Vitamin-D – All you need to know

Vitamin D is an essential fat-soluble vitamin that aids in the absorption of calcium and phosphorous from the intestine and is crucial to the development and maintenance of sturdy bones. It is also required for healthy nerve and muscle function, a strong immune system and successful cellular growth, repair and differentiation.

The body is able to manufacture Vitamin D through a complex pathway. Initially, a non-active version called calciferol is produced in the skin on stimulation by ultraviolet (UV) light from the sun. Calciferol is then altered first in the liver and then in the kidneys to its potent form called 1, 25 dihyroxyvitamin-D or Vitamin D3. It is this molecule that acts on cells to exert its effect.

Vitamin D can also be absorbed from then diet and is found in cod liver oil, salmon, tuna as well as fortified dairy and grain products. However, the sun remains as the primary source.

The amount of sunlight required to produce sufficient quantities of vitamin D can differ between individuals according to where they live, the strength of the UV rays, the colour of their skin, their dietary intake and the health and integrity of the kidneys and liver. For the average white adult, it only requires 15-20 minutes per day of pure sunlight (moderate UV strength) to 40% of the body with no sunscreen to produce the required 1200 IU of Vitamin D. However, most people are not getting this. Even in the summer, cloud, shade and sunscreen have now been shown to reduce vitamin D production by up to 60%. This surpasses the 40% reduction seen during the winter months in North America.

Formerly, the RDA (Recommended Daily Intake) for people up to 50 yrs old (excluding pregnant women) was only 400 IU/day. For those aged 51and above, higher doses are recommended depending on their lifestyle, health and bone quality. However, new studies confirming the importance of vitamin-D in disease and cancer prevention along with osteoporosis, combined with the realization that most individuals are not making sufficient quantities, has pushed the RDA up to 1000-1200 IU/day.

Why is Vitamin-D so important? The most obvious reason is bone health. Active Vitamin-D3 functions as a hormone because it sends a message to the intestines to increase the absorption of calcium and phosphorus. In addition, it helps to regulate the osteoclastic (bone absorbing) and osteoblastic (bone forming) processes in the body to ensure adequate bone mineralization. It is also involved in the coordination, growth, strength and control of muscles, and their repair following injury. These are all features important for everyone and particularly the athlete.

Vitamin-D has several other important roles that are less well known. It has been clinically shown to help regulate apoptosis (cell death) in neoplastic or cancerous cells in the colon, breast and prostate. A prospective study looking at nearly 48,000 men compared their calculated vitamin-D levels with their cancer risk. An increase of 25 nmol/L in the predicted blood level of vitamin D was associated with a 17 percent reduction in total cancer incidence, a 29 percent reduction in total cancer mortality and a 45 percent reduction in digestive system cancer mortality. Low levels of vitamin D were associated with higher risk.

The American Association for Cancer Research has found a strong correlation between low vitamin-D levels and risk of breast cancer. Individuals with more outdoor exposure or using supplements such as cod liver oil from a young age reduced their breast cancer risk by about 25%. Dr. C. Garland writes “There is a strong inverse dose-response relationship between the serum concentration of 25-hydroxyvitamin D and the risk of breast cancer”.

Vitamin D also plays a role in the control and regulation of glucose metabolism with deficiency being associated with insulin resistance and Type 2 diabetes. Another study also indicates that low levels of vitamin-D in infants constitutes a risk factor in the development of type 1 diabetes.

Vitamin-D deficiency is linked to obesity as insufficient levels impair the ability of the hormone leptin to tell the brain when the body has had enough to eat. Leptin is produced by fat cells and it signals the brain when fat cells are “full”, thereby reducing hunger and increasing fat burning. Leptin resistance is thought to be one of the important contributors to the growing obesity epidemic. In addition, because of it being deposited in body fat, obesity itself worsens vitamin-D deficiency by reducing the bioavailability of active vitamin-D3.

Vitamin-D helps reduce fatigue and depression. Activated vitamin-D regulates tyrosine hydroxylase, the rate-limiting enzyme necessary for the production of dopamine, epinephrine and norepinephrine in the adrenal gland. Low levels of these neurotransmitters results in depressed mood and fatigue and may partially explain the symptoms of seasonal affective disorder (depression during the winter months). This condition has been successfully treated with vitamin D and in a recent study covering 30 days of treatment, supplementation was more effective than use of “natural light boxes”.

Maintaining adequate levels of Vitamin-D is clearly important, particularly in the athlete. Current recommendations regarding supplementation vary but I would suggest 1000 IU daily, increasing to 1200 IU in the winter.

Wharton B and Bishop N, Rickets, The Lancet 2003;362:1389-1400)

April 5th 2006 issue of the Journal of the National Cancer Institute, Dr. Gary G. Schwartz of Wake Forest University, Winston-Salem, North Carolina
American Association for Cancer Research April 2006

Hypponen E, Laara E, Reunanen A, Intake of vitamin D and risk of type 1 diabetes: a birth-cohort study. Lancet. 2001 Nov 3;358(9292):1500-3.

Shi H, Norman AW, Okamura WH, Sen A, Zemel MB.1alpha,25-Dihydroxyvitamin D3 modulates human adipocyte metabolism via nongenomic action. FASEB J. 2001 Dec;15(14):2751-3

Puchacz E, Stumpf WE, Stachowiak EK, Stachowiak MK. Vitamin D increases expression of the tyrosine hydroxylase gene in adrenal medullary cells. Brain Res.Mol.Brain Res. 1996;36:193-6.