History of research
In 1936, a report in The
Lancet by Peller, a US Navy doctor, suggested an inverse relationship between
skin cancer and all other cancers. He observed that Navy personnel had eight
times the skin cancer rate of the rest of the population, but only forty per
cent of the total death rate from cancer. He proposed that the obvious
explanation for this was the greater amount of sunlight to which men serving in
the Navy were exposed.
Later, in 1941, Frank Apperley, well-known
pathologist, confirmed an inverse correlation between levels of ultraviolet
radiation in North America and mortality rates from cancers in nonskin sites and
proposed that sunlight somehow conferred “relative cancer immunity” to nonskin cancers. Although Apperley’s paper attracted little attention in its day, epidemiologists
rediscovered his fundamental insight half a century later. Many common cancers,
such as
cancers of the colon and prostate, display fascinating north –
south gradients, with rates that
increase systematically with increasing geographic latitude.
The increased risk with residence at northern latitudes and greater incidence
and mortality in persons with dark pigmentation recall the descriptive
epidemiology of rickets, the classic disease of vitamin D deficiency. These considerations led
several epidemiologists, including
Garland for the colon in 1980,
and Schwartz and Hulka for the prostate in 1990, to propose that vitamin D deficiency increased the risk for these cancers.
Similar claims later were made for cancers at other sites, e.g., breast, ovary,
and pancreas, so that vitamin D has become a prime candidate for cancer prevention.
Mechanism
Understanding how vitamin D could infl uence cancer risk requires an
understanding of vitamin D synthesis. The synthesis of vitamin D begins with
the production of vitamin D3
(cholecalciferol) after 7-dehydrocholesterol in the skin is exposed to
ultraviolet B radiation (wavelength = 290 – 315 nm). Because melanin is an
effective sunscreen, given
the same ultraviolet
exposure, blacks synthesize less vitamin D than whites, accounting for
the far higher prevalence of vitamin D deficiency among blacks. Vitamin D can also be obtained from the diet,
although the quantity of vitamin D in Western diets is generally small. To become biologically active, vitamin D undergoes two hydroxylations:
The first occurs in the liver at the 25-th carbon position, forming 25-hydroxyvitamin D [25(OH)D or
calcidiol], the prohormone and major circulating form of vitamin D; the second occurs at the 1α position, forming 1,25(OH)2D (calcitriol),
the hormonal form of vitamin D. Most of the biological effects of 1,25(OH)2D
are mediated by specific hormone receptors (vitamin D receptors, or VDRs).
Calcitriol provides numerous benefits against cancer. This form of
vitamin D encourages cells to either adapt to their organ or commit apoptosis
(cell suicide). Calcitriol also limits blood supply to the tumor and reduces
the spread of cancer.
Vitamin D and Pancreatic
Cancer
Several studies
found a correlation between vitamin D and pancreatic cancer. Among them:
- The study examined data from two large, long-term health surveys and found that taking the U.S. Recommended Daily Allowance of Vitamin D (400 IU/day) reduced the risk of pancreatic cancer by 43 percent. By comparison, those who consumed less than 150 IUs per day experienced a 22 percent reduced risk of cancer. Increased consumption of the vitamin beyond 400 IUs per day resulted in no significant increased benefit. The study was led by Halcyon Skinner, Ph.D., of Northwestern University.
“Vitamin
D has shown strong potential for preventing and treating prostate cancer, and
areas with greater sunlight exposure have lower incidence and mortality for
prostate, breast, and colon cancers, leading us to investigate a role for
Vitamin D in pancreatic cancer risk. Few studies have examined this
association, and we did observe a reduced risk for pancreatic cancer with
higher intake of Vitamin D", commented Skinner.
- St George’s Hospital showed that the provision of 400IU’s of vitamin D reduced pancreatic cancer by 34 percent.
Dosage recommendations
for Vitamin D
According to the
Office of Dietary Supplements, or ODS, children and adults up to 70 years old
need to take in at least 600 IU, or 15 mcg, of vitamin D daily. Only a few
foods are natural sources of this vitamin, including oily fish, such as salmon,
mackerel or tuna; eggs; cheese; and beef liver. Some dairy products, such as
milk and yogurt, as well as orange juice and many breakfast cereals are
fortified with vitamin D. In the U.S., the requirement for milk is 100 IU per
cup, while in Canada, it's 35 to 40 IU per 100 ml. Though still controversial,
some health researchers, such as those at the Linus Pauling Institute, recommend
that an adult dose of up to 2,000 IU, or 50 mcg, of supplemental vitamin D
daily.
The ODS
recommendation for older adults is at least 800 IUs, as they are less able to
absorb calcium efficiently and are more at risk for osteoporosis, or brittle
bones. People who have Crohn's or other malabsorption syndromes also might not
be able to effectively use vitamin D. Some people, such as the elderly who live
in nursing homes or babies, might not get enough direct sunlight to help them
produce vitamin D. Chronic liver disease and kidney failure are also risk
factors for vitamin D deficiency, as these damaged organs might not be able to
process the inert vitamin D into the useful form of the nutrient. Obesity and
dark skin are other risk factors.
How much are you getting from food?
- 110-120
IU in each 8-oz glass of milk. Soy milk also supplies vitamin D if it is
fortified. Cheese and yogurt do not necessarily count as sources;
most contain very little, although some yogurt is now vitamin D-fortified.
- 200
IU per day is added to your daily average if you eat at least 8 oz a week
of naturally fatty fish such as salmon and rainbow trout, recommended
because of their omega-3 fat for heart health. (A portion like a
deck of cards — three ounces cooked — contains from 450 to 650 IU of
vitamin D. Other fish, such as tuna, halibut and flounder also provide D,
but less than half as much.)
- 40
IU in each egg (if you eat the yolk).
- 100
IU in a typical 8-oz glass of juice that is fortified with vitamin D.
That’s if it says “25% of Daily Value” on the label’s Nutrition Facts panel.
- 40
IU in a typical ¾-1 cup serving of cereal fortified with vitamin D. Some
cereals contain more; others, such as the simple oatmeal that I often
recommend, does not contain any.
- 400 IU in one portabella mushroom piece or 4 large white mushrooms if you get the new “enriched mushrooms” that are exposed to ultraviolet light, which triggers vitamin D production within the mushroom. (Most mushrooms supply an insignificant trace of vitamin D.)
You can see most of us are unlikely to total 600 IU
(and certainly not 800 IU) through unfortified food alone, so consider taking
extra vitamin D supplements to reach recommended dosage levels.
Sources
and Additional Information:
