Sunlight exposure on the skin is the most important source of vitamin D.  The sun’s ultraviolet rays trigger vitamin D synthesis in the skin.

Vitamin D aids in the absorption of calcium from the food we eat, and in the mineralization of healthy bones and teeth.  In other words, Vitamin D helps calcium to be properly used in bones and muscle, and not just deposited abnormally in the joints.  Progression of degenerative arthritis is faster in people with low blood vitamin D levels.

However, vitamin D isn’t just important for healthy bones; it also acts as a regulator of cell growth and differentiation in a number of different cell types. Vitamin D deficiency has been linked to breast, prostate, colon and skin cancers, high blood pressure, high triglycerides and LDL cholesterol, auto-immune disorders such as rheumatoid arthritis, inflammatory bowel disease, multiple sclerosis, polycystic ovary syndrome, chronic fatigue, depression and obesity.

Vitamin D deficiency is also associated with insulin deficiency and insulin resistance, and may be a major factor in the development of type 1 diabetes in children.  Insulin resistance is a major contributing factor in heart disease and cancers. More heart attacks occur in the winter, and northern latitudes have higher incidence of heart disease.

In northern California, 80% of blood samples tested during winter months showed a D deficiency.  This problem increases dramatically in latitudes farther north. Many of us at this latitude are not exposed to enough sunlight in the winter to make adequate amounts of vitamin D.  This is especially true of those of us with indoor jobs.

SAD (Seasonal Affective Disorder) has been successfully treated with a supplemental vitamin D.  In a 30 day study comparing oral vitamin D and the 2 hour daily use of ‘light boxes’, depression completely resolved in the D group, but not in the light box group (1).

Sunscreens, especially those with an SPF of 8 or greater, block the UV rays that produce vitamin D. (I never use a sunscreen – I just make sure to not get burned, by limiting exposure and using organic coconut oil on my skin before exposure.)

With minimal sunlight exposure, the average older adult is found to need at least 600 – 1,000 IU of oral vitamin D daily.  (However, vitamin D is fat-soluble, thus it can accumulate over time in the body tissues, so it’s wise to stay at or below a total of 2,000 IU/day from all sources, over the long-term.) The best blood test for vitamin D status is 25(OH)D.

The highest natural food source of D is cod liver oil, which is what Scandinavians and other people of northern latitudes traditionally used to stay healthy in the winter.  For those living at latitudes above 30 degrees north, I recommend about 1 tablespoon cod liver oil daily from September to May.  The only non-rancid cod liver oil I’ve found that doesn’t leave a fishy after-taste is Carlson’s.  Cod liver oil also provides an abundance of the essential omega 3 fatty acids, vitamin A and vitamin K.  Other less potent sources of D are wild salmon, mackerel, sardines, liver, free-range eggs, sunflower seeds, butter and cream.


Am J Clin Nutr 1999; 69:  842-856.
Am J Clin Nutr 2001; 73: 288-294.
J Intern Med 2000; 247: 260-268.
Vitamin D vs. broad-spectrum phototherapy in the treatment of seasonal affective disorder. J Nutr Health Aging 1999; 3:5-7.