Did you know that in Seattle, from the months of October to April, you cannot synthesize any vitamin D? 

Anyone living above Latitude 35 degrees North is unable to synthesize vitamin D, owing to the angle of the sun; ozone in the upper atmosphere will completely block UVB radiation. 

In Seattle (47 degrees North) and London (52 degrees North), from October to April, UVB photons are blocked by the atmosphere, so one’s skin cannot make vitamin D. (The half-life of circulating vitamin D is approximately one month, which means the vitamin D you made in the summer will not last the winter.) 

This is essential information because we now know that “vitamin” D is actually a steroid hormone involved in more than 200 genetic processes. Every tissue in the body — including the brain, heart, muscles and immune system — has receptors for vitamin D, meaning that this nutrient is needed at proper levels for these tissues to function well. 

Vitamin D, often referred to as the “sunshine vitamin,” is actually a fat-soluble hormone that the body can synthesize naturally. There are several forms, including two that are important to humans: D2 and D3. Vitamin D2 (ergocalciferol) is synthesized by plants, and vitamin D3 (cholecalciferol) is synthesized by humans when skin is exposed to ultraviolet-B (UVB) rays from sunlight. The active form of the vitamin is calcitriol, synthesized from either D2 or D3 in the kidneys.

A lack of vitamin D can trigger infections including, but not limited to, the flu and cold virus; osteoporosis; autoimmune diseases (multiple sclerosis, Type 1 diabetes, rheumatoid arthritis and inflammatory bowel disease); cardiovascular disease; and breast, prostate, lung, pancreatic, ovarian and colon cancer. 

In fact, the American Journal of Clinical Nutrition noted a 75-percent reduction in the risk of breast cancer in women who take vitamin D supplement.

It also improves depression and mental state by regulating the release of neurotransmitters such as serotonin. The connection to vitamin D and diabetes is particularly strong and the rising incidence of Type 1 diabetes may be, in part, due to the current practice of protecting the young from sun exposure. When newborn infants in Finland were given 2,000 international units a day, Type 1 diabetes fell by 88 percent. 

How much vitamin D do I need to take? 

Most people in the modern world have lifestyles that prevent them from acquiring the levels of vitamin D that evolution intended us to have. 

A simple blood test (specifically, 25 hydroxy vitamin D) will reveal your level and allow you to know where you are starting, what dose you should take and if your levels are going up appropriately over time. 

A level less than 30 ng/ml is sufficient, but experts now consider 50 to 100 ng/ml to be the optimal level of vitamin D. Levels 101 to 150 ng/ml are excessive and those higher than 150 ng/ml to be potentially toxic.  

Doesn’t our fortified milk and foods give us enough vitamin D? 

The main dietary sources are wild-caught, oily fish (salmon, mackerel, bluefish and canned tuna), fortified milk and baby formula, cereal and orange juice. 

One would have to drink 200 8-ounce glasses of milk to obtain as much vitamin D as skin makes fully exposed to the noonday sun. 

How much should I take?

To reach a level of 50 ng/ml often requires taking a 2,000 to 5,000 IU/day vitamin D supplement. People with a serious vitamin D deficiency are often prescribed weekly doses of 50,000 units until the problem is corrected. 

The National Institute of Health recommends that children take 200 IU daily. 

Reassuringly, no adverse effects have been seen with supplemental vitamin D intakes up to 10,000 IU daily. 

Though the gray skies in Seattle may stay with us until spring, we can take the sunshine vitamin this winter and have a healthier and happier season.

DR. TAMI MERAGLIA, M.D., is owner and medical director of Vitality Medispa and Medical Clinic (www.VitalityMedicalClinic.net) in Queen Anne. She is the author of the upcoming book “The Hormone Secret.” To comment on this column, write to QAMagNews@nwlink.com.