A team of researchers led by Dr Claudia Gagnon, a fellow at the Western Hospital at the University of Melbourne in Australia, followed over 5000 people for five years to find out whether blood levels of vitamin D had any bearing on the risk of developing type 2 diabetes-the most common form of the disease.
All 5200 participants were free of diabetes at the start of the study, at which time their blood levels of vitamin D were measured. But five years later, around 200 people had develop-ed the condition.
The researchers discovered that twice as many people (six in 100) with low blood levels of vitamin D went on to develop diabetes compared with those whose blood levels were in the normal range (three in 100 people).
When the team took into consideration the usual risk factors for diabetes such as age, waist circumference and a family history of the disease, the increased risk due to low D levels translated to 57 per cent in relation to those with higher D levels.
"Higher serum 25OHD [blood vitamin D] levels . . . were associated with a significantly reduced risk of diabetes in Australian adult men and women," the researchers con-cluded. "Each 25 nmol/L [nanomoles per litre] increment in serum 25OHD was associated with a 24-per-cent reduced risk of diabetes" (Diabetes Care, 2011; 34: 1133-8).
This intriguing study is just the latest in a string of studies to suggest a role for the 'sunshine vitamin' in diabetes. More commonly known for its role in building and maintaining healthy bones, vitamin D is now being investigated for a wealth of additional benefits-from pre-venting cancer to fighting heart disease. Diabetes, it appears, is yet another chronic condition that is influenced by vitamin D.
As with the new Australian study, an investigation carried out in Finland also found a link between higher levels of vitamin D and a reduced risk of type 2 diabetes. The long-term 17-year study followed more than 4000 men and women, and reported that those with the highest blood levels of vitamin D were the least likely to develop diabetes, and vice versa.
After adjusting for age and gender, and accounting for the season during which each volunteer's blood was drawn (necessary because the body makes vitamin D in response to sun exposure), the researchers found that the risk among those with the highest D levels was 40-per-cent lower compared with the participants who had the lowest levels (Diabetes Care, 2007; 30: 2569-70).
Yet another study found that supplementing with vitamin D might ward off type 2 diabetes. In the US Nurses' Health Study, which included nearly 84,000 women with no history of diabetes at baseline, those who consumed 400 IU/day or more of vitamin D from supplements had a 13-per-cent lower risk of type 2 diabetes compared with women who consumed less than 100 IU/day. Moreover, a combined daily intake of more than 1200 mg of calcium and 800 IU of vitamin D-from either diet or supplements-was associated with a 33-per-cent lower risk of type 2 diabetes compared with women whose intake was half that or less (Diabetes Care, 2006; 29: 650-6).
However, another large-scale long-term US study in women-the Women's Health Initiative-found no reduced diabetes risk with vitamin D supplementation (Diabetes Care, 2008; 31: 701-7).
Nevertheless, other research suggests that vitamin D might be useful for people who already have the disease. Researchers at the National Nutrition and Food Technology Research Institute in Tehran, Iran, recently discovered that, when type 2 diabetics were given extra vitamin D as part of their daily diets for several months, their blood sugar levels began to drop.
In this study, a total of 90 diabetics were split into three groups: the first received plain yoghurt with no added vitamin D; the second received vitamin-D-fortified yoghurt (containing 500 IU of the vitamin); and the third received vitamin-D-fortified yoghurt with added calcium. After three months, the plain yoghurt group had an average increase of 9 per cent in blood sugar levels, while the two D-fortified groups experienced a decrease of 7 per cent.
"Daily intake of a vitamin-D-fortified yoghurt drink, either with or without added calcium, improved glycaemic status in [type 2 diabetic] patients," the researchers concluded (Am J Clin Nutr, 2011; 93: 764-71).
What about type 1?
Studies indicate that vitamin D also has a role to play in type 1 diabetes-the more severe form that usually affects children or young adults.
Too-low levels of the vitamin have been linked to an increased risk of developing the disease as well as a greater risk of death due to any cause in people who already have the condition (Curr Diab Rep, 2008; 8: 393-8; Diabetes Care, 2011; 34: 1081-5). What's more, vitamin D deficiency is more common in type 1 diabetics than in non-diabetics (Acta Diabetol, 2009; 46: 183-9).
Several studies have investigated the impact of vitamin D supplements on the risk of developing type 1 diabetes. In a Finnish study of more than 12,000 babies followed for more than 30 years, the children who regularly took vitamin D supplements had a lower rate of type 1 diabetes than those who did not, while those suspected of having rickets-a condition linked to vitamin D deficiency-had a threefold greater risk of having diabetes.
