What Actually Happens to Your Blood Sugar During a Marathon? A Mile-By-Mile Guide

Running 26.2 miles is an extreme psychological and physical challenge. For people living with diabetes, it can also be a remarkably complicated blood sugar experiment. The good news is today’s diabetes technology gives you more tools than ever to help navigate it safely and successfully. Still, there’s no way around it: Running a marathon with type 1 diabetes is a big, big deal. 

During any prolonged exercise, muscles need a steady supply of energy in the form of glucose. The body responds by increasing glucose uptake into working muscles while the liver releases stored glucose into the bloodstream. At the same time, hormones like glucagon, adrenaline and cortisol help maintain blood glucose as exercise continues. 

For someone without diabetes, insulin and other hormones automatically adjust to keep all of this in balance. But for people who use insulin that same process requires careful planning and a healthy dose of trial and error during the training process. 

So what typically happens to blood glucose levels over an entire marathon? Let’s look at what your body is doing and why blood glucose can behave so unpredictably along the way.

Mile 0: the starting line

The excitement and anticipation of race day may keep your blood glucose levels elevated even before you hit the starting line, due to increased adrenaline and cortisol. The key is balancing potentially higher starting blood glucose levels without causing a crash at mile 2. 

A healthy breakfast mixing protein and fat with a moderate amount of carbohydrates can help stabilize blood glucose levels, leaving you feeling steady without a spike or quick crash. 

Miles 1-3: muscles start using glucose 

Once the race begins (and your corral is finally called), your muscles will quickly start increasing their glucose uptake. During moderate-intensity aerobic exercise, muscles can take up glucose even when insulin levels are relatively low. Basically that means you can drop fast, even without a lot of insulin on board (IOB). Of course, your diabetes will vary, and not everyone will experience this. 

If your insulin levels are low enough—because you have little to no IOB, are in “activity mode” on your pump or ratcheted back your long-acting insulin the previous night—you may avoid a low altogether. 

If the starting intensity of your run is high enough to pump out adequate amounts of adrenaline, your blood glucose may even spike. 

Miles 4-6: carb-loading becomes important 

At this point, you’re probably about 30-45 minutes into the marathon. Your body is settling into a sustained energy demand (your brain knows today’s mileage goal, but your body doesn’t!). This is why eating some carbs, even if you’re not low, becomes very important. 

For people living with diabetes, eating carbs throughout the race serves two purposes: feeding your muscles and preventing low blood sugars too. 

Between miles 4-6, your muscles use a combination of stored glycogen, circulating glucose and fat for energy. Carbohydrate stores will be the most limited, which is why sustaining carb intake throughout the race is crucial. The recommended intake is 60-90g of carbs per hour.

Gels and liquids are the easiest to both consume and tolerate. Chews can work if you prefer carbs with some texture. 

• Monitor your blood glucose closely. Be sure to adjust insulin doses for your carb intake. Eating 90 grams of carbohydrates without the proper insulin dosage can be a recipe for a blood sugar disaster. 
• Hydrate! You can lose up to 2% of your body weight through sweat and dehydration when exercising, so it is crucial to drink water and plenty of electrolytes to prevent muscle cramping and weakness. Dehydration can also increase your risk of stubborn high blood sugars. Stop at every water table for some sips. 
•  Maintain healthy electrolyte levels. Aim for at least 300 mg of sodium per hour. Some gels and chews have sodium for this very reason. 

Miles 7-10: the liver dump 

When you get this deep into the first half of the marathon, be prepared for fatigue and a dump of glucose into the bloodstream from the liver. 

At this point, the liver is dumping glucose into your bloodstream—in people with and without diabetes—to maintain stable blood glucose levels. The difference is that In people without diabetes, their bodies also produce insulin to counteract any spikes. 

This is where your IOB becomes really important. Along with carb-loading, the liver glucose dump can cause blood glucose levels to skyrocket by miles 7-10. 

Make sure you’re counting carbohydrates and dosing your insulin appropriately (which will be greatly reduced while you’re actively running) to avoid sky-high blood glucose levels or, if you give too much insulin, a hard crash. 

