Vitamin D – Don’t let the sunshine vitamin fade during winter

As winter approaches and daylight hours continue to shorten, our intake of the sunshine vitamin becomes more critical than ever.  Vitamin D is referred to as a vitamin but is actually a hormone synthesized by our skin when exposed to ultraviolet B (UVB) rays or sunlight.  Despite the fact we can make vitamin D from sunlight and obtain it from food, it’s estimated 50% of children and adults worldwide are deficient causing a range of health problems.

Functions of vitamin D do in the body

·      Bone health and calcium absorption – Calcium absorption in the intestine is enhanced with the help of vitamin D allowing calcium to be better absorbed from food.  Vitamin D also increases bone strength by incorporating calcium into the matrix of the bone, strengthening the network of fibers within the bone itself.


·      Cellular differentiation – Vitamin D can regulate cell differentiation and growth by binding to the vitamin D receptors found in most body cells. Differentiation of cells leads to a decrease in proliferation or the multiplication of cells possibly reducing risk of cancer.  Observational studies have shown associations between low vitamin D status and increased risk of developing colorectal and breast cancer. When cells proliferate at a faster rate, this can increase the risk for developing mutations. 



·      Secretion of insulin – Studies have shown when there is insufficient vitamin D present, glucose intolerance and impaired insulin secretion are increased in people with type 2 diabetes.


·      Blood pressure regulation – Having adequate vitamin D is associated with a decreased risk for heart disease by improving blood pressure by decreasing parathyroid hormone levels (PTH) and releasing renin by the kidneys.



·      Immunity – Vitamin D helps to regulate many compounds involved with the immune system from the function of lymphocytes, the production of cytokines, and macrophage activity.  When vitamin D levels are maintained, it may lead to the prevention of autoimmune diseases.




Daily requirements of vitamin D



Infants – 0 to 12 months

400 IU or 10 mcg

Children – 1-13 years old

600 IU or 15 mcg

Adolescents – 14-18 years old

600 IU or 15 mcg

Adults – 19-70 years old

600 IU or 15 mcg

Adults – 71 and older

800 IU or 20 mcg


600 IU or 15 mcg


600 IU or 15 mcg

Institute of Medicine Dietary Reference Intake for Calcium and Vitamin D


Endocrine Society’s Guidelines on vitamin D status

Vitamin D status is determined by a blood test measuring the circulating form or concentration of vitamin D, 25(OH)D. 

Recommended Vitamin D Serum Level Standards








30 or >



Factors affecting vitamin D status

·      Aging – As people age, the ability to synthesize vitamin D decreases due to the reduced capability of the skin to utilize sunlight it is exposed to and the reduced absorption of UVB rays.  In addition, many elderly people may be homebound and are not outdoors enough to receive adequate sunlight.


·      Skin Pigmentation – The darker a person’s melanin is in the skin, the less UV radiation is penetrated.  Up to 99% of UV radiation that reaches skin cells can be reduced in very dark pigmented skin.

·      Sunlight exposure – Exposure to sunlight is the main way people make vitamin D and if the skin is not exposed to adequate sunlight and not supplemented with vitamin D, a deficiency is likely to occur.


·      Location – Where a person lives makes a difference in vitamin D status.  Those who live above the 37th degree latitude in the Northern Hemisphere are at a greater risk of developing vitamin D deficiency during the autumn and winter months.  Due to the angle of the sun at this time of the year, the UVB rays are prevented from being absorbed by the skin.  Even though vitamin D is a fat-soluble vitamin and is stored in our fat tissue, the storage form has a half life of approximately of 3 weeks so people living in northern latitudes can become deficient during the winter if they rely solely on the sun for vitamin D.




·      Diet – Very few foods contain vitamin D naturally. The lack of many rich sources of foods containing the sunshine vitamin pose a problem if a person is not consuming the few dietary sources of vitamin D available


·      Diseases causing malabsorption – Individuals with Crohn’s disease, cystic fibrosis, or celiac disease will have more difficulty having sufficient levels of vitamin D since they have trouble absorbing dietary fat (vitamin D requires fat to be absorbed).          



·      Liver and kidney disease – Both the liver and kidney are needed to make the active form of vitamin D and if they are not functioning properly, this function will be hampered.


·      Breastfed infants – Breastmilk is low in vitamin D therefore exclusively breastfed infants are usually supplemented with vitamin D with the advice of their pediatrician.



·      Medications – Certain medications can interfere with vitamin D status by decreasing absorption.  These include:









*Mineral oil


Possible increased risks due to low vitamin D status

·      Rickets or osteomalacia.  Rickets is the vitamin D deficiency in children while osteomalacia is a softening of the bone in adults

·      Risk of falls

·      Fracture or osteoporosis

·      Cardiovascular disease

·      Insulin sensitivity

·      Cancer – breast, colorectal, prostate

·      Autoimmune diseases such as multiple sclerosis, type 1 diabetes and rheumatoid arthritis

How to increase or maintain intake of vitamin D

There are essentially three ways to help increase the likelihood of obtaining an adequate vitamin D status:

1.     Sufficient sunlight

2.     Consuming food sources of vitamin D

3.     Taking a vitamin D supplement

Sufficient sunlight – Individuals living south of the 37th degree latitude will be able to synthesize vitamin D year round as long as they have sun exposure on their arms and legs or face and arms 3 times weekly between 11 am to 2 pm for at least 10 minutes.  Darker-skinned individuals will need to stay outdoors for up to 30 minutes or longer for synthesis of vitamin D.   Those living north of the 37th degree latitude can synthesize vitamin D during the spring/summer and part of fall but will not be able to during late fall and all of winter. 

Diet – Below are some of the best food sources of vitamin D.  Try to include vitamin D rich foods each day.

FOOD                                       AMOUNT                                      VITAMIN D (IU)

Pink salmon, canned                     3 ounces                                                360

Mackerel                                       3 ounces                                                345

Tuna, canned                                 3 ounces                                                200

Orange juice, fortified                   8 ounces                                                100

Cow’s milk, fortified                     8 ounces                                                 98

Breakfast cereal, fortified              1 cup                                                      40-100

Egg yolk                                         1 large                                                    21

Source: Office of Dietary Supplements, National Institutes of Health

Vitamin D supplementation – There are various opinions on what is an adequate amount of vitamin D supplementation to recommend.  The goal is to increase optimal blood levels for preventing chronic disease and other factors associated with low vitamin D status. For most adults, usually supplementation begins at 1000 IU up to 2000 IU daily to keep blood levels > 30 ng/ml but it is recommended to get the advice of your physician before self-dosing.  Some individuals may require a higher amount to boost their vitamin D status into the normal range.

In conclusion

This time of year, more than ever, is a good time to be thinking of vitamin D, getting your levels checked and doing what you can to correct or maintain a healthy level.  Keep the sunshine vitamin’s light shining brightly even when the sun is hiding on cloudy winter days.