One of the body’s vital homeostatic mechanisms i.e. the mechanisms that have evolved over millions of years to ensure that we function optimally and have our best chance of survival in the face of adversity, is to have the ability to store any excess food that we take in but cannot be utilised immediately.
The body’s primary storage facility is glycogen. Glycogen is a polymer of glucose and is manufactured by the cells of the liver by a process called glycogenesis. This process is promoted by insulin and when blood glucose goes up, particularly following a meal high in carbohydrate, insulin levels rise automatically in response.
The liver however is only capable of storing about 100g of glucose in the form of glycogen which in effect will release approximately 400 calories of energy. This storage facility therefore is fairly limited.
Because primitive man almost certainly went for days on end without any food, in particular when weather conditions where adverse, the body evolved to create a more long-term storage facility in the form of adipose tissue.
Any excess glucose that cannot be utilised immediately for energy and cannot be converted into glycogen because the glycogen stores are already at capacity is then converted into fatty acids by a process called lipogenesis. This process is promoted once again by insulin.
It is essential that the blood maintains a base level of glucose mainly because the brain has an obligation to use glucose as its primary energy source. It is only after approximately three days of surviving on baseline glucose levels that it is able to convert to its secondary energy source, namely ketone bodies.
The body once again has an important homeostatic mechanism in maintaining this minimum level of glucose and that is via a hormone called glucagon. This latter hormone is secreted by the pancreas and is responsible for the breakdown of glycogen into its constituent glucose molecules which can then be released into the blood stream. This process is called glycogenolysis.
Glucagon also promotes another important homeostatic process called gluconeogenesis which is the conversion of protein into glucose. However as previously mentioned the liver has a limited supply of glycogen and in effect will be completely depleted of its glycogen stores within a 24 hour fast. At this point blood glucose levels continue to be maintained by gluconeogenesis.
However, for the purposes of ongoing energy supplies, the body also calls upon its very important secondary energy source, namely fatty acids. If there are no fatty acids available immediately from triglycerides that are ingested via food then it will utilise the fatty acids that can be broken down from stored adipose tissue. This process is called lipolysis and is once again promoted by glucagon and also by the hormones adrenalin, cortisol and growth hormone.
The amount of energy that can be released from adipose tissue is clearly dependent on the amount of adipose tissue that the body has available in the average individual. Even for those who are within the normal BMI range, this would equate to several kilograms. Since a gram of fat releases approximately 9 calories of energy there is plenty of reserve!
An understanding of these mechanisms is vital in appreciating how both intermittent fasting and reducing carbohydrate intake is the most efficient and ultimately successful, and biologically proven, method of treating obesity. It is once again utilising the body’s own homeostatic mechanisms that have evolved over millions of years to ensure that it functions optimally and that it survives.