Bronwen Dalziel reports on findings that low fat, high carbohydrate diets may not be helping our waist lines, and may even have dangerous consequences.
Eating carbohydrates could prevent us from burning up our fat stores, according to a startling finding from new research at the University of Sydney.
This means that when nutritionists recommend a diet that is low in fat and high in carbohydrate, they could be inadvertently recommending one that turns us away from burning our fat and instead gives us dangerous rushes of sugar.
Over the past few years Prof Jennie Brand-Miller and her group have realised that not all carbohydrates have the same effect on our body. By ranking our food according to a glycaemic index (GI) - a measure of how quickly carbohydrates are digested and enter the blood as glucose - the researchers can rank foods from 0 to 100 into low or high GI foods, respectively.
Glucose is ranked the highest at 100 as it gives the maximum blood sugar level over 2 hours. White bread is ranked at around 70 as it will give about 70% of the response of glucose for the same amount of carbohydrate (Table 1). Most people with diabetes will be familiar with this ranking as it proves invaluable for controlling their blood sugar levels.
However, the Sydney group’s latest research shows a high GI diet could be important to all of us, not just patients with diabetes. One experiment found that fat cells are 10% larger in rats fed a rapidly-released high GI diet for 3 weeks. This surprised the Sydney group because the rats ate exactly the same amount of calories as a control group fed a low GI diet.
Every time we (or rats) eat a high GI meal, we send a flood of glucose into the blood stream. The body copes with this quickly by sending out a hormone called insulin. Insulin’s job is to put the glucose into cells where it can be stored, burned as fuel or even made into fat.
At the same time, insulin sends a signal to our fat cells to stop burning fat, so that all the glucose we ate can be used up first. In effect, it changes us into a carbohydrate burner rather than a fat burner.
This high GI sugar rush also causes the body to over-react so that it increases the amount of insulin released and our sugar levels come crashing down. This sudden change in blood sugar sets off alarms in the body so that new hormones tell the body to eat again.
Dr Sue Holt, a biochemist on the “GI team”, has shown that people eat more after eating high GI food. In her experiment the subjects ate rice cakes that were either high or low GI and after 2 hours they were allowed to eat from a buffet. She found the subjects ate more after high GI food. A similar experiment at Harvard University with obese teenage boys showed the same overeating effect.
This leads to the theory that eating high GI food can lead to a gain in weight. Not only is our fat being sent back into our fat cells, but we are inclined to eat more, stay full for less time and also burn less energy over the day because we have a lower metabolic rate.
In studies at Harvard, obese children were given the option of going on a low fat diet or a low GI diet. Those children on the low GI diet lost 10 times more weight than the others.
For Australians, the bad news is that many of the highly processed carbohydrates that we eat are classed as high GI. This includes most breakfast cereals and breads and starchy foods such as potatoes and rice. Our bodies are so good at breaking down these foods into glucose that we get a sharp rise in blood sugar after only 10 minutes.
This is cause for some concern, given current advice that is widely publicised as “the food pyramid”, to eat lots of breads and cereals and consume less fat.
Prof Simin Liu from Harvard stated at a recent nutrition symposium in Sydney: “Total fat intake has not yet been proven as a risk for heart disease in several large studies”. He said evidence from research in the USA has shown that a high GI diet is more of a risk factor for heart disease than fat consumption, with “a high glycaemic load consistently having an association with heart disease throughout many studies”.
Many readers might find it difficult to believe these conclusions. Certainly some doctors and the Heart Foundation have been happy to endorse low fat foods with a big red tick regardless of the type of carbohydrate. But this new evidence is compelling and the old advice is currently being questioned.
A recent article in Science documents the US government’s search to explain the increasing incidence of heart attacks in the 1970s. Public demand for action eventually prompted the government to give advice to consume a low fat diet based on opinions from only a handful of researchers. This started a 30-year, multi-billion dollar effort to prove that dietary fat was a major risk factor in heart disease. Unfortunately, they could only show that saturated fat is a contributor, while mono- and polyunsaturated fats are actually beneficial for our cholesterol profile and sensitivity to insulin.
Despite the lack of evidence for dietary fat’s contribution to heart disease, it is hard for health care workers to discard a 30-year belief.
Luckily, Australian nutrition research is leading the way with “GI Jennie’s” laboratory looking closely into the effects of a high GI diet. Investigations over the past 20 years have shown mounting evidence against eating a high GI diet. Not only is our lifestyle of highly processed foods increasing our chances of obesity, there is also increased risk of getting diabetes and heart disease.
Sydney sports dietitian Julie Ward believes “a well-balanced diet including lots of fruit and vegetables, whole grains and low fat meat, fish and diary products still holds true. What we should be encouraging is the inclusion of low GI foods as well, such as lentils, beans and wholegrain breads.”
Bronwen Dalziel is completing research for her PhD in obesity at the University of Sydney, Australia. © Control Publications 2002
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