Complicated term, yes. But it is simply the basis of how, what and when we use the different sources of energy for our body to function. And more importantly, how we lose fat as efficiently as possible (without insane restrictions and eating like a rabbit).
Oxidative priority is the order in which the body processes fuels coming in from the diet. At any moment, there isn’t much fuel in the blood. Normal blood glucose is 85 mg/dL, which is about 4 grams of glucose, or 16 calories’ worth. We know that very high blood glucose can be lethal, but an excess of any other fuel can be lethal as well. Alcohol, exogenous ketones, glucose (from carbohydrates), protein, and fat will all cause damage if allowed to build up in the blood.
When you eat a large meal (500 to 1,000 calories, or maybe even more), your body has to deal with all that food coming in and do it quickly so that you don’t die from hyperglycemia (high blood glucose), hypertriglyceridemia (high blood fat), or alcohol toxicity (if you have consumed alcohol). The body is smart; it deals with the different types of “meal inputs” you consume in reverse order of storage capacity. This means the fuel that it has no capacity to store—alcohol—is burned off first.
The second priority is exogenous ketones, which are ketone supplements. The body doesn’t like blood ketones to be too high and has almost no storage space for them, so it burns those off next. I don’t recommend taking ketone supplements, by the way; on a carnivore diet or Low Carb diet, your body will make all the ketones it needs from your fat stores.
The third priority is carbohydrate because the body’s storage capacity for it is relatively small. Carbohydrates are turned into glucose in the blood when digested (the body recognizes any carbohydrate whether it is bread, pasta, honey, chocolate, sugar as the same thing and turns each of these into glucose). That glucose is either burned for fuel or stored in the muscles or liver in the form of glycogen. You can store around 1,200 to 2,000 calories of glycogen in your muscles and liver. The glycogen in the muscles is locked away to be used by the muscles only during very intense exercise; a brisk walk around the block or a typical workout will not tap into your muscle glycogen stores in any significant way. Most people are really only replenishing liver glycogen on a typical day, which is only about 320 to 400 calories’ worth.
The fourth priority is protein, which is a little different. It is preferentially used to build lean mass (muscle, skin, and so on). The body is constantly turning over cells in the skin and other body parts, so it needs a steady supply of amino acids from complete protein sources to rebuild cells. Protein is used as a fuel only when no other fuels are present. If you were very lean and you ate only protein, you could run into problems as your body struggled to turn enough protein into fuel. This is known as rabbit starvation. Turning protein into fuel is an energy-intensive process so the body does it only when it has no other option.
The last priority is fat, which is the easiest of the five fuels to store. Our bodies have a theoretically unlimited fat storage capacity. There are people who have millions of calories of stored fat (1g of fat has 9 calories of energy. Someone with even only 1kg of stored fat, in theory, has 9000 calories of stored energy). Therefore, the body deals with all the other fuels and stores the fat to deal with later.
What does this look like in a typical meal? Let’s say you have a couple of beers with a meal high in protein, carbohydrates, and fat. Because your body can’t store the alcohol from the beer, it burns off as much of the alcohol as quickly as it can. While it is prioritizing the alcohol, your body stores the carbs (from both the food and the beer) and fat for later and sends the protein into muscle protein synthesis, which is the body’s process for building and rebuilding lean mass. Therefore, the majority of the carbs and fat you consumed go into storage due to the alcohol that is present. This is why drinking alcohol greatly limits fat loss. There are a couple of striking things to note. One is that in both overfed and underfed states, the energy expenditure for protein doesn’t really change. This reflects the body’s preference to use protein for building lean mass, not as a fuel. Then second thing is the drastic change in fat being used for fuel between the underfed and overfed states. The body’s requirement for energy doesn’t really change, but when carbohydrates are present in an overfed state, very little fat is used for fuel. In an underfed state, where less carbohydrates are coming in from the diet, the body oxidizes, or burns, much more fat for fuel. Dietary carbohydrates are simply displacing fat burning because they take oxidative priority over fat.
To sum up what I have written in this article, your body simply cannot burn away your fat stores in an overfed state (Calorie surplus) where carbohydrates and/or alcohol is present. Your body will even struggle to burn calories in a calorie deficit if fueled with too many carbohydrates and where alcohol is present because of the oxidative priority (your body still needs to burn through all stored glycogen and alcohol before it begins on burning through stored fat). See my post, “Why am I always hungry?” to learn more about satisfying your hunger in a calorie deficit that will help you finally lose fat.
So what do we need to do to enhance fat burning? to put it simply, prioritise protein from good quality animal sources (you will burn extra calories eating quality protein - this is called the thermogenic effect of food), limit fat to only necessary sources and to what comes with your animal protein (eg. fat in the pan to cook your meat, exclude all seed oils, limit dairy like cheese and yogurt), and try to eliminate all carbohydrates. To make this easier, I recommend an adaptation to something like what Tim Ferris suggests in his 4hr body book. Follow a reasonably strict carnivore or meat based diet during the week (I promise you wont be hungry if you follow this part right, even in a calorie deficit), and then have 1 day a week where you eat whatever you want. Highly cotroversial, I know. But it works.