Coffee Before Exercise: What the University of Granada Caffeine Study Reported

Researchers at the University of Granada put a familiar habit under a microscope: the cup of coffee people drink before a workout. Their trial was published in the Journal of the International Society of Sports Nutrition, and it measured how caffeine and time of day affected substrate use during a graded cycling test. Below is what the authors reported, what the numbers mean in context, and where the study stops. If you simply want fresh coffee for the taste of it, Buddha Beans Coffee roasts specialty grade beans in small batches.

What the University of Granada Team Set Out to Test

Scientists from the Department of Physiology of the University of Granada (UGR) studied caffeine (about 3 mg/kg, the equivalent of a strong coffee) taken half an hour before aerobic exercise, and reported changes in measured fat oxidation rates during that exercise. It stands as one of the first controlled examinations of caffeine and fat oxidation during exercise, and it put numbers behind a question athletes have argued about for years.

"The recommendation to exercise on an empty stomach in the morning to increase fat oxidation is commonplace. However, this recommendation may be lacking a scientific basis, as it is unknown whether this increase is due to exercising in the morning or due to going without food for a longer period of time," explains the lead author of this research, Francisco José Amaro-Gahete of the UGR's Department of Physiology. That question, morning versus simply going longer without food, is the one the study set out to untangle.

The research was published in January 2021 in the Journal of the International Society of Sports Nutrition, adding to the scientific literature on caffeine and exercise metabolism. It came out of the Department of Physiology at one of Spain's leading research universities, where scientists have spent years working on the intersection of nutrition, circadian rhythms, and exercise performance.

How the Study Was Designed

A total of 15 men (mean age, 32) participated in the research, completing an exercise test four times at seven-day intervals. Subjects ingested 3 mg/kg of caffeine or a placebo at 8am and 5pm (each subject completed the tests in all four conditions in a random order). The conditions prior to each exercise test (hours elapsed since last meal, physical exercise, or consumption of stimulant substances) were strictly standardized, and fat oxidation during exercise was calculated accordingly. That level of control is what lets the results point at caffeine rather than at some confounding variable.

Using a triple-blind, placebo-controlled, crossover experimental design, 15 active caffeine-naïve men (age: 32 ± 7 years) completed a graded exercise test four times at seven-day intervals. Triple-blind is the strict end of clinical research design: participants and researchers were blinded to treatment allocation, and the data analysts stayed blind during statistical analysis too. That rigor holds bias down within the trial, though it does not make a 15-person sample a large one.

Each participant served as their own control, completing tests under four different conditions: morning with caffeine, morning with placebo, afternoon with caffeine, and afternoon with placebo. Randomizing the order kept learning effects and adaptation from creeping into the results. Standardizing the pre-test conditions, including fasting duration and prior physical activity, removed variables that could have skewed the findings. Participants ate identical diets for 24 hours before each test, skipped exercise for 24 hours, and avoided caffeine for at least 48 hours, which gave the researchers a clean slate for measuring caffeine's acute effects.

The caffeine dose of 3 mg per kilogram of body weight translates to approximately 210 milligrams for a 70-kilogram (154-pound) individual. That is roughly a strong cup of coffee. The FDA states that healthy adults can consume up to 400 milligrams (mg) of caffeine a day, which is equivalent to about 4 or 5 cups of coffee, so the study's dosage sits well within that general guidance.

The researchers picked "caffeine-naïve" participants, meaning people with low habitual caffeine consumption. The choice was strategic. It took tolerance out of the equation and left a clearer view of caffeine's acute effects. It also means the sample was not representative of daily coffee drinkers, which the authors themselves flag as a limitation.

The Graded Exercise Test Protocol

A graded cycling test was performed. MFO and maximum oxygen uptake (VO2max) were measured by indirect calorimetry, and the intensity of exercise that elicited MFO (Fatmax) calculated. Graded exercise tests are the standard method for assessing metabolic responses to exercise. Each test started at low intensity and stepped the workload up every few minutes until participants hit volitional exhaustion or their maximum heart rate.

Throughout the test, participants wore a metabolic cart that continuously measured oxygen consumption and carbon dioxide production. Those measurements, run through stoichiometric equations, let researchers calculate how much fat versus carbohydrate participants used at each exercise intensity. Indirect calorimetry gives more direct substrate oxidation data than blood markers alone, since it captures fuel use in real time as intensity climbs.

The graded test also pinpointed "Fatmax," the exercise intensity at which measured fat oxidation reaches its highest rate before declining as carbohydrate oxidation takes over at higher intensities. Fatmax is a standard variable in exercise physiology research. In this trial, the authors reported that the caffeine conditions showed both higher peak fat oxidation values and a Fatmax at a slightly higher exercise intensity than the placebo conditions.