These data also showed that children who were regularly given the recommended dose of vitamin D (at the time, this was 2000 IU daily) had an 80-per-cent lower risk of developing type 1 diabetes compared with those who regularly received less than the recommended amount (Lancet, 2001; 358: 1500-3).
More recently, researchers from the Stockport NHS Foundation Trust in the UK pooled the results of five separate studies assessing the effects of vitamin D supple-mentation in infancy on type 1 diabetes risk. They found that the children given additional vitamin D were roughly 30-per-cent less likely to have type 1 diabetes later in life compared with those not given the supplement.
Although all of the pooled studies were observational and, as such, cannot prove a cause-and-effect relationship, the researchers concluded that "[v]itamin D supplementation in early childhood may offer protection against the development of type 1 diabetes". However, they also noted that "randomized control-led trials with long periods of follow-up are needed to establish causality and the best formulation, dose, duration and period of supplementation" (Arch Dis Child, 2008; 93: 512-7).
As with type 2 diabetes, there is also evidence to suggest that vitamin D supplements can help people already diagnosed with type 1 diabetes-or at least the ones who are also vitamin-D-deficient. In a study by a team from the King Fahad Armed Forces Hospital in Saudi Arabia, 80 type 1 diabetics with vitamin-D-deficiency (D blood levels of less than 50 nmol/L) were given 4000 IU/day of the vitamin for 12 weeks. At the end of the study, there was "an observed effect of vitamin D supplementation on glycaemic control". Patients were also more likely to achieve lower glycosylated haemoglobin levels (indicating better blood sugar control) at 12 weeks when they had higher vitamin D levels, the results showed (Ann Saudi Med, 2010; 30: 454-8).
D for diabetes-and more
Clearly, vitamin D is a promising candidate for the prevention and treatment of diabetes-both type 1 and type 2. Although much more research is needed (and is underway), the evidence so far suggests that not getting enough of this essential vitamin could play a crucial role in the development and progression of this increasingly common disease.
Given that the vitamin appears to have an impact on numerous other diseases and disorders-from osteoporosis and chronic pain to cancer and heart disease-and the fact that vitamin D deficiency is a growing problem worldwide, there are now more reasons than ever to make sure you get your daily dose of D.
Factfile: The D-diabetes connection
There are several plausible mechanisms for the suggested relationship between vitamin D and diabetes. In Type 1 diabetes, vitamin D appears to regulate the immune system which, in this form of the disease, is responsible for destroying the body's insulin-producing beta cells. In type 2 diabetes, however, vitamin D appears to act directly on the function of beta cells, thereby affecting insulin secretion and sensitivity. The vitamin is also thought to have an effect on inflammation and on parathyroid hormone, both of which are involved in type 2 diabetes (Diabetes Metab Res Rev, 2009; 25: 417-9).
Factfile: Getting enough D
Many of us-especially children and teenagers-aren't getting enough of this important vitamin (CMAJ, 2007; 177: 161-6), so how can we ensure that we're not vitamin D-deficient?
u Sunshine. The body naturally produces vitamin D when the skin is exposed to sunlight, and just 10-15 minutes of sunshine a day, without sunscreen, should be enough for most people (BMJ, 2003; 327: 1228). Sensible exposure along with supplementing with antioxidants, such as selenium, lycopene, beta-carotene, and vitamins C and E, will allow you to enjoy the sun without the need for potentially harmful chemical sunscreens (see also WDDTY vol 19 no 3, pages 20-1).
u Supplements. For those who don't get much sun, supplementing with 600-1000 IU/day of vitamin D3 (the natural form) is a good idea. Indeed, even regularly taking as much as 10,000 IU/day poses no risk of adverse effects in adults (Ann Epidemiol, 2009; 19: 441-5). In children, 800-1000 IU/day is recommended by US vitamin D expert Adrian Gombart for fighting against disease.
If you're a diabetic, supplements may help if you are deficient in vitamin D, so it's worth getting checked out by a qualified practitioner, who can also advise you on a safe dosage if you're interested in using vitamin D for diabetes control. However, so far, there don't appear to be any organizations recommending routine vitamin D supplementation for diabetics.
u Diet. Vitamin D is also found in a number of foods, including milk, yoghurt, eggs and fatty fish. However, watch out for the high mercury content in some fish (see WDDTY vol 17 no 10, pages 20-1,
for more information).
WDDTY VOL. 22 NO. 3