Reports say with riders burning up to 10,000 calories per day on the Tour de France as the race heads into the Alps this weekend, the right fuel is essential to avoid the dreaded 'bonk'.
It's the equivalent of marathon runners 'hitting the wall', when a lack of glycogen in their muscles results in severe fatigue.
The lack of power caused by a lack of food happened to Alberto Contador, a favorite to win this year's Tour, in March during the seventh stage of the Paris-Nice race.
"At 15km to go, my body was completely empty," remembers Contador whose "bonk" cost him his minute-long lead and eventually lost him the race.
"I was left without any strength. From that moment, the goal was just to finish."
For Tour team Cervelo, who have 2008 Tour winner Carlos Sastre as their leader, nutritionist Rob Child keeps an eye on his athletes' diets to ensure the riders reach their peak on the mountain stages.
Having worked with a wide range of athletes, including Britain's 2003 world champion swimmer Katy Sexton, plus Sydney 2000 Olympic gold medal-winning boxer Audley Harrison, Child is on his third major race with Cervelo.
He says a typical rider will come into the Tour de France carrying around seven percent body fat, a slim woman is around 20 percent, which drops to about four by the end of three-week endurance race.
"If a stage has three big climbs, we'd expect riders to burn off anything between 8,000 to 10,000 calories per day," said Child.
"But if you sit in the peloton all day with a heart rate of around 120 beats per minute, you may only burn off 300 calories per hour.
"So of course it depends greatly on the the stage and the rider."
The battle to load up with enough calories begins hours before the race start, when riders tackle mountains of pasta, cereals, eggs, rice and other energy-providing carbohydrates in varying quantities.
Seven-time Tour winner Lance Armstrong has been known to tuck into rice and eggs, while Child says Cervelo's chef Willy Balmat, who has cooked for Armstrong, specializes in omlettes and porridge packed with fruit and nuts.
"A lot of eating on Tour has a long tradition. For example, the chef may not be able to tell you why porridge is good for the riders, but the science behind the tradition is solid enough," he said.
"It is tried and tested and you have to respect that.
"Most riders like as much sleep as possible, while others have a set routine, but they will all have eaten at least three hours before the race."
In the last few hours before the stage's start, Child tests the riders urine to make sure they are hydrated enough and electrolyte drinks, rich in minerals to counter the effects of sweating, are handed out.
When the racing begins each stage has a feeding section when riders are handed 'musettes', bags full of fuel in the shape of small sandwiches, protein bars, gels, caffeine gels and bananas, which the riders eat in the saddle.
The evening meal is when most riders prefer to load up on carbohydrates, but, surprisingly, alchohol is on the menu.
"There are only a few things I don't like them eating," said Child.
"Cheese is the main thing, because it's high in fat and takes a long time to digest, but the riders are pretty good generally.
"For example, a few of them were eating chocolate after a mountain stage, but what is a few hundred calories when they have been burning off thousands?
"There is nothing wrong with a little wine and I don't think a regimented regime helps, if you are too hard on them they might go off the rails."
Cervelo's New Zealander Hayden Roulston says it is all about eating tried and tested food on Tour rather than experimenting.
"We all know our own bodies and know what to eat and when you are on the bike, you eat as much as you want really," he said.
"It's hard to say how much each rider burns, but you definitely chew through the calories when you are doing between five and six hours in a bike.
"The evening meal is the most important meal for me.
"It's important to eat the same thing as you do all year around."
And when this year's race is over, what will Roulston tuck into as his post-Tour treat?
"Sushi, barbecues and lots of wine, that is the main stuff I am missing most," grinned the Kiwi.