Alan Couzens, MS (Sports Science)
With the recent introduction of 2 new high altitude races to the Ironman calendar, the challenge of Ironman racing just reached a new level. Now, not only must athletes overcome the challenges of optimal pacing (over many hours), nutrition, heat, etc. but an additional variable has been thrown into the mix – the reduction of the partial pressure in O2 that comes with increasing altitude.
Considerable research has been conducted on the effect of altitude on high intensity performance. Much of this was inspired by the 1968 Olympics, which were held in Mexico City, at an elevation of ~8000ft. The general consensus of this research is that this reduced partial pressure results in a decreased O2 saturation within the blood and a concomitant decrease in VO2max. Specifically, for the non-acclimated, a decrease of ~2% per 1000ft of altitude after the first 1000ft (Perronet et al., 1991). For an altitude of 8000ft, a reduction in VO2max of 14%!
However, as we know, Ironman is not an Olympic event. In fact, the longest Olympic event, is only 2 and a bit hours long, a duration that is still greatly affected by VO2max. If we look at the men’s marathon in the Olympics, the winning time of 2:20 was a full 9% slower than the, then, world record of 2:09. In practice this leaves the impact of altitude on truly long duration performance still a little bit of a mystery. An important mystery to solve for those looking to get their best possible performance out of a race like Tahoe (~6200’) or Boulder (~5400’)
Long duration events like Ironman are fundamentally metabolically limited. The athlete who wins is not necessarily the one with the highest VO2max but rather the one who can sustain a high % of this VO2max for very long periods of time. A large part of this comes down to sparing their precious carbohydrate reserves and improving their ability to generate energy from fat – a virtually limitless energy substrate.
The impact of high altitude on Ironman performance, then, becomes a question of “how does altitude affect our body’s metabolism?” Or, more specifically, do we burn more carbohydrate for a given intensity as altitude increases? The limited research to date would suggest that the answer is a big “yes”!
In one of the most comprehensive data sets on the metabolic response to altitude in endurance athletes, Wehrlin & Allen (2005), found that the athlete’s respiratory quotient increased by an average .08 for the same absolute submaximal exercise intensity (55% vVO2max) when altitude was increased from 2600 to 9200 feet. This seemingly small change corresponds with an increase in carbohydrate oxidation from 53% to 82%! This has direct, linear implications on the athlete’s time to fatigue. In other words, for a given athlete, we would expect their time to fatigue in a metabolically limited activity at an altitude of 9200 feet to be almost half of what it is at sea level!
The data from that study along with the change in time to fatigue for a hypothetical athlete, with CHO stores of 2500kcal, & 200W of mechanical output at various altitudes is shown below.
While this presents a neat model for a hypothetical athlete, in reality, athletic response to altitude is quite varied. For example, the standard deviation in CHO burning in the study mentioned above was ~+/-8%!. Meaning that an average, non acclimated athlete could be generating anything from 74-90% of their energy from CHO at an altitude of 9200ft. Factors that have a significant impact here are..
· Gender/Size – females & smaller athletes appear to be less metabolically impacted by altitude than males or larger athletes (Amiel et al., 1993; Stary-Gunderson et al., 2000)
· Fitness – fitter athletes are more affected by altitude than less fit athletes (Gavin et al., 1998)
If we throw in the impact of acclimatization, this range broadens by an additional ~5%.
So, what is an athlete without a good amount of altitude experience (or altitude testing) to do? How should an athlete adjust their personal pacing plan when confronted with an unfamiliar altitude? How do they discern where on the spectrum of the metabolic impact of altitude he or she lies?
If we look at the Heart Rate column in the table above, there may be a clue. The difference in fat vs carbohydrate burning is strongly indicated by a difference in heart rate, even when the absolute intensity of the effort doesn’t change (remember, the intensity at all altitudes in the above study was a fixed 55% of sea level vVO2max). In fact, if we were to just look at the heart rate and fat burning columns in the above chart, it looks a whole lot like the results we would see in a progressive fuel test, where power (rather than altitude) is progressively increased. As HR (/stress) goes up, CHO expenditure goes up somewhat irrespective of whether that stress is caused by exercise intensity or environmental conditions.
Put simply, an athlete who is seeing half ironman heart rates shouldn’t expect to be able to hold that effort for an additional 5 or 6hrs just because they’re a few extra thousand feet up in the air!
Rather, the smart athlete will adjust their power/pace in response to this heart rate feedback.
The smarter athlete, still, will attempt to narrow that gap in altitude vs sea level heart rates by acclimatizing prior to the event. In fact, McLelland et al. (1998) found that carbohydrate utilization at altitude is the same as it is at sea level for the same % of relative VO2max. In other words, if you narrow the gap between your altitude and sea level VO2max via altitude training, you will also narrow the gap in CHO oxidation. While the details of acclimatization strategies extend beyond the scope of this article, the use of sleep tents, intermittent hypoxic exposure masks & altitude training trips close to the event can all have significant positive effect.
In conclusion, if you’re a sea level athlete racing an ironman at altitude…
· Pay extra attention to heart rate during the event. Recognize that increases in HR reflect a quicker usage of your body’s precious CHO fuel.
· Acclimatize! Non acclimated athletes will be at a significant disadvantage, both in terms of oxygen transport and metabolic efficiency.