Direct answer
What gives an energy drink its energy and alertness?
In most conventional energy drinks, sugar is the main energy-yielding ingredient, while caffeine is the principal ingredient responsible for increased alertness. Sugar-free energy drinks may still promote alertness when they contain an effective amount of caffeine, but they provide little or no energy from sugar. Caffeine-free versions may contain calories, but they do not provide the alertness associated with caffeine.
In a conventional sugar-containing and caffeinated energy drink, the two principal functional components have different roles: sugar provides nutritional energy, measured in kilocalories or kilojoules, while caffeine stimulates the central nervous system and can increase alertness and reduce sleepiness.
Caffeine does not provide calories and is therefore not a nutritional energy source. Sugar provides energy but does not reproduce the characteristic stimulant effect of caffeine.
Sugar provides nutritional energy
Sugars are carbohydrates. The body breaks down digestible carbohydrates into glucose, which cells can use to produce energy. In a conventional sugar-containing energy drink, sugar normally provides most of the drink’s calories. One gram of available carbohydrate provides approximately four kilocalories of nutritional energy.
Calories supplied mainly by sugar.
A stimulant effect produced primarily by caffeine.
A sugar-containing drink can provide metabolic energy even if it contains no caffeine. However, sugar alone should not automatically be expected to prevent sleepiness.
“A high sugar content, low caffeine drink does not alleviate sleepiness.”
A controlled study reported that the drink did not counteract sleepiness and was associated with slower reaction times after the initial period. [source ↗]
Caffeine provides alertness
Caffeine is a central nervous system stimulant. It reduces the effects of adenosine, a signalling substance involved in the development of sleep pressure and tiredness.
“In moderate doses [caffeine] increases alertness and reduces sleepiness.”
EFSA’s scientific assessment identified 75 mg of caffeine per serving as the minimum dose supporting its favourable opinion on increased alertness. Its assessment noted that the majority of studies using 75 mg or more showed a significant reduction in measures of reaction time. [source ↗] A favourable scientific opinion is not authorisation to use a health claim on an EU food label. The Commission rejected a separate application for intakes between 40 and 75 mg per serving in 2016; the European Parliament also objected to a distinct draft authorisation of caffeine alertness and concentration claims. Do not treat either scientific assessment as an authorised EU labelling claim. [source ↗] [source ↗] EFSA lists 80 mg as an approximate example for a standard 250 ml energy drink; actual products vary. EFSA serving-size example ↗
Natural and synthetic caffeine
Caffeine may come from botanical sources—including coffee beans, tea leaves, guarana seeds, yerba mate and kola nuts—or be produced synthetically.
Regardless of origin, pure caffeine is the same chemical compound: 1,3,7-trimethylxanthine, with the molecular formula C₈H₁₀N₄O₂. When chemical identity, purity and dose are equivalent, caffeine acts through the same physiological mechanisms.
“Natural sources of caffeine may have influences on the body similar to those of synthetic caffeine.”
A randomized, double-blind crossover study comparing 200 mg of caffeine from natural botanical extracts with synthetic caffeine found broadly similar pharmacokinetic and nervous-system effects. Botanical extracts may also contain other compounds that influence the overall product response; those effects should not be attributed to the caffeine molecule without evidence. [source ↗]
Sugar-free and caffeine-free variants
Sugar-free energy drinks
Sugar-free energy drinks replace sugar with low- or no-calorie sweeteners. They usually provide very little nutritional energy, but may contain the same amount of caffeine as a sugar-containing version. A sugar-free, caffeinated energy drink can therefore still increase alertness. It cannot automatically be described as less stimulating simply because it contains no sugar.
In a randomized crossover study involving 24 healthy young adults, a sugar-containing product produced greater improvements in two combined memory measures than the tested sugar-free version and placebo. This finding applies to those products, participants and tests; it does not prove that every sugar-containing energy drink is superior for every type of performance. [source ↗]
In another randomized crossover trial, neither a non-caloric nor a sugar-containing energy drink improved maximal oxygen consumption, time to exhaustion or peak power, although the non-caloric drink improved one resistance-type fatigue measure. [source ↗]
Caffeine-free energy drinks
A caffeine-free beverage may provide calories from sugar, vitamins, minerals, hydration and other ingredients. It does not, however, provide the central nervous system stimulation associated with caffeine. A product containing neither sugar nor caffeine provides neither meaningful energy from sugar nor caffeine-mediated alertness.
