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If you have been exercising hard (and that means more than half an hour to an hour of very intense muscle work), then your muscles need more glucose in order to metabolise the waste products of exercise, repair and build themselves stronger.

In this situation, eating carbohydrates will stimulate insulin to send glucose to your muscles so avoiding a catabolic (breaking down) state and encouraging an anabolic muscle building state.  

The best way to achieve this is by drinking a glass of fruit juice and eating a high protein snack such as fish or meat.  Fruit juice will provide the sugars as well as vitamins that are needed alongside the high quality protein to repair and build body tissues.

Glycaemc Index and Glycaemic Load of Foods

Glucose is the body’s main source of energy and is obtained directly from carbohydrates in the diet as well as from protein and fat stores in the body.

Despite wide variations in carbohydrate intake, the body manages to maintain glucose levels at a around 1 gram/litre of blood.  The organ responsible for this regulation is the pancreas.  

If blood glucose levels are too low, the pancreas makes a chemical called glucagon that releases stored glucose.  If blood glucose levels are too high, the pancreas releases insulin that removes the excess glucose from the blood and stores it in the muscles, liver or fat cells.  Any dysfunction in the insulin-glucose regulatory system will cause diabetes. More about insulin metabolism and sugar digestion.

Low GI foods are more nutritious and also encourage fat burning!

Although all carbohydrates can be analysed for the rate at which they raise glucose levels in the blood, carbohydrates are usually eaten with foods that contain protein and fat. The presence of the other food groups slows down the digestion of the carbohydrates. For example:

Medium-GI wholemeal bread plus turkey (protein) equals a low-GI meal.

Medium-GI mango plus plain yoghurt (protein) equals a low-GI snack. 

Acids reduce the GI number of a food, which is why sourdough bread has a lower GI score than ordinary bread. Adding acid to food also lowers the GI rating; so try dressing food with lemon juice or balsamic vinegar. 

The presence of viscous soluble fibre reduces the GI score of carbohydrates. Apples, which contain the soluble fibre pectin, have a lower GI rating than watermelon, which has no pectin. Porridge oats are also rich in soluble fibre, which is partly why porridge has a much lower GI score than a refined breakfast cereal such as Weetabix, which is low in soluble fibre. 

Added sugar affects the GI rating of carbohydrates. This means that canned fruit in syrup has a higher GI number than canned fruit in juice. 

What is the Glycaemic Index?

Are high GI foods ever a good choice?

High GI foods make you fatter!

How can I tell if a food has a low GI score?

All carbohydrates cause a glucose peak about 30 minutes after ingestion regardless of whether they are simple sugars or complex carbohydrates such as starch.  

However, there are dramatic differences in the levels of blood glucose they produce. The differences between different types of carbohydrate are measured in terms of the glycaemic index.  See pdf for chart of GI of common foods.

High glycaemic foods are those that break down almost as soon as they hit your stomach and include  foods like sweets, pastries, mashed potatoes, white bread, white rice, fruit juices, etc.. High GI foods are more likely to lead to fat deposition, pancreatic exhaustion, fatigue and hypoglycaemic attacks.

Low glycaemic foods digest more slowly in your body and provide slow release energy. Low GI foods include most fruits and vegetables, whole grain breads, oatmeal, sweet potatoes, etc.

Since low-glycemic carbs don't digest as quickly, when your body is looking for energy it will look to your fat cells for the fuel it needs. That means that a steady, regulated supply of the right amount of low-GI carbs will help you burn fat all day long while providing you with readily available energy when you need it.  

Low GI foods also tend to be inherently more nutritious (fruits and vegetables) so they also contain your important vitamins, minerals and fibre.

Since high-GI foods digest so quickly, they trigger your body to release insulin which forces your body to look to what you just ate or drank for fuel rather than using your fat stores as well as stimulating your fat cells to take up the excess sugar out of your blood so high GI foods trigger fat storage. So

Eating lots of high GI foods has two disadvantages:

1) The rush of energy given by high GI foods does not last and is soon followed by an energy lull. So you get hungry and want to eat more.

2) After eating high GI foods you will have a lot of readily available energy in your blood. Your body will use this energy first rather then other stores of energy like body fat. This makes it harder to lose weight.

How other foods affect the GI of a carbohydrate

Generally, the less processed a carbohydrate, the more likely it is to have a low GI score (for example, pumpernickel bread, which contains whole grains, has a low GI rating), but it is not always that simple. The following guidelines should help you to understand the GI of carbohydrates.  

Cooking and processing increases the GI number of a food, because it starts the process of breaking down the carbohydrate that would otherwise take place in the stomach. In cornflakes, for example, the corn grain has been ground down, mixed with sugar and baked, resulting in a high GI score. However, sugar-free muesli, which is made from whole grains of flaked cereals, still requires the digestive system to do a lot of work to break down the carbohydrate, so it has a low GI rating.

If the tough outer skin of a grain or pulse is still in place, it is harder for the digestive system to break it down. This means that multigrain bread has a lower GI number than white bread, which has a high GI score because the wheat husks have been removed by the milling process.

The type of starch in a carbohydrate also affects its GI rating. Rice contains two types of starch: amylose and amylopectin. Basmati rice is high in amylose starch, which has a complex structure that is difficult to digest. Standard white rice is high in amylopectin, which has a less complex structure and is easy to digest. This explains why basmati is a medium-GI carbohydrate food, while standard white rice is a high-GI food. 

Tightly packed starch grains are harder for the digestive enzymes to process than fluffy starch grains. This is why pasta, new potatoes and pitta bread all have lower GI ratings than gnocchi, baked potatoes and white bread. 

References

1. Ostman E1, Granfeldt Y, Persson L, Björck I. Vinegar supplementation lowers glucose and insulin responses and increases satiety after a bread meal in healthy subjects. Eur J Clin Nutr. 2005 Sep;59(9):983-8.

2. Björck I1, Liljeberg H, Ostman E.Low glycaemic-index foods. Br J Nutr. 2000 Mar;83 Suppl 1:S149-55.

Is it only carbohydrates that affect blood sugar levels?

It is important to note that large protein or fat meals will also contribute to blood sugar levels. Once the body's immediate protein or fat needs, respectively, have been satisfied, it will start to metabolise them into glucose for use as energy or for storage.

This effect has led to the development of the Insulin Index which measures the effect of foods on the insulin response.  Dairy products, particularly, score low on the glycaemic index but high on the Insulin Index (2).


View or download this chart showing the Glycaemic Index of common foods.

What is the Glycaemic Load?

An apple has a fairly low glycaemic index of 39.  Furthermore, due to its fibre content, an apple actually has a glycaemic load (a measure of the glycaemic effect of the actual carbohydrate content) of only 6.

The glycaemic index of sugar is 100 and its glycaemic load, since it is pure carbohydrate, would also be 100.

So sugar consumed as part of a whole fruit, vegetable or grain will have a lower glycaemic load and therefore also a lower and less dramatic insulin response.

However, the amount of a food eaten also affects its glycaemic load, so a pint of orange juice will have a larger glycaemic load than a teaspoon of sugar.

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