Effect of tart cherry juice on recovery and next day performance in well-trained Water Polo players

The researchers tested whether drinking tart Montmorency cherry juice concentrate (90 mL daily, diluted to 600 mL total beverage) for six days would improve recovery and next-day performance compared to a placebo drink that looked and tasted similar but contained no cherry anthocyanins. The intervention was a crossover design—each athlete did both conditions, separated by a five-week washout period.

The primary outcomes were:

**Performance:** In-water vertical jump height, 10-metre sprint time, repeat sprint ability (6 × 10 m sprints), and performance on the Water Polo Intermittent Shuttle Test (WIST, a water-based equivalent of the Yo-Yo intermittent recovery test)

**Blood markers of inflammation:** Interleukin-6 (IL-6) and C-reactive protein (CRP)

**Blood markers of oxidative stress:** Uric acid (UA) and F2-Isoprostane (F2-IsoP)

**Perceptual recovery:** Daily ratings of Total Quality of Recovery (TQR) and Delayed Onset Muscle Soreness (DOMS) for upper body, upper legs, and lower legs

The comparator was a placebo drink made from lime, cranberry, and raspberry cordials mixed with food colouring and maltodextrin to match the carbohydrate content (10 g/100 mL) and appearance of the cherry juice, but containing zero anthocyanins.

Nine highly-trained male water polo players from the Western Australian Institute of Sport (WAIS) Water Polo squad. Their average age was 18.6 years (SD ± 1.4), average body mass was 82.7 kg (SD ± 9.8), and average sum of seven skinfolds was 70.7 mm (SD ± 29.7). All were competitive athletes undergoing regular structured training. Participants under 18 provided parental consent. The study was approved by the University of Western Australia Human Research Ethics Committee.

**Vertical jump (VJ):** A customised Water Polo-specific Yardstick measured how high athletes could propel themselves out of the water, extending arms upward to displace vanes. Best of three trials recorded.

**10 m sprint:** Time to swim 10 metres between two ropes, filmed at 50 Hz with a Sony HDR-HC9 digital camera. Best of two trials analysed using Dartfish video analysis software.

**Repeat Sprint Test (RST):** Six × 10 m sprints departing every 17 seconds, filmed and analysed identically to the 10 m sprint.

**Water Polo Intermittent Shuttle Test (WIST):** Repeated 2 × 7.5 m shuttles (swim out and back) at progressively increasing speeds, with 10 seconds of treading water recovery between shuttles, controlled by audio signals. Lasts approximately 14 minutes.

**Blood lactate (BLa):** Capillary blood samples collected after RST and WIST, analysed using a Lactate Pro II analyser (Arkray, Japan).

**Venous blood markers:** High-sensitivity IL-6 and CRP (inflammation), uric acid and F2-Isoprostane (oxidative stress). Collected on day 1 (pre-supplementation, pre-testing), day 6 (pre- and post-match simulation), and day 7 (pre-testing, ~12 hours post-match simulation).

**Perceptual recovery:** Daily diary completed each morning, rating Total Quality of Recovery (TQR) and Delayed Onset Muscle Soreness (DOMS) on unstated scales (likely 1–10 or similar Likert-type scales).

**Training load:** Rating of Perceived Exertion (RPE) multiplied by session duration in minutes, producing arbitrary units (AU). Average daily training load was 561 ± 108 AU for cherry juice and 572 ± 89 AU for placebo (p > 0.05, meaning no significant difference between conditions).

**Study design:** Randomised, double-blind, repeated-measures, crossover design. Each participant completed two 7-day experimental trials—one with cherry juice, one with placebo—separated by a 5-week washout period.

**Randomisation:** The order of conditions (cherry juice first or placebo first) was randomised, though the paper does not specify the randomisation method (e.g., computer-generated random numbers, coin toss, etc.).

**Blinding:** Double-blind—neither the participants nor the researchers collecting data knew which drink was being consumed in each trial. The placebo was carefully formulated to match the taste, colour, and carbohydrate content of the cherry juice. Investigators were present at every supplementation session to ensure adherence, which could theoretically have compromised blinding if investigators could tell the drinks apart, but the authors report successful blinding.

**Washout period:** Five weeks between trials. This is a reasonable duration given that anthocyanins have a half-life of only a few hours and are cleared from the body within 24–48 hours. Five weeks is more than sufficient to prevent carryover effects.

