What Sports Nutrition May Get Wrong About Glycemic Index

🎧 Listen: What Sports Nutrition May Get Wrong About Glycemic Index

Rafal Nazarewicz Ph.D.

The spike-and-crash story is only a fraction of it. At the intersection of blood sugar, gut blood flow, and intestinal physiology, the real picture is far more complicated - and far more important.

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Glycemic index has a marketing problem. In sports nutrition, it usually gets reduced to a single soundbite: high-GI bad, low-GI good. Or the opposite, depending on who's selling what. The reality is more interesting, more nuanced, and more useful - if you're willing to sit with the complexity for a few minutes.

Because the glycemic index of what you eat interacts with your body differently depending on when you eat it, how hard you're working, how long you've been moving, and what's happening inside your gut at that exact moment. Understanding those interactions doesn't just make you a more informed consumer. It can be the difference between a strong finish and a miserable last hour.

Let's go through it properly.

 

 

Part One

What Glycemic Index Actually Measures - and What It Doesn't

Glycemic index ranks foods on a scale of 0 to 100 based on how quickly they raise blood glucose compared to pure glucose (which scores 100). A food scoring 55 or below is considered low-GI. Above 70 is high. The measurement is taken at rest, after a standardized serving, in healthy subjects - which immediately tells you something important: GI scores describe a controlled laboratory situation that has almost nothing to do with eating a gel at kilometer 35 of a marathon.

Still, GI is a useful starting point because it captures how fast carbohydrate structure is broken down and absorbed. The starch in whole basmati rice contains a mix of amylose and amylopectin. Amylose, the more linear chain, digests slowly, producing a gradual glucose curve. Heavily processed carbohydrates like maltodextrin are engineered to do the opposite: their highly branched, short-chain structure is absorbed almost instantly, with a glycemic index ranging from 95 to 136 depending on chain length. Rice syrup falls in between, at roughly 98, higher than table sugar.

The mechanism behind a high-GI response is well understood. Rapid glucose absorption triggers a sharp insulin response. Insulin efficiently drives glucose into cells, but the body tends to overcorrect. Blood sugar drops below the baseline it started from, creating reactive hypoglycemia. You've felt this: the energy crash roughly 30–60 minutes after eating something sweet at rest, the flat, foggy feeling that hits before you've even warmed up.

That crash is a real physiological event. But it's only one part of a much larger picture.

Part Two

How Exercise Changes the Equation and Where It Doesn't

Here's where it gets interesting. During exercise, the spike-crash cycle is blunted, but only above a certain intensity threshold. The reason is a protein called GLUT4, a glucose transporter that sits inside muscle cells. At rest, GLUT4 moves to the cell surface only in insulin response. But during exercise, muscle contractions trigger GLUT4 translocation independently of insulin. The muscles begin taking up glucose on their own, without waiting for an insulin signal.

This mechanism becomes meaningfully active at roughly 60–65% of VO2max, corresponding to about 65–75% of maximum heart rate, a pace where short sentences are possible but comfortable conversation isn't, an RPE of roughly 5–6 out of 10. Below that threshold, you're still largely insulin-dependent, and the high-GI spike-crash risk remains real. Above it, insulin becomes less of the story.

This gives us the first practical framework for thinking about fuel timing:

But this table, as shown above, tells only half the story. The blood sugar column improves as intensity rises. What it doesn't capture is that a completely separate set of GI problems gets dramatically worse at high intensity for entirely different reasons.

Part Three

The Gut Under Pressure: What Intensity Does to Digestion

As exercise intensity rises, the body makes a triage decision about blood flow. Working muscles demand oxygen. The cardiovascular system redirects blood toward them and away from the digestive organs. At efforts above 70–75% of VO2max, splanchnic blood flow (the circulation supplying the gut) can drop by 60–80%. The gut becomes functionally ischemic: oxygen-starved, compromised, and increasingly unable to do its job.

This produces a cascade of effects that have nothing to do with blood sugar:

Intestinal permeability increases.

The tight junctions between intestinal epithelial cells, the physical barrier between the gut lumen and the bloodstream, loosen under ischemic stress. This is measurable after hard efforts via biomarkers like intestinal fatty acid binding protein (I-FABP), which leaks into the blood when intestinal cells are damaged. In exercise science, this is sometimes called "leaky gut," and it creates a window for bacterial products and undigested food particles to enter circulation, triggering inflammatory responses that compound fatigue and discomfort.

Gastric emptying slows.

The stomach holds onto its contents longer during intense exercise. Anything consumed before or during a hard effort sits there, fermenting, rather than moving efficiently into the small intestine. This is why the gel you took at kilometer 30 is still in your stomach at kilometer 38, when the nausea arrives.

Gut motility becomes erratic.

