I took my model's cyclist and computed the energy output needed to cycle through each stage of this year's Tour de France. I then estimated the body's energy efficiency for cycling to be 20%. Some muscle groups are more efficient, some less efficient. Anyway, 20% is a reasonable estimate.
Keep in mind that what I plot in the above graph is the energy burned just to ride the bicycle. I do not include any estimate of normal metabolic rates and energy burns needed to stay alive. Based on our body temperature and energy exchange processes in our bodies, we are all similar to 100-W light bulbs. Burning energy at that rate is why we need something like 2000 Calories per day to maintain a constant body weight. The graph I give above does not include that 2000 Calories; it shows only the energy a cyclist needs to burn in his body to perform the mechanics of riding the bicycle through each Tour de France stage.
Think you can compete in the Tour de France? One slice of a 14-inch Pizza Hut pan pizza supreme contains 290 Calories. Over a three-week period, you would have to burn the equivalent of 425 slices of that Pizza Hut pizza, which works out to about 20 slices per day. Of course, top athletes don't eat from Pizza Hut while competing in the Tour de France, but they still have to replace all those Calories they burn. The two rest days during the race are necessary to help cyclists make up for any lost Calories.
Energy conversions in the body are complicated (see Chapter 9 of my book). Carbohydrates and fats are the usual sources of energy with the former delivering energy at a faster rate than the latter, which is why athletes like to burn carbohydrates. If, however, you could magically lose only fat while biking during the Tour de France, consider that a pound of fat contains roughly 3500 Calories of chemical energy content. Three weeks of Tour de France cycling would then burn off about 35 pounds of fat, if you had that much in reserve. The problem is that if you have 35 pounds of fat to lose, it's not likely you'll be able to ride in the Tour de France! Only the best of the best can burn calories like what you see in the above graph.
Is completing the Tour de France the toughest thing to do in sports?