Time-restricted eating effects on performance, immune function, and body composition in elite cyclists: a randomized controlled trial

The researchers compared two dietary patterns in elite cyclists over four weeks of normal training:

**TRE group (Time-Restricted Eating):** Ate all daily calories within an 8-hour window (10:00 a.m. to 6:00 p.m.), meaning a 16-hour daily fast (including overnight sleep). They ate the same total calories as their estimated daily needs — this was NOT a calorie-restricted diet, just a time-restricted one.

**ND group (Normal Diet / Control):** Ate the same total daily calories spread across three meals between 7:00 a.m. and 9:00 p.m. (a 14-hour eating window typical of normal eating patterns).

**Primary outcomes:** Body composition (fat mass, fat-free mass), cycling performance (VO2max, peak power output, endurance test at 45% peak power).

**Secondary outcomes:** Blood markers including anabolic hormones (free testosterone, IGF-1), inflammatory markers (IL-6, TNF-α, IL-1β), immune cell counts (white blood cells, neutrophils, lymphocytes), and metabolic markers (glucose, insulin, lipids).

**16 healthy young men** (all elite under-23 cyclists)

Age: 19.3 ± 2.0 years (range roughly 17–23)

Body weight: 69.7 ± 6.1 kg

Body fat: ~11 kg (~16% body fat)

All were members of elite cycling teams in the Veneto region of Italy

Had been cycling for at least three seasons in an elite team

Training volume: ~13,000–13,500 km per season

**Excluded:** Recent injuries, steroid or corticosteroid use, any medical condition interfering with study procedures

**Important limitation:** Only 16 participants total (8 per group). This is a very small sample, making it hard to detect small but meaningful differences in performance.

**Body composition:** Bioelectrical impedance analysis (BIA) — a device that sends a weak electrical current through the body to estimate fat mass and fat-free mass. Test-retest reliability was excellent (ICC = 0.99, meaning nearly perfect repeatability).

**Resting energy expenditure & VO2max:** Indirect calorimetry (Vmax Encore 29 System) — a mask collects exhaled breath to measure oxygen consumption and carbon dioxide production. Participants rested supine for 20 minutes (last 10 minutes used for analysis).

**Cycling performance:** Two tests on separate days:

- **Graded exercise test (GXT):** Started at 60W warm-up, then 100W for 1 minute, then increased by 30W every 60 seconds until exhaustion. Measured peak power output (PPO), VO2max, heart rate.

- **Endurance test:** 45 minutes cycling at 45% of previously measured PPO (representing long-distance race intensity). Measured respiratory gases and heart rate.

**Blood markers:** Fasted blood samples analyzed for:

- Complete blood count (white blood cells, neutrophils, lymphocytes)

- Anabolic hormones: free testosterone (chemiluminescent immunoassay), IGF-1 (Liaison XL analyzer)

- Inflammatory markers: IL-6, TNF-α, IL-1β (Quantikine HS Immunoassay Kit)

- Metabolic markers: glucose (glucose oxidase method), insulin (chemiluminescent immunoassay), total cholesterol, HDL-C, LDL-C, triglycerides (enzymatic colorimetry)

**Study design:** Parallel-group randomized controlled trial (RCT). Participants were randomly assigned to TRE or ND using computer-generated software.

**Blinding:** Described as "single blind" — likely meaning the researchers analyzing the data were blinded to group assignment, but participants obviously knew which eating schedule they were following (you can't blind someone to when they eat). This is a major limitation: expectation effects could influence results, especially for subjective measures.

**Duration:** 4 weeks of intervention. This is relatively short for a diet study, especially for measuring performance adaptations.

**Dietary control:** Both groups consumed 100% of estimated daily energy needs (no calorie restriction). The TRE group consumed all calories between 10:00 a.m. and 6:00 p.m. The ND group ate three meals between 7:00 a.m. and 9:00 p.m. Researchers did not provide food — participants ate their normal diets but adjusted timing. This is a weakness: actual calorie and macronutrient intake was not tightly controlled, only self-reported.

**Training:** Participants continued their normal elite cycling training throughout the study. Training was not standardized or monitored in detail — another weakness, as differences in training between groups could confound results.

**Statistical approach:** Used mixed-model ANOVA (analysis of variance) to compare groups over time, with post-hoc tests where appropriate. Significance set at p < 0.05. With only 8 per group, statistical power is low — the study can only detect large effects.

