Glycemic index

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Glycemic index and health

Digestion is the breaking down of large complex nutrient molecules into small simple molecules that can pass through the gut wall into the bloodstream. Fats bcome fatty acids and glycerol. Proteins become amino acids. Carbohydrates become sugars.

Nutrients that start off small, such as sugars like glucose and refined starch foods like white flour, pass through quickly because they need no digestion. This is good when you need a burst of energy when exercising. The downside is that it produces a spike in blood sugar. If not burned off rapidly this upsets the way the body produces insulin and can lead to, or aggravate, diabetes. That is why white sugar and flour are sometimes called 'killer whites'. Some people argue that calories alone are too simple an approach to losing weight. They say that obese people often have hyper-insulinism where the production of insulin is high and out of sync with blood sugar levels.

The World Health Organisation now recommends that no more than 5% of your daily calories should be from sugar. This is a turnaround as some time ago it suggested a maximum of 12% but withdrew that idea when the US sugar industry threatened its funding. In many western countries, particularly the US and the UK, some people have a far higher percentage than this. If you drink a lot of sweetened soft drinks, or eat sweetened processed stuff, you will exceed this and very likely become fat or suffer other illnesses.

An ideal diet will have some sugars, but will mostly comprise the more slowly digested nutrients. Glycemic index (GI) is a measure of the speed of digestion. Glucose is rapidly transferred into the blood so it is used as the standard and is given a GI value of 100. The speeds of other ingredients are compared with that, so one with a GI of 10 is very low and 90 is very high. There are some foods with a GI of more than 100. The concept was developed by Dr. David J. Jenkins and colleagues in 1980–1981 at the University of Toronto. Low GI foods result in lower insulin and lower weight. There is more information about this on the Montignac site (below) and in Wikipedia.

There has been a small change to the 'five-a-day' advice. Fruit and vegetables are both essential. However fruits contain natural sugars so ideally you should have more vegetables and fewer fruits in your five, and should try to avoid having only fruits. Oh yes, potatoes are now a respectable vegetable and part of the five. I heard a dietitian say that they were victim of a middle-class prejudice against chips.

And now the message is don't drink a lot of fruit juice. Taken in this way the fructose is less satisfying, so you drink more than you would eat. It is also absorbed more quickly and more likely to be turned into fat by the liver.

Accurate measurement of GI is a lengthy and expensive matter, involving blood tests on people. As a result it has not been done for all food ingredients. Published GI values are a grand mix of some that are carefully measured together with many that are approximate or are estimates based on similar ingredients.The most scientific sources give a range for each value. This shows that measurements have been made but, more important, that an ingredient varies a good deal. So GI is not an exact thing, but is a rough guide to 'good' and 'bad' carbohydrates. This does not matter, as for the most part we are just looking for an estimate. This is unlike calculating the energy in food for weight control, where reasonable accuracy is important. Michel Montignac devised a method of measuring GI and on his website it states, "GI estimates are ... necessarily approximate figures. "

GI is not the end of the story. To find the effect that a particular ingredient will have on your diet you need to include the percentage of carbohydrate in the ingredient and how much of it you eat. The carb percentage (carb%) can be found in ingredients tables as 'carbohydrate gram per 100 gram' or similar. Glycemic Load (GL) includes all three factors - GI, carb% and quantity - so is more meaningful than GI. The lower the GL the better. I think that it is most useful to know the GL for 100 gram of the food or ingredient, and you can find that using the upper formula below. You can then scale it up or down depending on the mass of the ingredient using the lower formula. If your food includes say 10 g of the ingredient then the GL will be 10/100 of the GLper100g value given.

GLper100g = GI * carb% / 100

GL for serving = (GLper100g) * (grams of ingredient) / 100

The rule of thumb is that the total GL per day should not exceed 100.

Glycemic index and load are of particular interest to people with diabetes but anyone wanting to eat a healthier diet should use it. It is important for non-diabetic people not to become obsessive. The low-GL rule is a guide to healthy living that may harmlessly be broken sometimes.

Anyone wanting to study diet further now has a wonderful free data source. In the Second World War, when Britain was near to starvation, McCance and Widdowson took on the job of analysing all known food ingredients to ensure that people got the key nutrients. The story can be read on Wikipedia. Their book, 'The Composition of Foods', became a standard. The data in it is now available as a free Excel workbook that may be downloaded from the UK National Archive, for non-commercial use on a single computer. It does not contain GI data. The address is www.food.gov.uk/multimedia/spreadsheets/cofids.xls

If you want to find the GI, or GL per 100g, for an ingredient please feel to use the tables below. I am a great believer in Open Source so this is my contribution. They are early versions of the tables and probably imperfect, but are more extensive than any that I have found. The data are my initial best attempt (a meta-analysis) from a wide range of sources. I intended to document the source of each GI datum and added a field for this. It soon became apparent that this would be far too onerous for what is not an exact set of data. If anyone has a better value for an ingredient, or additional ingredients, please get in touch with the data and the source. I will acknowledge the contribution. If you find these lists useful and have a website, please include the address in it as a link so it is higher in rankings, enabling others to find it more easily in searches.

You can download the three tables that I have produced as adobe files, on the understanding that they are first attempts that might well contain errors. Please remember that I take no responsibility for the consequences of using the data. Remember also that high GI foods are fine if you only have a small amount of them compared with the rest of your diet. No puritans here!

This contains the ingredients sorted alphabetically: adobefiles/GIGLAlpha.pdf

This has the ingredients sorted by GI value, highest first: adobefiles/GInumeric.pdf

This has the ingredients sorted by GL/100g value, highest first: adobefiles/GLnumeric.pdf

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Last edit 9 September 2015