Two and half times resting metabolic rate seems to be the limit of human endurance worked out by scientists.
The study by Duke University included analysing a 3,000 mile run, the cyclists in Tour de France and other elite events.
The 2.5 x metabolic rate is approximately 4,000 calories per day, with anything higher than that being unsustainable in the long term. The scientists believe that the 2.5 x is down to the digestive system rather than muscle, respiration or heart health.
They found the body cannot digest, absorb and process enough calories and nutrients to sustain a higher level of energy use.
The study found a pattern between the length of a sporting event and energy expenditure – the longer the event, the harder it is to burn through the calories.
So people can go far beyond their base metabolic rate while doing a short stints of exercise, it becomes unsustainable in the long term. The body can use up its own resources burning through fat or muscle mass – which can be recovered afterwards – in shorter events.
The study also shows that while running a marathon may be beyond many, it is nowhere near the limit of human endurance.
- One marathon – runners used 15.6 x their resting metabolic rate
- Cyclists during the 23 days of the Tour de France used 4.9 x their resting metabolic rate
- A 95-day Antarctic trekker used 3.5 x the resting metabolic rate
The body will seemingly endure intense stuff for a short time, but as the study showed, long-term it is not sustainable. This highlights the importance of timing and intensity of training regimens and competition and the real importance of scheduled rest periods.