Energy gels are not inherently harmful, but they carry real risks that depend heavily on how you use them, which product you choose, and how your individual gut responds. The most common problem is gastrointestinal distress, ranging from mild bloating to severe cramping and diarrhea, and about 10 to 20 percent of runners experience serious symptoms even under controlled study conditions. Beyond the stomach issues, there are legitimate concerns about gut barrier damage, wildly inconsistent product formulations, and questionable labeling accuracy that make the “are they safe?” question more nuanced than most sports nutrition brands would like you to think.
What Energy Gels Actually Do Inside Your Body
Energy gels are concentrated packets of simple carbohydrates, typically maltodextrin, glucose, or fructose, designed to be absorbed quickly and burned as fuel during prolonged exercise. The idea is straightforward: during activities lasting longer than about 60 to 90 minutes, your body’s stored glycogen starts running low, and topping up with external carbohydrate keeps your muscles fueled and your blood sugar stable. Research confirms this works. Cyclists who consumed energy gels during prolonged rides had higher blood glucose levels at the 60-minute mark compared to those consuming only water, and they covered meaningfully more distance in subsequent time trials.1Ovid / The Journal of Strength & Conditioning Research. Effects of Energy Gel Ingestion on Blood Glucose, Lactate, and Performance Measures During Prolonged Cycling
The format itself, gel versus drink versus chew, does not appear to matter much for absorption. A study comparing fluid, gel, jelly chews, and a mix of all three found nearly identical rates of carbohydrate oxidation across all formats when athletes consumed the same total amount.2PubMed. (13)C-glucose-fructose labeling reveals comparable exogenous CHO oxidation during exercise when consuming 120 g/h in fluid, gel, jelly chew, or coingestion So the gel itself is not a superior delivery system. It is simply convenient: small, portable, and easy to consume mid-stride without stopping. The risks come not from the format but from what happens when concentrated sugar hits a gut that is already under stress from exercise.
The Gastrointestinal Problem
This is the big one. If you have ever seen a marathon runner doubled over at mile 20, there is a decent chance a gel played a role. During hard exercise, blood flow shifts away from your digestive system and toward your working muscles, which leaves the gut vulnerable. Dumping a bolus of concentrated sugar into a blood-deprived stomach is a recipe for trouble.
The severity varies widely between individuals. In a controlled study of runners consuming high amounts of carbohydrate gel during a 16-kilometer run, the average gut distress scores were low, suggesting most people tolerated the gels reasonably well. But roughly 10 to 20 percent of runners experienced serious gastrointestinal problems, including nausea, cramping, and diarrhea.3International Journal of Sport Nutrition and Exercise Metabolism. The Effect of Carbohydrate Gels on Gastrointestinal Tolerance during a 16-km Run That is a meaningful minority. If you line up at a race with a thousand runners, a hundred or two hundred of them might have a genuinely bad time with their gel strategy.
What makes this worse is the enormous variation in gel formulations on the market. An analysis of commercially available gels found extreme differences in osmolality, which is a measure of how concentrated the solution is. The average osmolality across products was over 4,400 mmol/kg, but the range spanned from 303 to more than 10,000.4International Journal of Sport Nutrition and Exercise Metabolism. Extreme Variation of Nutritional Composition and Osmolality of Commercially Available Carbohydrate Energy Gels A highly concentrated gel sitting in your stomach draws water into the gut to dilute it, which can cause bloating, cramping, and diarrhea. If you are not drinking enough water alongside a high-osmolality gel, you are practically guaranteeing discomfort.
Gut Barrier Damage During Exercise
Beyond subjective discomfort, there is evidence that gel consumption during exercise can cause measurable damage to the intestinal lining. A study of runners exercising in the heat found that those consuming carbohydrate gel showed increased levels of endotoxin in their blood after exercise, along with elevated markers of intestinal cell damage and inflammation. The runners who consumed no carbohydrate did not show these increases.5PubMed. Carbohydrate gel ingestion during running in the heat on markers of gastrointestinal distress
Endotoxin leaking from the gut into the bloodstream is a sign that the intestinal barrier has become more permeable, sometimes called “leaky gut” in popular language. In the context of a single workout, this is usually temporary and resolves within hours. But the finding raises questions for athletes who train daily and consume gels repeatedly over months or years. The chronic effects of repeated low-grade gut barrier disruption during exercise are not well studied, and that gap in the research is worth noting honestly. Most sports nutrition guidance focuses on acute performance, not what years of concentrated sugar boluses do to the gut lining over time.
Why Hydration Changes Everything
One of the most underappreciated risks of energy gels is that they interact with your hydration status in ways that can amplify problems. A gel on its own is a thick, sticky concentrate. Your stomach needs fluid to dilute it before it can empty efficiently into the small intestine. Without enough water, the gel sits in your stomach longer, pulls fluid from surrounding tissue to dilute itself, and slows the very absorption it was designed to speed up.
