Vitamin D has in studies found to be associated with a reduced risk of a range of conditions including several forms of cancer, heart disease, multiple sclerosis and ‘auto-immune’ diseases such as Type 1 diabetes, multiple sclerosis and rheumatoid arthritis. There has, in recent years, been some interest in the role that vitamin D might play in obesity and associated problems such as metabolic syndrome, insulin resistance and Type 2 diabetes.
Awhile back one of my blog posts featured some research in which women with insulin resistance were treated with vitamin D (4,000 IU) a day for six months. Insulin resistance is a state where insulin’s effects in the body are somewhat ‘numbed’, and can lead to elevated blood sugar levels (as well as other biochemical abnormalities) and may progress to Type 2 diabetes in time. In this study, compared to placebo, the vitamin D improved their insulin sensitivity, signifying likely relative protection from Type 2 diabetes.
This is just one study so I was interested to read about a recent review of the evidence as it relates to vitamin D and Type 2 diabetes [1]. Part of the review looked as epidemiological evidence which is useful for assessing relationships between things. However, just because two things are associated, that does not mean that one is causing the other. Nevertheless, the evidence from 8 studies amassed together showed that individuals with the highest vitamin D levels (>25 ng/ml = 62.5 nmol/l) had a 43 per cent reduced risk of Type 2 diabetes compared to those with the lowest levels (<14 ng/ml = 35 nmol/l).
The review also assessed intervention trials – studies in which individuals were actually treated with vitamin D (or placebo). Some of these trials were in individuals with normal insulin sensitivity and, not surprisingly, did not yield positive results (if there’s no problem, you can’t fix one, right?) Other studies were very small and therefore unlikely to detect any benefit from vitamin D that might exist.
However, in two intervention trials in individuals with evidence of insulin resistance, there was evidence of benefit in the form of improved insulin sensitivity. The authors of the review call for more high-quality epidemiological evidence, though personally I feel this would be a waste or resources. They also call for ‘randomised controlled trials’ (a form of intervention studies) in which vitamin D levels and measures of blood sugar control are assessed.
One relevant study was recently published in the American Journal of Clinical Nutrition [2], and it’s lead author was the same as the review referred to above. In this study, 92 adults deemed to be at high risk of Type 2 diabetes were treated for 16 weeks with one of the following:
- Vitamin D (2000 IU per day) plus placebo
- Vitamin D (2000 IU per day) plus calcium (400 mg, twice a day)
- Calcium (400 mg, twice a day) plus placebo
- Placebo
An interesting finding of this study was that those who took vitamin D (with or without calcium) saw significant improvement in their production of insulin. Insulin is produced by so-called ‘beta cells’ in the pancreas. The function of insulin (insulin sensitivity) was not found to be improved by vitamin D in this study. Another measure that was assessed in this study was the ‘disposition index’ – a measure which includes both beta cell functioning and insulin sensitivity. This index is believed to be a good marker for risk of Type 2 diabetes. The index improved by 26 per cent in people taking vitamin D, and worsened by 14 per cent in those who were not. All-in-all, the evidence suggests that higher vitamin D levels offers some potential in combating diabetes.
References:
1. Mitri J, et al. Vitamin D and type 2 diabetes: a systematic review. Eur J Clin Nutr. 2011 Jul 6. doi: 10.1038/ejcn.2011.118. [Epub ahead of print]
2. Mitri J, Effects of vitamin D and calcium supplementation on pancreatic {beta} cell function, insulin sensitivity, and glycemia in adults at high risk of diabetes: the Calcium and Vitamin D for Diabetes Mellitus (CaDDM) randomized controlled trial. Am J Clin Nutr. 2011 Jun 29 [Epub ahead of print]
