Posted by: faithful | October 30, 2009

common glycemic index questions



What is the difference between glycemic index (GI) and glycemic load (GL)?

Your blood glucose rises and falls when you eat a meal containing carbs. How high it rises and how long it remains high depends on the quality of the carbs (the GI) and the quantity. Glycemic load or GL combines both the quality and quantity of carbohydrate in one ‘number’. It’s the best way to predict blood glucose values of different types and amounts of food. The formula is:

   GL = (GI x the amount of carbohydrate) divided by 100.

Let’s take a single apple as an example. It has a GI of 40 and it contains 15 grams of carbohydrate.
GL = 40 x 15/100 = 6 g

What about a small baked potato? Its GI is 80 and it contains 15 g of carbohydrate.
GL = 80 x 15/100 = 12 g

So we can predict that our potato will have twice the metabolic effect of an apple. You can think of GL as the amount of carbohydrate in a food ‘adjusted’ for its glycemic potency.

Should I use GI or GL and does it really matter?

Although the GL concept has been useful in scientific research, it’s the GI that’s proven most helpful to people with diabetes. That’s because a diet with a low GL, unfortunately, can be a ‘mixed bag’, full of healthy low GI carbs in some cases, but low in carbs and full of the wrong sorts of fats such as meat and butter in others. If you choose healthy low GI foods—at least one at each meal—chances are you’ve eating a diet that not only keeps blood glucose ‘on an even keel’, but contains balanced amounts of carbohydrates, fats and proteins.

We suggest that you think of the GI as a tool allowing you to choose one food over another in the same food group—the best bread to choose, the best cereal etc.—and don’t get bogged down with figures. A low GI diet is about eating a wide variety of healthy foods that fuel our bodies best—on the whole these are the less processed and wholesome foods that will provide you with carbs in a slow release form. So what’s the take-home message?

  • Choose slow carbs, not low carbs
  • Use the GI to identify your best carbohydrate choices.
  • Take care with portion size with carb-rich foods such as rice or pasta or noodles to limit the overall GL of your diet.

Do I need to eat only low GI foods at every meal to see a benefit?

No you don’t, because the effect of a low GI food carries over to the next meal, reducing its glycemic impact. This applies to breakfast eaten after a low GI dinner the previous evening or to a lunch eaten after a low GI breakfast. This unexpected beneficial effect is called the “second meal effect”. But don’t take this too far, however. We recommend that you aim for at least one low GI food per meal.

While you will benefit from eating low GI carbs at each meal, this doesn’t have to be at the exclusion of all others. So enjoy baking your own bread or occasional treats. And if you combine high GI bakery products with protein foods and low GI carbs such as fruit or legumes, the overall GI value will be medium.

Why do many high-fibre foods still have a high GI value?

Dietary fibre is not one chemical constituent like fat and protein. It is composed of many different sorts of molecules and can be divided into soluble and insoluble types. Soluble fibre is often viscous (thick and jelly-like) in solution and remains viscous even in the small intestine. For this reason it makes it harder for enzymes to move around and digest the food. Foods with more soluble fibre, like apples, oats, and legumes, therefore have low GI values.

Insoluble fibre, on the other hand, is not viscous and doesn’t slow digestion unless it’s acting like a fence to inhibit access by enzymes (eg. the bran around intact kernels). When insoluble fibre is finely milled, the enzymes have free reign, allowing rapid digestion. Wholemeal bread and white bread have similar GI values. Brown pasta and brown rice have similar values to their white counterparts.

Can I download or can you email me a full list of all GI food values?

Sorry but we have no such list available for download or emailing purposes. Instead, we invite you to search out the foods you are interested in finding on our free GI Database (see the menu link on the left). There you will find a brief explanation on how best to conduct the search. Another option is to purchase our pocket book which is updated annually and contains the latest values at the time of publication: The New Glucose Revolution: Shopper’s Guide to GI Values.

Does the GI increase with serving size? If I eat twice as much, does the GI double?

