Sports Drinks on the Edge of a New Era

This is a systematic review, not a single experiment. The authors synthesised historical and contemporary research on two main questions:

1. **Hydration and performance:** Does dehydration during exercise impair endurance performance, and if so, at what threshold? The review compares the "cardiovascular model" (which says dehydration reduces plasma volume, increases cardiac strain, and impairs performance) against the "central governor" and "anticipatory thermoregulation" models (which say the brain downregulates muscle recruitment to prevent heat stroke, and that dehydration is a *consequence* of high performance, not a cause of impairment).

2. **Carbohydrate delivery:** What is the optimal type, concentration, and delivery rate of carbohydrate (CHO) for endurance exercise? The review compares glucose-only drinks, maltodextrin-only drinks, and multi-transportable carbohydrate blends (maltodextrin + fructose) at various ingestion rates (0.5 g/min to 2.7 g/min).

Outcome measures included: exogenous carbohydrate oxidation rate (g/min), gastrointestinal comfort, running performance (finish time), core temperature, plasma volume changes, and percentage body mass loss due to dehydration.

This review synthesises data from dozens of studies spanning 1904–2018. Specific studies cited include:

**Wyndham and Strydom (1969):** 20 marathon runners during two races in hot conditions.

**Costill et al. (1970):** Participants running for 2 hours at 70% of maximal oxygen uptake (V̇O₂max).

**Jeukendrup et al. (multiple studies):** Trained cyclists and runners, sample sizes typically 8–12 per study.

**Systematic review by Stellingwerff and Cox (2014):** Multiple studies on CHO intake and running performance.

The populations are predominantly healthy, trained endurance athletes (runners and cyclists), aged approximately 18–45, with no reported medical conditions. No specific data on sex distribution, ethnicity, or baseline fitness levels are provided in this review.

The review does not report original measurements but summarises instruments used across included studies:

**Body mass loss:** Pre- and post-exercise body weight (kg), converted to percentage dehydration.

**Core temperature:** Rectal or oesophageal thermistors (continuous monitoring).

**Skin temperature:** Skin thermistors placed at multiple sites.

**Cardiac output:** Indirect methods (e.g., CO₂ rebreathing or Doppler echocardiography).

**Plasma volume:** Blood samples before and after exercise, haematocrit and haemoglobin concentrations.

**Exogenous carbohydrate oxidation:** Stable isotope tracers (¹³C-labelled glucose or fructose) measured in expired breath and blood samples.

**Gastrointestinal comfort:** Self-reported scales (specific instruments not detailed).

**Running performance:** Finish time (minutes) in competitive or laboratory-simulated events.

**Study design:** This is a narrative systematic review and historical perspective. It does not follow a formal PRISMA protocol or meta-analytic framework. The authors selected studies they consider landmark or representative, rather than conducting an exhaustive systematic search with explicit inclusion/exclusion criteria.

**Key studies discussed:**

**Wyndham and Strydom (1969):** Observational field study of marathon runners. No randomisation, no blinding, no control group. Measured body mass loss and core temperature during two races. Found fastest runner had 4.3–4.8% dehydration. **What this can prove:** Correlation between dehydration and performance in real-world conditions. **What it cannot prove:** Causation—the fastest runner may have been fastest *despite* dehydration, not because of it.

**Costill et al. (1970):** Crossover design. Participants ran 2 hours at 70% V̇O₂max on two occasions, ingesting either CHO-electrolyte drink or water. Order of conditions not specified as randomised. **What this can prove:** Within-subject differences in metabolic responses. **What it cannot prove:** Blinding was impossible (taste differences), so placebo effects cannot be ruled out.

**Jeukendrup et al. (multiple studies):** Randomised crossover designs with 8–12 participants. Typically single-blind (participants unaware of drink composition). Washout periods of at least 7 days between trials. **What this can prove:** Causal effects of CHO type and dose on oxidation rates. **What it cannot prove:** Generalisability to real-world competition conditions (laboratory cycling vs. outdoor running).

**Duration:** Individual studies ranged from 60 minutes to marathon distance (2–4+ hours). No study exceeded a single exercise bout.

**Statistical approach:** Not described in detail. The review reports ranges and maxima (e.g., "maximum of 1.8 g/min" exogenous CHO oxidation) rather than meta-analytic effect sizes with confidence intervals.

