Hypoxic training is a fancy-gym fad, along classic fad principles. Elite athletes, the thinking goes, train at altitude to improve their performance at sea level; ergo, if a non-elite athlete were to exercise in a hypoxic chamber (a sealed room in which the oxygen level has been decreased), then that person’s workout would improve.

In normal life, I would have gone to a Virgin Active, or Third Space in central London, done some cross-training in a hypoxic area, and reported back that it was incredibly hard. But just on a whim, I thought I’d check how it worked, which is how I fetched up in the hypoxic chamber of UCL’s Institute of Sport, Exercise and Health, with the intensive care consultant Dr Dan Martin.

“In gyms where they have a hypoxic area, if you go in there and do your normal training session – say you cycle for 20 minutes – you will find it much harder and you will feel wrecked when you come out,” Martin tells me. He fixes me with his straight-talking, intensive-care face. “You will feel like you worked harder, but you will not have worked harder. This is simple physics and physiology. You will feel terrible and your heart rate will be super-high, but that’s just because your heart is working harder to get your blood round your body.”

OK, but what about the whole rich sporting heritage of going to Kenya for a month, then coming back and winning the Olympics?

“There is evidence of a small benefit from living high and training low,” he says. That is, having a hypoxic area for resting, triggering a cascade of useful adaptations to improve the body’s uptake of oxygen (higher haemoglobin, more capillaries), and then training at low altitude. It is a world away from 20 minutes in a gym chamber, and also the opposite way around.

So why the hypoxic fad?

“Because athletes are desperate to try anything that’s legal.”

After 10 minutes in UCL’s chamber, my blood oxygen level was down 16% and I felt ropey – not breathless, but sluggish and oppressed. I came out starving and dumb, went into Yo! Sushi and had the same dish three times because I was outwitted by the conveyor belt. It’s a bit like being pregnant.

The persistent misconception that high altitude is good for you relates, I think, to the erroneous conviction that anything painful must be gainful. In that sense, the fitness world can – doesn’t always, but can – operate as a shadow science. It uses the same language, but it doesn’t mean the same thing – and often means nothing at all. This is probably quite obvious from far away; but the closer you get to these trends, the more convincing they are, simply because so much money is sunk into them, and so much sweat.

What I learned

If you spend time at serious altitude, you lose a huge amount of weight, however much you eat. But it’s all muscle