The above link should take you to a 5 minute video on cancer and improved recovery with targeted aerobic exercise. The 'zone' they discuss is optimum to train in is based on blood lactate accumulation vs clearance rates as measured in the blood while exercising at progressively increasing rates of work.
Warning. There is a full minute worth of commercials before the video.
Lactate testing and aerobic metabolic testing are the aspects of endurance training that I enjoyed studying the most as a coach. I haven't been able to get my hands on that type of equipment. We used an alternative protocol to successfully estimate those same zones without the blood testing. I think being able to do the full testing like in the video would be very interesting.
An athlete could use the Respiratory Quotient (RQ) to calculate what fuels were being used at particular rates of work. Makes for great targeted training, but more importantly, from the ratio of CO2 v O2 exhaled you can tell whether fat or carbs are being burned. It's an insight into the metabolic state of the mitochondria.
I had a test like this done on me in Vancouver before my diabetes diagnosis. It showed that despite my massive cycling endurance, I was a heavy carb burner, even at low efforts - over time (like several hours) I'd bet I had be getting into fat burning, but I only did metabolic tests lasting for 15 minutes or so. The maximum exertion tests are not very fun. Suffocating. Reaching maximum effort on a automated ramped protocol where the resistance increases by 30 watts every minute while cycling. While breathing through a tube.
So why do I want to have this tested again? I bet I would see a massive downward shift in my respiratory quotient. Ketones burn even cleaner than fats so I'd predict that my RQ would be significantly lower for each level of increasing work.
For training and racing I can use this info to figure out what fuels were being burned during an event so that I know what fuels (protein, fat, carbs..) to replace to optimize performance. The ratios of the fuels depends on the level of exertion for the intensity of each event. Athletes train for events of a variety of distances, therefore each event will have a different fueling plan. I'd also be curious to figure out basal metabolic rates by monitoring respiration at complete rest. With that information there would be a more complete picture as to the total daily energy expenditure.
That number could be used to figure out the amount of fuel (calories) that needs to be replaced. While I'm not a believer in the classical calorie in, calorie out philosophy I believe that the amount of energy obtained form each fuel source, whether it's from protein, carbohydrate or fat is variable depending on the state of the metabolism. Variable to a particular individual at a particular state of health.
Beyond athletic performance, what these numbers really are a look into is the metabolic health and efficiency of the mitochondria. Mitochondria are everything for health! I cannot emphasize that enough. They are our energy producing furnaces, treating them poorly can be a factor in nearly every chronic modern disease, especially those affecting the brain.
The shit of it all is that when something is affecting your brain, rarely are you able to notice it until it is too late. Having a measure of your mitochondrial efficiency is likely an insight into your overall health and vigor (endurance).