Blood sugar spikes from high GI foods - a problem?

Question

I read a lot of people saying you shouldn't fuel with sugary/high GI (Glycaemic Index)¹ foods, as you'll get a blood sugar spike, an insulin release to cope with it, and then a subsequent blood sugar crash afterwards that will leave you feeling crap and with little energy. I can understand this if you're just sat on a sofa doing nothing, but have there been any studies monitoring blood sugar in cyclists/athletes as they take in something like energy drinks and exercise at medium-high intensity?

The body can apparently absorb 60-90g of carbs an hour - is there a problem with taking on that amount of carbs in one go (e.g. a common combination of maltodextrin/glucose/fructose) and then cycling for an hour, rather than spreading it over an hour?

I've tried riding with low GI foods and they just didn't seem to give me enough oomph, maybe okay for relaxed touring but I tend to like making progress even if I'm not racing. The boost from an energy drink or handful of dates doesn't last too long, granted, but I can definitely feel it when it's there.

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Footnote 1: Glycemic index is a measure of how quickly food is digested and how quickly it affects blood sugar. The GI of pure glucose is 100, and foods are measured in relation to that. High GI foods include white rice, white bread, energy gels, and similar foods which have simple carbohydrates. Fiber, fat, and protein lower the GI. More information can be found on Wikipedia.

Answer 1

During a short ride you mostly fuel with pure sugars and as simple as possible - so mainly glucose in gels. For longer events you also want more complex sugars that take longer, but still mostly carbs - rice cakes, croissants, sandwiches, musli (granola) bars.

You are correct that you cannot sustain a long ride (a road race) just from the glucose gels, you will get hungry. But it is possible for short two hour or slightly longer events. Professionals in Ski Classics mostly only use energy gels and drinks for the long-distance skiing. Cyclocross events are one hour or less and you won't see any eating during that, just some drinks or perhaps a gel.

It is impossible to overeat when going full gas. Your concern is to give your body enough fuel in frequent small doses.

Large amounts of proteins and especially fats are too difficult to digest during the race, you have them later for dinner.

You were asking for studies and this one should be of interest for you: International society of sports nutrition position stand: nutrient timing

I select just one paragraph:

Extended (> 60 min) bouts of high intensity (> 70% VO2max) exercise challenge fuel supply and fluid regulation, thus carbohydrate should be consumed at a rate of ~30–60 g of carbohydrate/h in a 6–8% carbohydrate-electrolyte solution (6–12 fluid ounces) every 10–15 min throughout the entire exercise bout, particularly in those exercise bouts that span beyond 70 min. When carbohydrate delivery is inadequate, adding protein may help increase performance, ameliorate muscle damage, promote euglycemia and facilitate glycogen re-synthesis.

They recommend every 10-15 minutes.

It is supported in the article body:

For example, Fielding and colleagues [50] required cyclists to ingest the same dose of carbohydrate every 30 min or every 60 min over the course of a four-hour exercise bout. When carbohydrate was ingested more frequently, performance was improved.

Answer 2

For what it's worth, I participated in an ultraskate 2 years ago. The principle is easy enough: push on a skateboard as far as you can in 24h.

Skateboarding is less efficient than cycling, but I think the effort is comparable. You'd just go faster and further on a bicycle.

My goal was to push at least 200 miles (~320km). It's not easy to eat 4000+ kcal while riding a skateboard, so I decided to try out Tailwind. It's basically a powder with 99% glucose/sucrose and salts that you mix with water. There are other alternatives available (e.g. Skratch's Superfuel or Maurten's drink mix), but I didn't try them yet.

I drank half a liter every 30 minutes. Each liter had 75g sugar, for 300 kcal of energy. During training, I tried to drink 1l at the beginning of each hour, but it was too much at once.

I rode 50km every 3h, with 10 minutes of pause at the end of each 50km section.

I never had a too-full belly, I never had cramps and was never hungry. Just for curiosity, I tried to eat a sandwich after 10h but it really didn't feel good at all and I stopped after the first bite.

After 16 hours and 16 liters (and 1.2kg of pure sugar!), I was really fed up and couldn't drink anything anymore. On the other hand, I cannot think of any other liquid (even pure water), I'd happily drink 16l of. I finished my goal by riding 205 miles, and went to sleep.

I never felt a blood sugar spike, I didn't get bellyaches and didn't get diarrhea.

The next day, I was really hungry and ate 3 full menus in one afternoon.

Some friends tried the exact same product and they got diarrhea after 1l. Be sure to thoroughly test any nutritional product before using it during races, and try it while riding.

During Christmas, I was reminded that I feel really bad if I eat too much sugar while doing nothing else. So I definitely can get sugar spikes, but apparently not while exercising.

Answer 3

For what it’s worth: Energy gels are mostly glucose (sometimes a mix of glucose and maltodextrin). As far as I’m aware most professional athletes (my sister included) use such gels (or other glucose source) during competitions.

However, even on hard rides you shouldn’t need food if the ride lasts less than an hour. For rides longer than an hour it really depends on the intensity. I think most pros try to max out on the 60–90g carbs you’ve mentioned for longer competitions.

A lot can also be in your brain. I dimly remember a study where even pure glucose had a ~15 minute delay from ingestion until a significant rise in blood glucose levels. Yet you sometimes feel immediately better after a sugary snack.