**Major methodological weaknesses of this review:**

1. **No systematic search strategy:** The authors do not report databases searched, search terms, or inclusion/exclusion criteria. This introduces selection bias—they may have preferentially included studies supporting their preferred model.

2. **No quality assessment:** Studies are not rated for risk of bias. The review treats a 1969 observational field study and a 2010s randomised crossover trial as equally informative.

3. **No quantitative synthesis:** No forest plots, no pooled effect sizes, no heterogeneity statistics. Claims like "the optimum mix of CHO is a maltodextrin and fructose approaching a 1:1 ratio" are based on narrative summary, not meta-analysis.

4. **Historical framing bias:** The review is structured as a "progress narrative"—older ideas (drink to match sweat loss) are presented as naive, newer ideas (tolerable dehydration, central governor) as enlightened. This may overstate the evidence for newer models.

**Hydration and performance:**

The fastest runner in the Wyndham and Strydom (1969) marathon study lost 4.3% body mass in the first marathon and 4.8% in the second—the *most* dehydrated runner was the *fastest*.

The slowest runner lost only ~1.9% body mass—the *least* dehydrated runner was the *slowest*.

In a review of multiple studies plotting running speed vs. percentage dehydration, the best-performing runner was dehydrated by ~8%, while the runner who prevented body mass loss (only ~0.8% loss) was the slowest.

Current consensus (ACSM 2007) suggests "tolerable dehydration" up to ~2% body mass loss before performance is impaired—but the review notes this threshold is based on correlational data, not causal experiments.

Elevated skin temperature alone did not impair exercise performance; elevated whole-body temperature (core + skin) did impair performance.

**Carbohydrate delivery:**

Glucose-only ingestion at ~2.4 g/min yields exogenous CHO oxidation of ~1.1 g/min—the rest is not oxidised (likely stored or causes GI distress).

Maltodextrin + fructose in ~1:1 ratio increases exogenous CHO oxidation to a maximum of ~1.8 g/min—a **64% increase** over glucose-only.

This works because glucose is absorbed via SGLT1 and GLUT2 transporters, while fructose uses a separate transporter (GLUT5). Using both transporters simultaneously increases total absorption capacity.

Current ACSM guidelines recommend 30–60 g/h for exercise lasting 1–2.5 hours, and up to 90 g/h for ultra-endurance (>2.5 hours).

CHO beverages with 5.0–6.9% concentration are most beneficial for running performance. Above this concentration, GI discomfort increases due to physical distortion of the GI tract during running.

One study found no additional benefit of CHO intake above ~60 g/h when participants ate breakfast before exercise—suggesting pre-exercise nutrition modifies the effectiveness of mid-exercise CHO.

**Exogenous CHO oxidation:** Switching from glucose-only to maltodextrin-fructose (1:1 ratio) increases the rate from ~1.1 g/min to ~1.8 g/min. This means an additional ~42 grams of carbohydrate can be used as fuel per hour of exercise—roughly equivalent to an extra 168 calories of usable energy.

**Dehydration threshold:** The "2% body mass loss" threshold is not a precise cut-off. In the studies reviewed, athletes losing 4–8% body mass still performed at elite levels. The effect of dehydration on performance appears highly individual and context-dependent.

**GI discomfort:** Above 6.9% CHO concentration, GI issues increase noticeably during intense running lasting 60–90 minutes. The review does not provide exact prevalence rates.

**Acknowledged by authors:**

The review is a "perspective" rather than a formal systematic review—it aims to stimulate new research paradigms, not provide definitive conclusions.

The authors note that "deciding which [model] is closer to the truth is conceptually difficult because we use the brain to study the brain."

They acknowledge that the fastest, most dehydrated runners in historical studies *might* have benefited from better hydration—or might have benefited from greater dehydration (e.g., improved running economy).

**Not acknowledged but critical:**

**No systematic search:** Without a reproducible search strategy, the review cannot claim to be comprehensive or unbiased.

**Industry funding:** Sports drink research has historically been funded by companies with a commercial interest in promoting hydration products. The review does not disclose funding sources for included studies.

**Population limits:** Almost all studies used trained male endurance athletes. Results may not generalise to women, older adults, recreational exercisers, or athletes in non-endurance sports.

