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Vitamin D
What Is Its Role in Diabetes?

by Julie Lichty Balay, MS, RD

What is your vitamin D level? Some day — maybe soon — having your vitamin D level measured may become as routine as having your cholesterol checked. According to some advocates of routine vitamin D testing, the results could prove to be a useful piece of medical information.

The importance of adequate vitamin D levels in the body has been recognized for decades. In 1921, scientists proved that exposure to the sun hardens bones and prevents diseases of “rubbery,” weak bones called rickets (in children) and osteomalacia (in adults). It was later discovered that it is the vitamin D produced when the skin is exposed to sun that helps the body absorb calcium and strengthen the skeleton. Around the same time, cod liver oil was found to be a potent food source of vitamin D. Popular and scientific interest in vitamin D waned, however, once these diseases became relatively uncommon, particularly in the United States, where milk and some other foods have been routinely fortified with vitamin D since the 1930’s.

Recently, however, new research on the so-called sunshine vitamin has shown that it plays a role in many more bodily systems than just the skeletal system. In fact, a deficiency of vitamin D is thought to possibly play a role in the development of numerous diseases, from cardiovascular disease to multiple sclerosis to complications of pregnancy. All of this new attention has made vitamin D a very popular supplement and the subject of renewed scientific inquiry.

Vitamin D in the body
The human body was designed to get most of its vitamin D through exposure to the ultraviolet (UVB) rays of the sun — the same rays that cause sunburn and skin damage. “Inactive” vitamin D is derived from cholesterol and “waits” in the skin tissue until the skin is exposed to sun. (Cholesterol is produced by the liver, so consuming dietary cholesterol is not necessary for vitamin D production.) UVB rays set off a chain reaction in exposed skin that continues in the liver and finishes in the kidneys, where vitamin D is activated and sent out to tissues throughout the body. Active vitamin D is one of many body hormones, chemical messengers that regulate body functions.

Vitamin D can also be found in food (or supplements) and is absorbed like a dietary fat. Both dietary and sun-derived D are stored in fat tissue. This means the body has some capacity to store the vitamin for later use.

Active vitamin D, also known as 1,25-dihydroxycholecalciferol, calcitriol, or vitamin D3, has many functions. Some of the known functions are well understood, such as how vitamin D helps the body absorb calcium and how it hardens bones and teeth. Other functions, such as how it helps regulate the immune system, are still being studied and learned about.

What is known is that almost every cell in the body has vitamin D receptors, suggesting that it must have a role in all of them. Particularly interesting in diabetes is that the insulin-producing beta cells in the pancreas not only have the ability to accept vitamin D, but also to activate it, just as the kidneys do. This fact is made more provocative when combined with many observational studies showing that vitamin D intake and/or blood levels are somehow connected to the incidence of both Type 1 and Type 2 diabetes.

Vitamin D and diabetes
Deficiency of vitamin D has been associated with both Type 1 and Type 2 diabetes and with metabolic syndrome (a combination of high blood pressure, abnormal blood cholesterol and triglyceride levels, insulin resistance, and excess abdominal fat that often precedes or goes along with Type 2 diabetes).

While it is yet to be fully understood how vitamin D helps the body regulate blood glucose, there seem to be several ways. Because vitamin D is present in the insulin-producing beta cells, and insulin secretion is calcium dependent (and therefore indirectly vitamin D dependent), it is theorized that vitamin D has a direct effect on how much insulin the body makes. Both in vitro studies (studies done on cells in a laboratory) and animal studies have shown that vitamin D deficiency impairs insulin secretion and that correcting vitamin D status restores the function.

Furthermore, when insulin is released, it must make contact with insulin receptors on cells to lower blood glucose. Other laboratory studies have shown that vitamin D may help the body produce and activate these insulin receptors.

Finally, diabetes and other chronic conditions are associated with increased levels of inflammation throughout the body, which may be the root of some of their long-term complications such as atherosclerosis (hardening of the arteries). Vitamin D and calcium are both shown to reduce the production of inflammatory chemicals in the body called cytokines.

With so much evidence of a role for vitamin D in regulating blood glucose levels, the obvious question is whether vitamin D can prevent, cure, or treat diabetes.

