"This study is the first to definitively demonstrate that fast carbs do not make you fat," said study co-author Glenn Gaesser, PhD, professor of exercise science in the College of Health Solutions at Arizona State University.
"Contrary to popular belief, those who consume a diet of high-GI foods are no more likely to be obese or gain weight than those who consume a diet of low-GI foods. Furthermore, they are no less likely to lose weight."
The Grain Foods Foundation provided funding support for the study published in Advances in Nutrition.
Good carbs, bad carbs?
“High-glycemic index (high-GI) foods (so-called fast carbs) have been hypothesized to promote fat storage and increase risk of obesity,” wrote researchers in the study.
“Central to the hypothesized link between high-GI diets and excess body weight is the carbohydrate-insulin model of obesity. This model proposes that high-GI foods are particularly fattening because they elevate postprandial insulin secretion, which has direct effects on accelerating storage of fat. However, the validity of the carbohydrate-insulin model of obesity has been questioned,” noted researchers.
To investigate the connection between high-GI foods and excess weight gain, researchers analyzed data on 43 cohorts from 34 publications (comprising nearly two million adults in total) to assess if dietary glycemic index impacts body weight.
Researchers observed that from 27 cohort studies that reported results of statistical comparisons, 70% showed that BMI was either not different between the highest and lowest dietary GI groups (12 of 27 cohorts) or that BMI was lower in the highest dietary GI group (seven of 27 cohorts).
Results of 30 meta-analyses of randomized controlled trials from eight publications demonstrated that low-GI diets were generally no better than high-GI diets for reducing body weight or body fat.
“GI, as a measure of carbohydrate quality, appears to be relatively unimportant as a determinant of BMI or diet-induced weight loss,” said Gaesser.
Carbs, regardless of type, play an important role in healthy eating, say researchers
"The key takeaway is that carbohydrates, regardless of type, can be part of a healthy diet and have a place on a healthy plate," said study co-author Julie Miller Jones, PhD, LN, CNS.
But, just like any other healthy diet, moderation plays a role, added Jones.
"Over the past few decades, we've seen the blanket vilification of carbs, processed foods, and foods made with refined grains. Science has shown that these foods in the right balance can be part of a dietary pattern that can promote a healthy weight and reduce disease risk. The truth is that eating a wide variety of carbohydrates, from fast-carb white bread to slow-carb bran flakes and pairing them with smart choices from all the food groups can provide the nutritional benefits that healthy carbs, especially whole and enriched grain staple foods, can offer."
Study limitations, the question of satiety
Researchers of the study acknowledge several limitations including that studies analyzed did not mimic real-world situations where participants would have access to many more food choices in unlimited amounts; nor did it address the question of satiety.
As previous research has shown, diets rich in low-glycemic foods have been found to be generally more satiating (and therefore individuals are less likely to overeat) compared to a diet higher in high-glycemic foods.
“This may not represent real-world eating situations because most foods are rarely ingested singly and in prescribed amounts. The glycemic response to a meal with carbohydrate-containing foods can change depending upon the macronutrient composition and dietary fiber content of the meal, preparation of the food, and the time of day that the food is consumed,” noted researchers.
Additionally, the findings suggest that while high-GI foods may not lead to weight gain any more than low-GI foods, their consumption does not lead to weight loss, researchers clarified.
"The review questions the premise that low-GI diets lead to substantially better weight control outcomes and reminds us of the many other qualities of carbohydrates that are far more important to consider: for example, nutrient density, dietary fiber and whole grain content, and percentage of added sugar," added study co-author Siddhartha Angadi, PhD.