Energy Drinks and Your Gut: What Each Ingredient Actually Does

TikTok generates its topic pages from what people actually search. It currently has ones for why a Celsius makes your stomach hurt, whether it can make you constipated, and why it sends you to the bathroom. Search the same questions on Google and the first page is a supplement brand, another supplement brand, a third supplement brand, and a content farm. Not one major health publisher has written the answer down.

Which is a shame, because the answer is more interesting than “it is the caffeine” — and the study everyone reaches for to prove it is the caffeine does not actually prove that.

The labelStart with what is in the can

Take the standard 12-ounce Celsius, since it is the one in the search queries. The panel lists 200 milligrams of caffeine, ten calories, no sugar, and this ingredient order: carbonated filtered water, citric acid, taurine, guarana seed extract, caffeine, green tea extract, sucralose, calcium carbonate, ascorbic acid, glucuronolactone, ginger root extract, then the vitamins. The can carries its own instruction not to exceed two servings a day, and says it is not recommended for people sensitive to caffeine, children under 18, or women pregnant or nursing. The 16-ounce Essentials line runs to 270 milligrams and drops that to one serving a day, and adds acesulfame potassium alongside the sucralose. Alani Nu’s 12-ounce can is also 200 milligrams, also sugar-free, and also uses both sweeteners.

Note what is doing the work and what is decoration. Caffeine appears twice over if you count the guarana, which is itself a caffeine source. Taurine is second in the list by weight. And the whole thing is carbonated, which no one ever counts as an ingredient and which is the most mechanically obvious way to put gas into a stomach.

The caffeine study everyone citesAnd what it actually found

Nearly every page on this subject cites the same 1998 experiment, and nearly every one of them describes it wrong. Researchers at the University of Iowa threaded manometry catheters into 12 healthy volunteers and measured colonic motor activity after black coffee containing 150 milligrams of caffeine, decaffeinated coffee, plain hot water, and a 1,000-calorie meal. The headline result is genuinely striking: caffeinated coffee stimulated the colon about as much as the meal did, 60 percent more than water and 23 percent more than decaf.

Here is the sentence that never travels with the statistic. In the same abstract, the researchers report that decaffeinated coffee’s effect on colonic motility was not significantly different from either water or caffeinated coffee. Both coffees and the meal beat water. Caffeinated coffee beat decaf by 23 percent, but decaf was not statistically distinguishable from the caffeinated version. That is a study about coffee, which contains hundreds of compounds. Lifting it onto a carbonated, sucralose-sweetened can because both contain caffeine is a bigger leap than it looks, and it is the leap the entire first page of results is built on.

What caffeine itself doesA stranger finding, in the stomach

For caffeine specifically rather than coffee, the cleanest human work is a 2017 study in 13 healthy adults who swallowed pH capsules so researchers could watch stomach acid in real time. Caffeine in a gelatin capsule — swallowed without being tasted — stimulated acid secretion, cutting the time the stomach took to re-acidify by 23 minutes against a water control. But when the same participants drank a caffeine solution, acid was delayed; when they tasted it and spat it out, delayed further still. The mechanism the researchers identified is a bitter taste receptor, TAS2R43, sitting not on the tongue but in the stomach lining.

So caffeine does act on the stomach, and the route it arrives by changes the direction of the effect. The older 1975 work in the New England Journal of Medicine pointed the same way with a caution attached: caffeine alone produced a maximal acid response of 8.4 mEq per hour, while regular coffee produced 20.9 and decaf 16.5. The authors’ own conclusion was that caffeine’s effects “may bear little relation to the actual observed actions of coffee.” Fifty years later that is still the most useful sentence in the literature on this topic, and the one the internet has most thoroughly ignored.

The sweetenerReal evidence, narrower than the headlines

Sucralose is in every can discussed here, and it has been through a proper trial. A randomised controlled study published in Cell in 2022 took 120 adults who had never consumed non-nutritive sweeteners, split them into six groups, and gave four of them sucralose, saccharin, aspartame or stevia for two weeks — sucralose at 34 percent of the acceptable daily intake. Every sweetener distinctly altered the stool and oral microbiome and the plasma metabolome. Saccharin and sucralose significantly impaired glycaemic responses. And when the researchers transplanted the participants’ microbiomes into germ-free mice, the mice reproduced their donors’ glucose responses — which is the part that turns a correlation into something closer to cause.