Caffeine, glucose and cognitive performance
Research examining individual ingredients supports the need to distinguish their effects. In a randomized, double-blind study, Giles and colleagues examined caffeine, glucose, taurine and their combinations.
“Caffeine enhanced executive control and working memory, and reduced simple and choice reaction time.”
Glucose produced different effects, while the effects of taurine were less consistent. Ingredients should therefore be assessed separately as well as in combination. [source ↗]
Physical performance
Caffeine can support certain aspects of physical performance, particularly endurance, but results depend on dose, timing, training status, test method and individual response.
A University of Texas at Austin randomized, double-blind crossover study involved 12 trained cyclists. Participants consumed placebo or 500 ml of an energy drink containing 160 mg caffeine and 54 g carbohydrate before a simulated time trial. Performance time improved after the energy drink, while perceived exertion did not differ significantly.
“Consuming a commercially available ED before exercise can improve endurance performance.”
The result applies to the tested product, dose, participants and cycling protocol—not every energy drink or every form of exercise. [source ↗]
Can energy drinks improve performance? What the studies found
Yes, some controlled studies found improvements in specific tests of running, simulated driving, cycling, reaction time, metabolism, memory and attention after consumption of a particular energy drink. The results concern the tested formulation and participants; they do not establish that every energy drink produces the same benefit.
Study-by-study comparison
Each row links to the original source and states the tested population, dose, comparator, result and main limitation. Positive and null results are shown together.
| Primary source | Population | Exposure | Comparison / design | Observed result | What it cannot prove |
|---|---|---|---|---|---|
| Prins et al., 2016 · PMID 26937774 ↗ | 18 recreational runners | 500 ml caffeinated, sugar-containing drink | Caffeine-free, sugar-free placebo; crossover | 5 km treadmill: 1413 vs 1444 seconds (p=.016) | Multiple ingredients differ; cannot isolate caffeine or sugar. |
| Mets et al., 2011 · PMID 21063868 ↗ | 24 healthy volunteers | 250 ml drink after 2 hours of simulated driving | Similar beverage without functional ingredients; crossover | Less lane weaving in simulated hours 3 and 4 | Driving simulator; no road-crash outcome; sleep not replaced. |
| Ivy et al., 2009 · PMID 19403954 ↗ | 12 trained cyclists | 500 ml, 160 mg caffeine, 54 g carbohydrate | Placebo; crossover | Faster simulated cycling time trial | Small athlete sample; caffeine/carbohydrate not separated. |
| Chtourou et al., 2019 · DOI 10.3390/nu11050992 ↗ | 19 physically active young men | Caffeinated drink before testing | Placebo; double-blind crossover | Better selected reaction-time and maximal-performance measures | Only men in a small, short-term laboratory study. |
| Scholey et al., 2004 · PMID 15549275 ↗ | Healthy adults; short-term testing | Whole caffeine/glucose drink and ingredient fractions | Placebo and individual fractions | Better secondary-memory and speed-of-attention factors | Task-specific effects, not lasting cognitive improvement. |
| Banks et al., 2024 · PMID 38197606 ↗ | 30 physically active men | Sugar-containing or non-caloric drink | Matched non-caloric placebo; crossover | No improvement in VO₂peak, time to exhaustion or peak power | A resistance-fatigue measure improved for one product; both raised resting systolic BP. |
| Krieger et al., 2025 · PMID 41374082 ↗ | 60 adults (33 men, 27 women) | Drink with 200 mg caffeine, acutely and for 28 days | Placebo; randomized double-blind | Higher energy expenditure at some times; fat oxidation on day 1 | Laboratory metabolism is not proof of sustained weight loss. Funded by the tested drink's manufacturer. |
Running performance — University of Pittsburgh co-authors
In a double-blind crossover trial, 18 recreational runners completed a five-kilometre treadmill time trial after either 500 ml of a caffeinated, sugar-containing energy drink or a non-caffeinated, sugar-free placebo. They finished the run faster after the energy drink. This supports the measured conclusion that an energy drink may improve five-kilometre running performance in some recreational runners. Because the beverages differed in several ingredients, the trial cannot establish whether caffeine, carbohydrate or their combination was responsible. [source ↗]
Driving performance — Utrecht University
Twenty-four healthy volunteers took part in a placebo-controlled crossover study using a highway-driving simulator. After two hours of driving, participants drank the tested energy drink or placebo before continuing. During the later driving periods, the energy drink group showed less lane weaving and reported less sleepiness. Thus the tested drink improved some measures of prolonged simulated driving performance; the study did not test actual road crashes. Caffeine does not make driving while dangerously tired safe, and sleep remains essential. [source ↗]
