What is Hypoglycaemia?

The total amount of glucose in the body at any one time is very small indeed, rarely exceeding 20g in total and generally less than half that amount. This would ordinarily be sufficient to keep the brain and red cells, the only major glucose‐requiring tissues, supplied with energy for a little more than an hour or so were it not constantly being replenished either by food or, in the interval between meals, by glycogen stores in the liver. Even these stores contain sufficient carbohydrate to meet requirements for only 24 hours or so yet we know from experimental and clinical evidence that most people can survive, providing they have ready access to water, for days or even weeks without food and without their blood glucose level falling to below 75% of their ordinary overnight fasting level. This is because under conditions of dietary carbohydrate deprivation the body has the capacity, by means of a series of interrelated mechanisms, to reduce, to an absolute minimum, the utilisation of glucose as a fuel by all the tissues of the body save the brain and red cells and at the same time increase glucose production in the liver. In between meals and during longer fasts, fat becomes the major fuel of all the tissues of the body apart from the brain and red cells.