Thinking about running a marathon

Hi guys! I’m T1 for about 8 years now, controlled pretty well on a CGM and insulin pens.

I run regularly, and ran a half marathon about 3 years ago, and I’m thinking about training to run a full marathon next fall. Typically when I run (usually 2-4 miles), I see my blood sugar spike, and then it comes down slowly later. I ran the half in about 2.5 hours, so looking at around 4.5-5 hours for a full. That just seems like such a long time that I’m obviously thinking about lows.

Just wondering if any of you guys have run full marathons and what advice you have for someone who’s thinking about giving it a shot.


I haven’t run a full, but am in training for a half-triathlon (next summer is the target). Have to learn to swim first haha (or at least be more like a fish and less like a stone). So, I can only speak from personal experience, but I’ve had both issues you have had (or worry about):

  1. Blood sugar rising from exercise: you might look up information on resistant starch and Type 1 diabetes. Personally, I’ve found that 30g of RS a half hour before starting exercise completely flattens the exercise spike.

  2. Blood sugar falling form extended exercise: mine tanks at about 45 to 60 minutes, reliably. I see it the most with cycling, which is just steadily intense enough at pace to reliably burn through muscle glycogen like mad. The solution for me has been to do a ton of BG testing while riding (no CGM, I just use finger sticks), charting my BG, and using Clif gels periodically to stabilize my BG. I’m sure it’s different for everyone, but I can handle a full gel pack (24g of carbs, 12g of sugar and 12g of starch) at 60 minutes into a ride without rising more than 10 mg/dL, if I’ve really been pushing it. I take one every 60 minutes thereafter if I’m continuing to ride. I also end up doubling or tripling my post-ride carbs in my next meal (not quite to the “eat pasta after riding/running/lifting” stage, but I’ll have 40-50g including fiber after working out). It works for me, but took a lot of testing to figure out the break even points.

What does not work for me is “carb loading.” If I try to take dextrose or any other “simple” carb before working out, my BG will absolutely skyrocket. I know it works for some people, but I really don’t want my BG at 150 or 200 mg/dL before starting exercise, since my exercise bump will send it another 50 mg/dL higher yet. I’ve found it’s best to time my carbs after I begin exercise, so that I’m digesting them as my muscles need them.

Both of the above strategies only work while very well hydrated for me, with plenty of electrolytes (magnesium and potassium, in particular). If I really want to screw up my BGs (high or low), getting dehydrated is the worst.

My biggest challenge right now is figuring out exercising in heat. I don’t know what it is, but even mild exercise will tank my blood sugar when the temperature is above 80 degrees F. Yesterday, it was 95 here, and I was working on some fencing for my yard. Feeling a bit wobbly, tested and was 60 mg/dL, down from 95 mg/dL an hour after lunch…


I’ll be figuring this out too. I just got diagnosed last month (4 days in the hospital w dka) but I’ve run marathons pre-diagnosis. I haven’t run more than 2h since- but no insulin/1 gu pre run led to my bg going to 500 by the end of the run then crashing later. insulin+2gu/1bar kept my bg more stable on the next long run.

What marathon are you running? I have 3 on my calendar for the rest of the year. I’m sure there will be trial+error. Target will be sub 3:30 at CIM in December…

1 Like

Wow thanks for this! This is seriously so helpful. <3

1 Like

Oh man, sorry to hear about your recent diagnosis. This community has been really helpful to me though so I’m glad you’re here.

I’m thinking about the NYC marathon! But not until 2018

The Diabetic Athlete’s Handbook was very helpful for me to figure out my daily workouts. I highly recommend it!

Go for it. Although I’d never try it before or after dx, the NYC Marathon is quite a feat. Good luck with the lottery! Hope you get a slot.

1 Like

I’ve run several half’s and a full. I usually start with my bg a little high (on purpose) and then take gels each 50 min or so. I’ve also had a family member meet me along the course in the later miles with my glucometer.