**Duration:** Each trial lasted 7 days. Supplementation occurred on days 1–6. Day 1 included baseline performance testing and blood sampling. Days 2–5 involved fixed training sessions (weights, swimming, skills, game play). Day 6 included a fatiguing match simulation (8 × 5-minute quarters) with pre- and post-exercise blood sampling. Day 7 included final blood sampling and repeat performance testing.

**Statistical approach:** The paper reports using repeated-measures ANOVA or similar (not explicitly stated in the abstract, but implied by the crossover design). They compared blood markers across time points (pre-exercise day 1, pre-exercise day 6, post-exercise day 6, day 7) and between conditions (cherry juice vs. placebo). Performance measures were compared between day 1 and day 7 and between conditions. Significance was set at p < 0.05.

**What this design can prove:** The crossover design is a strength—each participant serves as their own control, reducing the influence of individual differences in fitness, diet, sleep, and genetics. Double-blinding reduces placebo effects and experimenter bias. The fixed training schedule ensures that any differences between conditions are due to the supplement, not differences in training load. The design can prove whether cherry juice causes changes in blood markers and performance within this specific population under these specific conditions.

**What this design cannot prove:** The small sample size (n=9) means the study has low statistical power to detect small-to-moderate effects. The design cannot prove whether cherry juice works in other populations (e.g., female athletes, older adults, recreational exercisers, athletes in weight-bearing sports like running or basketball). The 7-day duration cannot rule out effects that might emerge with longer supplementation (e.g., 2–4 weeks). The design cannot prove mechanisms—it can show associations between supplementation and outcomes, but not the precise biological pathways. The single match simulation on day 6 may not replicate the cumulative fatigue of a tournament or heavy training block.

**Major methodological weaknesses:**

1. **Very small sample size (n=9):** With only 9 participants, the study can only detect large effects. If cherry juice has a small but real benefit (e.g., 2–3% improvement in recovery), this study would miss it.

2. **No power calculation:** The authors do not report a priori power analysis to justify the sample size.

3. **Retrospective trial registration:** The trial was registered on 11/08/2016, after data collection had presumably begun (the paper was published in 2016). Retrospective registration is a red flag for potential reporting bias, though the null results here make selective outcome reporting less likely.

4. **Single dose:** Only one dose of cherry juice (90 mL concentrate daily) was tested. Higher doses or different timing might produce different results.

5. **No measure of compliance beyond investigator presence:** While investigators watched participants drink the supplements, there is no mention of measuring anthocyanin levels in blood or urine to confirm absorption.

6. **Limited generalisability:** Highly-trained male water polo players are a very specific population. Results may not apply to other sports or fitness levels.

**Performance outcomes (all null):**

Vertical jump: No significant difference between cherry juice and placebo conditions. Exact values not reported in abstract; full text states "no differences were found for any performance... measures."

10 m sprint: No significant difference between conditions.

Repeat Sprint Test: No significant difference between conditions.

Water Polo Intermittent Shuttle Test: No significant difference between conditions.

**Blood markers of inflammation:**

**IL-6:** In both conditions, day 6 post-exercise IL-6 was significantly higher than pre-exercise day 6 and day 7 (p < 0.05). This means the match simulation successfully induced an inflammatory response. However, there was **no significant difference between cherry juice and placebo** at any time point.

**CRP:** In both conditions, day 7 CRP was significantly greater than day 6 pre- and post-exercise (p < 0.05). This reflects the delayed inflammatory response typical after exercise. Again, **no difference between conditions.**

**Blood markers of oxidative stress:**

**F2-Isoprostane:** In both conditions, day 7 F2-IsoP was significantly lower than day 1 and day 6 (p < 0.05). This suggests an adaptive response to the training week, but **no difference between cherry juice and placebo.**

**Uric acid:** Remained unchanged across all time points in both conditions (p > 0.05). **No difference between conditions.**

**Perceptual recovery:**

Total Quality of Recovery (TQR): No significant difference between conditions.

Delayed Onset Muscle Soreness (DOMS): No significant difference between conditions for upper body, upper legs, or lower legs.

**Training load:**

Average daily training load was similar between conditions: 561 ± 108 AU (cherry juice) vs. 572 ± 89 AU (placebo), p > 0.05. This confirms that any differences in outcomes would not be due to differences in training volume or intensity.

Because all comparisons between cherry juice and placebo were non-significant, the effect size is essentially zero for all measured outcomes. The authors do not report Cohen's d or similar effect size metrics, but the p-values were all above 0.05 for between-condition comparisons.

To put this in context: if cherry juice had a meaningful effect on recovery, you would expect to see at least a trend toward lower IL-6 or CRP in the cherry juice condition post-exercise. Instead, the inflammatory markers rose identically in both conditions. The match simulation increased IL-6 and CRP as expected, and cherry juice did nothing to blunt that response.

