What does a GI value of 70 mean? It means that on any given day, an exchange of white bread will have approximately 70 percent of the glycemic impact of an exchange of glucose sugar; that is, it will send sugar into the bloodstream 70 percent as fast as glucose sugar.
The authors of the Tufts study erroneously concluded that the GI value of the very same bread varied from 44 to 132. That's not correct. The bread didn't vary: Its GI value remained 70. The GI is a property of the food; the person is simply the instrument of measurement.
The Tufts researchers did not follow GI testing protocol, which is carefully designed to compensate for the variability inherent in using a person as a test instrument. As a result, all the Tufts study measured was the natural variation in individual responses to food at different times. Unfortunately, they mistook that for variation in the bread itself.
It's true that any one person's glycemic response varies from day to day and that glycemic response also varies from person to person. Nevertheless, the relative glycemic differences between foods are maintained. A person with diabetes can be confident that a high GI food (GI value of 70 or more) will produce a significantly higher glycemic response than a low GI food (GI value of 55 or less) 95 percent of the time. Just as the height of high tide and low tide varies from day to day and place to place, we still know that high tide will be higher than low tide on any one day at any one place. It's not the absolute level, but the difference, that's important.
Any one person's glucose tolerance may vary from day to day by as much as two-fold. (In people with diabetes, this variability is actually less than in normal subjects.) Why this occurs is not clear, but we can point to varying beta-cell responsiveness and insulin sensitivity, factors that are beyond anyone's control. The beta cells just work better on some days than on others. This variability among and within people must be managed carefully in order to detect true differences in the glycemic potential of the carbohydrates in different foods.
That's why GI testing has such a strict protocol. Ten subjects are used, each of whom is given the reference food (glucose) on three separate days. Each time, the overall fluctuation in their blood sugar is determined by testing their blood glucose eight times over a period of two hours. The findings from those three days of testing are averaged to find each person's usual response to the reference food, glucose. Next, his or her glycemic response to the test food is measured once, using the same two-hour testing protocol. Then each person's response to the test food is expressed as a percentage of their average response to the reference food. Finally, the relative responses of all ten subjects to the test food are averaged. This is the published GI value. The GI value of bread (70) means that the overall fluctuation in blood glucose after eating an exchange of white bread will be about 70 per cent of the effect of pure glucose (GI value of 100).
When it's done properly, there's nothing crude about GI testing. By taking the average of ten subjects, each of whom has undergone this painstaking process, we are simultaneously compensating for both within-subject and between-subject variability. If we were to test white bread over and over again using this protocol, we'd get the same result: a GI of 70.
The Tufts researchers did not satisfy the strict GI testing protocol. They repeatedly compared one test of the reference food (glucose) with one test of the test food (white bread). Inexplicably, they did this three times with three different groups of people. In a small sub-group of subjects who participated in all three groups (that is, who received the reference food on three occasions), the Tufts authors were able to calculate the GI according to the standard protocol. And lo and behold, they arrived at 70!
The Tufts study's within-subject variability was also heightened by the fact that it used venous sampling, not fingertip capillary sampling. Capillary testing is associated with much less variability than venous testing. We insist on capillary testing in the Australian Standard for GI testing (which is under review by the International Standards Organization).
The Tufts study uses the variability inherent in individual glycemic responses to criticise the GI. Were that a legitimate criticism, then that same variability could be used to denigrate carbohydrate counting as well. Can we be sure that 15 grams of carbohydrate in white bread will always give half the glycemic response of 30 grams? No, we can't. It will also vary for the same reason: day-to-day within-subject variability. Yet carb counting is considered a cornerstone of good diabetes self-care.
Meta-analyses show that a diet based on low GI carbohydrate foods (compared to a conventional low fat diet) will reduce A1c's by an average of 0.6 of a point, about the same amount as a serious exercise program. Other meta-analyses show that low GI diets improve blood lipids and weight control. Longer studies are still required, but long term prospective cohort studies indicate that diets with a low GI are likely to reduce the risk of chronic disease.
– also published online
in Diabetes 3.health October 18, 2007The world is fat – some food for thought from Prof. Barry Popkin Prof Barry Popkin
- ‘The long-held philosophy of agricultural experts is that once a country produces enough grains and tubers, it should massively subsidise its livestock, poultry and fish industries. The result has been a major reduction in the prices of animal-source foods.’
- ‘The drop in the cost of vegetable oils and animal-source foods combined with the recent increases in personal incomes in China, India and other developing nations, has led to a consumer revolution. People are rapidly abandoning their traditional low-fat high fibre diets and switching to meals of calorie-rich fats, sweeteners and refined carbohydrates.’
- ‘The world is getting fatter, and the annual rates of increase are higher today than they were 15 years ago. Representatives of the food industry have long insisted that governments should not restrict an individual’s dietary choices. Their solution is to teach people how to control their diets and become more physically active.’
- ‘This strategy ignores the vast social, technological and structural changes that are pushing millions of people into debilitating lives of obesity. If left unchecked, the nutrition transition will cause horrendous increases in illness and devastating reductions in life expectancy.’
– Scientific American
Barry Popkin’s book, The World Is Fat
is in press and due for publication May 2008 (Penguin Press).Are we sleepwalking towards obesity
The technological revolution of the 20th century has led to weight gain becoming inevitable for most people, because our bodies and biological make-up are out of step with our surroundings, says the latest report from Foresight, the UK Government's futures think-tank.
‘Foresight has for the first time drawn together complex evidence to show that we must fight the notion that the current obesity epidemic arises from individual over-indulgence or laziness alone,’ said Sir David King, the Government's Chief Scientific Adviser and head of the Foresight Programme. ‘Personal responsibility is important, but our study shows the problem is much more complicated. It is a wake-up call for the nation, showing that only change across many elements of our society will help us tackle obesity. Stocking up on food was key to survival in prehistoric times, but now with energy-dense, cheap foods, labour-saving devices, motorised transport and sedentary work, obesity is rapidly becoming a consequence of modern life.’
The Foresight project was an in-depth two-year study by almost 250 experts and scientists to examine the causes of obesity and map future trends to help the UK Government plan effective policies both now and in the future.
– Department For Innovation, Universities And Skills News ReleaseDownload the report
: 'Tackling Obesities: Future Choices'