The GI always remains the same, even if you double the amount of carbohydrate in your meal. This is because the GI is a relative ranking of foods containing the “same amount” of carbohydrate. But if you double the amount of food you eat, you should expect to see a higher blood glucose response – ie, your glucose levels will reach a higher peak and take longer to return to baseline compared with a normal serve.

If testing continued long enough, wouldn’t you expect the areas under the curve to become equal, even for very high and very low GI foods?

Many people make the assumption that since the amount of carbohydrate in the foods is the same, then the areas under the curve will finally be the same. This is not the case because the body is not only absorbing glucose from the gut into the bloodstream, it is also extracting glucose from the blood. Just as a gentle rain can be utilised better by the garden than a sudden deluge, the body can metabolise slowly digested food better than quickly digested carbohydrate. Fast-release carbohydrate causes “flooding” of the system and the body cannot extract the glucose from the blood fast enough. Just as water levels rise quickly after torrential rain, so do glucose levels in the blood. But the same amount of rain falling over a long period can be absorbed into the ground and water levels do not rise.

Why doesn’t the GI of beef, chicken, fish, tofu, eggs, nuts, seeds, avocadoes, many fruits (including berries) and vegetables, wine, beer and spirits appear on the GI database?

These foods contain no carbohydrate, or so little that their GI cannot be tested according to the standard methodology. Bear in mind that the GI is a measure of carbohydrate quality. Essentially, these types of foods, eaten alone, won’t have much effect on your blood glucose levels.

Some vegetables like pumpkin and parsnips appear to have a high GI. Does this mean a person with diabetes should avoid eating them?

Definitely not, because, unlike potatoes and cereal products, these vegetables are very low in carbohydrate. So, despite their high GI, their glycemic load (GI x carb per serve divided by 100) is low. Vegetables contain only small amounts of carbohydrate but loads of micronutrients and should be considered as “free foods”. Eat them all you like!

Can you tell me the GI of alcoholic beverages (beer, wine and spirits)?

Alcoholic beverages contain very little carbohydrate. In fact, most wines and spirits contain virtually none, although beer contains some (3 or 4 grams per 100 mL). A middy of beer (10 ounces) contains about 10 grams of carbohydrate compared with 36 grams in the same volume of soft drink. For this reason, a beer will raise glucose levels slightly. If you drink beer in large volumes (not a great idea) then you could expect it to have a more significant effect on blood glucose. As for enjoying an occasional drink, researchers from the University of Sydney found that a pre-dinner drink tends to produce a ‘priming’ effect, flicking the switch from internal to external sources of fuel and keeping blood-sugar levels low.

Why does some variability occur in the GI for the same food types? For example, Special K cereal shows values from 54 to 84.

The GI database confirms the reproducibility of GI results around the world. White and wholemeal bread, apples, cornflakes, breakfast cereals etc give the same results wherever/whoever tests them. Where there is variability, there are four possible explanations:

  1. Some GI testing groups are not as experienced/accurate as ours. They use venous blood which gives more variability than capillary blood. If we test a product over and over again, we get the same result +/- 5%. That’s as good as nutrient data such as protein, fat, fibre etc.
  2. The variability among different types of potatoes, rices, and oats is REAL. They contain different types of starch (amylose, amylopectin) and that affects the degree of starch gelatinisation. When it comes to sugars like fructose, the concentration of the solution makes a difference to the rate of gastric emptying and therefore the glycemic response. A more dilute solution, say 25 g fructose in 500 mL water will have a higher GI than 25 g fructose in 250 mL. But fructose has a very low GI whichever way you consume it.
  3. Sometimes the manufacturer may change the formulation of their product by reducing the fat content for example. Reducing the fat can increase the GI. Manufacturers may have their products retested if they make significant changes to the formulation, or source ingredients from different suppliers.
  4. Some foods have been tested in people with type 2 diabetes. These values may be higher than that seen in the normal population. Follow the food links in the GI database to find more information on the testing conditions.

