Is Caffeine’s ‘Fat-Burning Boost’ Time-of-Day Dependent?

If you’re anything like me, you relish a daily cup of coffee (or two) as a jump-start to the morning, a mid-afternoon pick-me-up, or maybe even a nighttime “wind down” drink (decaf might be best for that.) Most people drink coffee for the caffeine, but the warmth, aroma, and taste are also something to be enjoyed.

In addition to being a cognitive stimulant, caffeine is one of the most well-evidenced “ergogenic” (performance-enhancing) supplements for exercise. Caffeine has been shown to improve strength, aerobic, and anaerobic exercise performance — it’s probably one of the few sports “supplements” actually worth using.

There is another perhaps surprising benefit to ingesting caffeine — at rest but also during exercise. That benefit being enhanced fat burning.

Caffeine acts on multiple different pathways in the body to increase the release, breakdown, and utilization of fatty acids (which come from our stored body fat.)

Effects of caffeine on skeletal muscle and adipose tissue. de Silva et al. (2017)

As such, some have proposed that caffeine intake alone, or when paired with aerobic exercise, could be a strategy to enhance fat loss and, for athletes, improve endurance by reducing their reliance on glucose and glycogen for energy.

One looming question is whether the effects of caffeine intake on fat metabolism during exercise are different in the morning compared to the afternoon/evening. Why is this relevant? It is well known that in general, strength, endurance, and fat metabolism are all higher in the afternoon compared to the morning, due to a variety of hormonal and metabolic factors.

Could consuming caffeine diminish the drastic difference in performance seen between morning and evening exercise? When is the best time to pair exercise and caffeine to optimize fat metabolism and performance?

All of these questions were investigated in a study recently published in the Journal of the International Society for Sports Nutrition (JISSN.)

Study aims and hypothesis

The primary aim of this study was to investigate the effects of caffeine on the diurnal (day/night) variation in fat oxidation during exercise and maximal oxygen uptake (VO2 max). The authors hypothesized that caffeine intake would elevate maximal fat oxidation, the intensity of exercise where max fat oxidation rate occurred, and increase VO2 max during a treadmill exercise test.

Brief study methods

This study was a randomized crossover study where the participants, study staff, and data analysis team were all blind to the intervention being received by the participants (known as a triple-blind study.)

The participants (15 in total) were men with an average age of 32 who had a history of endurance training and exercised an average of ~4 days per week. All participants were “caffeine naive,” meaning that they consumed less than 50mg of caffeine per day.

On four separate occasions, participants completed one of the following conditions

• Morning exercise + caffeine intake (3mg/kg)
• Morning exercise + placebo intake
• Afternoon exercise + caffeine intake (3mg/kg)
• Afternoon exercise + placebo intake

Based on the average weight of participants (80kg), they were ingesting approximately 240 mg of caffeine before exercise on each of the relevant visits. Caffeine (and placebo) were ingested 30 minutes before each exercise test session — which involved a submaximal cycling exercise bout and a maximal graded exercise test (to exhaustion.)

During each exercise test, the rate of fat oxidation was measured to determine maximal fat oxidation (MFO.) VO2 max was also assessed as one of the primary outcome measures. This study also looked at something called Fat max — the intensity of exercise where one achieves their maximal rate of fat oxidation.

Results

• Max fat oxidation was significantly higher in the afternoon vs. the morning
• Caffeine increased max fat oxidation by 10.7% in the morning and by 29% in the afternoon, compared to the placebo

• Max fat oxidation occurred at a higher intensity of exercise in the afternoon vs. the morning
• Caffeine increased Fatmax by ~11% in the morning and ~13% in the afternoon

• VO2 max was higher in the afternoon compared to the morning by ~3%
• Caffeine increased VO2 max by 3.9% in the morning and 3.2% in the afternoon

These results indicate that, regardless of the time of day, caffeine has beneficial effects on fat metabolism during exercise and on oxygen uptake — as both were increased following caffeine ingestion.

Though max fat oxidation and fat max increased at both times of day, the values for each were still higher in the afternoon compared to the morning — caffeine did not “erase” the diurnal variation in these outcomes. However, VO2 max was similar in the afternoon and the morning after caffeine intake. Essentially, caffeine can allow you to perform just as well in the morning as you would in the afternoon.

Related to this are the effects of caffeine on fat oxidation during exercise. In the morning after caffeine ingestion, max fat oxidation rates were similar to those observed in the afternoon without caffeine intake. Again, this says something about how caffeine could compensate for the morning vs. evening differences in fat metabolism.

A final point relates to the use of different fuel sources during exercise. Fat max, which we talked about earlier, refers to the intensity of exercise where max fat oxidation occurs. This is usually somewhere around 50–65% of VO2 max in humans, since we have a greater percentage of fat at a lower intensity of exercise. As intensity increases, we burn a higher proportion of carbohydrates to fuel exercise and rely less on fat.

In this study, caffeine increased the intensity of exercise where max fat oxidation occurred. This suggests that caffeine impacts substrate utilization during exercise, and may allow one to burn fat at a higher exercise intensity at the expense (or perhaps the salvation) of glucose and glycogen. For endurance athletes, this is a crucial factor, as glycogen stores are limited and when depleted, contribute to fatigue and reduced power output.

What are some practical applications of these findings?

For one — consuming caffeine pre-workout may help you to burn a little bit more fat for fuel. Whether this translates to a “significant” loss in fat is another topic entirely.

If you want to maximize fat burning and performance, exercise in the afternoon. If you’re a morning exerciser, caffeine can help to offset some of the morning reductions in fat metabolism and performance.

Finally, to maximize fat metabolism and aerobic performance, pairing caffeine with afternoon exercise is the best tool for the job.

I love reading studies like this that confirm my personal experiences. This applies to both the performance and caffeine effects observed in this study. I have always found my performance (strength, speed, power) to be higher in the afternoon, along with a greatly reduced perception of effort. I’m generally a morning exerciser, and never go without at least a few sips of caffeine pre-workout. Often, it’s a few shots of espresso (150–200mg?) to get me mentally and physically primed to perform at my best — well, at least my best for a morning workout, apparently.

Study cited

Ramírez-Maldonado M, Jurado-Fasoli L, del Coso J, R. Ruiz J, Amaro-Gahete FJ. Caffeine increases maximal fat oxidation during a graded exercise test: is there a diurnal variation? J Int Soc Sports Nutr. 2021;18(1):5.