Closeup of young sporty man holding sweet donut and fresh apple in hand.
(Photo: Getty Images)

Sugar Substitutes and the Gut

Modest amounts of artificial sweeteners are unlikely to cause much gut distress, but sugar alcohols can wreak havoc.

Closeup of young sporty man holding sweet donut and fresh apple in hand.

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The first man-made sweetener was discovered—or more accurately, stumbled upon—over a century ago by a graduate student named Constantin Fahlberg at Johns Hopkins University. As the story goes, Fahlberg noticed a sweet taste on the bread he was eating at dinner one evening in 1879. Earlier that day, Falhberg had been investigating the chemistry of coal tar derivatives and spilled some on his hands. He subsequently figured out that saccharin was the chemical responsible for the delightfully sweet taste of his dinner rolls. Just think, saccharin may have never been happened upon if Fahlberg was of a more hygienic disposition.

Today, numerous artificial as well as natural low-calorie sweeteners are used by food producers throughout the world. According to one representative survey of Americans that was done from 2009 to 2012, 25 percent of kids and 41 percent of adults were consuming low-calorie sweeteners, and these intake rates represented a 200 percent surge in consumption for kids and a 54 percent increase among adults from just 10 years earlier. Although similar data in athletes is hard to come by, it’s almost a certitude that they are, on average, consuming more artificial and low-calorie sweeteners than in years past.

In large part, the surging use of these sugar replacers is a byproduct of the obesity epidemic. At the beginning of the 1960s, the prevalence of obesity among adults was 13 percent in the United States. Fast forward 50 years, and the prevalence had skyrocketed almost threefold, to just over 35 percent. Perhaps even more disturbing is that about one in five children and adolescents is now obese. While there are lots of complex theories about why this rise in overweightness has occurred in such a short period, on the most basic level it is due to an imbalance in energy intake and energy expenditure on a population-wide scale.

People have all sorts of questions and concerns about these supposedly guilt-free sweeteners. Alas, we don’t have time to debate all the pros and cons of said sweeteners. Instead, the aim of this article is to review how these sweeteners can potentially impact gut function and symptoms from an athlete’s perspective.

Sugar Substitutes Are Not One and the Same

It turns out there are many different energy-free and low-calorie sweeteners, so before we can talk about how they might affect your gut, it will be valuable to briefly review some of them individually.

Saccharin, the original artificial sweetener, is several hundred times sweeter than table sugar (sucrose), but it also has a somewhat unpleasant aftertaste. Sweet’N Low® is probably the most recognized brand of saccharin-based sweetening products. Aspartame is another popular synthetic sweetener that was unearthed in 1965. Interestingly, its discovery story is akin to saccharin’s; while working on anti-ulcer drug candidates and related chemicals, a chemist named Jim Schlatter accidently detected aspartame’s sweetness after licking his finger before picking up a piece of paper. Aspartame goes by the brand names Nutrasweet® and Equal® and is about 200 times sweeter than regular-old sugar.

Another artificial sweetener, sucralose (aka Splenda®), is even sweeter than saccharin and aspartame, coming in at 600 times the potency of sugar. Several other artificial sweeteners are approved by the Food and Drug Administration (e.g., acesulfame potassium, neotame, etc.), but we will focus on saccharin, aspartame, and sucralose because they are the most popular.

Outside of these manmade sweeteners, natural low-calorie sweeteners are also found in some foods and beverages that athletes consume. Stevia is one such natural sweetener and is often touted as being safer than artificial sweeteners. Stevia sweeteners usually come from the Stevia rebaudiana Bertoni plant that is native to South America and, like aspartame, they are about 200 to 300 times sweeter than sugar.

Sugar alcohols—otherwise known as polyols—are an additional type of sweetener used to replace real sugar in certain foods. Many polyols are considered natural because they are found in plant foods such as fruits and vegetables. In contrast to the other sweeteners mentioned so far, most polyols provide some energy, albeit less than sugar. Most contain about 1.5 to 3.0 kilocalories per gram, whereas sugar supplies 4 kilocalories per gram. When it comes to sweetness levels, polyols tend to be somewhat less potent than sugar.

Context and Dosage Are Key

It’s important to acknowledge there are numerous aspects of physiology that influence overall gut health. Energy-free/low-calorie sweeteners could impact anything from gut hormone secretion to intestinal motility to the microbiome (i.e., the population of microbes living in your gut). That means we should avoid making overly simplistic conclusions about whether these sugar substitutes are good or bad for your gut’s overall fitness.

With that caveat aside, let’s look at some of the relevant research on these sweeteners. In animals, feeding artificial sweeteners like sucralose and aspartame sometimes increases the secretion of gut-produced hormones (e.g., gastric inhibitory peptide, peptide YY), which impact hunger and intestinal motility. In addition, a study that fed mice sucralose, aspartame, or saccharin for 11 weeks found alterations in the gut microbiome, which caused intolerance to a standardized glucose feeding. This isn’t an isolated finding, as other studies have found compositional shifts in the gut microbiome with artificial sweetener consumption.