The osmolality data makes this concrete. When a gel has an osmolality several times higher than your blood, consuming it without adequate water creates a hypertonic environment in your stomach that delays gastric emptying and can trigger nausea.4International Journal of Sport Nutrition and Exercise Metabolism. Extreme Variation of Nutritional Composition and Osmolality of Commercially Available Carbohydrate Energy Gels By contrast, some newer gel formulations use polymers that are markedly hypotonic, and these appear to empty from the stomach much faster. One such formulation had a half-emptying time of about 17 minutes, versus roughly 33 minutes for a standard glucose solution, and delivered more than twice the carbohydrate to the small intestine in the first 10 minutes after ingestion.6PubMed. Improved gastric emptying rate in humans of a unique glucose polymer with gel-forming properties
The practical takeaway is blunt: if you are going to use gels, you need to drink water with them, and the amount matters. Most product instructions say something like “consume with water,” but they rarely specify how much, and many athletes skip the water entirely during races when aid stations are inconvenient. That mismatch between how gels should be used and how they actually get used in the real world is a significant source of avoidable problems.
You Can Train Your Gut to Handle Them Better
One of the more encouraging findings in recent sports nutrition research is that the gut is adaptable. If you practice consuming carbohydrate during training, your tolerance tends to improve substantially. A two-week gut-training protocol found that gastrointestinal symptoms dropped by roughly 60 percent when athletes repeatedly consumed carbohydrate during exercise, along with reduced signs of carbohydrate malabsorption and improved blood glucose availability.7PubMed. Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance
A systematic review pooling results from multiple studies confirmed the pattern: repetitive carbohydrate feeding during exercise over two weeks reduced gut discomfort by an average of about 47 percent and lowered carbohydrate malabsorption by 45 to 54 percent, though the review also noted that some studies found unclear or nonsignificant improvements.8PubMed Central. The Effect of Gut-Training and Feeding-Challenge on Markers of Gastrointestinal Status in Response to Endurance Exercise: A Systematic Literature Review The mechanism likely involves upregulation of intestinal transporters, essentially your gut gets better at absorbing sugar quickly because you have been asking it to do so regularly.
This means one of the biggest mistakes athletes make is trying gels for the first time on race day. If you have never consumed concentrated carbohydrate during exercise, your gut is unprepared, and you are rolling the dice on a bad outcome at the worst possible moment. The evidence strongly supports practicing your fueling strategy in training, starting with small amounts and building up gradually.
Individual Tolerance and Fructose Sensitivity
Not everyone’s gut responds the same way, and fructose is a major reason why. Many energy gels use a glucose-to-fructose blend because the two sugars use different intestinal transporters, which theoretically allows more total carbohydrate to be absorbed per hour. Research confirms that high intake rates, around 120 grams per hour using combined glucose and fructose, can push exogenous carbohydrate oxidation above 1.5 grams per minute.9PubMed Central. Increased exogenous but unaltered endogenous carbohydrate oxidation with combined fructose-maltodextrin ingested at 120 g h −1 versus 90 g h −1 at different ratios
But fructose absorption capacity varies enormously between individuals. When fructose is not fully absorbed, it stays in the intestine, draws in water, and gets fermented by gut bacteria, producing gas, bloating, and cramping. People with functional gastrointestinal disorders are particularly susceptible. Research on low-fructose gel formulations has highlighted that breath hydrogen responses, a marker of malabsorption, vary widely from person to person, underscoring the need for individualized fueling strategies.10Heliyon. Development and feeding tolerance assessment of a low-fructose carbohydrate gel for exercise If you know that fruit juice or honey tends to upset your stomach, high-fructose gels may be a particularly poor choice for you, and a glucose-only or low-fructose formulation could be worth trying.
What Is Actually in the Packet
Even if you find a gel that works well for your gut, you might not be getting what the label says. A laboratory analysis of commercially available carbohydrate gels found that products contained significantly less energy than their labels claimed. Most products ran a bit under, but one brand was an extreme outlier: it contained roughly 71 percent less energy and 72 percent less carbohydrate than stated on the label, delivering only about 53 calories instead of the advertised 180.11International Journal of Sport Nutrition and Exercise Metabolism. What’s (Not) in Your Supplement? An Energy and Macronutrient Analysis of Commercially Available Carbohydrate Gels
This matters because athletes who plan their fueling strategy based on label information may be consuming far less carbohydrate than they intend. If you time your gels to deliver a specific amount of fuel per hour and the product delivers a third of what it claims, you will bonk even if your strategy is otherwise sound. Energy gels occupy a regulatory gray area in most countries: they are classified as food supplements rather than pharmaceutical products, which means manufacturing standards and label verification are less rigorous than you might assume. The takeaway is not to avoid gels entirely, but to be skeptical of claims on the packaging and ideally stick with brands that have been independently tested.
