What is the glycaemic load (GL)?

Glycaemic load (GL)The concept of the glycaemic load is one that is becoming more used and referred to in the food industry. We may have all heard of the glycaemic index (GI) – a value that is obtained by measuring the effect that a carbohydrate containing food has on blood sugar levels, compared to the effect of the same amount of pure sugar on blood sugar levels. However, the problem with the GI is that it doesn’t provide an accurate picture of the entire blood sugar raising potential of the food. The blood sugar response depends on both the quantity and quality of carbohydrates consumed. The GI only provides us with an idea of how rapidly a carbohydrate turns into sugar, but not how much of that carbohydrate is in a food serving.

The glycaemic load (GL) is an extension of the GI, taking into account the quantity of carbohydrates as well. The GL is obtained by multiplying the GI value by the carbohydrate content of the food. This provides us with a more accurate picture of the overall effect that the food product has on blood sugar levels. Clinical studies have shown that the dietary GL is linked to risk factors for heart major vessel disease, and obesity, although data is still being collected.


How is the glycaemic load calculated?

The glycaemic load of a food ranks the effect of a specific serving size of that food on the blood sugar levels. It is calculated by multiplying the GI value of the food by the amount of carbohydrate the serving of food contains, divided by 100. The GL uses white bread as a baseline standard, where each unit of GL reflects the glycaemic effect of 1g of carbohydrate from white bread. If we take a common fruit such as an apple for example, the GL is determined as follows:

 GI of a standard apple = 40
 Carbohydrate content of a standard apple = 15
 GL  = (GI x Carbohydrate content) / 100
 = (40 x 15) / 100
 = 6

If we take two foods with the same GI value, but Food A contains 5% carbohydrates and Food B contains 80% carbohydrates, Food B has a much higher GL and should be eaten in lesser proportions. In general terms, foods with high carbohydrate levels and low fibre contents have high GI and GL values, whereas those with high fibre contents have lower GLs.


Factors affecting the glycaemic load

hotdogThere are various factors that affect the GL of a food. Many of these are similar to those that also influence the GI of a food product. These include: the types of sugar and starches in the food, the way it is prepared, its fat, fibre and carbohydrate content, and the serving size. The rate of absorption and digestion of food products also influences the GI and GL.

The following ranges are usually applied to determine the GL of a particular food:

  • Low GL – 10 or less
  • Medium GL – 11 to 19
  • High GL – 20 or more

The following values are applied to define the GL per day:

  • Low GL – less than 80
  • High GL – more than 120

Foods with a low GL means that they cause a steadier and lower rise in blood sugar levels. These foods include many fruits and vegetables. Foods with a high GL means that they cause a faster and higher rise in blood sugar levels.

High GL foods include white rice and refined snack foods such as chips and sweetened drinks. By consuming foods that have a low GL, there is an overall slower and lower rise in blood sugar levels.

The usefulness of the GL has been demonstrated, particularly in relation to diabetic patients – if you suffer from diabetes, the GL has been shown to be a more accurate and reflective predictor on blood sugar levels. By adhering to a diet with a low GL, diabetics can reduce their average blood sugar levels and lower their risks of developing complications such as eye, kidney and nerve damage.


Table 1: List of foods and their GL

Glycaemic load Fruits / Vegetables Carbohydrates Others
Low Fruits: Apples, pears, oranges, grapes, peaches, strawberries, tomatoes, watermelon, cantaloupe

Vegetables: Beans, peas, carrots, sweet corn

All grain cereal

Bread: Whole wheat, grain, multi-grain, whole grain crackers

Dairy products: Low fat yoghurt, whole/low fat/skim milk

Eggs, Milo

Medium Fruits: Bananas, apple juice, orange juice

Vegetables: Navy beans, sweet potatoes

Wholemeal bread, white bread, brown rice, rice cakes, Cornflakes Coke, chocolate
High Sultanas, dates, raisins, tomato juice, potatoes Bagels, spaghetti, macaroni, white rice, refined cereal products Chips/french fries, jellybeans, lamingtons

 

 


Metabolism and glycaemic load

The GL is a bit more complex than the GI. Carbohydrates vary in terms of quantity and quality (ie GI values) in different foods. Let us compare the GI values of foods with different carbohydrate densities.

The GI of a baked potato is classified as medium-high, which places the potato on the list of foods to avoid or eat less of. The GI of watermelon is also high. However, a serving of watermelon contains high levels of water and a low level of carbohydrates – a 100g serving of watermelon contains 5g of carbohydrates. This gives the watermelon a low GL value. The potato is packed with more carbohydrates, which gives it a high GL value. Therefore, consuming a serving of watermelon has a much smaller impact on blood sugar levels than a serving of potatoes.

In summary, the GI of potatoes is not a misleading value because potatoes are carbohydrate dense, thus their GL is also high. Watermelon on the other hand contains only a small percentage of carbohydrates, so although they have a high GI level, their GL is low.


Medical conditions and glycaemic load

Looking at over 75,000 healthy middle aged women involved in the Nurse’s Health Study, the impact of carbohydrate consumption on long term heart and major vessel health was assessed. They found that diets with a high GL value correlated with increased risk of heart and major vessel disease, regardless of whether the women had conventional risk factors for heart disease or not (ie high blood pressure, high cholesterol levels, family history, smoking, etc). There was also a strong association with weight – particularly in overweight women, a strong positive association was seen between dietary GL and cardiovascular disease. In lean women, there was little association between the two. Another study was performed in 280 postmenopausal women, which showed that dietary GL was inversely related to the level of good cholesterol (HDL) and directly related to bad cholesterols such as LDL and triglycerides.

If you have diabetes, blood sugar levels and insulin resistance is important in understanding and management of your disease. A chronically high blood sugar level after meals has been linked to increased insulin resistance and decreased production of insulin. Some studies performed in patients show that there is a lower rise in blood sugar levels if a meal with a low glycaemic index or glycaemic load is consumed. In the Framingham Study, diets high in total fibre and whole grain were associated with decreased insulin resistance. Elevated dietary glycaemic index and glycaemic load were associated with insulin resistance. A higher glycaemic intake and lower intake of fibre were also shown to be linked to an increased risk of the metabolic syndrome.


Lowering dietary glycaemic load

Some ways to achieve a lower dietary GL include:

  • Replace carbohydrates with protein;
  • Eat low GI carbohydrate foods instead of high GI carbohydrate foods;
  • Try and have at least 3 low GL foods throughout the day, with each meal.

More information

Nutrition For more information on nutrition, including information on types and composition of food, nutrition and people, conditions related to nutrition, and diets and recipes, as well as some useful videos and tools, see Nutrition.

References

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