Effects of antioxidant-rich foods on altitude-induced oxidative stress and inflammation in elite endurance athletes: A randomized controlled trial

The researchers tested whether doubling the usual intake of common antioxidant-rich foods (not supplements) would reduce the oxidative stress and inflammation that typically occurs when athletes train at high altitude (2,320 metres / ~7,600 feet).

**Intervention group:** Received 750 ml fruit-vegetable-berry smoothie, 50 g dried berries and fruits, 40 g walnuts, and 40 g dark chocolate (70% cocoa) daily — providing 21.2 mmol of antioxidants per day (measured by FRAP method).

**Control group:** Received 220 ml milkshake, 330 ml recovery beverage, 90 g salty and sweet crackers, and 50 g white chocolate daily — providing only 2.8 mmol of antioxidants per day.

Both groups received isocaloric foods (~1,000 kcal/day) that replaced their usual snacks. Neither group knew which condition they were in.

**Primary outcomes measured:**

Blood antioxidant capacity (FRAP, uric acid-free)

Oxidative stress (8-epi-PGF2α in urine)

Systemic inflammation (15 different cytokines and micro-CRP in blood)

**Secondary outcome:**

Acute cytokine response to a maximal exercise stress test (VO2max ramp test or 100 m swimming) before and after altitude exposure

31 elite endurance athletes (23 male, 8 female)

Mean age 23 ± 5 years (range approximately 18–35)

All were Norwegian national team athletes, including 7 World Championship medalists

4 Paralympic athletes, 27 Olympic athletes

Mean VO2max: ~67 mL/kg/min (extremely high aerobic fitness)

Mean training volume: 17–20 hours per week

All attended the Norwegian Olympic Sports Centre's annual altitude training camp in Sierra Nevada, Spain

**Key limitation for generalisability:** These are not recreational athletes or general population — they are world-class competitors with exceptional physiological baselines.

**Blood antioxidant capacity:** Ferric Reducing Ability of Plasma (FRAP) — a lab test that measures how well blood can neutralise free radicals. They used a version that subtracts the contribution of uric acid (which is itself an antioxidant) to isolate the effect of dietary antioxidants.

**Oxidative stress:** 8-epi-PGF2α measured in urine — this is a stable breakdown product of lipid peroxidation (fat molecules being damaged by free radicals). It's considered a gold-standard biomarker for oxidative stress in humans.

**Inflammation:** A panel of 15 cytokines measured in blood using multiplex technology (Luminex) — including IL1β, IL2, IL5, IL6, IL7, IL10, IL12p70, IL13, IL17, IFNγ, TNFα, MCP-1, IL1α, IL1RA, and micro-CRP (high-sensitivity C-reactive protein).

**Stress test:** Before and after altitude, athletes performed either a VO2max ramp test on a treadmill/cycle ergometer (land sports) or a 100 m maximal swim (swimmers). Blood was drawn immediately after to measure the acute cytokine response.

**Timing:** Fasting blood and urine samples were collected at 4 time points: 7 days before altitude, after 5 days at altitude, after 18 days at altitude, and 7 days after returning to sea level.

**Study design:** Parallel-group randomised controlled trial (RCT) with 1:1 allocation ratio.

**Randomisation:** Computer-generated random sequence, stratified by sport and gender. The researcher who performed randomisation was not involved in participant enrolment or group allocation — this is good practice to prevent selection bias.

**Blinding:** All researchers involved in testing and sample analysis were blinded to group allocation. Participants were not told which group they were in (though complete blinding of food interventions is difficult — participants likely noticed whether they were eating chocolate vs. crackers, but the researchers did not reveal the study hypothesis about which was "active").

**Duration:** 3-week altitude training camp at 2,320 m, with pre- and post-altitude testing at sea level. Total hypoxic exposure: 440–480 hours.

**Statistical approach:** Mixed models for repeated measures — this is appropriate for longitudinal data with multiple time points. It accounts for the fact that measurements within the same person are correlated. They tested for group × time interactions (i.e., did the two groups change differently over time?).

