What are the limits of human endurance? The Science of "Not Being Able to Run Forever": The Metabolic Ceiling Even Top Athletes Can't Break

On October 26, 2025, a new study in Current Biology reported via Cell Press brought a conclusion to a longstanding debate. The upper limit of sustainable metabolic output for humans over a long period is approximately "about 2.5 times the Basal Metabolic Rate (BMR)"——the so-called **"metabolic ceiling"**, which even elite ultra-athletes cannot surpass. The research team tracked **14 elite athletes** participating in ultra-running, multi-day races, and triathlons, and directly estimated long-term total energy expenditure using **doubly labeled water (deuterium and oxygen-18)**. Although short-term burning can reach 6-7 times BMR, over a span of 30 to 52 weeks, it averaged out to about 2.4 to 2.5 times BMR.SciTechDaily

How it was measured: Doubly labeled water and "year-scale" observation

The key to measurement is the doubly labeled water method, which involves "mixing" heavy isotopes into the body's water. By analyzing the behavior of isotopes in urine, the CO₂ production rate is determined, and thus the actual "calories burned" can be reconstructed as a period average. Unlike conventional snapshots of single races, this study is valuable for tracking changes over several weeks. Among the participants, there were instances where, in extreme races like the Cocodona 250, the rate temporarily spiked to 6-7×BMR (7,000-8,000kcal/day), but when viewed over the long term on an annual basis, it settled near 2.5× almost without exception.SciTechDaily

Why the "2.5 times barrier" exists: The constraint of digestion and absorption as the "entry point"

What determines the "limit"? Often, heat dissipation (thermoregulation) is considered a candidate, but an analysis presented in 2019 suggested that the essential constraint is the "entry point" of "digestive and absorptive capacity". In the short term, the body can burn stored energy (muscle and fat), but in continuous exercise over several weeks to months, it cannot continue to use more than the "incoming energy", eventually leading to **body composition breakdown (emaciation)**. The 2025 follow-up study also advanced this view by showing that the long-term average ultimately converges to 2.5×BMR.PubMed Central

Updates from the new study: One-year tracking of 14 elite athletes and "individual differences"

The 2025 Current Biology paper (by Best et al.) tracked 14 top-tier athletes for up to 52 weeks, reaffirming the "effective upper limit" that, despite high short-term peaks, the average over 30/52 weeks is about 2.4×BMR. An interesting aspect is the individual differences. Some athletes, with outstanding absorption and metabolic processing abilities, can "push up" to around 2.7×, but even then, the average ceiling remains near 2.5×BMR. Andrew Best, the lead researcher, stated that "to average about 2.5×BMR over a year requires a level of 'activity' equivalent to running about 18km (11 miles) a day for a year," cautioning that for the general public, injuries would likely precede reaching the limit.Cell The nostalgic debate: Is pregnancy a "super long-term endurance event"?

A 2019 report also showed that the metabolic load in late pregnancy (about 2.2×BMR) approaches the human endurance limit

, sparking significant discussion in the media and on social media. The metaphor of a "40-week marathon" gained traction, but the focus is on **the "long-term sustainable limit" rather than "short-term peaks"**. The 2025 study, although designed with athletes in mind, consistently supports the framework of a long-term ceiling of about 2.5×BMR
.

today.duke.edu Implications for the field: Race design, nutrition strategy, and recovery1)

Be aware of the upper limit of nutritional intake
: Since there is a limit to the amount that can be "taken in" through the intestines,

short-term peak deficits (drawing from body stores)

must be repaid in the long term

. In consecutive races, long FKTs, or stage races, pacing that matches the absorbable amount

is rational.科学.org 2) Recovery planning determines success: How and where to repay the "debt state" that exceeds the 2.5×BMR ceiling. Annual plans that incorporate rest blocks and waves of intensity

will ultimately increase the total amount

.
Cell 3) It's not about heat or grit: While coping with extreme heat is important, the upper limit itself is mainly explained by the constraint of the "entry point (digestion and absorption)". Training the gastrointestinal system (alignment of solids, gels, and electrolytes) can become as much a "performance factor" as pacing strategy.

科学.org SNS Reactions: Praise, Misunderstanding, and Realistic RecalculationRegarding the 2025 update and the ongoing "2.5×BMR" debate since 2019, three main tones are repeatedly observed on social media. The convinced (practical) group: Sharing the practical sense that "for long FKTs or split long runs, one should optimize intake plans

with the 2.5×BMR barrier in mind," aligning with the context of increased gastrointestinal issues in the latter half of races.

Reddit

• Misunderstanding→Correction group: Addressing the metaphorical confusion of "pregnancy = marathon" by explaining the **"long-term average limit"** within the community.Reddit

• Number-crunching group: Highlighting the danger of fixed numerical values like "2.5×BMR = 4,000kcal/day" due to individual differences in BMR, and encouraging not to confuse long-term and short-term.Reddit

• Additionally, Cell Press's official X announced the paper, and specialized media aligned with the explanation that "when viewed over a year span, it hits a ceiling around 2.5×". Incorporating the 2019 discussion, the latest one-year tracking data reinforced the robustness of the upper limit.X (formerly Twitter)

  Making "limits" your ally: Tactical notes for amateursWhen setting **yearly "total volume goals,"** use the 2.5×BMR ceiling as a "backstop." During highlight (peak) periods, allow short-term deficits while ensuring the average stays within 2.5× through build→taper→recovery.

Strengthening gastrointestinal tolerance

(distribution of solids and gels, synchronization of salt and water, intake practice at race intensity) is a pillar equivalent to pace training

.

"Running endlessly" is not the goal

. Knowing the limit provides a basis for the "courage not to overdo it."

• Conclusion

The "wall" is not pessimism.
• Knowing the limit improves planning

  . While dreaming of short-term peaks, keep the annual average within 2.5×BMR. This is the scientific "manual" for getting stronger without breaking down.

SciTechDaily

• Cell

Reference Article

"How Far Can the Body Go? Scientists Find the Ultimate Limit of Human Endurance"

Source: https://scitechdaily.com/how-far-can-the-body-go-scientists-find-the-ultimate-limit-of-human-endurance/

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