Will Racing Fuel Give You Diabetes? The Internet Says Yes. The Science Says LOL.

If you've spent any time in the corners of the internet where dudes with six-pack profile pics dispense unsolicited nutrition advice, you've probably encountered The Take: that high-carb race fueling, the gels, the drinks, the carefully calculated 90-120g of carbs per hour that elites are now crushing, is going to give you diabetes.

The claim shows up on Reddit threads, in low-carb influencer content, and most spectacularly, in blog posts from supplement companies with names like "High Carbohydrate Athletic Fueling: A Fad Metabolic Dumpster Fire." (Yes, that's a real title. From a company that sells low-carb sports fuel. Coincidence? In this economy?) The argument goes something like this: sugar is toxic, chronic high blood sugar causes insulin resistance, therefore sucking down gels during your ultra is basically speedrunning your way to Type 2 diabetes.

It sounds plausible if you squint. It also happens to be almost entirely unsupported by actual evidence.

The Study That Launched a Thousand Reddit Posts

Here's where this idea gained its scientific veneer. In 2016, a researcher named Felicity Thomas published a pilot study of ten sub-elite endurance athletes wearing continuous glucose monitors. She found that some of them spent a surprising amount of time with elevated blood glucose, and three of the ten had fasting glucose in the "prediabetic" range.

The study was interesting! It was also a pilot study of ten people, with no control group, no dietary intervention, and massive variation between subjects. One athlete ate over 4,000 calories a day; another ate 1,800. One consumed three times the recommended sugar intake and had normal blood glucose. Another ate below carbohydrate recommendations and showed prediabetic markers. The athletes with the highest blood sugar weren't necessarily eating the most carbs or training the least.

Thomas herself has been careful to note that her data doesn't actually support the diabetes panic, but that nuance got lost somewhere between "pilot study" and "time to build a brand around metabolic dumpster fires."

What Actually Happens When You Eat Carbs While Running

Here's the thing the fear-mongers skip over: exercise fundamentally changes how your body handles glucose.

When you're sitting on your couch eating gummy bears, glucose uptake into your muscles depends heavily on insulin. Your pancreas secretes it, insulin receptors activate, and glucose transporters (specifically GLUT4) move to the cell surface to let sugar in. Chronic overreliance on this system—combined with excess body fat and inactivity—is indeed part of how insulin resistance develops.

But when your muscles are contracting? Totally different pathway. Exercise triggers GLUT4 translocation through insulin-independent mechanisms. Your working muscles are basically vacuuming up glucose at 50-100 times the rate of resting muscles, and they don't need insulin to do it. The signals come from muscle contraction itself, AMPK activation, calcium signaling, mechanical stress. Your muscles become glucose sinks that operate outside the insulin system entirely.

This is why exercise is literally prescribed as a treatment for insulin resistance and Type 2 diabetes. It's not that exercise "burns off" the sugar before it can do damage, it's that exercising muscle has an entirely separate mechanism for glucose disposal that doesn't tax the insulin system at all.

The Actual Epidemiological Evidence

If high-carb endurance athletics caused diabetes, you'd expect to see it in the data. Instead, we see the opposite.

A 2014 study of nearly 400 former elite Finnish athletes who competed internationally between 1920 and 1965 found that compared to non-athlete controls, former athletes were 42% less likely to have impaired glucose tolerance and 31% less likely to have diabetes. The endurance athletes specifically? They had a 47% reduction in diabetes prevalence, the lowest risk of any athletic group studied.

Similar patterns show up in studies of ultramarathon runners, who report dramatically lower rates of virtually all chronic diseases compared to the general population. The overwhelming scientific consensus remains that endurance exercise substantially reduces diabetes risk.

As Javier Gonzalez, an exercise metabolism researcher at the University of Bath, put it: "I'm open to the idea [that endurance athletes could be at elevated risk], although I currently see very little, if any, good evidence to support it."

What About CGM Weirdness in Athletes?

Here's where it gets nuanced, because athletes do sometimes show funky blood glucose patterns on continuous glucose monitors. But context matters enormously.

Endurance athletes commonly experience transient hyperglycemia during and immediately after intense exercise. This is a normal stress response, cortisol and catecholamines mobilize glucose to fuel the work. It's adaptive, not pathological. Athletes may also have elevated HbA1c (the marker of long-term glucose exposure) because they have longer-lived red blood cells than sedentary people, which accumulate more glycation over their extended lifespan. Again: not the same as developing diabetes.

Some researchers have suggested that what we're seeing in athletes with elevated glucose markers isn't metabolic disease at all, it's the body's orchestrated attempt to support repeated high-intensity exertion. The patterns resolve when athletes take breaks from intense training. Their fasting insulin and HOMA-IR scores (actual markers of insulin resistance) typically remain excellent.

The Real Metabolic Threat: Not Eating Enough

You know what does reliably mess with athletes' metabolic health? Underfueling.

Relative Energy Deficiency in Sport (RED-S) affects an estimated 44.7% of athletes, according to a recent meta-analysis. When the body senses chronic energy deficit, it down-regulates metabolism, disrupts hormone production, and, here's the kicker, can actually cause elevated HbA1c and markers that look like prediabetes.

An underfueled athlete might show up at their doctor's office with elevated LDL cholesterol, rising blood glucose, and metabolic markers that scream "eat less and exercise more!" Which is, of course, exactly the wrong advice. The body isn't showing signs of overconsumption; it's showing signs of a system under energetic strain, desperately conserving resources.

Kylee sees this constantly: athletes confused by lab work that suggests metabolic dysfunction when the actual problem is chronic undernutrition. The irony of the "carbs give you diabetes" fear is that it might push athletes toward restriction that genuinely does harm their metabolic health. (Listen to our blood sugar episode for more!)

The Tradeoff That Doesn't Exist

Let's be real: taking in 120g of carbs per hour while running 100 miles is a pretty extreme thing to do. It's not "normal" in any general-population sense. Neither is running 100 miles.

But the question isn't whether race fueling is "natural" or "what our ancestors did." The question is: given that you're choosing to do the extremely abnormal thing of running for 20+ hours, what fueling strategy balances performance and health?

And on that question, the evidence is pretty clear. Underfueling during prolonged endurance exercise increases risk of bonking, impaired performance, compromised recovery, and potentially long-term metabolic issues from chronic energy deficiency. Adequate carbohydrate fueling supports performance, protects muscle glycogen, and takes advantage of your body's insulin-independent glucose uptake mechanisms that are already revved up from the exercise itself.

The dumpster fire isn't the gels. It's the conflation of sedentary metabolic disease with athletic performance nutrition, and the financial incentive for low-carb supplement companies to make you afraid of the fuel that actually works.

The Bottom Line

If you're an endurance athlete worried about metabolic health, here's what the evidence actually supports:

Your working muscles handle glucose through pathways that don't require insulin. Exercise dramatically reduces diabetes risk. The study that launched the "athletes get prediabetes" panic involved ten people and doesn't support the conclusions being drawn from it. Transient blood glucose elevation during and after exercise is normal and adaptive. Underfueling is far more common than overfuleing among athletes and poses genuine metabolic risks.

Eat the damn gels.

This post is based on research including: Richter & Hargreaves (2013) on exercise and GLUT4; Laine et al. (2014) on diabetes prevalence in former elite athletes; Thomas et al. (2016) CGM pilot study; Mountjoy et al. (2023) IOC consensus on RED-S; Gallant et al. (2025) meta-analysis on low energy availability in athletes.