**What this design can prove:**

Causal relationships (because of randomisation, differences between groups can be attributed to the intervention)

Whether antioxidant-rich foods specifically alter blood biomarkers compared to control foods

Temporal effects (changes over 3 weeks at altitude)

**What this design cannot prove:**

Whether the effects translate to real-world outcomes like infection rates, performance, or recovery (these were not measured)

Whether the effects persist beyond 3 weeks

Whether the effects occur at lower altitudes or in non-athletes

Whether individual foods (e.g., just berries vs. just chocolate) are responsible — the intervention was a package of foods

Whether higher or lower doses would work better

**Major methodological weaknesses:**

1. **Small sample size** (n=31) — limits statistical power to detect differences, especially for cytokines with high variability

2. **No true placebo** — the control foods were different in texture, taste, and appearance, making perfect blinding impossible

3. **Swimmers tested 13 days post-altitude** (instead of 7 days) due to logistical issues — this introduces inconsistency

4. **Multiple comparisons** — they tested 15+ cytokines without correction, increasing the risk of false positives

5. **No performance outcomes** — we don't know if biomarker changes mattered for actual athletic performance or illness

6. **Industry sponsorship** — foods were donated by Bama (fruit/vegetable importer) and TINE (dairy company), though authors state sponsors had no role in study design or analysis

**Primary outcomes:**

**Blood antioxidant capacity (FRAP):** Increased more in the antioxidant group than controls (p = 0.034). This confirms the intervention successfully raised circulating antioxidant levels.

**Oxidative stress (8-epi-PGF2α):** Increased significantly across the whole study population during altitude (p = 0.033), regardless of group allocation. The antioxidant-rich foods did NOT prevent altitude-induced oxidative stress.

**Inflammatory markers:**

- **IL13:** Decreased in the antioxidant group while increasing in controls (p = 0.006) — a clear group difference

- **IL6:** Showed a similar trend but did not reach statistical significance (p = 0.062)

- **micro-CRP:** Larger decrease in the antioxidant group compared to controls (β = -0.62, p = 0.02)

- **All other cytokines (IL1α, IL1β, IL1RA, IL2, IL5, IL7, IL10, IL12p70, IL17, IFNγ, TNFα, MCP-1):** No significant differences between groups

**Secondary outcome (stress test response):**

The acute cytokine response to maximal exercise was significantly larger after altitude training compared to before — for IL1β, IL6, IL7, IL13, IL12p70, and TNFα (all p < 0.05)

However, there were NO differences between the antioxidant and control groups in this stress response

**Summary of what worked and what didn't:**

✅ Antioxidant-rich foods raised blood antioxidant capacity

✅ They reduced some markers of chronic inflammation (IL13, micro-CRP)

❌ They did not prevent altitude-induced oxidative stress

❌ They did not blunt the acute inflammatory response to maximal exercise

❌ Most cytokines were unaffected

**Antioxidant capacity:** The FRAP increase was statistically significant but the paper does not report the raw difference in mmol/L — only the p-value. Given the large difference in dietary antioxidant content (21.2 vs 2.8 mmol/day), the effect was likely modest in absolute terms.

**micro-CRP reduction:** The β coefficient of -0.62 means that, on average, the antioxidant group's micro-CRP dropped by about 0.62 units more than controls. For context, micro-CRP is measured in mg/L; a change of 0.6 mg/L is considered a small-to-moderate effect. To put it in perspective: this is roughly the difference between someone with optimal cardiovascular risk (CRP < 1 mg/L) and average risk (CRP 1–3 mg/L).

**IL13:** This cytokine decreased in the antioxidant group while increasing in controls. IL13 is involved in allergic inflammation and immune regulation. The magnitude of the difference was not reported as a raw value, only as a significant group × time interaction.

**Oxidative stress (8-epi-PGF2α):** Increased by an unspecified amount across both groups at altitude. The fact that the antioxidant intervention did NOT blunt this suggests that altitude-induced oxidative stress is driven by mechanisms (e.g., mitochondrial reductive stress, xanthine oxidase activation) that dietary antioxidants cannot easily counteract.

**Practical translation:** If you're an athlete training at altitude, eating antioxidant-rich foods might slightly lower your baseline inflammation (especially CRP and IL13), but it won't prevent the oxidative damage caused by high altitude itself. The effect is real but modest — not a game-changer.