What the Researchers Measured

"The results of our study showed that acute caffeine ingestion 30 minutes before performing an aerobic exercise test increased maximum fat oxidation during exercise regardless of the time of day," explains Francisco J. Amaro. The authors also reported diurnal variation in measured fat oxidation during exercise, with values higher in the afternoon than in the morning for equal hours of fasting.

Put in numbers, the trial reported maximal fat oxidation values roughly 11% higher in the morning caffeine condition and roughly 29% higher in the afternoon caffeine condition, each compared with its matching placebo condition. Those are laboratory measurements taken during a single graded cycling test in 15 caffeine-naïve men. They describe what the metabolic cart recorded inside that test. They are not a prediction of what any individual will experience, and they are not a statement about weight, body composition, or health.

MFO, Fatmax and VO2max were significantly higher in the afternoon than in the morning (all P < 0.05). That pattern is consistent with the broader observation in physiology research that measured values shift across the 24-hour cycle rather than holding flat. It also cuts against the assumption, common in gym advice, that fasted morning exercise is automatically the higher fat oxidation condition, at least within the four conditions this trial compared.

The authors discuss circadian differences in circulating adrenaline as a possible explanation for the higher afternoon values. Core body temperature, sympathetic nervous system activity, and hormone levels all vary across the day in published physiology research, and the researchers point to that variation rather than to any single cause. It remains a proposed explanation, not a demonstrated one.

Understanding Maximal Fat Oxidation Rate (MFO)

The study focused on maximal fat oxidation rate, or MFO, a laboratory measure in exercise physiology that represents the highest rate at which fat oxidation is recorded during exercise. MFO typically occurs at moderate exercise intensities, generally between 45-65% of VO2max for most individuals, though it varies with training status, diet, and metabolic flexibility. MFO is a research variable used to describe fuel use at different intensities. It is not a health score, and it is not a diagnostic number.

Using graded exercise tests in active men, scientists examined the effect of caffeine on the oxidation of fat in morning and afternoon workout sessions. Because graded tests raise intensity progressively, researchers can identify the intensity at which measured fat oxidation peaks before declining as carbohydrate oxidation takes over. It yields precise, quantifiable data about substrate use during exercise, well beyond what subjective measures or single-intensity protocols can offer.

The participants were active men with endurance training experience, which matters when reading the size of the numbers. Their baseline MFO values reflected that training history. Other research cited in this literature has reported larger caffeine-related differences in sedentary or untrained participants than in trained and recreational athletes. That is a comparison between study populations, and it does not tell any individual reader what to expect.

Morning Versus Afternoon in This Trial

The authors also reported that the morning caffeine conditions produced measured fat oxidation values similar to the afternoon condition without caffeine. Their stated conclusion is that the combination of acute caffeine intake and aerobic exercise performed at moderate intensity in the afternoon was the condition with the highest measured values among the four tested.

That comparison is worth sitting with, because it runs against the standard advice to train first thing in the morning on an empty stomach. Within these four lab conditions, afternoon testing produced higher measured values than morning testing at matched fasting duration, and adding caffeine raised measured values in both time slots. Whether that ordering holds outside a graded cycling test, across other populations, or over weeks of training was not something this trial examined. For a look at how people actually structure a training-day routine, see our post on building a morning routine.

The Mechanisms the Researchers Discuss

The authors and the wider caffeine literature describe several proposed mechanisms. Caffeine is described as promoting lipolysis (the process by which fats are broken down) through a greater release of adrenaline, with fatty acids released into the blood and transported to muscle for use as energy. That cascade is the textbook explanation researchers offer, and it is a description of proposed physiology rather than a promise attached to a cup of coffee.

In that account, caffeine stimulates the release of catecholamines, including epinephrine (adrenaline) and norepinephrine, which act on hormone-sensitive lipase, the enzyme that breaks stored triglycerides into free fatty acids and glycerol. Those free fatty acids enter the bloodstream, where they can be transported via albumin binding to muscles and other tissues. More circulating free fatty acids means more available substrate for oxidation, at least as the model is drawn in the literature.

Adenosine receptor antagonism is the mechanism most often described as caffeine's primary mode of action. Adenosine promotes sleep and suppresses arousal; caffeine and adenosine compete for the same receptors, so when caffeine is present adenosine binds less. Researchers connect that to central nervous system stimulation, and to reported changes in perceived exertion and focus during exercise. This is pharmacology as described in research papers, not an effect we claim for our coffee.

