A pro cyclist in full flight climbing a mountain is miraculous to behold. I was once slogging my way up a sun-blasted hill in Lanzarote when three Rabobank team members swept by on a training ride, chatting breezily to each other without a tremor of strain. Their passing was so sudden and chirpy it was like being divebombed by swifts.

On a bike these guys are elegant, but dismounted they peck awkwardly like feeding birds — a bundle of spindly angles. You sense their limbs could snap at any moment. Michael Rasmussen, the Danish road racer ditched from the Rabobank team in 2007 after missing dope tests, always had the haunted look of man on the edge of starvation.

He’s 5ft 9in and when racing weighed just 9 stone 4lb with a body fat percentage of less than 5% — hence his nickname “the Chicken”. Despite his clouded record, a lot of amateur cyclists regard Rasmussen with special distinction — he was King of the Weightweenies, the anorexics of the bike world.

Why was he so obsessed by weight? Does it make that much difference when cycling uphill for miles at a time?

Any weight reduction in body or kit will result in an energy saving. If a reasonably fit 12 stone chap with a body fat percentage of 14% (a bit better than the national average) rode a stage of the Tour de France with 2½ miles of climbing, how much easier would it be for a chap with a body fat percentage of just 5%?

The calculation by Tim Lawson, of Science in Sport, is that a rider with 5% body fat would expend 6% less energy. His equation is: work (joules) = mass x gravity x vertical distance. Dragging that extra sack of fat makes a huge difference when climbing.

Another benefit of low fat is that you’re less prone to overheating. When I was climbing that hill in Lanzarote, I was puffing like a boiler about to blow. And that’s what was happening. With the extra layer of fat I was a well-lagged boiler overheating in the afternoon sun. A pretty ugly sight, but not as ugly as the experience.

If you want to explore the far galaxies of the weight obsession have a look at www.weightweenies.starbike.com . If you’re coming at this as an innocent, forget the carbon fibre bottle cages. The most cost-effective upgrade is good, lightweight wheels — and quitting the Snickers bars. On the other hand, you can console yourself that you’ll go faster on the descents. Gravity works both ways.