Shortly before the start of the 2011 TdF it was quite interesting to notice the claim of one of the participants, David Zabriskie, who would try and cope with the energy expenditure of the stages, around 8000 calories daily, with a vegan diet. 

Unfortunately, Zabriskie was forced to retire after nine stages as a result of a crash, so we'll never know if he could've completed the whole Grand Tour successfully, implementing such an extreme nutritional choice. 

The energy expenditure of pedaling depends on the average wattage produced during the effort and the duration of it. 
It is therefore sufficient to multiply the AVERAGE WATTS by 3.6, in order to get the amount of Kcal spent per hour of effort: 

- 150w correspond to an expenditure of 540 Kcal/h 
- 200w correspond to an expenditure of 720 Kcal/h 
- 250w correspond to an expenditure of 900 Kcal/h 
- 300w correspond to an expenditure of1080 Kcal/h 

A flat stage at the Tour rarely requires more than 200-220 average w/h, for a rider of 70kg of body weight: for a duration of 5 hours, the total Kcal expenditure will be about 3600-3960. 

An Alpine or Pyrenean stage usually requires an average power of 240-260w: for a 6h course, the amount of Kcal gets up to 5200–5600. 
The highest average power expressed by a professional cyclist over 6h that I could personally verify was 280w (4.11 w/kg), equal to 6000 Kcal expenditure. 
A good amateur cyclist of 70kg can get up to 3.00 w/kg in a 5-6h Gran Fondo, which equals to about 210 average watts, a total of 3800-4500 Kcal. 

In addition to the energetic expenditure of pedaling we also need to consider the cost of basal metabolic rate (the calories needed to maintain life) and that of normal activities (including food digestion): for a trained athlete, this amounts to the body weight multiplied by 24. 
A 70kg cyclist therefore needs about 1680 Kcal in addition to the cost of pedaling alone: the daily total caloric expenditure at the TdF will be between 5280 and 7280 Kcal, according to the typology of the stages. 

These caloric requirements will be met by the daily intake of: 

- 840g (12g/kg bodyweight) of carbohydrates, equal to 3360 Kcal 
- 140g (2g/kg bodyweight) of protein, equal to 560 Kcal 
- 150g–370g (2-5g/kg bodyweight) of fats, equal to 1350-3330 Kcal 

If the athlete is unable (or unwilling) to ingest the expended calories he will inevitably lose weight, using the body reserves of subcutaneous fat and/or muscle protein. 
A 100 Kcal deficit will correspond to an 11g reduction in body fat or a 25g muscle mass decrease.