Several readers have written asking for an explanation of the glycaemic index (GI). With the festive season in full swing and a diet for most of us consisting of plenty of alcohol and rich, sugary, refined carbohydrates, there's no time like now to consider the index. Even if you do over-indulge, once the principles of the index are understood, the damage can be limited. Carbohydrates are an essential source of fuel for the body, especially the brain, which is why they are always present in the bloodstream as blood sugar. But not all carbohydrates were created equal, there are good ones and bad ones. The bad - alcohol, sugar, foods containing it, and those based on white rice or white flour - cause the level of glycaemia or sugar in the blood to become too high too quickly. When this happens the brain sends an emergency signal for help to the pancreas. This organ secretes the hormone insulin, one of whose many roles is to keep blood sugar at a safe level. The excess is stored in the liver and muscles and insulin plays a key role in both the burning and storage of fat too. Unfortunately, the surge of insulin drives levels too low, causing hypoglycaemia, or low blood sugar, making the person suddenly hungry again. This is the sugar 'hit' followed by the slump after a sugary snack. It also explains rebound hunger after a Chinese meal with white rice. Insulin is the hunger hormone and is also stimulated by alcohol (hence the munchies), coffee and monosodium glutamate (MSG). Low blood sugar leads to cravings for sugary, fatty food, for an instant sugar boost, which makes you fat. Insulin is also called the fattening hormone. The glycaemic potential of each carbohydrate is measured by the glycaemic index, topped by glucose (sugar) at 100. The higher the GI, the greater a carbohydrates potential to raise blood sugar levels and stimulate insulin production. Processing, refining and cooking can dramatically change the GI of foods. For example, cornflakes are 85, but corn is 70. Instant mashed potatoes are 95, boiled potatoes 70. The amount of fibre and protein in a carbohydrate also affects the GI. For example: burger roll scores 85, white bread or baguette 70, whole wheat bread 40, while white rice 70 and brown rice 50. All you need to worry about is eating more good carbohydrates of GI 50 and under, and fewer bad, high GI ones. The baddies are sugar in all forms and in all foods be it alcohol, brown sugar, honey or maple syrup along with white bread, white rice, corn and potatoes. The effect of eating low GI foods such as wholewheat pasta (40) cooked with vegetables, protein or fruit is a low, steady blood sugar level. This aids concentration, good mood and keeps hunger at bay. Conversely, hypoglycaemia or low blood sugar, caused by excess amounts of insulin responding to high GI food causes fatigue, sudden tiredness, anxiety, poor concentration, poor decision making skills, headaches, yawning and mood swings. Think twice before you skip a meal or snack on rubbish again. The good carbohydrates are whole flours, wholemeal pasta, brown rice, and particularly legumes such as lentils and kidney beans, most fruits and all vegetables that have fibre such as cabbage, broccoli, cauliflower, peas, but not potatoes or corn. What this means at Christmas is that the turkey, sprouts (10), boiled not roast potato and unsugared cranberry sauce (40) won't make you crave chocolate, booze and plum pudding afterwards. Neither will the tangerines (40) or walnuts (15) or even peanuts (20). It's a question of balancing your meals, if you must have roast potatoes (95), make that your only high GI food at that meal. Limit alcohol, which is a sugar and never drink on an empty stomach or you'll devour every crisp (80) in sight. This is not about deprivation but common sense. And the best news of all is that really good dark chocolate, of 70 per cent cocoa butter or more, has a GI of 22. Enjoy. The many books on the GI include The GI Factor, The Glucose Revolution by Dr Anthony Leeds and three other authors, published by Hodder. The writer is a specialist counsellor in eating disorders and weight control.