That is a real result and it deserves to be taken seriously. It is also not the result it usually gets reported as. The trial measured the microbiome and blood sugar. It did not report that anyone felt bloated, gassy or unwell, and no human trial has shown sucralose causing gastrointestinal symptoms at the amounts in a can or two. The trials that looked hardest for microbiome effects at ordinary doses largely did not find them: a crossover trial in 17 adults at 20 percent of the acceptable daily intake found no differences in bacterial taxa or short-chain fatty acids, and a trial in 34 men at a considerably higher dose for a week found the microbiome unmodified at phylum level. So: something measurable happens, the evidence disagrees about how much, and none of it is the explanation for a stomach ache.

The ones with nothing behind themTaurine, guarana, and the rest

Taurine is the second ingredient by weight in two of these cans, and there is no human evidence at all that the taurine in an energy drink affects digestion — in either direction. Searches of the human trial literature return nothing. What exists is mechanistic and animal work, some of it pointing in genuinely unhelpful directions for marketing purposes: taurine is used as a substrate by certain gut bacteria that produce hydrogen sulfide as a byproduct, which has been studied in mice, not in people drinking energy drinks. If you have seen a brand blog claiming taurine soothes the gut or helps with IBS, there is no study under it.

Guarana, ginseng, glucuronolactone and the B-vitamin stack are in the same position for digestion specifically: nothing in humans. Guarana is worth singling out only because it is a caffeine source, so it belongs in the caffeine column rather than in a column of its own.

Which leaves the ingredient nobody lists. These are carbonated drinks, frequently consumed fast and often on an empty stomach before a workout. Carbonation puts gas into the stomach directly, and that is not a subtle mechanism requiring a trial to detect. It is not a very satisfying answer, which may be why no one selling anything wants to lead with it.

The regulatory pictureWhat changed in 2026

Two developments this year, both worth reporting precisely. On 29 June 2026 the FDA published its 2026 Human Foods Program guidance agenda, and among the new topics is “Labeling Caffeine Content in Foods and Beverages; Draft Guidance for Industry.” That is a draft guidance of best practices for labelling added caffeine, and the agency notes on the same page that guidance documents do not impose legally enforceable requirements. It is a signal of attention rather than a rule.

Separately, on 4 June 2026 the Texas attorney general announced an investigation into Celsius Holdings, which owns Alani Nutrition, over whether the companies misled consumers about the safety of their drinks for teenagers and children under the state’s deceptive trade practices act. The release states that the family of a 17-year-old from Weslaco, Texas is suing the distributor of Alani Nu after the teenager died from an enlarged heart allegedly caused by excessive caffeine consumption, and that the lawsuit claims the product failed to provide adequate warnings about its caffeine content. That is an investigation and an allegation in a lawsuit, not a finding; we are reporting the state’s own words and nothing beyond them.

For context on the guidance that already exists: the FDA’s consumer page cites 400 milligrams a day for most adults as an amount “not generally associated with negative effects” — carefully not the same word as safe — while noting wide variation in sensitivity, and listing digestive problems among the effects of too much caffeine in children and teens. The American Academy of Pediatrics has been blunter since 2011: caffeine and other stimulants in energy drinks “have no place in the diet of children and adolescents.” Two 12-ounce cans reach that adult 400 milligram figure on their own, before any coffee.

The ledgerWhat is actually going on

How common this is, for scale: an analysis of national nutrition survey data covering 12,103 adults aged 20 to 39 found energy drink consumption rose from 0.5 percent in 2003 to 5.5 percent by 2015–2016, and that people who drank them averaged 278.7 milligrams of total daily caffeine against 135.3 for everyone else. That gap, not the taurine, is the difference that has evidence attached to it.

So the honest decoder reads roughly like this. Caffeine stimulates stomach acid and, as part of coffee at least, gets the colon moving about as much as a full meal — and 200 milligrams arriving cold, fast, fizzy and often on an empty stomach is a meaningful dose delivered in an unusual way. Carbonation adds gas mechanically. Sucralose measurably changes the microbiome and glucose handling in trials, with no evidence it produces symptoms at these amounts. Taurine and the botanicals have nothing behind them for digestion. The interesting part is that the two ingredients with the best evidence are the two nobody markets, and the ones on the front of the can are the ones with none.

If you are trying to work out your own pattern, the ingredient list is a better starting point than the brand, and the variable easiest to change is rarely the drink itself — it is whether it arrives on an empty stomach and how fast. Symptoms that keep coming back, or that arrive with bleeding, weight loss, or pain that will not settle, belong with a clinician rather than a comment section, and anyone with a heart condition or who is pregnant has a different conversation to have about caffeine entirely. Our piece on coffee covers the other half of this mechanism, and the frequency checker shows what the population data actually looks like. This article is educational, reports what the cited research found, and is not medical advice.