The Tour de France is one of the world's most demanding events of physical endurance. In its 100th edition, this year's Le Tour encompasses 21 stages, totalling 3479km, including six mountain stages with climbs as high as 2000m.
The time taken to complete individual stages ranged from 51 minutes and 24 seconds (stage nine: a flat time trial over 38km, with a mean speed of 48.4km/h) to five hours, 42 minutes and 46 seconds (stage 12: a medium mountain-stage over 226km, with a mean speed of 39.5km/h) with an overall mean race-pace of 39.9km/h.
Given such speed and consistency of pace hydration and nutrition are crucial.
Most Tour cyclists will need to consume 2 litres of fluid an hour to stay hydrated.
Excessive fluid loss can lead to a loss of power, so it's recommended that any athlete engaged in vigorous exercise minimises fluid losses to no more than 2-3 per cent of body mass during exercise; electrolytes (such as sodium and potassium) are also lost in the sweat and should be replaced if sweating is heavy and exercise prolonged.
But here's where things get a bit tricky.
In a sport where a 1 per cent change in performance could be the difference between wearing the coveted yellow jersey and second place, fluid over-replenishment may also equate to carrying unnecessary weight. A 70kg rider who has lost two litres of sweat (about 3 per cent of body mass) has less mass to push uphill. So that rider's performance should be better on climbs, as long as his power output is not compromised by the mild dehydration.
Consuming enough dietary carbohydrates before, during and after exercise is also crucial. We store carbohydrates predominantly as muscle and liver glycogen, and a tiny amount as blood glucose. These are the key fuels for muscle contraction during intense exercise: glycogen depletion below a critical threshold will result in a dramatic decline in performance (known in cycling as "bonking" or "hitting the wall").
Carbohydrate loading can super-compensate glycogen to beyond normal levels, thus minimising the chances of bonking.
Consuming carbohydrates during exercise can also reduce the rate of glycogen decline, enabling an athlete to maintain a given exercise intensity for longer before the onset of fatigue.
To support optimum performance over the three-week Tour, riders also need to consume enough daily energy to offset their high expenditures and adequate dietary protein to support muscle recovery.
Negative energy balance (such as when the total daily energy from food and drink is less than that expended) or inadequate dietary protein can lead to metabolic changes and cannibalism of existing body proteins to produce additional energy for cells.
Tour riders attempt to minimise the risk of impairment by consuming as many kilojoules (kJ) of energy as they expend.
In the pros' pockets?
Well before the advent of sports nutrition, riders competing in early Tour editions were left to fend for themselves, including foraging for food and drink along the way.
Times have changed, although there is very little published data on the dietary practices of modern-era Tour riders.
In one of the few studies to offer clues as to what professional riders eat and drink, an investigation of five 1988 Tour de France competitors published in 1989 found that, on average, they consumed 24,700kJ per day and expended 25,400kJ.
That's around two-and-a-half times more energy than a typical sedentary male of similar size might expend. Fluid intake averaged 6.7 litres a day (the largest daily intake was 11.8 litres) with 61 per cent consumed during cycling.
Dietary carbohydrates accounted for about 61 per cent of their energy intake, fats about 23 per cent and proteins about 15 per cent (nearly 230 grams a day for a 70kg rider).
Half of their daily energy was consumed during exercise, mostly as carbohydrate (about 94 grams an hour), and sports drinks alone contributed 15 per cent of their total daily energy. Interestingly, sweet cakes eaten during rides were the major energy provider in the diets.
High in simple sugars and fats, these were energy dense and clearly palatable for the riders.
Will winning or finishing second in the 100th Tour hinge on a well-timed drink or bite to eat?
We'll never know, but we do know that the sports nutrition behind the scenes of professional cycling has moved a long way from foraging for food and water in the French countryside.