**Laboratory vs. field:** Many CHO oxidation studies were done on stationary cycles in climate-controlled labs. Real-world conditions (heat, hills, pacing variability, competition stress) may alter results.

**No blinding in early studies:** The 1969 marathon study and 1970 Costill study had no blinding. Placebo effects cannot be ruled out.

**Duration limits:** No study exceeded a single exercise bout. Effects of repeated-day hydration or CHO strategies (e.g., during multi-day events) are not addressed.

**Individual variability:** The review does not discuss genetic or microbiome differences that might affect CHO absorption or hydration needs.

For someone running their own n=1 experiment:

### What to test (specific intervention and dose)

**Primary test:** Compare a maltodextrin-fructose sports drink (~6% concentration, 1:1 ratio) against a glucose-only sports drink (same concentration) during endurance exercise lasting 60–120 minutes.

**Dose:** Aim for ~60 g carbohydrate per hour (e.g., 1 litre of 6% solution per hour, consumed in 150–250 ml servings every 15–20 minutes).

**Optional test:** Compare drinking to thirst vs. drinking to replace 100% of sweat losses. Use the "2% body mass loss" as a reference point—try to stay above or below this threshold and see which feels better.

### Minimum meaningful duration

**For CHO comparison:** At least 3–5 sessions per condition (total 6–10 sessions) to account for day-to-day variability in performance, GI comfort, and hydration status.

**For hydration comparison:** At least 5 sessions per condition, ideally during similar weather conditions (temperature within 5°C, humidity within 10%).

**Session length:** Minimum 60 minutes at moderate-to-high intensity (70–80% max heart rate). Longer sessions (90–120 minutes) will show larger differences.

### What to measure (specific metrics)

**Primary outcome:** Perceived exertion (RPE, 6–20 Borg scale) every 15 minutes. A difference of 1–2 points is meaningful.

**Secondary outcomes:**

- GI comfort (1–10 scale, 1 = no issues, 10 = severe nausea/cramping)

- Power output or pace (if using GPS or power meter)

- Heart rate (average and drift over session)

- Body weight before and after (to estimate sweat loss)

- Thirst sensation (1–10 scale)

- Subjective energy levels (1–10 scale)

**Optional:** Blood glucose finger-prick test before, during (at 30 min intervals), and after exercise.

### Key confounds to control for

**Pre-exercise nutrition:** Eat the same meal 2–3 hours before each session (same composition and timing). The review notes that breakfast before exercise reduces the benefit of mid-exercise CHO.

**Hydration status:** Start each session euhydrated (same pre-exercise body weight within 0.5 kg). Urine colour should be pale yellow.

**Time of day:** Test at the same time of day (±1 hour) to control for circadian effects on performance and digestion.

**Temperature and humidity:** Record weather conditions. Differences >5°C or >20% humidity will confound results.

**Sleep and stress:** Log sleep quality (hours and subjective rating) and daily stress (1–10 scale). Poor sleep or high stress can independently impair performance.

**Blinding:** Use opaque bottles and have someone else prepare the drinks so you don't know which condition you're in. If taste differs, add a flavour mask (e.g., sugar-free cordial) to both drinks.

**Order effects:** Randomise the order of conditions (e.g., coin flip for each session). Do not do all glucose sessions first, then all maltodextrin-fructose sessions.

### What a positive result would look like

**For CHO comparison:** You consistently report lower RPE (by ≥1 point) and higher energy (by ≥1 point on a 1–10 scale) with the maltodextrin-fructose drink compared to glucose-only, with no increase in GI discomfort.

**For hydration comparison:** You find that drinking to thirst (rather than forcing fluids to match sweat loss) results in similar or better performance, with less GI discomfort and no increase in perceived heat stress. Your body weight loss after exercise is 1–3% rather than 0–1%.

**Practical benchmark:** If the maltodextrin-fructose drink allows you to maintain pace or power output in the last 20 minutes of a 90-minute session when you would normally fade, that's a meaningful improvement.

**Caveat:** Individual responses vary. Some people absorb glucose well and see no benefit from fructose addition. Some people are "salty sweaters" and need electrolyte replacement more than CHO. Run the experiment for yourself—don't assume the group average applies to you.