Morning Training, Empty Stomach: What New TRE Data Shows
Research on time-restricted eating and early-morning cycling performance reveals surprising effects on fat oxidation and power output.
Cyclists love debating fasted training, but a new study from the University of Copenhagen just added hard data to the conversation. Published May 22 in the Journal of Applied Physiology, the research tracked 32 trained cyclists following time-restricted eating (TRE) protocols for eight weeks, measuring performance during early-morning interval sessions before breaking their overnight fast. The results challenge the conventional wisdom that fasted high-intensity work inevitably degrades performance.
Time-restricted eating compresses all daily food intake into a 6-10 hour window, typically from noon to 8 PM, leaving a 14-18 hour fasting period overnight and into the morning. The Copenhagen study compared three groups: TRE with morning training (eat 12-8 PM, train at 7 AM), TRE with afternoon training (eat 12-8 PM, train at 5 PM), and a control group eating normally across 12-14 hours with morning training.
Performance Outcomes
After eight weeks, all three groups improved their 20-minute power by 4.2-5.8%, with no significant differences between groups. The TRE-morning group—riding fasted 15-16 hours after their last meal—matched the performance gains of the fed groups. Their average power at the end of the study was 318 watts, compared to 322 watts for the TRE-afternoon group and 320 watts for the control group. The differences weren't statistically significant.
VO2max improvements followed the same pattern: all groups gained 2.8-3.4% over eight weeks, with no advantage to training in a fed state. However, the subjective experience differed dramatically. Athletes in the TRE-morning group reported higher perceived exertion during the first three weeks of the protocol, rating interval sessions 7.8 out of 10 on the RPE scale compared to 6.9 for the fed groups. By week six, perceived exertion had equalized as athletes adapted to training fasted.
Fat Oxidation and Metabolic Flexibility
The TRE-morning group showed significantly higher fat oxidation rates during submaximal riding. At 65% of VO2max, they burned an average of 0.68 grams of fat per minute compared to 0.52 g/min for the control group—a 31% increase. This adaptation occurred despite no change in body weight or body fat percentage across the groups, suggesting improved metabolic flexibility rather than simple body composition effects.
Mitochondrial enzyme activity also differed. Muscle biopsies taken before and after the eight-week protocol showed that the TRE-morning group increased citrate synthase activity by 22%, compared to 14% for the control group. Citrate synthase is a key enzyme in the Krebs cycle and serves as a marker of mitochondrial density. More mitochondria mean greater capacity for aerobic energy production, which should translate to improved endurance.
The mechanism likely involves cellular stress signaling. Fasting and exercise both activate AMPK (AMP-activated protein kinase), a metabolic sensor that triggers mitochondrial biogenesis when energy availability is low. Training in a fasted state combines both stressors, potentially amplifying the adaptive signal. However, the Copenhagen researchers note that this benefit comes with a ceiling—excessive fasted training without adequate recovery can backfire into overtraining.
High-Intensity Limitations
The TRE-morning group's success came with an important caveat: interval intensity was capped at threshold and below. When researchers tested sprint power and neuromuscular function, the fasted group showed 6-8% lower peak power output during 15-second max efforts compared to the fed groups. Their average sprint power was 1,182 watts versus 1,274 watts for the control group.
This aligns with existing research on glycogen availability and high-intensity performance. Sprints and efforts above FTP rely heavily on anaerobic glycolysis, which requires readily available carbohydrate. After 15+ hours of fasting, muscle glycogen stores sit at 60-75% of capacity, and liver glycogen is depleted. While that's sufficient for threshold intervals, it limits the explosive power needed for sprints, attacks, and short maximal efforts.
Practical Application Windows
The Copenhagen protocol divided training into three intensity categories with specific fueling recommendations:
Zone 1-2 endurance rides: Fasted training fully supported. Athletes maintained power output and showed the greatest fat oxidation benefits. Rides up to 90 minutes performed well fasted; longer rides required intra-workout carbohydrate after 90 minutes to prevent performance decline.
Threshold intervals (FTP and below): Fasted training supported with adaptation period. First 2-3 weeks showed elevated RPE, but performance matched fed training by week 4. Session duration should stay under 90 minutes total.
VO2max and sprint work: Fasted training not recommended. Performance declined 6-11% for efforts above 105% FTP, and athletes reported difficulty completing prescribed intervals. Pre-workout carbohydrate intake (50-75g consumed 60-90 minutes before training) eliminated the performance gap.
Individual Variation
Not all athletes responded identically to TRE-morning training. About 25% of participants in that group struggled to adapt even after eight weeks, continuing to report high RPE and showing smaller performance gains (2.1% FTP improvement versus 5.8% for best responders). Genetic factors related to fat metabolism and individual differences in hormonal responses to fasting likely explain this variation.
Women made up 38% of the study participants, and subgroup analysis showed no significant sex-based differences in performance outcomes. However, female athletes in the TRE groups reported more difficulty with training consistency around their menstrual cycle, particularly during the luteal phase when basal metabolic rate increases. The researchers recommend relaxing TRE windows during high-hormone phases to maintain energy availability.
Body Composition Surprises
Despite popular claims that TRE promotes fat loss, the Copenhagen study found no significant body composition changes in any group. All athletes maintained weight within 0.5 kg of baseline, and body fat percentage changed less than 1% across the board. The TRE groups naturally reduced daily calorie intake by 150-200 calories due to the compressed eating window, but this deficit wasn't large enough to produce measurable fat loss over eight weeks.
The takeaway: TRE isn't a body composition hack. If you're trying to lose weight, you still need a meaningful calorie deficit, whether that comes from time restriction or traditional calorie counting. TRE's benefits for cyclists lie in metabolic flexibility and training adaptation, not fat loss.
What This Means for Your Riding
If you train in the morning before work and find it impractical to eat beforehand, the TRE-morning protocol offers a validated framework. You can build fitness effectively with fasted threshold intervals and endurance rides, as long as you fuel properly the night before and post-workout. Aim for 6-8 g of carbohydrate per kg of body weight in your eating window to ensure glycogen stores recover fully.
Don't apply fasted training to every session. Save it for endurance and threshold work, and fuel properly before VO2max intervals, sprint sessions, or races. Expect a 2-4 week adaptation period where fasted sessions feel harder than usual—push through unless you're experiencing warning signs like persistent fatigue, irritability, or declining power numbers.
If you try TRE, commit to at least six weeks before evaluating results. The metabolic adaptations take time, and judging the protocol after one or two difficult fasted workouts doesn't capture the full adaptation curve. Track your subjective session RPE and compare it to power data to monitor whether you're adapting or struggling.