The coordinated muscular contractions that move food through the gut peristalsis are disrupted when blood flow is compromised, and the enteric nervous system is competing with stress hormones. The result is cramping, bloating, and the urgent, unscheduled bathroom situations that define many race experiences.

Absorption capacity drops.

Even carbohydrates that successfully reach the small intestine face reduced uptake because the mucosal transport systems that absorb glucose and electrolytes depend on adequate blood flow to carry them away from the intestinal wall. A compromised circulation means nutrients accumulate in the lumen rather than being absorbed, adding to the osmotic load.

The picture this creates is different from the one most athletes have been given. The gut doesn't become more tolerant as intensity rises because blood sugar instability drops. The gut becomes less tolerant overall,  just for different, more structurally fundamental reasons.

The net risk never fully disappears. It only shifts the mechanism from metabolic (blood sugar) at low intensity to vascular and structural (ischemia, permeability, motility) at high intensity.

Part Four

How Carbohydrate Type Amplifies or Reduces These Problems

Once you understand the ischemic gut problem, the question of carbohydrate type becomes less about glycemic index and more about osmolality the concentration of dissolved particles in a solution, which determines how much water the gut needs to move to dilute them.

Highly processed carbohydrates like maltodextrin and concentrated rice syrup create high-osmolality solutions in the gut. The intestinal lumen pulls water from surrounding tissue to dilute this concentration, adding a significant fluid burden to an already-stressed system. The result is additional luminal distension, increased permeability pressure, and the kind of acute, mid-race GI distress that ruins finishing times and leaves athletes hunched over at the side of the road.

Whole food carbohydrate sources, such as rice, dates, oats, and fruit, have a more complex matrix. The carbohydrates exist within a food structure that includes fiber, water, protein, and fat in their natural proportions. This complexity moderates osmolality, slows the rate of digestion and absorption, and presents a less concentrated challenge to a gut that's already working under duress.

There's also a microbiome dimension that rarely gets discussed in sports nutrition. Research has found that maltodextrin can promote the growth of pathogenic gut bacteria, suppress beneficial species, and encourage the adhesion of harmful organisms to intestinal epithelial cells. Over a training season, where athletes may be consuming processed fuel products daily, that's a cumulative effect on the gut ecosystem that has implications well beyond race day performance. The gut microbiome is central to immune function, mood regulation, and nutrient absorption, a system worth protecting, not degrading one gel at a time.

Part Five

The Long-Effort Problem: When Everything Compounds

For efforts running four hours and beyond, ultramarathons, Ironman events, long mountain days a third layer of complexity arrives. Fatigue progressively raises perceived exertion relative to actual output. A pace that felt like 65% of VO2max at hour two may feel like 80% at hour five. Gut blood flow drops accordingly, even without any increase in speed.

At the same time, cumulative high-GI fueling across many hours creates metabolic fatigue that compounds with physical fatigue. Each blood sugar spike accelerates insulin release; each crash signals the body to down-regulate fat oxidation and lean harder on glucose. Well-trained endurance athletes invest enormous effort in training their bodies to oxidize fat efficiently to become metabolically flexible, able to sustain effort for hours without constantly feeding the glucose-insulin cycle. A fueling strategy built on high-GI carbohydrates partially undermines that adaptation in real time, forcing greater glycogen dependence precisely when glycogen stores are most depleted.

The brain suffers too. Blood glucose variability impairs decision-making, reduces perceived effort tolerance, and degrades mood, the phenomenon athletes describe as the "dark patch" that arrives in the back half of a long event. It's not always bonking in the classical sense. Often it's the accumulated effect of minor blood sugar fluctuations over many hours, each individually small, collectively destabilizing.

The practical prescription for long efforts: use lower-GI whole food carbohydrates as your fueling foundation, taken consistently and in moderate amounts, maintaining a stable glucose baseline. Reserve higher-GI options for specific tactical moments, a climb that demands a brief spike, the final push through the last hour, rather than as the primary substrate across all hours.

Putting It Together

A Working Framework for Smarter Fueling

The GI of your fuel matters most at the margins before exercise, during long slow efforts, and cumulatively across multi-hour events. Here's how to apply what the physiology actually tells us:

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The sports nutrition industry has spent decades simplifying the story of carbohydrates into one that's easy to market. Fast energy. Rapid absorption. Optimized delivery. What it has been slower to communicate is that your gut is not a passive fuel tank. It's a complex physiological system that operates under real constraints during exercise, constraints that are amplified by the very ingredients designed to fuel you.

Whole food carbohydrates don't just avoid the crash. They work with the architecture of the human gut rather than against it. They moderate osmolality, support the microbiome, reduce the ischemic burden on a system already under stress, and provide the metabolic stability that allows trained fat-burning capacity to do its job. That's not a marketing story. It's physiology.

 

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