Why does pasta have a low GI?

Pasta has a low GI because of the physical entrapment of ungelatinised starch granules in a sponge-like network of protein (gluten) molecules in the pasta dough. Pasta is unique in this regard. As a result, pastas of any shape and size have a fairly low GI (30 to 60). Asian noodles such as hokkein, udon and rice vermicelli also have low to intermediate GI values.

Pasta should be cooked al dente (‘firm to the bite’). And this is the best way to eat pasta – it’s not meant to be soft. It should be slightly firm and offer some resistance when you are chewing it. Overcooking boosts the GI. Although most manufacturers specify a cooking time on the packet, don’t take their word for it. Start testing about 2-3 minutes before the indicated cooking time is up. But watch that glucose load. While al dente pasta is a low GI choice, eating too much will have a marked effect on your blood glucose. A cup of al dente pasta combined with plenty of mixed vegetables and herbs can turn into three cups of a pasta-based meal and fits easily into any adult’s daily diet.

Most breads and potatoes have a high GI. Does this mean I should never eat them?

Potatoes and bread, despite their high GI, can play a major role in a high carb/low fat diet, even if your goal is to reduce the overall GI. Only about half the carbohydrate needs to be exchanged from high to low GI to derive health benefits. Of course, some types of bread and potatoes have a lower GI and these should be preferred in order to lower the GI as much as possible.

The good news for potato lovers is that a potato salad made the day before, tossed with a vinaigrette dressing and kept in the fridge will have a much lower GI than potatoes served steaming hot from the pot. There are a couple of simple reasons for this. The cold storage increases the potatoes’ resistant starch content by more than a third and the acid in the vinaigrette whether you make it with lemon juice, lime juice or vinegar will slow stomach emptying.

What about flour? If I make my own bread (or dumplings, pancakes, muffins etc) which flours, if any, are low GI? What about sprouted grain breads?

To date there are no GI ratings for refined flour whether it’s made from wheat, soy or other grains. This is because The GI rating of a food must be tested physiologically that is in real people. So far we haven’t had volunteers willing to tuck into 50 gram portions of flour on three occasions! What we do know, however, is that bakery products such as scones, cakes, biscuits, donuts and pastries made from highly refined flour whether it’s white or wholemeal are quickly digested and absorbed.

What should you do with your own baking? Try to increase the soluble fibre content by partially substituting flour with oat bran, rice bran or rolled oats and increase the bulkiness of the product with dried fruit, nuts, muesli, All-Bran or unprocessed bran. Don’t think of it as a challenge. It’s an opportunity for some creative cooking.

Bread made from sprouted grains might well have a lower blood-glucose raising ability than bread made from normal flour. When grains begin to sprout, carbohydrates stored in the grain are used as the fuel source for the new shoot. Chances are that the more readily available carbs stored in the wheat grain will be used up first, thereby reducing the amount of carbs in the final product. Furthermore, if the whole kernel form of the wheat grain is retained in the finished product, it will have the desired effect of lowering the blood glucose level.

Some high fat foods have a low GI. Doesn’t this give a falsely favourable impression of that food?

Yes it does, especially if the fat is saturated fat. The GI value of potato chips or french fries is lower than baked potatoes. Large amounts of fat in foods tends to slow the rate of stomach emptying and therefore the rate at which foods are digested. Yet the saturated fat in these foods will contribute to a much increased risk of heart disease. It is important to look at the type of fat in foods rather than avoid it completely. Good fats are found in foods such as avocadoes, nuts and legumes while saturated fats are found in dairy products, cakes and biscuits. We’d all be better off if we left the cakes and biscuits for special occasions.

Why not just adopt a low carbohydrate diet (like the Atkins diet) to keep my blood glucose levels and weight down?

