The idea that all carbohydrates are not created equal has become the foundation of many popular diets.
The idea that all carbohydrates are not created equal has become the foundation of many popular diets. Some argue that foods like white bread and potatoes, which have a high so-called glycemic index because they spike blood sugar and insulin, should be avoided in favour of more healthful carbs like whole grains.
But rigorous new research from the National Institutes of Health suggests that for people who already follow a healthful diet, the glycemic index may not be very important.
The study, published in JAMA, found that diets containing low glycemic foods did not lower cholesterol and other heart disease risk factors compared to diets containing mostly high glycemic foods. Nutrition experts argue that low glycemic diets improve blood sugar levels and insulin sensitivity. But the new study found that low glycemic diets actually made insulin sensitivity worse.
Evidence from studies has been mixed, and many question its usefulness of glycemic index. The 2010 Dietary Guidelines for Americans encourage people to eat whole grains and “naturally occurring carbohydrates”. The guidelines caution that “strong evidence” shows no relationship between the glycemic index of foods and body weight.
In the new study, researchers wanted to find out whether diets that were similar in calories and carbohydrates but composed of either high or low glycemic foods had different effects on cardiovascular health. So they recruited 163 people who were mostly overweight and had high blood pressure, putting them at greater risk of heart disease and Type 2 diabetes.
The subjects were rotated through four diets for five weeks at a time, with all of their food provided to them. The researchers said they devised each diet to be heart healthy, with plenty of fruits, vegetables, beans, fish, poultry, lean meat and grains.
The researchers then altered the types of carbs they contained. The low glycemic diets included things like whole grain bread and cereal, apples, steel-cut oats, and non-starchy vegetables. The high glycemic diets allowed things like white bread, carrot and bran muffins, instant rice and instant oatmeal, and sweet snacks like honey, bananas, and apricots in heavy syrup.
When the overall amount of carb intake was lowered, cardiovascular risk factors like cholesterol, triglycerides and blood pressure moved in the right direction. But when two diets had similar amounts of carbs and calories, the low glycemic approach did not improve insulin sensitivity, cholesterol or blood pressure levels.
Dr Frank M Sacks, the lead author of the study, said other trials carried out were even longer had reached similar results. He said people should eat whole grains, fresh produce and high fibre foods because of the nutrients they contain. But unless someone has diabetes and must monitor their blood sugar levels, people who are already following a healthful diet do not need to worry about the blood-sugar impact of one type of fruit or grain versus another.
– By Anahad O’Connor