Seasonal variation in blood glucose levels   (December 19, 2008)

A while back, I received an e-mail inquiring about seasonal variation in blood glucose levels. The author wrote:

I'm seeing if you are able to help answer a question, as I cannot find anything on any website. I use Lantus insulin and have found that when the cold seasonal weather begins to turn into spring (usually about March or April), I require less insulin and vice versa for the change from autumn (fall) to winter. This has caused me hypos and high blood sugar until I change the insulin dosage and begin to get it under control. I am trying to tell my employer this is the case. Are you able to advise of any studies about this phenomenon? I am aware of anecdotal evidence but my employer is not willing to accept this.

I must admit that I was unaware of any studies that might directly address this issue, although I'm not surprised, as increased activity in warmer weather would seem likely to require less insulin than would be used during the colder and presumably less active months of the year. But it turns out there's plenty of published material to support the hypothesis.

I searched PubMed using two terms simultaneously: "insulin" and "seasonal", and found lots of articles with these two words. (If you try, be aware that lots of the articles that are found are about a different subject, namely that the number of cases of diabetes that are diagnosed varies by season... And there are lots of other articles that are totally irrelevant!)

Here are some articles of interest:

1) Seasonal patterns in monthly hemoglobin A1c values. The authors reviewed variations in monthly A1C values over a 2 year period among 285,705 US diabetic veterans, and found A1C values were higher in winter and lower in summer. Regions with colder winter temperatures had larger winter-summer contrasts than did those with warmer winter temperatures. The abstract doesn't mention whether the patients needed changes in diabetes treatment to compensate for the changes in A1C.

2) Seasonal changes in body composition and blood HbA1c levels without weight change in male patients with type 2 diabetes treated with insulin. This article looks interesting, based on the title, but there was no abstract. Phooey.

3) Seasonal variation of HbA1c in intensive treatment of children with type 1 diabetes. The authors of this study evaluated whether there is a seasonal variation in A1C and insulin dose used in the intensive treatment of children with type 1 diabetes, and whether such variation is related to severe hypoglycemia. They followed 114 intensively treated type 1 diabetic patients less than 19 years of age, and noticed lower A1C levels were seen in spring and summer, and higher in autumn and winter. Patients reporting severe hypoglycemia had a seasonal variation in A1C and a tendency to seasonal variation in insulin dose, while patients not reporting severe hypoglycemia did not vary in A1C or insulin dose. The authors concluded that adjustment of insulin doses to seasonal change does need to be taken into account in intensively treated children, with regard to the risk for worsened metabolic control after the summer and increased severe hypoglycemia in spring and early summer. (Sure sounds like the same situation the person who wrote to me had encountered!)

Well, it turns out there are some scientific articles supporting the concept that diabetes does vary by season. I learned something, and the writer of the question now has some ammunition to show his employer.