Training in heat and at altitude: what acclimation really does

Heat and altitude are the two environmental factors most likely to humble a fit athlete, and the two most surrounded by gimmicks. Cooling vests, altitude tents, expensive masks that promise thin-air gains at sea level: most of it oversells. The real adaptations are well understood, genuinely powerful, and mostly free. They just require time and patience rather than a purchase.

Heat: a real, fast, useful adaptation

Heat is brutal because your body has to cool itself and fuel the effort at the same time, and cooling wins. Blood diverts to the skin, heart rate climbs at any given pace, and performance falls. The good news is that heat acclimation is one of the most reliable adaptations in sport. Train in the heat for one to two weeks and your body responds: you sweat earlier and more efficiently, your blood plasma volume expands, and your core temperature runs lower for the same effort.

You get there by exposure. Repeated sessions in the heat, starting easier and shorter and building up, drive the change within about two weeks. Manage it sensibly, hydrate well, respect the early sessions, and back off if you feel genuinely unwell rather than merely hot.

Heat acclimation is not a gadget you buy. It is two weeks of sensible exposure, and it works remarkably well.

Altitude: real, slower, and easy to get wrong

At altitude the air holds less oxygen, so everything feels harder and your sustainable power drops until you adapt. The body's response is to build more red blood cells to carry oxygen more effectively, but this takes weeks, not days, and needs meaningful time actually spent up high. A long weekend at a race does not deliver the blood adaptation; it mostly just makes you slower while you are there.

A few honest points cut through the marketing:

• Arrive early or arrive late. For an event at altitude, either get there long enough to adapt, a couple of weeks or more, or arrive right before it, within a day, before the worst of the short-term decline sets in. The awkward middle is the worst of both.
• Ease off on arrival. Your first days up high are for adjusting, not for hard training. Cut intensity and let your body catch up.
• Be skeptical of sea-level masks. Restricting airflow is not the same as breathing thin air, and it does not reliably reproduce altitude's blood adaptations. It mostly trains your breathing muscles, which is a modest thing.

Plan for conditions, do not just endure them

The through-line for both is preparation over suffering. If you know a hot or high event is coming, build the adaptation deliberately in the weeks before, and pace conservatively when you get there, because your usual numbers will not apply. Conditions are not something to grit through blindly; they are something to train for.

Train Fondo accounts for heat and altitude rather than pretending every day is temperate at sea level. It can build a heat-acclimation block into the weeks before a hot event and adjust your targets and expectations when you are training in demanding conditions, so you adapt on purpose instead of just getting cooked.