Several large observational studies have shown that vitamin D intake might prove to be a preventative factor in diabetes. For example, a study of over 10,000 Finnish children showed that taking supplements of 2,000 IU per day of vitamin D (5 times the current US recommendation) resulted in an 80% reduction of risk of Type 1 diabetes. Further strengthening the notion of this relationship, the children in the study who had suspected rickets had the highest rates of Type 1 diabetes. Although this magnitude of risk reduction sounds amazing, remember that Finland gets relatively little sunlight due its northern latitude (so people may need more vitamin D from food and/or supplements). Also, because this was only an observational study, one cannot assume this tremendous risk reduction from supplements for all children.

Apparent risk reduction has also been shown with Type 2 diabetes. In 2006, the journal Diabetes Care published a report from the Nurse’s Health Study, which followed over 80,000 women for 20 years. Women in the study who had both a calcium intake of more than 1,200 mg and a vitamin D intake of more than 800 IU had a 33% lower risk of developing Type 2 diabetes.

These are promising reports, but to prove that vitamin D helps prevent diabetes, clinical trials, or studies on humans, are still needed. Clinical trials will show whether the relationship between vitamin D and diabetes is direct or indirect. In other words, they will determine whether it’s vitamin D deficiency or some other factor, such as genetics, that causes the high rate of Type 1 diabetes among Finnish children. They will also determine whether high levels of vitamin D protect women from Type 2 diabetes, or whether women who lead the types of healthy lifestyles that help prevent Type 2 diabetes (getting lots of outdoor activity, for example) just happen to have higher levels of vitamin D.

So far, it doesn’t appear that diabetes can be cured with vitamin D supplementation. The few intervention studies in which vitamin D has been administered in hopes of reversing diabetes have not been successful.

But what about using vitamin D to help manage diabetes? Observational research shows that blood glucose levels rise in the winter, which is when vitamin D levels fall. This doesn’t prove cause and effect, since many factors could contribute to higher blood glucose during the winter, but it raises the possibility of a connection. The few human intervention studies that have been done to try to establish cause and effect have been inconclusive. But most have been done over short periods with only small numbers of subjects, and some critics believe the amounts of vitamin D taken in the studies were too low to make a difference.

Currently, therefore, it cannot be definitively said that vitamin D deficiency causes diabetes or that raising vitamin D levels helps manage it, but more clinical trials are under way, so the relationship may become clearer over time.

Vitamin D in food and nature
Vitamin D is found naturally in very few foods. When it is present, it is primarily found in its active form, vitamin D3, and only mushrooms have small amounts of D2, which can be converted to D3 in the body.

The best natural sources of vitamin D are fatty fish like salmon, trout, and sardines, which have between 46 IU and 794 IU per 3-ounce serving, or 12% to 199% of the Daily Value (DV), which is 400 IU (10 micrograms). One egg yolk has about 25 IU (6% DV), and 3.5 ounces of beef liver contain 46 IU (12% DV), but many people avoid these foods due to their high fat and cholesterol content. Fortified milk is the primary source of vitamin D in the US diet, with 100 IU (25% DV) in each 8-ounce serving. Other foods such as breakfast cereals and energy bars may also be fortified with vitamin D during processing.

Vitamin D is called the sunshine vitamin because even modest exposure to the sun can produce up to 20,000 IU in a single day. When produced via sun exposure, this amount of vitamin D poses no risk for toxicity. Interestingly, the indigenous populations of far northern regions such as Alaska cannot synthesize adequate vitamin D from sun exposure — it is too cold and/or dark most of the time to expose enough skin, and the sun reaches them at a steep angle, which weakens its rays. But they get ample amounts from their traditional diets, which are primarily composed of fatty fish.