Miles 11-13.1: the halfway point and insulin sensitivity

By the time you reach the marathon’s halfway point, you’ve been running for a few hours. Glycogen stores are mostly depleted, so carbohydrate intake becomes increasingly important as the race continues. At this point, insulin sensitivity is high, but adrenaline is also pumping. 

This can result in a few things: if you’re dosing insulin for carb intake, you’re more likely to crash from increased insulin sensitivity. 

At the same time, adrenaline is pumping, stimulating the liver to release additional glucose, sometimes faster than the muscles can use it up. This can result in higher-than-ideal blood glucose levels. 

It’s a lot to think about, right?

Miles 14-17: a balancing act

The second half of the marathon will increasingly test your body’s ability to maintain energy availability. Oh and your knees are probably starting to hurt. 

Carbohydrates are even more critical in the second half of a marathon. Your energy stores are being completely depleted and muscles are starting to burn fat for energy.

• Monitor your blood glucose closely. If you’re using a continuous glucose monitor (CGM), keep track of anything trending down or dangerously high.
• Don’t wait to treat lows or highs. Get ahead of low or high blood sugars. At this point in the race, things can progress quickly into a crisis. Diabetes technology can be a game-changer here—automated systems that adjust insulin in real time take some of the guesswork off your plate when you’re mid-race and can’t stop to think through every number. Closed-loop AID systems, like the MiniMed 780G system, can help you catch a trending low or high before it becomes a problem. 
• Remember that your goal is safety, not perfection. 

Miles 18-20: the infamous “wall”

You know something is famous when there’s a Wikipedia page all about it. Remember the phrase, “it’s a marathon, not a sprint?”  It turns literal right about here. 

“The wall” is the part of an endurance race when runners experience fatigue and a sudden loss of energy. 

Combined with depleted glycogen stores, people with diabetes may be running into low blood glucose levels at this point, making running all that much harder. 

Continued carbohydrate intake, hydration and lowering the intensity of your run can increase your stamina and bring you over the finish line. 

Miles 21-24: AHHHHHHHHHHHH

Everything hurts. Every little thing hurts. And then there’s still diabetes to worry about. 

• Slow your pace and take an inventory of how you’re actually feeling. If anything is hurting too much or you think you’ve injured yourself, there is no shame in cutting out early and finishing your marathon another time. 
• Be gentle with yourself. Know that this is when fatigue and fuel demands are at their all-time high. If your blood glucose levels are holding steady, adjust your pace and intensity as needed. It’s okay to slow down or even to walk!
• Know when to stop. If your blood glucose levels remain too low for too long, if you’ve spiked above 250 mg/dL, you’re dehydrated or you fear you may fall into DKA, know when to call it quits. There will always be another marathon, but never another you. 

If you need to stop early, notify an emergency contact or let a race official know that you have diabetes and that you need assistance ASAP.

Miles 25-26.2: the finish line aka the promised land

You can see the finish line basically in the distance. This will test you physically and mentally. You may be feeling on top of the world thanks to an adrenaline rush, or you may feel completely tapped out. 

• Keep a close watch on blood glucose levels. Depleted glycogen and glucose stores may have you crashing, or the last-mile adrenaline rush may surprisingly cause a spike. 
• Stay steady. Some runners may be tempted to increase their pace as the finish line approaches, but depending on your blood glucose, it’s important to stay steady and finish strong. This will help you avoid last-minute blood glucose crashes or injuries.

You made it!

You’ve just run a marathon! That’s something less than 1% of the world’s population has accomplished. 

The physical work is over, but keep a close watch on your blood glucose levels for hours and even days afterward. Your body just did something super human and will take time to recover. Adjust your insulin dosages accordingly. 

As your muscles start to replenish glycogen, insulin sensitivity will remain high for a long time.

This can increase the risk of delayed hypoglycemia, including overnight. Eat plenty of carbohydrates, fat, and protein, and drink plenty of water and drinks containing electrolytes. 

And cheers to a job well done! Both you and your diabetes just accomplished something remarkable. tyow 

Thinking about running your marathon? Stay up to date on the latest diabetes technology to help you along the way!