Cycling performance — University of Texas at Austin
In a double-blind crossover trial with 12 trained cyclists, participants completed a standardized simulated cycling time trial faster after consuming a drink containing 160 mg caffeine and 54 g carbohydrate than after placebo. Their perceived exertion did not differ significantly between conditions. The finding indicates that the tested energy drink may improve endurance-cycling performance under these conditions; it cannot be generalized to caffeine-free drinks or every cyclist. [source ↗]
Reaction time and short-term physical performance — University of Sfax
Researchers tested a caffeinated energy drink against placebo in a randomized, double-blind crossover experiment involving physically active young men. After the tested drink, they observed faster simple visual reaction times and improvements in the measured short-term physical-performance tasks, including handgrip and a 30-second cycling test. This suggests that an energy drink may improve reaction speed and some short-term exercise measures in similar test settings, but not necessarily in every sport or population. [source ↗]
Energy expenditure and fat oxidation — Lindenwood University
A randomized trial funded by the manufacturer of the tested drink examined responses to a caffeine-based energy drink after a single serving and after 28 days of use. Compared with placebo, the researchers observed higher measured energy expenditure at certain post-consumption time points; greater fat oxidation was observed at some time points on the first day. The authors also reported that some energy-expenditure effects remained at day 28. Therefore the tested drink may temporarily increase energy expenditure and, under some conditions, fat oxidation. These laboratory measurements do not demonstrate lasting fat loss or make an energy drink a weight-loss treatment. Sponsorship should be considered when assessing the independence of this product-specific evidence; it does not by itself negate the measured results. [source ↗]
Memory and attention — Northumbria University
In a controlled experiment, researchers compared a whole caffeine-and-glucose drink with placebo and with separate ingredient fractions. The whole drink improved performance on the study’s secondary-memory and speed-of-attention measures compared with placebo. The result suggests that some energy drink formulations may improve specific short-term memory and attention tasks; it does not demonstrate lasting cognitive benefits or equal effects in every person. [source ↗]
These studies measured selected short-term performance outcomes, not the long-term safety of energy drinks or their effects in children. Different formulations, doses and people may yield different results. The supplied infographic additionally names a “Washington University” calorie-expenditure and fat-burning study without identifying a publication, authors or DOI. We cannot verify that attribution and therefore do not present it as an established finding.
Individual response matters
Caffeine’s effects are not identical in every person. Body mass, habitual intake, sensitivity, genetics, sleep status, food consumption, medication use, pregnancy, health status, dose and timing can all influence response.
An energy drink may improve alertness or a specific performance measure in one situation while producing little benefit—or unwanted effects—in another. More caffeine does not necessarily produce a greater benefit.
Safety and performance
EFSA concluded that, for healthy adults, single caffeine doses up to 200 mg do not raise safety concerns. The same amount does not raise safety concerns when consumed less than two hours before intense exercise under normal environmental conditions. Habitual caffeine intake up to 400 mg per day does not raise safety concerns for non-pregnant adults.
These amounts include caffeine from all dietary sources combined, including coffee, tea, cola, chocolate, supplements and energy drinks. They are safety reference values, not performance targets or recommended intakes. [source ↗]
Key conclusions
- Energy drinks are designed to provide nutritional energy, increased alertness or both.
- In conventional energy drinks, sugar is usually the main source of calories.
- Caffeine is the principal ingredient responsible for increased alertness and reduced sleepiness.
- Caffeine provides stimulation but not nutritional energy.
- Natural and synthetic caffeine are the same chemical compound and act through the same physiological mechanisms when purity and dose are equivalent.
- Sugar-free energy drinks can still promote alertness if they contain sufficient caffeine.
- Caffeine-free drinks do not provide caffeine-mediated alertness.
- Sugar alone should not be expected to reliably counteract sleepiness.
- Performance effects depend on the exact product, dose, consumer and task.
- One product or study should not automatically be generalised to the entire category.