What does 'fat burns in a carbohydrate flame' mean?

Answer 1

This means that you need a sufficient supply of glucose (carbs) in your body as glucose to sustain exercise and once you are depleted of carbohydrates (blood glucose, stored glycogen), any intensity of exercise will become difficult. Even though typical 'fat burning exercise' is lower intensity (fats are metabolised greatly at lower (up to moderate) intensities of exercise), you still need blood glucose to keep exercise pace without fatigue. So in other words, without glucose, you have restriced chances of utilising fats because glycogen and glucose depletion are what cause enhanced local and whole-body symptoms of fatigue during exercise.

A: Burning Fat in a "carbohydrate flame" is more than just maintaining the BLG and Glycogen stores to maintain exhersion over longer periods of time. This statement acknowledges that fact that there are common pathways that both fat and carbohydrate substrates are oxidised and that fat metabolism is dependent on a background level of carbohydrate catabolism.

If you consider the Citric acid cycle, many of the intermediates are products of glucose catabolism; eg oxaloacetate. Both carbohydrates and fats must be broken down to acetyle CoA before they enter this stage of aerobic metabolism. If you deplent your body of adequate carbohydrates many of the intermediates of the Citric acid cycle will be "borrowed" in synthesise glucose (gluconeogenesis) to maintain BGL's. It is intuative then to assume that if you remove the intermediates that are required for oxidation of fat there will be a reduced capacity to burn fat.

Moral of the story: low carbohydrate diets will ultimately fail in maintaining lasting weight loss because your ability to burn fats is blunted because fat oxidation is dependent on background carbohydrate catabolism.