Reviews in this literature also discuss enzymes involved in moving fatty acids into mitochondria, including carnitine palmitoyltransferase (CPT-1), and possible effects on mitochondrial function. Those remain proposed mechanisms under investigation. None of them has been established as an outcome you can expect from drinking coffee, and nothing in this research changes what coffee is: a beverage.

Dose in the Study: 3 mg/kg

A meta-analysis in this area reported a dose-response pattern, indicating that more than 3.0 mg/kg appeared necessary to reach a statistically significant difference in measured fat oxidation during exercise. Lower doses have been studied mainly for central nervous system effects. The threshold is a research finding about study protocols, not a dosing recommendation for readers.

The University of Granada study's use of exactly 3 mg/kg lands on that threshold, chosen to balance measurable effect with tolerability. Doses in the 4-6 mg/kg range have been studied and generally show diminishing differences alongside a higher rate of unwanted side effects. One reported figure in this literature: a caffeine dose of only 100 mg has been shown to increase resting energy expenditure by 3%-4%, although fat oxidation was not measured in that work.

A separate line of research in healthy active women reported increases in measured fat oxidation of 26.5% with 3 mg/kg and 24.7% with 6 mg/kg at an intensity producing maximal fat oxidation. Doubling the dose did not double the measured difference in that work, which is the practical reason researchers tend to stay near the lower end of the range rather than chase larger doses.

Individual responses to caffeine vary with genetics, body composition, tolerance, and metabolism rate. Some people are "fast metabolizers" who clear caffeine quickly due to genetic variation in the CYP1A2 enzyme, while "slow metabolizers" hold elevated caffeine levels for longer. Slow metabolizers generally do better keeping doses conservative and expecting effects to last longer into the evening.

What 3 mg/kg Looks Like in a Cup

The trial used caffeine powder for precise dosing, so translating it into coffee is arithmetic plus a little brewing knowledge. At 3 mg/kg, a 150-pound (68 kg) person is at roughly 204 mg and a 180-pound (82 kg) person at roughly 246 mg. A typical 8-ounce cup of drip coffee contains 95-165 mg of caffeine. Espresso runs approximately 60-70 mg per 1-ounce shot. Cold brew concentrate can carry 200+ mg per serving. So the study dose corresponds to a strong 12-16 ounce drip cup, 2-3 espresso shots, or 8-10 ounces of cold brew concentrate, adjusted for body weight.

Caffeine absorption occurs primarily in the small intestine, with peak plasma levels typically reported 30 to 60 minutes after consumption. The study's 30-minute window before exercise was chosen with that timing in mind. Individual variation is wide, which is one reason the researchers standardized timing rather than leaving it to participants.

Brewing method changes extraction and the finished cup. French press and cold brew use longer contact times and pull more caffeine and more oils into the cup. Pour-over gives you control over grind, temperature, and ratio, which is useful if you want flavor and strength dialed in together. Drip sits in the middle and is the most predictable day to day. Sticking to one method and one coffee is the simplest way to keep your cup consistent.

Freshness, Roast, and Flavor

Freshness sets the ceiling on flavor. Oxidation begins immediately after roasting and progressively degrades the volatile aromatic compounds that give coffee its smell and its brightness in the cup. Coffee that has sat on a shelf for months, especially pre-ground (which accelerates oxidation because of the increased surface area), tastes flat and papery next to a recently roasted bag. Buddha Beans Coffee roasts in small batches and ships quickly so bags arrive tasting like coffee rather than like storage.

Roast level shifts both the flavor profile and the chemistry. Lighter roasts hold more of the acids present in green coffee and read as brighter and more fruit-forward. Darker roasts develop caramelized and roasty flavors through Maillard reactions, along with the melanoidins that give dark roast its color and heavier body. The caffeine difference between light and dark is modest by weight, though more pronounced by volume because of bean density, so pick a roast by taste.

Coffee also carries hundreds of compounds besides caffeine, including chlorogenic acids, trigonelline (which converts to niacin during roasting and contributes to aroma), caffeic acid, and diterpenes such as cafestol and kahweol that carry through in unfiltered brews. These are the chemistry behind what you taste. We are not making any health claim about them, and none of them is why we sell coffee. We sell coffee because it tastes good.

Who the Study Did Not Include

The authors acknowledge that the present study used only a small cohort, and was only performed in active men who typically consumed only a small amount of caffeine. The study will need to be repeated in a wider cohort that includes women, people who are less active, and those who regularly consume caffeine before anyone can say whether the same pattern appears. That caveat comes from the researchers, not from us, and it is the most important sentence in the paper.

