Mercury Rising: Ambient Temperature and Blood Sugar

No matter which part of the world you're in as you read this, chances are that in the preceding 60 days you read local news warnings of how local temperatures are higher than previous averages for this time of the year. In fact, July 2023 has been the hottest month ever recorded in this planet's history. Concomitantly, we have seen hundreds of heat-related deaths across the world this year. But did you know that heat waves can have particularly deleterious effects in people with diabetes?

Heat-Related Morbidity and Mortality in People With Diabetes

Previous studies from Australia and the US have shown that people with diabetes have up to a 30% higher risk of being admitted with heat exhaustion and heat stroke. During the 1996 heat wave in New York City, the death rate in people with diabetes increased by 117% (Schuman; Faunt et al; Semenza et al). It is thought that people with especially poorly controlled diabetes have impaired autonomic thermoregulation, increasing the risk for hyperthermia with concomitant fluid and electrolyte imbalances.

Even a mild increase in body temperature leads to an increase in counterregulatory hormones and also levels of free fatty acids, 3-hydroxybutyrate, glycerol, and lactate. This can lead to significant variation in blood glucose levels. People with diabetes are asked to exercise at least 150 minutes per week, which can often lead to an increased risk for dehydration and potential worsening of glycemic control. In addition, they might be more likely to be on medications, such as sodium-glucose cotransporter 2 inhibitors or diuretics, thereby further exacerbating the risk for dehydration. Taken together, all of these factors may increase the risk for hyperglycemia and even diabetic ketoacidosis by as much as 60%.

Considerations for People on Insulin Therapy

There are special considerations for people on insulin therapy during higher ambient temperature conditions. We know that insulin is best stored at about 4 °C (39 °F) with a certain shelf life at room temperature. At ambient temperatures > 30 °C (86 °F), there can be a decrease in potency of insulin in vials by about 5% in 4 weeks. Exposure to heat may lead to dissociation of insulin hexamers and formation of fibrils. This also increases the risk for immunogenicity to insulin therapy. If insulin within fibrils is injected subcutaneously, bioavailability is reduced because stable bonds of insulin molecules within fibrils provide less insulin monomers available for resorption. Hence, it is recommended that people discard insulin especially if it shows visible impurities.

Interestingly, newer studies with insulin pens have shown that exposure to temperatures as high as 37 °C (98 °F) for 4 weeks have no significant effect on the glucose-lowering efficacy of basal insulin (Kongmalai et al, 2022; Kongmalai et al, 2021)

Similarly, heat may also affect absorption of insulin that is injected subcutaneously. Studies show that changes in temperature from colder to warmer may increase the rate of disappearance of rapid-acting insulin from the subcutaneous site after injections. Studies in people with type 1 diabetes show that the rate of absorption of insulin can be accelerated by up to 60% at an ambient temperature of 35 °C (95 °F) compared with 20 °C (68 °F), related to increased blood flow in the skin with vasodilatation.

In addition, exposure to elevated temperatures can affect the accuracy of testing strips, particularly at low glucose levels, and hence the test results may not be reliable at the time of hypoglycemia. This effect may potentially extend to continuous glucose monitoring (CGM) sensors also, although studies are lacking for newer-generation test strips as well as for CGM sensors. Unfortunately, some of the symptoms of hypoglycemia (eg, sweating, tiredness, dizziness, headache) might be mistaken for effects of exposure to high ambient temperatures, and hence episodes of hypoglycemia may not be treated in a timely manner in many individuals.

Yes, we recently saw the hottest month in Earth's history, but looking at the trends, chances are that it might not be too long before this record gets broken again. It is imperative that we consider all direct and indirect health effects that global warming brings in its wake. People with diabetes will especially benefit from increased education on hydration and more frequent glucose monitoring during heatwaves.

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