Carbohydrates and the Glycaemic Index

| June 29, 2014 | 0 Comments

Food provides fuel for our body in the form of fat, protein, carbohydrates   and alcohol but carbohydrates are the body’s preferred fuel source.   Carbohydrate-containing foods include bread, breakfast cereals, rice, pasta,   legumes, corn, potato, fruit, milk, yoghurt, sugar, biscuits, cakes and   lollies.

Digesting and absorbing carbohydrates
The digestive system breaks down carbohydrate-containing foods into simple   sugars, mainly glucose. For example, both baked beans and soft drink will be   broken down to simple sugars in your digestive system. This simple sugar is   then carried to each cell via the bloodstream. The pancreas secretes a   hormone called insulin, which helps the glucose to migrate from the blood   into the cells. Once inside a cell, the glucose is ‘burned’ along with oxygen   to produce energy.

The body converts any excess glucose from food into another form called   glycogen. This is stored inside muscle tissue and the liver, ready to   supplement blood sugar levels should they drop between meals or during   physical activity.

The glycaemic index
Carbohydrate-containing foods are rated on a scale called the glycaemic index   (GI). This scale ranks carbohydrate-containing foods based on their immediate   effect on blood sugar levels. The GI compares carbohydrate-containing foods   gram-for-gram of carbohydrate.

Carbohydrate-containing foods are compared with glucose, which is given a GI   score of 100. Carbohydrates that break down quickly during digestion have the   highest glycaemic indexes (GI more than 70). These high GI carbohydrates give   a ‘quick hit’. The blood glucose response is fast and high.

Carbohydrates that break down slowly release glucose gradually into the   bloodstream. They have low glycaemic indexes (GI less than 55). The blood   glucose response is slower and flatter.

glycaemic index chart

Choosing between high and low GI foods
Which carbohydrate foods are best to eat? That depends on the situation. For   example, the rate at which porridge and cornflakes are broken down to sugar   is different. People with type 2 diabetes, or impaired glucose tolerance,   cannot produce insulin rapidly enough to match the quick breakdown of   cornflakes to simple sugars. This means their blood glucose levels may rise   above the normal level. Porridge is digested to simple sugars much more   slowly so the body has a chance to respond with production of insulin and so   the rise in blood glucose levels is less. Porridge is a better choice of   breakfast cereal than cornflakes for people with type 2 diabetes. It will   also provide more sustained energy for other people as well.

How much you eat is also important
The amount of the carbohydrate-containing food eaten will also affect the   blood glucose levels. For example, even though pasta has a low GI it is not   advisable for people with diabetes or impaired glucose tolerance to have a   large serve. This is because the total amount of carbohydrate, and therefore   the kilojoules, will be too high.

GI and exercise
Eating low GI foods two hours before endurance events, such as long distance   running, may improve exercise capacity. It is thought that the meal will have   left your stomach before you start the event but remains in your small   intestine releasing energy for a few hours afterwards. On the other hand,   high GI foods are recommended during the first 24 hours of recovery after an   event to rapidly replenish muscle fuel stores (glycogen).

High GI foods are influenced by low GI foods
Generally, eating low GI foods slows down the absorption of glucose from any   high GI foods eaten at the same time. This is important, as most foods are   eaten as part of a meal and this affects the GI value of foods. For example,   eating cornflakes (a higher GI food) with milk (a lower GI food) will reduce   the effect on blood sugar levels.

If a person with diabetes experiences a ‘hypo’, where the blood glucose   levels fall below the normal range of 3.5-8mmol/L, then they need to eat   foods with a high GI to restore the blood sugar levels to normal quickly. For   example, eating five jellybeans will help to raise blood glucose levels   quickly.

GI scale examples
Some examples of the GI rating of various carbohydrates include:

  • Low GI (less than        55) – soy products, beans, fruit, milk, grainy bread
  • Medium GI (55 to        70) – sugar, orange juice, oats
  • High GI (greater        than 70) – potatoes, wholemeal and white bread, rice.

Factors that affect the GI of a food
Factors such as the size, texture, viscosity and ripeness of a food affect   its GI. For instance, an unripe banana may have a GI of 30 while a ripe   banana has a GI of 52. Both ripe and unripe bananas have a low GI.

Fat, protein, soluble fibre, fructose (a carbohydrate found in fruit) and   lactose (the carbohydrate in milk) also generally lower a food’s glycaemic   response. Fat and acid foods (like vinegar, lemon juice or acidic fruit) slow   the rate at which the stomach empties and so slow the rate of digestion,   resulting in a lower GI. Other factors present in food, such as phytates in   wholegrain breads and cereals, may also delay a food’s absorption and thus lower   the GI.

Cooking and processing can also affect the GI – food that is broken down into   fine or smaller particles will be more easily absorbed and thus have a higher   GI.

GI symbol on packaged foods
A food packaging symbol, G – Glycaemic index tested, for comparing the   effect of different foods on blood sugar was launched in Australia in   July 2002. The GI symbol indicates the GI rating of packaged food products in   supermarkets. It ranks food products based on the speed at which they break   down from carbohydrate to sugar in the bloodstream.

The GI symbol will only appear on food products that meet certain nutrient   criteria for that food category. High and intermediate GI soft drinks,   cordials, syrups, confectionery and sugars are excluded. Jams, honey and   other carbohydrate-containing spreads are not necessarily excluded.

GI symbol

Using the GI as a guide to healthy eating
The GI can be used as a guide to healthy eating, as long as you are aware of   the limitations. For example, the GI of some fruits, vegetables and cereals   can be higher than foods that are considered to be treats, such as biscuits   and cakes. This does not mean we should replace fruit, vegetables and cereals   with treats, because the former are rich in nutrients and antioxidants and   the latter are not.

It is not always possible or necessary to choose all low GI foods. There is   room in a healthy diet for moderate to high GI foods and many of these foods   can provide important sources of nutrients. If you mix a low GI food with a   high GI food, you will get an intermediate GI for that meal.

Tips for healthy eating
Some practical suggestions include:

  • Use a breakfast        cereal based on oats, barley and bran.
  • Use grainy breads or        breads with soy.
  • Enjoy all types of        fruit and vegetables.
  • Eat plenty of salad        vegetables with vinaigrette dressing.
  • Eat a variety of        carbohydrate-containing foods. If the main sources of carbohydrates in        your diet are bread and potatoes then try lentils, legumes, pasta,        basmati rice and pita breads.

Expert medical supervision
If you have a medical condition, such as diabetes, it is important to seek   the advice of your doctor or specialist before making any changes to your   diet.

Where to get help

  • Your doctor
  • An accredited        practising dietitian (APD)        website.

Things to remember

  • The glycaemic index        (GI) rates carbohydrates according to how quickly they raise the glucose        level of the blood.
  • The glycaemic load        (GL) rates carbohydrates according to the glycaemic index and the amount        of carbohydrate in the food.
  • A low GI rating of a        food does not mean you can eat a larger serve of that food – the total        amount of carbohydrate consumed is still important.
  • Choose a diet        containing plenty of fruits, vegetables and legumes but with smaller        helpings of potatoes and less highly refined grain products and        concentrated sugar.
You might also be interested in:
Diabetes   and healthy eating.
Diabetes   explained.
Sporting   performance and food.
Weight   loss and carbohydrates.
Filed Under: Nutrition

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