Why Exercise Can Drop Your Blood Sugar Too Low
Working out with diabetes isnât just about getting fit-itâs a balancing act. When you move, your muscles grab glucose from your blood without needing insulin. Thatâs great for lowering high blood sugar, but if youâre on insulin or certain diabetes meds, it can send your levels crashing. For many people with type 1 diabetes, fear of a low during a workout is the number one reason they skip exercise altogether. And itâs not just fear-hypoglycemia during or after physical activity is real, common, and sometimes dangerous.
Hereâs the simple truth: exercise makes your body more sensitive to insulin for up to three days. That means even if you took your insulin hours ago, itâs still working hard when youâre lifting weights, running, or cycling. Add in the fact that your muscles are pulling glucose out of your bloodstream like a sponge, and youâve got a perfect storm for a low.
When to Check Your Blood Sugar Before You Start
You wouldnât drive a car without checking the fuel gauge. Donât start a workout without checking your blood sugar. The American Diabetes Association says if your level is below 90 mg/dL, you need to eat carbs before you begin. Between 90 and 150 mg/dL? You still need a snack, but you might be okay to start moving. Anything above 150 mg/dL? Youâre probably in the clear-unless youâre doing something intense or long.
Hereâs what works in real life: check your glucose 15 to 30 minutes before you start. If itâs under 100 mg/dL, eat 15 to 20 grams of fast-acting carbs. Thatâs about half a banana, four glucose tablets, or 6 ounces of regular soda. Wait 15 minutes, check again. If itâs still low, eat another 15 grams. Donât rush. A low blood sugar during a workout can make you dizzy, shaky, or even pass out. Thatâs not worth the risk.
Carbs Before, During, and After: The Right Amount
How many carbs you need isnât one-size-fits-all. It depends on your weight, how hard youâre working, and how much insulin you have on board. A good rule of thumb: aim for 0.5 to 1 gram of carbs per kilogram of body weight during moderate exercise.
Letâs say you weigh 70 kg (about 154 pounds). That means you might need 35 to 70 grams of carbs during a 60-minute bike ride. Thatâs a lot-but you donât have to eat it all at once. Spread it out. Eat 15 grams every 30 to 45 minutes. Keep glucose tabs or gels in your pocket, your bike bag, your gym locker. Donât wait until you feel bad to reach for them.
And donât forget after. Exercise can keep lowering your blood sugar for hours-even overnight. If you worked out in the afternoon, check your glucose before bed. If itâs under 120 mg/dL, eat a small snack with carbs and protein: a slice of peanut butter toast, a cup of yogurt, or a handful of nuts with an apple. This helps prevent nighttime lows, which are scary and common. One study found that 42% of people with type 1 diabetes have at least one severe low at night after daytime exercise.
Timing Matters: Avoid Peak Insulin Hours
When you take insulin affects how your body responds to exercise. If you take rapid-acting insulin before meals, that dose peaks about 60 to 90 minutes later. If you hit the gym right then, youâre asking for trouble. The same goes for long-acting insulin-it doesnât peak, but itâs always working. Thatâs why consistency matters.
Try to exercise at the same time each day. If you always run at 5 p.m., your body learns to expect it. Your insulin needs become more predictable. If youâre on an insulin pump, you can lower your basal rate by 50 to 75% starting an hour before exercise. If you use injections, reduce your pre-workout bolus by 25 to 50%. These arenât guesses-theyâre evidence-backed adjustments recommended by the ADA.
And hereâs a pro tip: always check your insulin-on-board (IOB) before you start. If you have 2 units of active insulin in your system, and youâre about to run, thatâs like having 3 or 4 units working against you. Your pump or app can calculate this for you. Use it.
Not All Workouts Are Created Equal
Hereâs something most people donât know: not all exercise causes low blood sugar. In fact, some types can actually protect you from it.
Aerobic workouts-like jogging, swimming, or cycling-tend to lower glucose steadily. But resistance training? Weight lifting? Thatâs different. A 2018 study showed that doing 45 minutes of strength training before a 45-minute cardio session cut the drop in blood sugar almost in half. Why? Because lifting weights triggers your liver to release glucose. Itâs like your body hits the brakes on the drop.
Even better: short bursts of high-intensity effort. Just 10 seconds of all-out sprinting-on a bike, treadmill, or even stairs-before your workout can stabilize your glucose for the next hour. People who added this to their routine reported fewer lows, sometimes cutting them from four times a week to once every two weeks.
High-intensity interval training (HIIT) also helps. A few 15-second sprints during your workout keep your blood sugar from falling too far. Circuit training, though, can be risky if youâre going nonstop with little rest. Itâs easy to burn through glucose fast. Know your limits.
Technology Is Changing the Game
Five years ago, managing exercise with diabetes meant guessing. Today, itâs data-driven. Continuous glucose monitors (CGMs) like the Dexcom G7 now have an âexercise mode.â When it detects movement, it lowers alert thresholds by 20 mg/dL so you donât get false alarms. Itâs smarter, more accurate, and built for active people.
