A growing body of epidemiological, animal and clinical studies has linked insufficient levels of vitamin D and calcium to a broad range of health problems such as osteoporosis, cancer and autoimmune diseases.
Vitamin D is found in low concentrations in some food, and most vitamin D is manufactured in the skin on exposure to sunlight. Recent studies have shown that sunshine levels in some northern countries are so weak during the winter months that the body makes no vitamin D at all.
Indeed, at the European Congress of Endocrinology in Glasgow yesterday, Professor Roger Bouillon from the University of Leuven reported that more than a billion people of all ages worldwide needed to up their vitamin D intake.
The new study, published in the journal Diabetes Care (Vol. 29, pp. 650-656), used data from the Nurses Health Study, and related the vitamin D and calcium intake of 83,779 registered nurses to the incidence of type-2 diabetes.
None of the women had diabetes at the start of the study, and the average body mass index was 24 kilograms per square metre (meaning that very few of the women were overweight or obese, factors that are strongly associated with increased risk of diabetes).
After 20 years of follow-up, during which time dietary and supplementary intakes were measured using validated food frequency questionnaires every two to four years, the authors concluded: "A combined daily intake of more than 1,200 milligrams of calcium and more than 800 international units (IU) of vitamin D was associated with a 33 per cent lower risk of type-2 diabetes."
Interestingly, lead researcher Anastassios Pitas, from the Tufts-New England Medical Center, found that dietary intake of vitamin D did not result in a statistically significant benefit. But women who consumed from that 400 IU vitamin D per day from supplements had a 13 per cent lower risk of diabetes, compared to women who consumed less than 100 IU per day.
Both dietary calcium and supplements were associated with significant decreased risks of type 2 diabetes, with women who had total daily intakes of calcium greater than 1,200 milligrams had a 21 per cent lower risk compared to women who had intakes less than 600 mg per day.
"For both vitamin D and calcium, intakes from supplements rather than from diet were significantly associated with a lower range of type 2 diabetes," said Pittas.
The mechanism as to why vitamin D and calcium may reduce the risk of diabetes is not clear, but the researchers proposed that the two nutrients work together. Vitamin D facilitates calcium absorption in the intestine, while calcium is reported play a role in normalising glucose intolerance.
The researchers did not rule out a direct role of vitamin D, independent of calcium, noting that studies have reported that vitamin D insufficiency had been linked to insulin resistance and reduced function of pancreatic beta-cells.
Strengths of this study included the large-scale and long-term follow-up, but the authors note the inability of the study design to measure all the possible confounders. Also, no blood samples were taken to measure serum vitamin D levels.
"If these results are confirmed in prospective studies or in randomised trials, they will have important health implications because both of these interventions can be implemented easily and inexpensively to prevent type 2 diabetes," concluded the researchers.
An estimated 19 million people are affected by diabetes in the EU 25, equal to four per cent of the total population. This figure is projected to increase to 26 million by 2030.
In the US, there are over 20 million people with diabetes, equal to seven per cent of the population. The total costs are thought to be as much as $132 billion, with $92 billion being direct costs from medication, according to 2002 American Diabetes Association figures.