The human body is the most sophisticated being on the planet and has many wonderful mechanisms that, as previously discussed in other presentations, have evolved over millions of years. One of these mechanisms is its ability utilise different food substrates for the purposes of cellular energy production. In this respect the billions of cells within our body, irrespective of their function, have a common pathway namely the Krebs cycle. For further details about the chemical processes that make up the Krebs cycle please see the presentation with this title.
There is one exception and that is the cells of the central nervous system ie the brain and spinal cord. Fatty acids are unable to enter the cell membrane known colloquially known as the blood brain barrier, and therefore have an obligatory requirement for glucose as the main substrate. And this of course explains the importance of the biological process known as gluconeogenesis which is the generation of glucose from non-carbohydrate sources, but not fatty acids.
Gluconeogenesis occurs in the liver and the main substrate is protein. It is specifically designed to maintain the blood glucose within very strict parameters, thus ensuring that the brain which, as previously discussed has an obligatory requirement for glucose, does not become starved of this essential element during periods when carbohydrate is in short supply such as in periods of actual starvation.
However as discussed in the presentation entitled ‘ketone bodies’, the cells of the central nervous system are able to switch to utilising these chemicals after about three days of starvation. So in effect this is the failsafe mechanism in the event that the restriction of carbohydrate might extend for a protracted period. Ketone bodies are generated from triglycerides in the liver.
All other cells are able to switch from glucose to fatty acids as an energy substrate with considerable ease. And this of course is the rationale behind intermittent fasting and in particular restricting refined carbohydrate in everything that we eat. All carbohydrate is converted to glucose in the gut and is absorbed as such. If you don’t give the body carbohydrate then effectively you do not give the cells the opportunity to use glucose as its primary substrate.
The cells are also able to utilise protein as an energy substrate in the form of amino acids and certain amino acids can enter the Krebs cycle at various points of the cycle. So there is the opportunity also for dietary protein to be used directly.
But one has to be aware that a diet that has a substantial intake of protein, runs the risk of switching off lipolysis and thus losing the opportunity for burning stored adipose tissue. Protein is certainly required for some very important body functions namely maintaining muscle mass and the production of some important hormones but the requirement for this is somewhere in the region of 60-80 grams per day so in fact relatively little compared to what a lot of people are ingesting.