Since most people are unlikely to eat like an Eskimo, what is the best way to get vitamin D? Dr. Michael Holick, director of the General Clinical Research Unit and professor of medicine, physiology, and biophysics at Boston University Medical Center, has done extensive research on the topic and suggests several methods. One is “sensible sun exposure,” which means exposing skin to the sun while taking caution never to burn. Dr. Holick points out that sunscreen with an SPF of 30 reduces vitamin D production by 99%, so he recommends that those who live in a northern latitude such as Boston expose their arms and legs to the sun without sunscreen for 5–15 minutes, two to three times each week in the summer months to encourage natural vitamin D production. He also supports the MyPyramid guideline to consume 2–3 servings of milk daily and strongly believes that supplements are also necessary for most people (although not everyone agrees with him on this point).

How much vitamin D do you need?
The amount of dietary vitamin D needed varies from person to person. Accounting for these differences are both the varied amount of sun exposure and the wide variations in internal production among individuals. One small study of young people living in Hawaii showed that approximately half had levels under 30 ng/ml (nanograms per milliliter), which is considered insufficient by many doctors. This was the case even though they averaged almost 24 hours of sun exposure weekly with no sunscreen. It appears, therefore, that sun exposure is not sufficient for everyone, which is why some experts consider dietary D a more reliable source.

The dietary recommendations for vitamin D were set in 1997 (see “‘D’ by the Numbers”) and have recently been criticized as being far too low. They are currently under scrutiny by the Institute of Medicine’s Food and Nutrition Board, and may be increased soon.

Until then, it is agreed that there are several populations at particular risk for vitamin D deficiency who likely need a vitamin D supplement. Among them are breast-fed infants (breast milk is very low in D), the elderly (who do not synthesize it as efficiently and often spend little time outside), dark-skinned individuals, and those who never expose their skin to the sun (for religious or other reasons). Obesity is also a risk factor. Since vitamin D is stored in body fat, a person with more body fat will tend to store more vitamin D rather than having it circulating in the bloodstream doing its many jobs. Because of this, it may be that obese individuals need two or three times the amount of vitamin D as people with less body fat.

It is estimated that as many as half of Americans are deficient in or have insufficient levels of vitamin D in their bodies in the winter. However, taking any nutrition supplement poses some risk for toxicity, and vitamin D is no exception. The primary concern with overconsumption of vitamin D is an abnormal increase in blood calcium, which may lead to hardening of soft tissues and issues such as kidney stones. Because the question of the necessity and safety of vitamin D supplementation is currently controversial, it is wise to discuss testing and whether and how much to supplement with your doctor.

Keep in mind that this test may or may not be covered by your insurance plan, and out-of-pocket costs will vary greatly among labs. Also be aware that there is not yet a standard definition of vitamin D deficiency (as there is for iron, for example), nor is there currently a standard protocol to correct suspected deficiency, so the advice you get will depend on your caregiver. Even the laboratory tests have proven almost as controversial as the vitamin itself. Some analyses have shown that results vary greatly among different labs and testing methods, which means some people might be advised to take vitamin D unnecessarily, and others will have their deficiencies go undiagnosed.

Although there is no unanimous agreement about what optimal vitamin D levels are — and it’s possible that optimal levels may be different for people with diabetes — it is generally accepted that a level above 30 ng/ml is sufficient. Keep in mind that vitamin D status may change seasonally, particularly if you live above 35 degrees latitude, or in the northern two-thirds of the United States, so take note of what month you are checked. Levels will most likely be lowest in the spring and highest in the early fall. According to Dr. Holick, the body can store sun-derived vitamin D for about one month and the supplemental form for only two weeks, so he believes that supplementation is necessary in the fall, winter, and spring months in northern states. He points out that even in the southernmost states, vitamin D production drops drastically in winter months.

If you choose to take vitamin D supplements, be aware that you will find it available in both the inactive D2 form and the active D3 form. It is arguable whether one is better utilized or safer than the other, but most experts suggest seeking the D3 form in a supplement. Dr. Holick suggests either taking a daily dose of up to 2,000 IU or taking up to 20,000 IU once every two weeks, since single doses this large have not been found to be toxic, and the body can store a two-week supply.

The jury is out on whether vitamin D is a miracle cure-all for sure, but the message is clear that everyone needs it and many people do not get enough. Getting your vitamin D is yet another good reason to eat more fish and to get out and enjoy the outdoors.

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Also in this article:
"D" by the Numbers

 

 

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