In the world's first study of its kind, University of Sydney researchers have found conclusive evidence that diets with a high GI (Glycemic Index - a measure of how different foods affect your blood glucose levels) leads to a higher risk of common lifestyle diseases.

The study, conducted by Alan Barclay, Joanna McMillan Price, Professor Jennie Brand-Miller and colleagues, found high blood glucose led to high risk of type 2 diabetes and heart disease, and is also linked to gall stones and some types of cancer.

In the study, published in the latest edition of the American Journal of Clinical Nutrition, Barclay and his colleagues looked at the diets of nearly two million (1,950,198) healthy men and women worldwide in a systematic analysis of 37 key studies.

"The key message from this study," says lead author Alan Barclay, "is that the GI of your diet is a powerful predictor of disease risk. Grandma was right, you are what you eat."

Because GI ranks carbohydrates according to their effect on our blood glucose (sugar) levels, Barclay said it's not surprising to find a link between high GI diet and diabetes. "If you have constantly high blood glucose (sugar) and insulin levels due to a high GI diet, you may literally 'wear out' your pancreas over time. Eventually it may lead to type 2 diabetes in older age."

He was more surprised by the "strong relationship" between GI and some of the other common diseases. "There's good evidence from the studies that have been done that high GI diets are linked to cancer as well. This is because constant spikes in blood glucose that cause the body to release more insulin also increase a related substance called 'insulin like growth factor one' (IGF-1).

"Both these hormones increase cell growth and decrease cell death, and have been shown to increase the risk of developing cancer.

"Other research shows that a high GI diet tends to reduce 'good' HDL cholesterol levels and raise triglycerides levels; bad news for cardiovascular diseases. And people with low HDL cholesterol and high triglyceride levels are more prone to gall stones. Up until now, people have considered fats and salt, not carbohydrates, to be the major contributor to heart disease.

"Low GI foods have benefits for everybody - they can keep you feeling full longer, help you maintain a healthy weight, and provide you and your brain with more consistent energy throughout the day. They can also have a major effect on whether or not you develop health problems."

The study is published in the latest edition American Journal of Clinical Nutrition March.

Glycemic Index explained

High GI foods are carbohydrate foods (sugars or starches) that break down quickly during digestion, causing blood glucose levels to increase quickly, and stay higher for longer. Examples include most breakfast cereals, smooth-textured bread, short-grain rice, potatoes and most biscuits and crackers.

Low GI foods are carbohydrates that breaks down slowly during digestion, releasing glucose gradually into the bloodstream. Examples include some grainy breads, natural muesli and most fresh fruit and some starchy vegetables.

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