Recent studies show that low carb diets such as the Atkins diet produce faster rates of weight loss than conventional low fat diets. The probable mechanism is lower day-long insulin levels – allowing greater use of fat as the source of fuel – the same mechanism underlying the success of low GI diets. We believe that low carb diets are unnecessarily restrictive (bread, potato, rice, grains and most fruits are restricted) and may spell trouble in the long term if saturated fat takes the place of carbohydrate. Low GI diets strike a happy medium between low fat and low carb diets – you can have your carbs, but must choose them carefully.

Is there a GI plan for nursing mothers?

A low GI diet is ideal while you are breastfeeding. Breastfeeding requires a lot of energy and theoretically this additional energy comes from the body fat laid down during pregnancy. Of course in reality it doesn’t all get used up and most have to make a concerted effort to work off the baby weight. To do this though it is important that you don’t go on a low calorie diet or any sort of extreme measure such as the low carb diets popular in the press. Since breastfeeding tends to increase your appetite (the body’s way of ensuring you have the energy required to produce milk) this is good news as staying on such a diet would be a nightmare! This is what makes the low GI approach so successful – forget about trying to count calories or even your portions of food.

First and foremost focus on the sorts of foods you are eating. Low GI foods are the wholegrains, fresh fruit and vegetables and legumes. By eating these foods as the mainstay of your meals you can trust your appetite and eat to satisfaction while you are breastfeeding. Also get back to some exercise – even if it’s just a daily walk with the pram/carriage. You should then find that the weight slowly starts to shift – realistically give yourself at least that first six months to get back to your pre-pregnancy weight.

How relevant is the GI for athletes?

The GI can be a useful tool to help athletes select the right type of carbohydrates to consume both before and after exercise. Studies have consistently reported that a low GI pre-exercise meal results in a better maintenance of blood glucose concentrations during exercise and a higher rate of fat oxidation. This is likely to result in reduced muscle glycogen utilisation during prolonged exercise and possibly improve endurance performance. Eating high GI meals before exercise may result in plasma glucose concentrations peaking before the onset of exercise and then hypoglycemia occurring within the first 30 minutes of the exercise period. There is little data available on the effect of the GI of carbohydrates eaten before intermittent, power or strength related sports.

During recovery from exercise, muscle glycogen resynthesis is of high metabolic priority. The eating of high GI carbohydrates after exercise increases plasma glucose and insulin concentrations and this facilitates muscle glycogen resynthesis. If however, you are exercising for weight loss purposes or are involved in weight restricted sports, low GI carbohydrates after exercise may be more beneficial as the lower glucose and insulin concentrations will not suppress fat.

I have recently been diagnosed with celiac disease (gluten sensitivity). It’s extremely hard to find both low GI and wheat-free foods. Any suggestions?

This is not as hard as you may think! There are low GI gluten-free foods in four of the five food groups.

Fruit and Vegetables

  • Temperate climate fruits – apples, pears, citrus (oranges, grapefruit) and stone fruits (peaches, plums, apricots) – all have low GI values. Tropical fruits – pineapple, paw paw, papaya, rockmelon and watermelon tend to have higher GI values, but their glycemic load (GL) is low because they are low in carbohydrate.
  • Leafy green and salad vegetables have so little carbohydrate that we can’t test their GI. Even in generous serving sizes they will have no effect on your blood glucose levels. Higher carb starchy vegetables include sweet corn (which is actually a cereal grain), potato, sweet potato, taro and yam, so watch the portion sizes with these. Most potatoes tested to date have a high GI, so if you are a big potato eater, try to replace some with lower GI starchy alternatives such as sweet corn, yam or legumes. Pumpkin, carrots, peas, parsnips and beetroot contain some carbohydrate, but a normal serving size contains so little that it won’t raise your blood glucose levels significantly.