Additives and Your Gut Microbiome
Beyond the sugar itself, many gels contain emulsifiers, thickeners, and preservatives that have raised separate health concerns. Carboxymethylcellulose, polysorbate 80, various carrageenans, and gums are common in processed food products, and some appear in gel formulations as well. Research using a model of the human gut microbiome found that several of these additives had a lasting detrimental impact on the composition and function of gut bacteria, including increased expression of pro-inflammatory molecules. Carrageenans and gums showed particularly strong effects, while lecithin, for comparison, did not significantly alter the microbiome in the same model.12PubMed Central. Direct impact of commonly used dietary emulsifiers on human gut microbiota
It is worth keeping this in perspective. The doses of additives in a single gel packet are small, and the research on emulsifiers has mostly used concentrations relevant to a broader processed-food diet rather than isolated sports nutrition products. Still, endurance athletes who consume gels regularly over years are getting repeated low-dose exposure to these compounds during a time when their gut barrier is already compromised by exercise-induced blood flow redistribution. Whether that combination matters for long-term gut health is an open question that the sports nutrition field has not adequately addressed.
Real Food Works Just as Well
If gels bother your stomach or you would rather avoid processed concentrates altogether, the performance data is reassuring: real food can do the same job. A study of trained cyclists found that potato purée consumed during a two-hour ride followed by a time trial produced identical performance outcomes to commercial gels. Both the potato and gel conditions significantly outperformed water alone, and there was no difference between the two carbohydrate sources in blood glucose maintenance or time trial completion.13PubMed Central. Potato ingestion is as effective as carbohydrate gels to support prolonged cycling performance
That said, format does matter for comfort at high intensities. A comparison of drinks, gels, bars, and a mix found that solid bars produced notably more nausea, stomach fullness, and abdominal cramps than gels, along with a measurable drop in peak power output. Gels and drinks were essentially equivalent for both comfort and performance.14International Journal of Sport Nutrition and Exercise Metabolism. Solid, Gel, and Liquid Carbohydrate Format Effects on Gut Comfort and Performance So the issue with solid food during hard efforts is not that it lacks the right nutrients but that chewing and digesting a solid takes more work from an already-stressed gut. For lower-intensity or longer-duration events where pace allows for more digestion time, real food is a perfectly viable alternative.
Hydrogel Technology and Newer Formulations
The sports nutrition industry has been trying to solve the GI problem rather than just telling athletes to tolerate it. One approach gaining traction is hydrogel technology, where sodium alginate or similar compounds encapsulate the carbohydrate in a gel matrix that passes through the stomach more quickly and releases sugar gradually in the small intestine. The idea is to decouple the concentrated sugar from the gastric distress it causes by essentially sneaking it past the stomach.15PubMed Central. Impact of sodium alginate energy gel on the marathon performance of amateur runners: a randomized controlled study
The concept has physiological plausibility, and early gel formulations using similar polymer technology showed faster gastric emptying than standard glucose solutions.6PubMed. Improved gastric emptying rate in humans of a unique glucose polymer with gel-forming properties Whether the commercially available hydrogel products live up to the marketing claims is harder to assess. The research is still relatively thin, and some of the most prominent hydrogel brands have faced skepticism from independent researchers. As with most sports nutrition innovations, the technology is probably ahead of the evidence, and early-adopter athletes are functioning as informal test subjects.
The Placebo Effect in Sports Nutrition
One underappreciated dimension of the energy gel question is how much of their perceived benefit is psychological. A systematic review of placebo and nocebo effects in sports and exercise found that nutritional interventions produced large placebo effects, with an average effect size of 0.86, which is substantial.16PubMed Central. Placebo and Nocebo Effects on Sports and Exercise Performance: A Systematic Literature Review Update This does not mean gels do not work physiologically; the blood glucose and oxidation data clearly show they do. But it does mean that part of the performance boost athletes feel from gels may come from believing they are doing something helpful, and part of the gut distress some athletes experience may be amplified by anxiety about consuming gels during a race. The nocebo effect, where expecting something to cause harm actually makes you feel worse, is well documented in exercise contexts.
This has practical implications. If you have had one bad experience with a gel and now dread using them, your body may produce more gut symptoms than the gel itself would cause on a neutral day. Conversely, if you are convinced a particular brand is superior, you might perform marginally better on it even if the formulation is identical to a cheaper product. None of this should discourage you from using gels if they work for you, but it is worth being aware that your expectations shape your experience more than most athletes realize.
How Much Is Too Much
Current guidelines for endurance athletes suggest carbohydrate intake during exercise of up to 90 grams per hour for events lasting over two and a half hours, with some recent research pushing that ceiling to 120 grams per hour using glucose-fructose blends. Studies at that higher rate have shown increased exogenous carbohydrate oxidation without a corresponding increase in endogenous carbohydrate use, suggesting the extra fuel is genuinely being burned rather than just sitting in the gut.9PubMed Central. Increased exogenous but unaltered endogenous carbohydrate oxidation with combined fructose-maltodextrin ingested at 120 g h −1 versus 90 g h −1 at different ratios But more is not automatically better for everyone. The athletes in these studies are typically well-trained, habituated to high carbohydrate intake during exercise, and consuming adequate fluid alongside. For a recreational runner doing a half marathon, slamming three gels per hour would be overkill and a near-guaranteed GI disaster.
The dose-response relationship between gel intake and gut distress is not linear, and there is probably a sweet spot for each individual that depends on exercise intensity, heat exposure, hydration, and training history. Starting lower and building up during training blocks is far safer than copying the fueling strategy of a professional triathlete from an Instagram post.