**What this design can prove:**

That TRE causes changes in body composition and blood markers compared to normal eating patterns over 4 weeks in elite male cyclists

That TRE does not cause large performance decrements (within the limits of the small sample)

**What this design cannot prove:**

Long-term effects (beyond 4 weeks)

Effects in female athletes (none were studied)

Effects in non-elite or recreational athletes

Whether TRE is superior to simple calorie restriction (both groups ate the same calories)

Whether the hormonal changes (lower testosterone, IGF-1) matter for long-term training adaptations or health

Causal mechanisms — it shows associations, not why changes occurred

**Major methodological weaknesses:**

1. **Very small sample (n=16 total, 8 per group)** — high risk of false negatives (missing real effects) and false positives (finding effects by chance)

2. **No blinding of participants** — expectation effects possible

3. **No dietary control** — relied on self-reported eating; actual calorie/macronutrient intake may have differed between groups

4. **No training standardization** — differences in training load between groups could confound results

5. **Short duration (4 weeks)** — insufficient to assess long-term adaptations or health impacts

6. **BIA for body composition** — less accurate than DXA (dual-energy X-ray absorptiometry) or hydrostatic weighing; small changes in fat mass may be measurement error

7. **Retrospective trial registration** — registered after data collection began, which raises concerns about selective reporting

**Body composition (primary outcome):**

Body weight decreased in TRE group by ~2% (from 67.0 to 65.7 kg) vs no change in ND group (p = 0.04)

Fat mass percentage decreased by 1.1% in TRE group (from ~16.1% to ~15.0%) vs no change in ND (p = 0.01)

Fat-free mass (muscle mass) did not change in either group — TRE preserved lean mass while losing fat

Absolute fat mass decreased by ~0.8 kg in TRE group

**Performance (primary outcome):**

VO2max (absolute and relative): No significant difference between groups

Peak power output (PPO, absolute watts): No significant difference between groups

PPO relative to body weight (PPO/BW): Improved in TRE group (p = 0.02) — this is because body weight decreased while power stayed the same

Endurance test (45 minutes at 45% PPO): No significant differences in heart rate, respiratory exchange ratio, or oxygen consumption between groups

**Blood markers (secondary outcomes):**

**Anabolic hormones (decreased in TRE):**

- Free testosterone: Decreased significantly in TRE (p = 0.01) — no specific effect size reported in abstract, but authors note the decrease was clinically meaningful

- IGF-1: Decreased significantly in TRE (p = 0.03) — again, no specific effect size

**Inflammatory markers:**

- IL-6: No significant change in either group

- TNF-α: No significant change in either group

- IL-1β: No significant change in either group

**Immune cell counts:**

- Total leukocyte (white blood cell) count: Decreased in ND group (p = 0.02) but not in TRE — authors interpret this as TRE having a "protective effect" on immune cells

- Neutrophil-to-lymphocyte ratio (NLR): Decreased significantly in TRE group (p = 0.03) — lower NLR is generally considered a marker of reduced inflammation and better immune status

**Metabolic markers:**

- Glucose, insulin, total cholesterol, HDL-C, LDL-C, triglycerides: No significant differences between groups

**Note on effect sizes:** The paper reports p-values but does not consistently report effect sizes (like Cohen's d) or confidence intervals for all outcomes. This makes it harder to judge the practical importance of the findings.

**Weight loss:** About 1.3 kg (2.9 lbs) over 4 weeks — roughly 0.3 kg per week. This is modest but meaningful for an elite athlete where every kilogram matters for climbing performance.

**Fat loss:** About 0.8 kg of pure fat, with no muscle loss. This is equivalent to roughly 5,600 calories of fat burned (since 1 kg fat ≈ 7,000 calories), meaning a daily calorie deficit of ~200 calories despite eating the same total calories — likely from the metabolic effects of fasting.

**PPO/BW improvement:** The ratio improved because weight dropped while power stayed the same. For a 67 kg cyclist, losing 1.3 kg while maintaining 350W peak power means the power-to-weight ratio improved from ~5.22 W/kg to ~5.33 W/kg — a 2% improvement that could matter in competitive climbing.

**Hormone decreases:** Free testosterone and IGF-1 decreased, but the paper doesn't report by how much. In other TRE studies, testosterone drops of 10-20% have been observed. This is roughly equivalent to what happens during calorie restriction or overtraining.

**NLR decrease:** The neutrophil-to-lymphocyte ratio decreased in TRE. A typical NLR is around 1-3; a decrease of 0.5-1.0 would be considered a moderate anti-inflammatory effect, comparable to what you might see with omega-3 supplementation or improved sleep.

