Almost all foods of plant origin contain carbohydrates which are broken down into sugars – mainly into glucose – during the digestive process. Absorbed in the gut, glucose is a major source of fuel for the body. Some can be stored in the liver and muscles as glycogen, which can be broken down to release glucose when energy reserves are low.

When exercise helps

Glucose tolerance is improved by spreading carbohydrate intake over several small, rather than a few larger meals. Regular physical exercise, which helps remove glucose from teh bloodstream to the muscles, also boots glucose tolerance, as does a reduction in weight if you are particularly overweight.  Starchy foods as well as fruit and vegetables help to slow down the rate of glucose absorption.

Blood glucose levels are regulated by hormones, particularly insulin, and following a meal high in carbohydrate, their levels rise. In response to these rising levels of glucose, the pancreas secrets insulin, which stimulates the removal of glucose from the bloodstream into the liver and muscles.  When too much insulin is secreted, blood sugar levels rebound and this can cause temporary symptoms of Hypoglycemia (low blood sugar). Hypoglycemia is treated by eating small amounts of glucose.

Glucose is also the major source of fuel for the brain, which cannot store glucose and so depends on a steady supply from the blood.  A sudden drop in glucose levels can trigger mood changes, irritability and coma or even death.  However, the brain can adapt, to some extent, by using ketone bodies as an alternative fuel – produced by the liver’s metabolism of fat when the body’s supply of carbohydrates is low. These keotone bodies, with an odour similar to nail varnish, can be detected on the breath.