Video summary

How Many Carbs Do You Really Need? (Lab Test Results!)

Main summary

Key takeaways

Science and Nature

Scientific concepts, discoveries, and nature/physiology phenomena

Goal: personalize carbohydrate needs for marathon fueling

  • The key question is: how many grams of carbohydrate per hour (carbs/hour) should marathon runners consume, and whether this value should be universal or individualized.
  • Two runners with different performance levels are tested at Liverpool John Moores University, in the Science in Sport lab.

Testing methodology (lab protocol)

1) Lactate threshold (LT) test

  • Pace increases every 4 minutes
  • Blood samples are taken after each stage to measure lactate responses.

Key concepts

  • Lactate production and clearance
    • At slower paces, lactate tends to clear.
    • At faster paces, lactate builds up.
  • LT1
    • A harder pace that can be maintained for a few hours.
  • LT2
    • An even higher build-up pace maintainable for about 30–60 minutes.

Interpretation

  • Lactate spikes occur before exhaustion, making them useful for predicting endurance and performance.

2) VO₂ max test

  • Begins near lactate threshold 1 pace and increases until the runner can’t continue.

Key concept

  • VO₂ max = maximum oxygen uptake and use during exercise (mL O₂ per kg per minute).
    • Higher VO₂ max suggests greater fitness potential.

Notes

  • This is a maximal and very demanding test because the runner must push until they cannot run faster.

3) Carbohydrate oxidation test

  • The runner exercises for 60 minutes at a steady effort close to LT1 pace.

Fueling during the test

  • Carbohydrate ingestion: 40 g gel every 20 minutes
  • Total mentioned for one runner: 120 g/hour

Measurements

  • GI (gastrointestinal) symptoms are rated during the test.
  • CO₂ (carbon dioxide) is measured every 15 minutes.

Key concept

  • When the body burns carbohydrates, it produces CO₂.
  • By tracking CO₂ production, the lab estimates:
    • grams of carbohydrate used per minute
    • how that reliance changes over time (shifting between fat vs carb use).

Findings: individual fueling needs differ by runner physiology and over time

Mark (6-hour marathon runner)

  • Predicted marathon capability: about 2:50 marathon
  • At race pace in fresh conditions:
    • Carbohydrate cost: 4.3 g/min
    • Total carbs for the marathon: ~645 g
  • Fueling shift over the race:
    • Fresh state: about 80% carbohydrate / 20% fat
    • Later at same speed: about 50% carbohydrate / 50% fat

Tom (sub-3-hour marathon runner)

  • Predicted marathon capability: about 5:09 marathon (based on test results)
  • At test race pace:
    • Carbohydrate cost: 3.1 g/min (about 28% less than Mark at his pace)
    • Total carbs for the marathon time: ~957 g
  • Evidence of depletion:
    • Later in the test, at the same speed, carbohydrate burning dropped:
      • from ~3.1 g/min to ~2 g/min
    • Interpreted as under-fueled, despite ingesting 120 g/hour
  • Expected consequence if under-fueling continues:
    • Increased reliance on fat (70–80%)
    • Likely harder running due to:
      • greater oxygen demand to burn fat at the same speed
      • higher rating of perceived exertion (RPE) / “heavy legs”

Practical fueling recommendations derived from the lab data

Ballpark intake

  • At least ~90 g carbohydrate/hour for both runners.
  • With experience, potentially up to ~120 g/hour.

Upper limit / risk of over-fueling

  • Presumed upper practical limit: around ~120 g/hour (≈ 2 g/min).
  • Exceeding it is associated with more gastrointestinal symptoms.

Timing frequency

  • A simple rule: fuel every 15–20 minutes
  • Example framing using gel doses:
    • 80 g/hour ≈ 2 gels/hour
    • 120 g/hour ≈ 3 gels/hour

Pre-race nutrition emphasis

  • The video stresses that breakfast and prior-day carbohydrate intake may matter more than during-run fueling.
  • If someone under-eats the day before or at breakfast, the race-day “damage” is already done (less glycogen available).

Sex differences (women vs men)

  • The video states females have:
    • less carbohydrate availability at race start due to lower glycogen storage
  • Therefore, females would likely benefit even more from carbohydrate ingestion than males.

Researchers or sources featured

  • Professor James Morton — Chief Science Officer, Science in Sport (at Liverpool John Moores University)

Original video