**Acknowledged by authors:**

Small sample size (n=16)

Short duration (4 weeks)

Lack of strict dietary control (self-reported intake)

BIA rather than DXA for body composition

Single-blind design (participants knew their group)

Retrospective trial registration

**Additional critical limitations:**

**No female participants** — results cannot be generalized to women, who may respond differently to fasting due to hormonal cycles

**No calorie restriction control group** — cannot determine if effects are due to time restriction or the small calorie deficit that occurred naturally

**No measurement of sleep quality or duration** — fasting can affect sleep, which independently impacts performance and hormones

**No measurement of training load or recovery** — differences in training between groups could explain results

**No long-term follow-up** — don't know if effects persist, reverse, or if hormonal changes lead to overtraining or injury

**Industry funding not disclosed** — but no obvious conflict of interest

**Elite athlete population** — results may not apply to recreational athletes or non-athletes who have different metabolic profiles

**No measurement of hunger, mood, or adherence** — important practical factors for anyone considering TRE

**The "protective effect" on immune cells is speculative** — the leukocyte count decreased in the control group but not in TRE; this could mean TRE prevented exercise-induced immune suppression, or it could be a statistical artifact with such small groups

For someone running their own n=1 experiment:

### What to test

**Intervention:** Time-restricted eating with an 8-hour eating window (e.g., eat only between 10:00 a.m. and 6:00 p.m.) and a 16-hour daily fast (including overnight sleep). Do NOT reduce total calories — eat your normal daily intake, just within the window.

**Dose:** 8-hour feeding window daily. Some people use 10:00 a.m. to 6:00 p.m., others prefer 12:00 p.m. to 8:00 p.m. — choose what fits your schedule.

**Comparator:** Your normal eating pattern (e.g., 3 meals between 7:00 a.m. and 9:00 p.m.) for the same duration.

### Minimum meaningful duration

**At least 4 weeks** (same as this study). Hormonal adaptations to fasting take 2-3 weeks to stabilize. For body composition changes, 4 weeks is the minimum to see measurable fat loss.

**Ideal duration:** 8-12 weeks to see if effects plateau or continue, and to assess whether hormonal changes (lower testosterone) affect training adaptations long-term.

### What to measure (specific metrics)

**Body weight:** Weigh yourself daily at the same time (morning, after bathroom, before eating). Track weekly averages.

**Body composition:** If possible, get a DXA scan before and after (most accurate). Otherwise, use calipers (skinfold measurements) or a smart scale (less accurate but tracks trends). Measure waist circumference.

**Performance:** Pick ONE test you can repeat reliably:

- For cyclists: 20-minute time trial power output (average watts), or a set hill climb time

- For runners: 5K time trial, or 1-hour distance

- For gym: 1-rep max on a key lift, or a set workout (e.g., 5x5 squats at fixed weight)

- Track power-to-weight ratio if possible (watts per kg)

**Blood markers** (if accessible): Fasted blood draw before and after — focus on free testosterone, IGF-1, complete blood count (especially neutrophils and lymphocytes), and inflammatory markers (IL-6, TNF-α, hs-CRP)

**Subjective measures:** Daily ratings of energy (1-10), hunger (1-10), sleep quality (1-10), mood, and training readiness. Use a simple app or notebook.

**Training load:** Track volume (hours, distance, or reps) and intensity (heart rate, perceived exertion) to ensure training is consistent between phases.

### Key confounds to control for

**Calorie intake:** This is the BIG one. TRE only works for body composition if you don't overeat during the feeding window. Track calories for the first week to ensure you're eating the same as usual. Many people unconsciously eat less on TRE (which is fine for weight loss but confounds the comparison).

**Training consistency:** Keep training volume and intensity IDENTICAL between TRE and control periods. If you train harder during one phase, you can't attribute changes to diet.

**Sleep:** Fasting can disrupt sleep in some people. Track sleep duration and quality. If sleep changes, it could explain performance or hormonal differences.

**Timing of training relative to eating:** In this study, cyclists trained in the morning (fasted) or afternoon (fed). Be consistent about when you train relative to your eating window.

**Hydration:** Fasting can lead to dehydration if you forget to drink water during the fast. Drink water freely during the fasting period (black coffee and tea are usually allowed).

**Menstrual cycle (for women):** Hormones fluctuate across the cycle, which affects metabolism and performance. If you're female, run the TRE phase and control