Type 1 in the heat: why holidays make you run low, and how to keep insulin from spoiling
Hot-country holidays: my insulin absorbs faster and I keep going low, plus how do I keep insulin from cooking in the heat?
Heat tends to make people with type 1 run low, for two reasons at once: warmth widens your blood vessels so insulin is absorbed faster, and holidays usually mean more walking and swimming, which burns glucose. So expect more hypos, check more often, and keep fast sugar on you. Heat also spoils insulin, so it needs keeping cool but never frozen. And sweat and pool water loosen sensor and pump adhesive. Being unwell or dehydrated in the heat can push you the other way, upward, so it moves both directions.
- Warmth speeds insulin absorption, so it can act faster and stronger than you are used to, making lows more likely.
- Holiday activity adds to that: sightseeing, swimming and heat itself all use glucose, and a low can arrive hours later, including overnight.
- Insulin is damaged by heat: keep spares in a fridge or a cooling wallet, and never let it freeze, which ruins it too.
- Sensors and pump sites peel in sweat and water, so overpatches and good skin prep save you a lost sensor mid-holiday.
- Illness, sunburn or dehydration can push glucose up, so heat is not only a low story, and ketones still matter.
Why hot weather tips you towards lows
When you are warm, your blood vessels widen to shed heat, and that includes the small vessels under the skin where injected or pumped insulin sits. More blood flow there means insulin is picked up and put to work faster, so it can hit sooner and harder than the same amount does at home. On top of that, a holiday is rarely a rest for your body: walking a city all day, swimming, and simply coping with heat all pull glucose out of your blood.
Put together, that is why so many people notice they drift low abroad when nothing about their food has changed. As with any activity, the low does not always come at the time: muscles keep drawing in glucose for hours afterwards, so a big day in the sun can show up as a low that evening or overnight. This is a pattern to expect, and any change to insulin to allow for it is a conversation for your diabetes team before you travel.
Keeping your insulin from cooking
Insulin is a protein, and heat breaks it down, so insulin that has cooked can quietly stop working properly, which shows up as unexplained highs. The general rule is that the pen or vial you are using is fine at normal room temperature for a few weeks, but spares should be kept cold, ideally in a fridge between about 2 and 8 degrees. What ruins it is extremes: leaving it in a hot car, a glovebox, direct sun, or on a beach, and, at the other end, letting it freeze, which damages it just as surely.
For travel, a cooling wallet that works by evaporation (a well-known brand is Frio, though others exist) keeps insulin below room temperature without a fridge or ice, and is ideal for days out. If you use a cool bag with ice packs, keep the insulin from touching them directly, so it does not freeze. Carry all of it in your hand luggage on flights, never the hold, where it can freeze and where your bag might not arrive with you.
Keeping sensors, pumps and treatment working
Adhesive is the other thing heat tests. Sweat, sun cream and swimming all work a sensor or pump site loose, so many people use an overpatch or adhesive wipe, and put sensors on clean, dry, cream-free skin. It is worth carrying spares, because a sensor that peels off on day two is a real nuisance far from home. Sensors and meters also have temperature limits of their own, so keep them out of direct sun and do not leave your phone or receiver baking on a sun lounger.
Your hypo treatment needs to survive the heat too. Chocolate and some sweets melt into a useless mess, so pack glucose tablets, jelly sweets or juice cartons that cope better. Injecting into sunburnt or very hot skin can sting and absorb unpredictably, so give burnt areas a miss. And drink plenty of water, because staying hydrated helps everything, including how you feel and how your kidneys clear glucose.
What changes how heat hits you
A gentle warm week is not the same as a scorching, humid one, and the effect builds the hotter and more active you are. Swimming and sea days combine cooling water with a lot of movement, which can drop you quickly. Going the other way, sunburn, a holiday tummy bug or getting dehydrated can raise glucose and bring on ketones, so heat is genuinely a both-directions story. Everyone responds a little differently, so your own pattern over the first day or two abroad tells you more than any rule.
How heat shows up abroad
Examples, not instructions or doses.
You walk around a hot city all morning, eat as usual, and dip low by mid-afternoon. Warmth speeding your insulin plus hours on your feet did it, and the same day can bring a second low that evening.
A spare pen spends the afternoon in a bag on the beach, and for the next few days your numbers run high for no clear reason. Heat had quietly damaged it, so it was no longer working properly.
Between sun cream, sweat and swimming, your sensor lifts at the edge and falls off early. An overpatch and cream-free skin at the start would likely have kept it on.
Worth paying attention to
Questions that make an appointment useful
When it's urgent
A severe hypo is the emergency to plan for, and it matters more away from home. If a low leaves someone confused, unable to swallow safely, fitting, or unconscious, they cannot treat themselves: this needs someone else to give glucagon (an emergency injection) if it is available and to call for emergency help. In the UK that is 999; across most of Europe it is 112, and elsewhere check the local emergency number before you travel. Heat exhaustion can look similar, with confusion and feeling faint, so glucose still gets checked, but if someone cannot safely swallow, do not delay the call. Tell the people you are with what a hypo looks like and where your treatment is, before you ever need them to know.