Other work in this literature has reported broadly similar caffeine responses in men and women, including the study in active women noted above, which reported increased measured fat oxidation from 30 to 60% of VO2max with reduced carbohydrate oxidation at those workloads, independently of dose. Some research also reports possible differences in caffeine metabolism tied to hormonal fluctuations across the menstrual cycle. These are separate studies with separate samples, and reading them together is interpretation rather than proof.

On tolerance, the available research is mixed but leans toward performance-related effects persisting with habitual use, while tolerance develops to some of caffeine's other effects. As one review put it, the limited information available suggests that caffeine non-users and users respond similarly and that withdrawal from caffeine may not be important. People with very high habitual intake (6+ cups daily) may see blunted responses.

What This Study Does Not Show

This is the part usually left out of the headline. Even at the higher afternoon rate reported, about 0.4 grams of fat oxidized per minute, roughly 42 hours of exercise would correspond to one kilogram of body fat. One kilogram of body fat holds approximately 7,700 calories, and vigorous exercise burns only 400-600 calories per hour for most people, with fat contributing a fraction of that total.

So a percentage difference measured inside a single cycling test is not a weight loss result, and this trial did not measure weight, body composition, or any health outcome. It measured substrate use in a lab over the length of one graded test. Nothing here shows that drinking coffee causes fat loss, changes body composition, or affects any disease or condition. Body composition changes come down to cumulative energy balance and consistent habits over months, which is boring and remains true.

The other honest limit: this is 15 caffeine-naïve men on a stationary bike, in one lab, in 2021. Independent groups have reported directionally similar findings, including a meta-analysis of 19 studies concluding that pre-exercise intake of a moderate dose of caffeine may increase fat oxidation during aerobic exercise, especially after a fasting period of at least five hours, and a 2020 meta-analysis reaching the same conclusion for submaximal aerobic exercise after a fast of 5 or more hours. Converging findings raise confidence. They do not turn a measured lab difference into a benefit.

Caffeine and Caution

Excessive caffeine consumption can cause unpleasant side effects, including insomnia, jitteriness, anxiousness, a rapid heart rate, nausea, and headaches. Side effects tend to be dose-dependent, showing up more often and more severely at higher doses. Caffeine agrees with most people and not with everyone.

Anyone with an underlying health condition should talk to a healthcare professional before using caffeine deliberately, rather than taking cues from a study. That includes people with irregular heart rhythm, arrhythmias, tachycardia, or uncontrolled hypertension, since caffeine acutely raises heart rate and blood pressure. Current guidelines advise pregnant women to limit caffeine to 200 mg daily or less, which puts the 3 mg/kg dose used in this study above that limit for most pregnant individuals. People with GERD, ulcers, or irritable bowel syndrome, and people with anxiety or panic disorders, often already know how coffee sits with them. A study is not a reason to override that.

Other Findings in the Caffeine Literature

Fat oxidation is a narrow slice of what has been studied. Across the sports science literature, caffeine has been reported to improve endurance exercise performance by roughly 2-4% on average, to extend time to exhaustion by approximately 15-20%, and to lower ratings of perceived exertion by about 6%, meaning the same workload was rated as feeling easier. Research has also reported effects on reaction time and focus during prolonged exercise. Those are pooled findings from exercise performance research, reported as such.

Findings span exercise modalities including distance running, cycling time trials, swimming, rowing, team sports involving intermittent high-intensity efforts, and resistance training. The effective dose range in that research typically falls between 3-6 mg per kilogram of body weight, with 3-4 mg/kg most common when balancing measured effect against tolerability. Doses below 3 mg/kg appear insufficient for the metabolic measures, while doses above 6 mg/kg raise side effect risk without a proportional difference in results.

Research has also reported that people rate exercise as more enjoyable after caffeine. That may matter more than any metabolic number, because adherence, actually showing up week after week, predicts long-term outcomes better than the fine-tuning of any single session. Coffee 30 minutes before you train is also just a ritual: grinding, brewing, and a few quiet minutes that mark the transition into your workout. That part needs no study.

The Bottom Line on the Research

The University of Granada trial is a well-controlled, small study that measured higher fat oxidation during cycling in its caffeine conditions, with higher values in the afternoon than the morning, and published the numbers with its own limitations stated plainly. That is the whole finding. It is interesting physiology and it is not a health claim.

What we sell is coffee: specialty grade beans, roasted in small batches, shipped fresh. Buddha Beans Coffee makes claims about flavor, freshness, and roast craft, and stops there.