In a new Spanish study that was published in the European Journal of Nutrition the role of caffeine in fat burning (oxidation) was investigated (1). The idea that caffeine stimulates fat oxidation dates back to the 1970s, but studies have produced very mixed results. The new study from Juan del Coso’s research group (a very well conducted study), showed improvements in fat oxidation by 27%. But does this matter? Is it important?
Greater fat mobilisation with caffeine
The studies by David Costill in the late 1970s (2) showed that caffeine can increase the mobilisation of fatty acids from their stores. Fat is stored in several tissues but mostly in adipose tissues. The fatty acids are released from adipose tissue, can be transported to the muscle and be used as fuel. Some, but not all, studies show also an increase in fat oxidation with this elevation in fatty acids. However, it is not always true that an elevation in fatty acids levels in the blood also results in increased fat oxidation. This increase in fat oxidation was originally used to explain the performance benefits but it is now generally accepted that the increase in fat oxidation, does not spare muscle glycogen and improve performance. The performance enhancing effects of caffeine are predominantly through effects on the central nervous system.
New study finds increased fat oxidation with caffeine
The authors conducted a placebo controlled double blind study. Participants conducted two experimental trials. Before one of these they ingested 3 mg/kg of caffeine and before the other trial they ingested the same amount of a placebo. The capsules were ingested 1 hour before exercise. The participants (12 in total) exercised at fatmax intensity. This means that in an earlier test, the researchers had established the exercise intensity where these participants oxidised most fat. This is highly individual but on average was around 50% VO2max, a low-to-moderate exercise intensity.
During the hour of cycling, the subjects oxygen uptake and carbon dioxide production were measured and fat and carbohydrate oxidation were calculated from this. What the researchers found was an average total fat oxidation of 19.4 grams for the hour with placebo and 24.7 grams with caffeine. That is a whopping 27% more fat oxidation. They also found that there was no difference in energy expenditure and thus the increased fat oxidation meant that carbohydrate use was used (or carbohydrate was spared).
The researchers found an average total fat oxidation of 19.4 grams for the hour with placebo and 24.7 grams with caffeine. That is a whopping 27% more fat oxidation with caffeine.
The findings raise two questions. How does this relate to the rest of the literature, because we don’t want to make the mistake of drawing a conclusion on the basis of a single published study. The second question is what are the practical implications, if any?
The finding that caffeine can increase fat oxidation is not new. Several studies have shown this before, but it is important to realise that there are also many studies that did not see any effect of caffeine on fat oxidation. The different findings can at least party be explained by the exercise intensity used in the studies. A previous study by Juan del Coso's research team (2) showed that effects of caffeine on fat oxidation could be observed at lower intensities but not at higher intensities (>80%VO2max).
There are other factors that could affect the outcomes as well, such as how trained the tested individuals were, the duration of the activity, the control of the diet before the trials and so on. But overall the findings of the study are in line with several previous studies and I would conclude that caffeine CAN in some situations increase fat oxidation. I was maybe a little surprised by the magnitude of change in this study, which seems a little greater than other studies. Of course, the effects may have been exaggerated by the fact that these studies were conducted in the fasted state (no breakfast), when fat oxidation is naturally high. As soon as we ingest a breakfast or eat carbohydrate, fat oxidation may be reduced by 30%. But the study is solid and well done and the authors need to be complimented. The issue is not the study, but may be the interpretation of the results by others.
Does it matter?
The million dollar question: does it matter? What are the practical implications of these findings? The authors are very careful not to speculate too much here. They just say that if people use caffeine for weight loss or for reducing body fat, they need to consider the negative effects of caffeine as well. I have seen social media reports who also linked the use of caffeine and the 27% increase in fat oxidation to weight loss. But let's be clear: Weight loss will only occur when the energy expended exceeds energy intake. Caffeine did not increase energy expenditure, so you would still have to reduce intake to lose weight regardless of whether you burn more fat or not. The second observation is of course that even in this study where the effects on fat oxidation seem to be rather large, we are talking about a difference of only 5 grams of fat per hour of exercise. This means that in order to lose 1kg of body fat, one would have to exercise 200 hours!!
.....we are talking about a difference of only 5 grams of fat per hour of exercise. This means that in order to lose 1kg of body fat, one would have to exercise 200 hours!!
If an 80 kg person would like to lose 2% fat, it would take them a year, IF they exercised for an hour every day! So in terms of weight loss these findings are probably not important. Where the findings may potentially be important is that we stimulate fat oxidation more with caffeine, we therefore stress fat oxidation pathways more and we may adapt more. But how often would we need to do this in order to get significant adaptations that also translate to a performance or health effect? Without longer term studies such questions are difficult to answer.
Ruiz-Moreno, C., Gutiérrez-Hellín, J., Amaro-Gahete, F.J. et al. Caffeine increases whole-body fat oxidation during 1 h of cycling at Fatmax. Eur J Nutr (2020). https://doi.org/10.1007/s00394-020-02393-z
Costill DL, Dalsky GP, Fink WJ. Effects of caffeine ingestion on metabolism and exercise performance. Med Sci Sports. 1978 Fall;10(3):155-8. PMID: 723503.
Gutiérrez-Hellín, J; Del Coso, J. Effects of p-Synephrine and Caffeine Ingestion on Substrate Oxidation during Exercise, Medicine & Science in Sports & Exercise: September 50(9): 1899-1906, 2018. doi: 10.1249/MSS.0000000000001653