Bread and Cereals

  • Opt for breads made from chickpea or legume based flours. For example chapattis made with besan (chickpea flour) have a low GI. If you make your own bread, try adding buckwheat kernels, rice bran and psyllium husks to lower the GI. Most gluten-free breads seem to be better toasted than used to make sandwiches.
  • Breakfast cereals containing pysllium husks are likely to have a lower GI – you could also add a teaspoon or two of pysllium to you usual cereal. To date there are just a few gluten-free breakfast cereals on our database that have a low GI. If you do have a higher GI gluten-free cereal, combine it with lots of fruit and low fat yoghurt or low fat milk, to lower the GI.
  • Noodles are a great stand-by for quick meals, a good source of carbohydrate, provide some protein, B vitamins and minerals and will help to keep blood glucose levels on an even keel. There are several low GI gluten-free options available fresh and dried: buckwheat (soba) noodles; cellophane noodles, also known as Lungkow bean thread noodles or green bean vermicelli, are made from mung bean flour; rice noodles made from ground or pounded rice flour, are available fresh and dried.
  • Gluten-free pastas based on rice and corn (maize) tend to have moderate to high GI values so opt for pastas made from legumes or soy. As for wholegrains, try buckwheat, quinoa, low GI varieties of rice such as basmati and sweet corn. Currently there are no published values for amaranth, sorghum, and tef. Millet has a high GI.
  • Minimise refined flour products and starches irrespective of their fat and sugar content such as crispy puffed breakfast cereals, crackers, biscuits, rolls, most breads and cakes or snack foods. Limit high GI snacks such as corn and potato chips, rice cakes, corn thins and rice crackers.

Legumes (pulses) including beans, chickpeas and lentils
When you add legumes to meals and snacks, you reduce the overall GI of your diet because your body digests them slowly. So make the most of beans, chickpeas, lentils, and whole and split dried peas.

Although nuts are high in fat (averaging around 50 per cent), it is largely unsaturated, so they make a healthy substitute for foods such as biscuits, cakes, pastries, potato chips and chocolate. They also contain relatively little carbohydrate, so most do not have a GI value. Peanuts (actually a legume) and cashews have very low GI values.

Low fat dairy foods and calcium-enriched soy products
Low fat milk, yoghurt and ice-cream or soy alternatives provide sustained energy, boosting your calcium intake but not your saturated fat intake. Check the labels of yoghurts, icecream and soymilks as many contain wheat-based thickeners. If lactose intolerance is a problem, reach for live cultured yoghurts and lactose-hydrolysed milks. Even ice-cream can be enjoyed if you ingest a few drops of lactase enzyme first.

Is a low GI diet suitable for vegetarians?

The low GI diet is just as easy for a vegetarian to follow – in fact, teaching vegetarians to follow the low GI diet can be easier because most are eating many of the best low GI foods already. For the vegetarian, the same principles apply: substitute your plant protein sources for the meat. Eat more beans, lentils and other legumes – all among the lowest GI foods we have tested. Quorn is also a great meat substitute with no GI as it has almost no carbohydrate (2 g/100 g).

Some additional points:

  • The GI only applies to foods containing significant amounts of carbohydrate. Most vegetables have small amounts of carbohydrate and those that provide more usually have a low GI, with the exception of potatoes. You can therefore tuck into your veggies without considering the GI for every one – and benefit from antioxidants and all the micronutrients they supply!
  • Legumes should be a daily part of any vegetarian diet for your protein – happily these are also a mainstay of a low GI diet.
  • Almost every low GI food we talk about in the book is suitable as part of a vegetarian diet. Animal products are usually high in fat, protein or both and therefore do not have a GI.
  • The range of protein and carb intake that is healthy is fairly broad – as a vegetarian you will inevitable have a higher carb intake and slightly lower protein intake. This makes the GI important for you but easy to adapt if you choose wholegrain cereal products and legumes as your carbohydrate base.
  • Coffee has no carbohydrate (unless you add sugar and/or milk and the GI response comes from these foods) and hence it is not in the GI tables. Neither does it contain calories so has little impact on weight control.


  1. Wow, What an educative article and just
    saying thanks will not suffice, we r indebted for educating us on managing blood glucose levels. When such informations are freely available, we wish
    people educate themselves before meeting their Doctors.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s


%d bloggers like this: