Take eight elite cyclists up to 2,340 meters and test them on their first day there. Their VO2max, the most oxygen their body can actually use during hard exercise, drops about 13 percent. Their staying power drops twice as far. At sea level these riders could hold a hard effort for about 43 minutes before they had to stop. On day one at altitude, the same effort ran out at around 32 minutes.
A 2007 study tested those riders at sea level and then again after one, seven, fourteen and twenty-one days at altitude. Everybody discusses altitude in terms of oxygen, because oxygen is what scientists measure. But nobody crosses a finish line on their VO2max. You cross it on how long you can keep going, and that is what altitude goes after hardest.
Which makes "just add a few minutes" bad advice for a marathon in Mexico City or Bogotá. The lab number tells you what your body can take in. The finish clock tells you how long it can keep working, and in thin air those two answers drift a long way apart.
What the data actually says
There is a rule of thumb among runners that altitude only starts to matter above 1,500 meters. Below that, the thinking goes, you are basically at sea level.
The data does not show a line like that. One study put eight endurance-trained athletes in a hypobaric chamber, a sealed room that mimics thin air without leaving the lab, and tested them at simulated heights from 300 meters all the way up to 2,800. At 800 meters, a height most runners would never think to worry about, VO2max had already dropped measurably and the athletes tired sooner. The decline from 300 to 1,300 meters ran at the same rate as the decline from 1,800 to 2,800.
No cliff edge, no free zone underneath it. Just a steady slope that starts almost as soon as you leave sea level. This is one study of eight men, so hold the exact figures loosely. But the shape is worth keeping: altitude is a slope, not a switch.
Five cities, ranked by what altitude takes from you
One warning before the list. Nobody has timed a field of sea-level runners in each of these cities and compared the results. Every percentage below is instead built by taking two published decline rates, one for VO2max and one for marathon finish times, and stretching them across each city's elevation. That makes these figures a planning range, not a prediction.
5) Medellín, Boulder and Denver, around 1,500 to 1,600 meters. The shallow end. The model puts the finish-time cost somewhere near 16 or 17 percent, which turns a four-hour sea-level marathon into something closer to 4:40. Maratón Medellín sits at this height, along with the Colfax Marathon in Denver and the BolderBOULDER 10K just up the road. You will feel it. It will not wreck you.
4) Puebla, 2,140 meters. This is where it starts to bite. The model points to roughly 23 percent on the clock, pushing that same four-hour runner past 4:55. Maratón Puebla sits close enough to Mexico City in elevation that the two races ask almost exactly the same thing of you.
3) Mexico City, 2,240 meters. Here the evidence forks, and the two branches disagree by more than a factor of three.
One answer comes from a theoretical model published by Péronnet and colleagues in 1991, built around world-record-level performance. It says the marathon cost at this elevation is about 7 percent.
The other answer comes from actual results. Lara and colleagues looked at sixteen marathons ranging from sea level up to 2,800 meters, and instead of studying the winners they studied the runners finishing 21st through 100th, on the reasoning that elites mostly race low anyway. Every 1,000 meters of elevation added about 11 percent to men's finish times and about 12 percent to women's. Stretch that to Mexico City and you get something near 24 percent.
One source says 7 percent. The other says 24. Both are careful pieces of work by serious researchers, which leaves an obvious problem: how can they both be right?
They can, because they are answering different questions. Péronnet's model describes a physiological ceiling, what thin air does to a body running at the outer limit of human performance, paced perfectly, on a good day. Lara's number describes what actually happened to real people in real races. The first is what altitude costs you in theory. The second is what it costs you in practice, and the space between the two is where bad pacing, dehydration, unfamiliar heat, and a body that trained for a course nothing like this one all quietly add their own tax.
The tempting conclusion is that ordinary runners get punished harder than elites do. Resist it. Those two figures were produced by completely different methods, and nobody has run a study that compares abilities head to head. There is even an argument for the reverse. The chamber study noted that very well-trained athletes already run their blood oxygen low at sea level, which leaves them less margin, not more.
Underneath all that uncertainty sits one usable number. If you run four hours at sea level, plan for something between 4:45 and 5:00 at the Maratón de la CDMX Telcel. It is an estimate. It is a far better estimate than four hours.
2) Bogotá, about 2,620 meters. The model climbs to roughly 28 percent on the clock, which turns a four-hour marathon into something past 5:05. Both the Media Maratón de Bogotá and VCHALLENGES42K Bogotá sit up here, and a sea-level runner arriving with their usual training paces in mind is preparing for a race that will not happen.
1) Quito and La Paz, past the edge of the map. Quito is around 2,850 meters. La Paz is around 3,640. Both sit beyond the highest elevation either study tested, so there is no number worth quoting. The body's response to thin air does not necessarily keep sloping in a neat straight line forever, and pushing the math out that far would dress a guess up as data. Maratón de La Paz is the race where the truthful answer is that nobody knows, which is itself worth knowing before you enter.
Day one is the worst day
Back to those cyclists at 2,340 meters. Day one was rock bottom. From there they clawed back ground in installments that got smaller each time: a decent recovery across the first week, another much like it across the second, and then, across the third, essentially nothing. That final week moved both measures by under two percent, small enough that the researchers could not separate it from noise.
A 2013 review of when endurance athletes should arrive at altitude turned that curve into a scheduling rule. Fourteen days gets you close to everything acclimatization has to offer. A shorter stay, anywhere from two to seven days, gets you a real but partial version of it. Beyond fourteen days you are mostly just spending time.
And you never get all the way back. That same review says it flatly: even after acclimatizing, oxygen uptake and performance do not return to sea-level values no matter how long you stay up there. Altitude takes a cut permanently. Adaptation only shrinks it.
Most runners picture acclimatization as their body quietly building extra red blood cells to haul more oxygen around. That process is real, but it is slow, and it is not what makes you feel better in week one. The response that arrives almost immediately is ventilatory, meaning how hard and how fast you breathe. The blood-building side takes days, weeks, or longer.
So two weeks at altitude does not hand you a bigger oxygen tank. It teaches you to breathe more efficiently out of the tank you already have. Worth knowing before you book a fortnight in the mountains expecting a transfusion's worth of benefit.
For everyone who cannot spend two weeks in Bogotá
Almost nobody can clear fourteen days before a marathon. The strategy built for that constraint is not a compressed version of the fourteen days. It runs the other way entirely: get there as late as you possibly can.
That same 2013 review describes professional and national soccer teams flying in as close to kickoff as the schedule allows, sometimes only hours ahead. The logic is that if you cannot outrun the altitude penalty, you can at least avoid stacking other problems on top of it: broken sleep, the drop in blood plasma volume, the chemistry shifts that come with sitting at altitude in an unadapted body. The review lists the cities where teams do this, at heights between 2,100 and 3,600 meters: Mexico City, Bogotá, Toluca, Quito and La Paz.
The catch is stated plainly there: the fly-in approach has not been rigorously tested. It was borrowed from the logistics of professional sport, not validated on distance runners, and it means racing on the exact day the data marks as your worst.
Which leaves two defensible plans. Arrive within a day of the gun and take the day-one hit in exchange for a body that has not been worn down by a week of poor sleep at 2,600 meters. Or commit to the full fourteen days and collect most of what adaptation can give you.
The middle ground is not useless. Three or four days out still buys a partial adaptation. But it is a fraction of the fourteen-day version while carrying all of the travel disruption, so go in knowing you bought a slice rather than the whole thing.
None of this changes the arithmetic from the countdown. A runner landing in Mexico City the day before the Maratón de la CDMX Telcel is not escaping the penalty, only choosing which day to pay it on. Altitude does not care whether your schedule allowed for preparation. It just asks you to move the same oxygen through thinner air, and then it takes roughly twice as much from your endurance as it takes from your capacity.
The number to plan around was never the one on the lab report. It is the one on the finish clock.
Further reading
The five papers this piece is built on, if you want the originals.
- Linear decrease in VO2max and performance with increasing altitude in endurance athletes (Wehrlin and Hallén, 2006). The hypobaric chamber study, and where the 1,500 meter rule of thumb falls apart.
- Timing the arrival at 2340 m altitude for aerobic performance (Schuler and colleagues, 2007). The eight cyclists, tested from day one through day twenty-one.
- Altitude is positively correlated to race time during the marathon (Lara and colleagues, 2014). Sixteen marathons and the finish-line data behind the 24 percent.
- A theoretical analysis of the effect of altitude on running performance (Péronnet and colleagues, 1991). The model that says 7 percent.
- Timing of arrival and pre-acclimatization strategies for the endurance athlete competing at moderate to high altitudes (Chapman and colleagues, 2013). The fourteen day rule, and the fly-in, fly-out strategy.
