The strongest fat-burning effects of consuming caffeine before exercise are seen in the afternoon, according to a new study published in Journal of the International Society of Sports Nutrition.
It has long been known that consuming caffeine before exercising can help increase fat burning — by a measure called the maximal fat oxidation rate, or MFO — as well as increase a person’s aerobic capacity, meaning how well their body uses oxygen during exercise. Both of these functions, without caffeine, tend to be lower in the morning compared with exercising in the afternoon. For this study, researchers were interested in whether consuming caffeine before exercise would lead to a bigger improvement in fat burning or aerobic capacity in the morning, when they tend to be lower without caffeine — or in the afternoon, when they tend to be higher without caffeine.
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The researchers designed a small study involving 15 men with an average age of 32, who didn’t consume caffeine prior to the study. It was a triple-blind design, meaning that the participants weren’t told if they were taking caffeine or a placebo (inactive substance), the people administering doses didn’t know if they were giving each participant caffeine, and the researchers didn’t learn which participants received caffeine until after they had calculated the results for each person. These steps are meant to reduce the possibility of bias at every stage. Of course, they can’t actually eliminate bias, especially since people may feel different when they take caffeine and perform differently when they exercise as a result. But, in a sense, this could also be considered a normal effect of caffeine that is within the scope of the study.
Participants were given graded exercise test — designed to evaluate their exercise capacity — using a treadmill and other equipment. This was done every seven days, four times total. Participants completed this test under four different conditions, in a random order: after consuming 3 milligrams of caffeine per kilogram of body weight at both 8 a.m. and 5 p.m., and after consuming a placebo at both times. The researchers measured both fat oxidation and maximum oxygen uptake during exercise.
Caffeine and fat burning
Not surprisingly, fat burning and maximum oxygen uptake during exercise — without caffeine — were found to be significantly greater when participants exercised in the afternoon, consistent with previous studies. But the effects of consuming caffeine before exercise were also significantly greater in the afternoon. Compared with taking a placebo, caffeine increased fat burning by 10.7% in the morning and 29.0% in the afternoon. Taking caffeine also increased the intensity of exercise that led to fat burning by 11.1% in the morning and 13.1% in the afternoon.
These results indicate that rather than helping people overcome the typical “sluggish” fat-burning response seen when exercising in the morning compared with the afternoon, taking caffeine before exercise actually leads to an even greater gap in fat burning between morning and afternoon exercise. So while drinking coffee before you exercise in the morning may help you burn fat, it won’t help nearly as much as if you drink coffee before exercising in the afternoon — if these results apply to the broader population. More research is needed to know if these findings apply beyond a small group of younger men without a history of regular caffeine consumption.
Want to learn more about coffee and diabetes? Read “Is Coffee Good for Diabetes?”
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