Insulin pumps are catching up too. The Tandem t:slim X2 pump, approved in March 2023, has an âExercise Impactâ feature. It uses machine learning to predict how your glucose will drop based on your past workouts, your insulin on board, and your current activity. Then it automatically reduces your insulin delivery to keep you safe.
And itâs not just hardware. New research is testing artificial pancreas systems that deliver glucagon during exercise. Glucagon is the hormone that tells your liver to release sugar. In a recent NIH trial, a dual-hormone system reduced exercise-related lows by over 50%. This isnât science fiction-itâs coming soon.
What to Do When You Still Go Low
Even with all the planning, lows still happen. Youâre human. Your body changes. One day you do the same run at the same time with the same insulin dose-and youâre fine. The next day, youâre at 60 mg/dL halfway through.
Thatâs normal. Donât panic. Hereâs your action plan:
- Stop exercising immediately.
- Check your glucose. If itâs under 70 mg/dL, eat 15 grams of fast-acting carbs.
- Wait 15 minutes. Check again.
- If itâs still low, eat another 15 grams. Repeat until youâre above 100 mg/dL.
- Donât restart until youâre stable. Pushing through a low can lead to a crash-or worse.
Keep fast-acting carbs with you at all times. Donât rely on vending machines or gym snack bars. Carry your own. Glucose tabs are compact, reliable, and donât melt in your pocket.
Real People, Real Results
On diabetes forums, people share what works. One user on Reddit said he did the same 5K run three days in a row-and his glucose was 70, 180, and 120. Same time. Same distance. Same insulin. Why? Stress. Sleep. Weather. Hormones. Your body doesnât follow a spreadsheet.
But hereâs what worked for others:
- 78% of users on T1D Exchange use temporary basal rate reductions before exercise-and 63% say it helps.
- One woman added a 10-second bike sprint before every workout. Her lows dropped from four a week to one every two weeks.
- A man with type 1 diabetes started lifting weights before his runs. His average glucose during cardio went from 105 to 124 mg/dL. No more mid-run crashes.
Thereâs no magic fix. But there are tools. There are patterns. And thereâs hope.
Your Next Steps
Start small. Pick one workout you already do. Track your glucose before, during, and after for a week. Note your insulin dose, carbs eaten, and how you felt. Look for patterns. Did you go low after morning runs but not evening walks? Did lifting before cardio help?
Then make one change. Lower your basal rate. Eat a snack before your workout. Add a 10-second sprint. See what happens.
It takes 3 to 6 months to get good at this. Youâll have bad days. Youâll have lows. Youâll have highs. But youâll also start to feel stronger, more confident, and more in control. Exercise isnât the enemy. Itâs part of your treatment. And with the right plan, you can do it safely-for life.
12 Comments
This is đ„ honestly. I used to panic before every run, but now I just do a 10-second sprint first and boom - no more crashes. Game changer.
Iâve been using Dexcomâs exercise mode for 6 months now. The alerts are so much smarter - no more false alarms during yoga. Also, always carry glucose tabs. Not gummies. Not candy. Tabs. đ§Ș
i just started tracking my bg before and after workouts and wow. i had no idea my morning runs were dropping me to 65 every time. now i eat half a banana and iâm golden. you got this!! đȘ
The idea that âexercise is part of your treatmentâ is dangerously oversimplified. Many of us are on fixed insulin regimens. You canât just âadjustâ like itâs a thermostat. This article reads like a marketing brochure for Tandem pumps.
I actually think Ashley has a point. I tried reducing my basal rate before running and ended up with a 220 mg/dL spike because my pump didnât account for my cortisol surge. The tech isnât perfect. You still need to test. A lot.
Let me tell you something - lifting before cardio isnât just a hack, itâs a goddamn revolution. I went from needing a granola bar mid-run to crushing 10Ks without a single drop. My glucose didnât just stabilize - it danced. And yeah, Iâm still alive to brag about it.
The evidence supporting pre-exercise resistance training is robust. The 2018 study referenced demonstrated a statistically significant attenuation of hypoglycemic events with a p-value of 0.013. However, individual variability remains substantial due to circadian hormonal fluctuations and insulin pharmacokinetics
I must confess, I find the casual tone of this article almost offensive. One does not simply âeat half a bananaâ and expect physiological equilibrium. The human endocrine system is a cathedral of complexity - not a vending machine to be tinkered with by the uninitiated. Your insulin is not a suggestion. It is a covenant.
You say âone size doesnât fit all.â But you gave exact carb amounts. Contradiction.
In India, we donât have access to Dexcom G7 or Tandem pumps⊠we use glucose strips and hope⊠and sometimes, we just run anyway⊠and pray⊠and pray⊠and prayâŠ
This whole thing is just another liberal wellness scam. Real Americans donât need fancy pumps. We just eat less sugar and lift heavy. If you canât handle a jog without a snack, maybe you shouldnât be jogging.
you guys are overthinking this so much like its rocket science bro just eat a candy bar and run dont be so weak
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