**Acknowledged by authors:**

Small sample size limits statistical power

Swimmers' post-altitude testing was delayed (13 days vs 7 days)

No assessment of dietary compliance beyond self-report and food provision

The study was not registered in ClinicalTrials.gov before enrolment began (registered retrospectively)

The intervention was a package of foods — cannot isolate which component was responsible

**Additional critical limitations:**

**No performance or illness outcomes:** The study measured biomarkers only. We don't know if the reduced inflammation translated to fewer infections, better recovery, or improved performance — which is ultimately what athletes care about

**Short duration:** 3 weeks at altitude is typical for training camps, but longer-term effects are unknown

**Elite athlete population only:** Results may not apply to recreational athletes, non-athletes, or people with different baseline diets

**No true blinding of participants:** While they weren't told the hypothesis, eating chocolate vs. crackers is distinguishable

**Multiple comparisons problem:** Testing 15+ cytokines without correction means some "significant" results could be chance findings (the IL13 finding, while p=0.006, should be interpreted cautiously)

**No washout period:** This was a parallel design, not crossover — so individual variation between groups could influence results despite randomisation

**Baseline diet not tightly controlled:** Athletes ate their usual meals at the training centre; only snacks were controlled. Differences in baseline antioxidant intake could have diluted the effect

**Funding source:** Food donations from commercial companies (Bama, TINE) create at least the appearance of conflict of interest, even if authors state no role in study design

For someone running their own n=1 experiment (e.g., an athlete or fitness enthusiast planning an altitude training camp):

### What to test

**Intervention:** Double your usual intake of antioxidant-rich whole foods — specifically berries (fresh or dried), dark chocolate (≥70% cocoa), walnuts, and a fruit/vegetable smoothie. Aim for approximately 20 mmol/day of total antioxidants (roughly equivalent to: 1 cup mixed berries + 30g dark chocolate + handful walnuts + 500ml green smoothie)

**Comparator:** Your normal diet or a matched-calorie control (e.g., crackers, white chocolate, milkshake)

**Dose:** The study used ~1,000 kcal/day of antioxidant foods — this is substantial. A scaled-down version (half the dose) might still be worth testing

### Minimum meaningful duration

**At least 3 weeks at altitude** (the study duration)

**Measure before, during (at 1 week and 3 weeks), and 1 week after returning to sea level**

The effects on CRP and IL13 appeared by day 18 at altitude — shorter camps (<2 weeks) may not show effects

### What to measure

**Primary:** High-sensitivity CRP (hs-CRP) — this is available through standard blood tests and is the most clinically relevant marker they found affected

**Secondary:** Subjective recovery scores, illness incidence (colds, sore throats), training quality (how workouts felt)

**Optional but harder to access:** Blood antioxidant capacity (FRAP test), urinary 8-epi-PGF2α (oxidative stress), cytokine panel (expensive and not routinely available)

**Track:** Daily energy intake, sleep quality, training load, and any illness symptoms

### Key confounds to control for

**Total calorie intake:** The study matched calories between groups — if you eat more antioxidant foods without reducing other snacks, you'll gain weight, which independently affects inflammation

**Training load:** Altitude training camps typically increase training volume and intensity. Track your training load (e.g., heart rate, RPE, duration) to ensure any changes aren't due to training differences

**Sleep quality:** Altitude disrupts sleep, and poor sleep increases inflammation. Measure sleep (e.g., with a wearable) as a potential confound

**Hydration status:** Dehydration concentrates blood markers. Standardise hydration before blood draws

**Iron status:** Altitude training affects iron metabolism. If you're iron-deficient, this could independently affect inflammation and performance

**Time of day for blood draws:** Cytokines have circadian rhythms. Always test at the same time of day, fasted

### What a positive result would look like

**CRP:** A decrease of ~0.5–1.0 mg/L from your pre-altitude baseline, while your usual diet at altitude would show no change or an increase

**Subjective:** Fewer upper respiratory tract infections during and after the camp, better perceived recovery between training sessions

**Antioxidant capacity:** A measurable increase in blood FRAP (if you can access this test)

**Important caveat:** Even if you see these changes, the study suggests you should NOT expect protection against altitude-induced oxidative stress (measured by 8-epi-PGF2α) — so don't be discouraged if that marker doesn't improve

### Bottom line for self-experimenters

This is a low-risk, high-reward intervention. Eating more berries, dark chocolate, and walnuts is generally healthy, and the study provides reasonable evidence that it may reduce altitude-induced inflammation (especially CRP and IL13). The effect is modest — don't expect dramatic performance improvements. But given that altitude training already stresses the immune system, and that infections can derail a training camp, this dietary strategy is worth trying if you're planning a 3+ week stay at moderate altitude (2,000–3,000 m). The key is to replace other snacks (not add calories) and to measure CRP before, during, and after to see if you personally respond.