 98 IU
- Fortified Breakfast Cereals 40-100 IU
The RDA (Recommended Dietary Allowance) for adults ages 19-70 is 600 IU of vitamin D a day, and for adults over 70, the RDA is 800 IU. (For an explanation of RDA, refer to my post on Alphabet Soup). The UL (tolerable upper intake level) for all adults is 4,000 IU (4).
Dark skin, increased age (individuals aged 65 and over generate only 1/4 as much vitamin D as individuals in their 20’s do), and lack of exposure to sunlight inhibit the synthesis of vitamin D from sunlight. Research indicates that people who live above 37 degrees northern latitude have insufficient duration and intensity of sunlight exposure for optimal synthesis (2).
There is much more that can be said about vitamin D, such as medication interactions, disease states, risks of toxicity, etc. but that’s a bit much to fit into one blog post. I hope you’ve enjoyed learning a little bit about this vitamin!
(2) Harvard Health Publications
(3) Nelms, Sucher, and Long. Medical Nutrition Therapy and Pathophysiology.
(4) Office of Dietary Supplements