The use of glycaemic index (GI) ranks carbohydrates according to their ability to affect blood glucose. A measure of the quality and quantity of carbohydrate intake, the concept of glycaemic load (GL) was created, accepted by many experts as the better measure, but said by some to complicate an already complicated measure for many consumers.
And the use of GI/GL has been criticised by some as being "unrealistic" by some scientists.
The general 'take-home' message for consumers therefore has been "low GI good, high GI bad."
Such statements could change however if researchers, like the Sydney team, continue to report positive results from well-controlled intervention trials.
To test the effects of different diets in a free-living young population, the researchers from the University of Sydney recruited 129 overweight and obese people (average age 32.3, average BMI 31.2 kg per sq. m) and randomly assigned them to one of four test diets; Diet 1: High carbohydrate (55 per cent), average protein (15 per cent), high GL (127 grams). Diet 2: The same as diet 1 but with low glycaemic load (75 grams). Diet 3: High protein (25 per cent), reduce carbohydrates (45 per cent), high GI (87 grams). Diet 4: Same as diet 3 but with low glycaemic load (54 grams).
The subjects consumed the diets for 12 weeks, and blood samples were taken at the start, mid-way and end points to measure blood levels of glucose, insulin, leptin, total cholesterol, HDL-cholesterol and triglycerides.
Dual-energy X-ray absorptiometry was also used at the start and end of the trial in order to assess changes in body composition.
Lead researcher Jennie Brand-Miller and her colleagues found that there was no significant differences in weight loss between the diet groups (averaging 4.2 to 6.2 per cent).
However when the researcher looked at the proportion of individuals who lost five per cent or more, significant differences were seen, with 66 per cent of people following diet 3 losing five per cent or more. Diets 1, 2 and 4 lead to weight loss of five per cent or more for 31, 56, and 33 per cent of the subjects, respectively.
In terms of the markers for cardiovascular disease, namely blood lipid levels, no significant differences between the groups were seen. However, levels of total and LDL-cholesterol increased for people in the third diet group (five and eight per cent increases, respectively).
Interestingly, in the reduced carbohydrate, low GL diet 2 group led to reductions in these lipid values over the 12 weeks. (four and six per cent for total and LDL-cholesterol, respectively).
"The favourable result for diet 2 could not simply be the result of its low-fat content, because the other high-CHO group [diet 1] also achieved a very low-fat intake yet the slower rate of fat loss," wrote the authors.
The results are published in the Archives of Internal Medicine (Vol 166, pp. 1466-1475).
Women participants and volunteers with high triglyceride levels also appeared to benefit more from following the high-carb, low-GI, diet 2 regime.
"Diets based on low-GI whole grain products (in lieu of whole grains with high-GI) maximise cardiovascular risk reduction, particularly if protein intake is high," concluded the researchers.
In an accompanying editorial from Simin Liu, MD, from the University of California, Los Angeles, said that the findings provide a "welcome insight" into a field that is currently "shrouded in confusion".
"My best recommendation is to move forward incrementally. For example, for a first step, we should encourage the use of GI and GL concepts in conjunction with caloric density and nutrient composition, especially for ranking high-carbohydrate starchy foods," said Liu.
The GI/GL diet does have its critics, like, for example, Dr Glenn Gaesser, co-chair of the US Grain Food Foundation's clinical advisory board who said last summer that the growth in popularity was being pushed by the industry.
"With the GI craze we have a case of the tail wagging the dog- everyone is following along for fear of losing market share.
"The utility of the glycaemic index and glycaemic load (GL) with regard to health and weight control is overstated and not backed by a fair amount of published research," he said.
CVD causes almost 50 per cent of deaths in Europe, and is reported to cost the EU economy an estimated €169bn ($202bn) per year. According to the American Heart Association, 34.2 per cent of Americans (70.1m people) suffered from some form of cardiovascular disease (CVD) in 2002.