Humans can briefly extract energy from fat and carbohydrate without needing oxygen in a process called anaerobic metabolism, but this process can only continue for a very short time before it causes so much lactic acid to accumulate in tissue that further metabolism becomes impossible. We need oxygen to do it right and ultimately our ability to burn fat and carbohydrate is totally dependent upon oxygen. This dependency requires that oxygen consumption be directly proportional to calories burned in exercise such that if we can accurately measure oxygen consumed, we can determine calories burned. This is known as indirect calorimetry.Another useful parameter is carbon dioxide output.
When fat or carbohydrate is "burned" it ultimately invloves combining carbon atoms in the fuel with oxygen to produce carbon dioxide that is then breathed out from the lungs. In a very general way therefore, carbon dioxide output is proportional to oxygen consumption, but with a caveat (and this is VERY important): Carbon dioxide output per liter of oxygen consumed is slightly greater when we are burning carbohydrate than when we are burning fat . This difference means that if we carefully measure how much oxygen is going into and how much carbon dioxide is coming out of a person who is exercising, we can, from the ratio of carbon dioxide out and oxygen in, get a pretty good idea of exactly what percent of the calories burned are coming from fat and what percent from carbohydrate.
This ratio of carbon dioxide-out to oxygen-in is called the "Respiratory Exchange Ratio" or RER.
Armed with indirect calorimetry through oxygen consumption and the RER, we can then set out to determine what types and intensities of exercise work best to burn the most calories and fat. Instead of guessing or theorizing, we can use real data to get real answers. We'll explore that data and those answers in the next section.
As described on the previous page, the Respiratory Exchange Ratio or RER provides a way to actually measure how much fat and carbohydrate we are burning during exercise. From this we can answer some important questions that directly help determine the best exercise for weight loss. These questions can be summarized as follows:
Low intensity and moderate intensity exercise burns mostly fat, high intensity exercise burns very little fat and mostly carbohydrate. More precisely, we burn mostly fat during exercise as long as the intensity of that exercise doesn't require us to consume more than about 65-70% of our maximum oxygen consumption capacity (see chart below).
Duration affects the amount of fat burned, not the rate or burning. So the simple answer is that the longer the duration, the more fat is burned. This comports well with the fact that the fat burning zone occurs at a relatively low level of exercise intensity.
"Type" of exercise only matters insofar as "type" relates to "intensity". In other words, whatever you do, keep it fairly slow and go a LONG time.
Most of us don't have the ability to actually measure our oxygen consumption. Fortunately, there is a nearly linear relationship between oxygen consumption (as percent of maximum) [%Vo2max]and heart rate (as percent of maximum) [%HR max]. This means that we reach our maximum oxygen consumption about when we reach our maximum heart rate and 50% VO2max at about 50% HRmax.
There is no perfect calculation to determine your maximum heart rate because each us us is slightly different. The most familiar formula is called the "Fox and Haskell Formula" and states thatHRmax = 220 - Age(in years)Most experts feel this is a poor formula and reccomend that people NOT use it.The most widely verified and esteemed calculation was developed at Oakland University and takes data from the heart rates of 132 people over 25 years of their lives. This formula reads:HRmax = 191.5 - (0.007 × [age Squared])