Background

Exercising in the morning may be less beneficial than exercising later in the day,[1][2][3] especially if your goal is to burn fat.[4] But does this hold true if you take caffeine as a fat loss?

The study

This randomized crossover trial enrolled 15 active men (aged 32 on average) who consumed less than 50 mg of caffeine per day.

The participants took either caffeine (3 mg/kg) or a placebo 30 minutes before completing a cycling session. Each participant completed two sessions — one at 8 a.m., the other at 5 p.m., in random order — separated by a 1-week washout.

Each session comprised two cycling tests separated by a 5-minute rest.

  • The first test was a graded submaximal cycling test. The workload increased every 3 minutes until the participants reached a respiratory exchange quotient of 1.0.
  • The second test was a maximal graded test to exhaustion. The workload increased every minute and VO2max was measured.

The primary outcome was the rate of maximal fat oxidation (MFO) during exercise, and the secondary outcomes were VO2max and fatmax, with VO2max being measured in milliliters of oxygen used in one minute per kilogram of body weight (mL/kg/min) and fatmax being the exercise intensity that elicited MFO, in % of VO2max.

The results

Caffeinated afternoon exercise gave the best results.

  • Compared to morning placebo, morning caffeine resulted in greater MFO (+10.7%) and fatmax (+11.1%).

  • Compared to afternoon placebo, afternoon caffeine resulted in greater MFO (+29%) and fatmax (+13.1%).

  • Compared to morning caffeine, afternoon caffeine resulted in greater MFO (+18.3%) and fatmax (+2%).

Maximal fat oxidation during exercise (mg/min)

MorningAfternoon
No caffeine280 (baseline)310 (+11%)
Caffeine310 (+11%)400 (+43%)

Fatmax during exercise (% of VO2max)

MorningAfternoon
No caffeine36.9 (baseline)42.0 (+14%)
Caffeine41.0 (+11%)47.5 (+29%)

VO2max was higher in the afternoon (by up to 3.9%). Proportionally, caffeine increased VO2max equally in the morning and in the afternoon (by 1.5 mL/kg/min).

Note

Performance and core body temperature peak in the afternoon.[2] Moreover, exercise increases catecholamines more in the evening than in the morning.[4] Increases in core body temperature and catecholamines lead to increases in fatty-acid oxidation, which helps explain the effect of exercise timing in this study.

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This Study Summary was published on May 4, 2021.

References

  1. ^Mladen Savikj, Brendan M Gabriel, Petter S Alm, Jonathon Smith, Kenneth Caidahl, Marie Björnholm, Tomas Fritz, Anna Krook, Juleen R Zierath, Harriet Wallberg-HenrikssonAfternoon exercise is more efficacious than morning exercise at improving blood glucose levels in individuals with type 2 diabetes: a randomised crossover trialDiabetologia.(2019 Feb)
  2. ^Weipeng Teo, Michael J Newton, Michael R McGuiganCircadian rhythms in exercise performance: implications for hormonal and muscular adaptationJ Sports Sci Med.(2011 Dec 1)
  3. ^Nizar Souissi, Nicolas Bessot, Karim Chamari, Antoine Gauthier, Bruno Sesboüé, Damien DavenneEffect of time of day on aerobic contribution to the 30-s Wingate test performanceChronobiol Int.(2007)
  4. ^Hyeon-Ki Kim, Masayuki Konishi, Masaki Takahashi, Hiroki Tabata, Naoya Endo, Shigeharu Numao, Sun-Kyoung Lee, Young-Hak Kim, Katsuhiko Suzuki, Shizuo SakamotoEffects of Acute Endurance Exercise Performed in the Morning and Evening on Inflammatory Cytokine and Metabolic Hormone ResponsesPLoS One.(2015 Sep 9)