The F2-Isoprostane finding (lower on day 7 than day 1 in both conditions) suggests that the training week itself may have induced an adaptive antioxidant response, but this happened equally with or without cherry juice. This could mean either that (a) cherry juice has no antioxidant effect beyond the body's natural adaptation, or (b) the exercise stimulus was too mild to create measurable oxidative stress that cherry juice could mitigate.

**Acknowledged by authors:**

The non-weight-bearing, intermittent nature of water polo may not create sufficient inflammatory or oxidative stress for cherry juice to have a detectable effect.

The 7-day supplementation period may be too short.

The sample was limited to well-trained male water polo players, limiting generalisability.

**Additional critical observations:**

**Sample size (n=9):** Extremely small. With only 9 participants, the study has approximately 80% power to detect only very large effects (Cohen's d > 1.0). Smaller but still meaningful effects (e.g., d = 0.5, equivalent to a moderate improvement) would be missed.

**No female participants:** Results cannot be generalised to female athletes, who may have different inflammatory and oxidative stress responses.

**No dietary control:** Participants were not placed on a standardised diet. Differences in background dietary antioxidant intake (e.g., from berries, tea, dark chocolate) could mask or dilute any cherry juice effect. The authors do not report assessing or controlling for dietary anthocyanin intake.

**Single dose:** Only one dose (90 mL concentrate daily) was tested. Previous positive studies have used similar or slightly higher doses, but the optimal dose for water-based sports is unknown.

**Timing of supplementation:** Participants consumed cherry juice in two doses (200 mL before morning training, 400 mL in the evening). The evening dose may have been too far from the next day's testing to provide acute benefits.

**Match simulation vs. real competition:** The simulated team game activity, while designed to replicate water polo demands, may not produce the same physiological stress as a real competitive match with psychological pressure, variable pacing, and opponent interactions.

**No measure of muscle damage:** The study measured inflammation (IL-6, CRP) and oxidative stress (UA, F2-IsoP) but not direct markers of muscle damage such as creatine kinase (CK) or lactate dehydrogenase (LDH). Cherry juice has been shown to reduce CK in some studies, so this is a notable omission.

**Industry funding not disclosed:** The paper does not state whether Cherry Active (the supplement manufacturer) provided funding or product. This is not necessarily a flaw, but transparency would be helpful.

**Retrospective registration:** The trial was registered on 11 August 2016, after data collection had likely started. This increases the risk of selective reporting, though the null results make this less concerning.

For someone running their own n=1 experiment:

### What to test

Test tart Montmorency cherry juice concentrate at a dose of 90 mL daily (diluted to ~600 mL total beverage), split into two servings—one in the morning and one in the evening. Alternatively, if you do weight-bearing exercise (running, jumping, team sports on land), you might see different results than this water polo study.

### Minimum meaningful duration

Run the experiment for at least 7–14 days of supplementation before a fatiguing event, and continue for 2–3 days after. This study used 6 days of loading before the fatiguing protocol. However, some positive studies have used longer loading periods (10–14 days). For an n=1, consider 10 days of loading to be safe.

### What to measure (specific metrics)

**Performance:** If you do a sport with a repeatable test (e.g., 1RM squat, 5 km run time, vertical jump, repeated sprint test), measure it before and after the supplementation period. Use the same warm-up, time of day, and equipment.

**Recovery markers:** Rate your muscle soreness daily on a 1–10 scale (1 = no soreness, 10 = worst imaginable). Rate your perceived recovery on a 1–10 scale (1 = not recovered at all, 10 = fully recovered). Measure these at the same time each morning before eating or exercising.

**Inflammation (optional, if you have access):** If you can get blood tests, measure high-sensitivity CRP and creatine kinase (CK) 24 hours and 48 hours post-exercise. CRP should be < 1 mg/L at baseline and may rise to 2–10 mg/L after intense exercise. CK can rise to 200–1000+ U/L after muscle-damaging exercise.

**Sleep quality:** Cherry juice has been linked to improved sleep in some studies due to its natural melatonin content. Track sleep duration, sleep onset latency (how long to fall asleep), and subjective sleep quality using a sleep diary or wearable device.

### Key confounds to control for

**Dietary anthocyanins:** Avoid other high-anthocyanin foods during the experiment (blueberries, blackberries, raspberries, red cabbage, purple grapes, red wine, dark chocolate). These could dilute or mask the cherry juice effect.

**Training consistency:** Keep your training volume