Still, mice aren’t humans, and many of these animal experiments used dosages (relative to body weight) that were way higher than the amounts most people consume on a daily basis. As a result, it is incredibly hard to say how well some of these findings apply to people who consume modest amounts of manmade sweeteners. For example, although some animal research suggests that artificial sweeteners could negatively affect hunger, body weight, and glucose metabolism, most experiments in humans have not found this to be true. In fact, a meta-analysis of clinical trials that was published this year found consuming non-caloric/low-calorie sweeteners in place of sugar led to modest reductions in body weight in humans. Likewise, another review of human experiments found that replacing sugar-sweetened beverages with artificially-sweetened versions prompted modest decreases in body weight, body fat, blood triglycerides, and liver fat.

On the other hand, eating loads of artificial sweetener could potentially lead to problems metabolizing carbohydrate in the body, but these effects probably only occur at very high doses. In one study, volunteers ate the equivalent 10 Sweet’N Low packets’ worth of saccharin per day for a week, yet only four of seven volunteers developed any carbohydrate metabolism issues at this extreme dose. The point is, dosage matters.

Overall, if sugar in your diet is replaced with a reasonable amount of artificial sweetener, it is unlikely that your metabolic health will be negatively impacted in a major way. And although artificial sweeteners can certainly alter the gut microbiome, we don’t yet know what the long-term implications of those changes are.

Are Sugar Substitutes a Source of Gut Symptoms in Athletes?

The surest thing we know about this topic is that eating loads of polyols is akin to dropping a bomb in your gut. About half to three-quarters of ingested polyols get absorbed, meaning that a portion of them hang around in the gut’s lumen. The fact that polyols are incompletely absorbed is precisely why they contain less energy than sugar. While a gram or two of unabsorbed polyols isn’t a big deal, larger amounts sitting in the gut can wreak havoc. In one experiment, about half of people who ate chocolate bars containing 32 grams of a polyol called sorbitol suffered from diarrhea, flatus, stomach aches, and bloating. Likewise, in a study of children who ate hard candies with 25 grams of the polyol isomalt, stomach aches and watery stool were more frequent in comparison to when they ate candies with regular sugar.

These are just a couple of the many studies that have shown polyols can be disagreeable to the gut if 20 grams or more are eaten in a single sitting. From personal experience, I can still vividly recall being transformed into a human flatus factory after I ate an entire bag of sugar-free gummy bears as a kid. It turns out the gummy bears had maltitol as a sugar replacer. (Apparently, I’m not the only one who has had enteric distress from sugar-free gummies; you can check out some horror stories here.) On a mechanistic level, unabsorbed polyols draw water out of your body and into your gut lumen, resulting in loose, watery stools. Bacteria in the latter half of your gut also ferment these unabsorbed polyols, provoking gas and bloating.

Clearly, eating a good dose of polyols isn’t a wise choice in proximity to exercise. Very few of the classic sports drinks marketed to endurance athletes contain polyols, so you probably need not worry about them when drinking those beverages. Other products like diet and meal replacement bars, however, are much more likely to be chock-full of polyols. One potential red flag to look for is a product that markets itself as being “sugar free” but that still has a semi-sweet flavor like chocolate, vanilla, or strawberry. Look at the ingredients list to see whether any polyols are listed. For some products, you can figure out the exact quantity by looking for sugar alcohols under the total carbohydrate section of the Nutrition Facts panel.

As for Stevia, there is little reason to think that the amounts found in most products would cause notable gut issues. In an experiment involving eating cookies (how fun!), adding Stevia didn’t result in any digestive complaints in comparison to standard Stevia-free cookies. Likewise, there is little evidence from animal or human experiments that consuming modest amounts of artificial sweeteners like aspartame or sucralose causes gut discomfort, gas, or stool abnormalities. There are always exceptions, though, and a small subset of athletes may want to avoid artificially sweetened foodstuffs before and during exercise if they know from their experience that they are triggered by them.


Eating excessive amounts of artificial sweeteners like aspartame, saccharin, and sucralose can have undesirable effects on the body, but the dosages required for this to occur are quite high. In other words, downing a six-pack of diet soda or a dozen Sweet’N Low packets every day is much more likely to lead to problems than a once-a-day diet soda habit. Scientists can’t provide a 100 percent guarantee that there will be no long-term adverse effects from eating modest amounts of artificial sweeteners, but the probability is pretty low for most people that they will cause lasting harm.

When it comes to natural sweeteners, polyols are the type you should be most mindful of as it relates to preventing gut issues. Because they are incompletely absorbed in the gut, these sugar alcohols can cause stomach pain, loose stools, gas, and bloating, especially if 20 or more grams are eaten at one time.

From PodiumRunner Lead Photo: Getty Images

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