Like most food products yeasts are labeled with nutritional values for calories, carbs, etc. When yeast is 'activated' it is said to be 'reproducing' (or making more yeast). So, my question is,
"If I start with a tablespoon of yeast in bread, does the nutritional value change as the yeast grows?" .and if so
Well, it's a vanishingly small amount of sugar lost to the CO2. Nevertheless, from first principles, I would say that the answer is definitely yes: the nutritional value of the yeast-plus-bread food complex will change as the yeast consumes the sugar. All metabolic activity comes at the cost of energy, and while the nutritional values of the other ingredients will not, from first principles, be altered merely by being consumed into a yeast cell, once inside the yeast cell, they will be broken down by metabolic activity; and in the case of sugars, the metabolic byproduct of sugar is CO2, the gas which causes bread to rise.
Thus, one rough measure of how much CO2 has been produced would be to measure the volume of dough at the beginning of yeast activation, before and after any punching-down of the dough, and then at the end. Once getting those measurements, you'd take the total increase in volume over the total baking process, assume that that volume is equivalent to the volume of pure CO2 produced inside the bread, and then calculate what number of moles of CO2 would be required to take up such a volume at the relevant temperatures measured. You'd then calculate the number of moles of sugar that needed to be fermented (not fully-metabolized) in order to produce that much CO2, and convert the moles likely first to dry weight and then to calories, and voila! You have your number, specifically (according to the below comments) 3.75g of sugar per liter of CO2.
One caveat about this measure (beyond merely its internal assumptions about CO2 volume) is that it assumes the yeasts are primarily using glucose/fructose as a power source. The reality is that the metabolism of unicellular organisms is complicated, and there may be other compounds other than these in there. However, these minor metabolic side-effects will be even smaller components of the nutritional loss than that of the sugar, negligible in scope.
