There is no globally accepted definition of carbohydrate quality – no equation, algorithm, or succinct way of identifying the quality of a carbohydrate-containing food, meal, or diet. Unlike protein and fat – where deficiency can lead to serious and even life-threatening situations – the physiological impacts of carbohydrate food intake are much more complex and individualized. Effects can vary with one’s lifestyle, dietary habits, health status, and age – among many other factors. Despite this complexity, carbohydrate dietary guidance often heavily relies upon a single, simplistic marker – Glycemic Index (GI) – despite its notable flaws and application limitations.
The Alliance for Potato Research and Education interviewed a select group of carbohydrate experts to better understand how the carbohydrate quality debate got to where it is today. This is the second of two articles exploring this topic – read the first article here.
Glycemic Index is an oversimplified approach with unintended consequences
Since it was developed in the 1980’s, the GI has been used as a simple surrogate for glycemic response – which was then made a measure of carbohydrate quality. Yet, over the decades, numerous scientific studies and nutrition experts have indicated that although the GI has its utility in a clinical or laboratory setting, this metric has significant shortcomings that make it a less-than-optimal measure of carbohydrate quality for consumers.
For one, experts argue that GI is a narrow indicator. Jamie Baum, PhD from the University of Arkansas states that “GI alone is not the right tool to determine the quality of a carbohydrate food. Among other things, it fails to account for total nutrition – the vitamins, minerals, fiber, and protein – in a food.” According to Julie Miller Jones, PhD, professor emeritus from St. Catherine’s University, negative health consequences can arise from selecting foods solely based on GI. “Some low-GI foods are high in fat and fructose, and elevated intake of many of these foods is not advisable. Meanwhile, avoiding breads and cereals – especially whole-grain versions – based on their high GI value might lead to low intake of cereal fiber, a dietary component associated with lower risk of cardiovascular disease and obesity. While avoiding potatoes due to GI concerns means missing out on a food that has a high satiety value and is a low-cost source of fiber and potassium.”
Even as a research tool, GI has its limitations. Several studies suggest that the GI is difficult to replicate, even in the same person following the same protocol. While researchers have gotten better over the years with standardizing procedures to minimize variability, people in the real world don’t live in the regimented way of a laboratory setting. Human routines often differ from day-to-day or week-to-week. Consequently, the GI results gathered under rigid lab conditions rarely reflect the actual response in free-living situations.
Dr. Brooke Devlin, PhD from Australian Catholic University pointed out several confounders that make reliance on GI a dubious endeavor. “In the laboratory, the GI of a food is assessed by testing that food in isolation, and not as part of a meal. This is not how people eat in the real world. In addition, GI is usually tested in healthy subjects in the morning, limiting its utility for people with type 2 diabetes or in assessing its effects when eaten at different times of the day, including before or after meals. Finally, GI in the lab is based on the body’s response to 50 grams of available carbohydrate in a food. In order to consume 50 grams of available carbohydrate, one would have to eat an entire mini watermelon or unreasonable quantities of many other foods. The volume of food required to obtain a GI value could certainly produce a false perspective of the food’s impact on the body in normal circumstances.”
Nick Bellissimo, PhD from Ryerson University in Toronto further elaborates on the concern with applying the GI to real-life settings, where mixed meals are typically eaten, and different preparation techniques are used. He explains, “a small amount of butter or oil added to a high GI food can greatly reduce the glycemic response to that food and, consequently, its GI. Additionally, evidence shows that two forms of the same food can lead to highly different glycemic responses – such as choosing a store-bought frozen potato product versus a homecooked potato. This makes GI an inadequate tool for representing the highly varied food options chosen by consumers today.”
For all these reasons, Richard Bruno, PhD, RD – a nutrition scientist at The Ohio State University – says, “I don’t put too much stock in the GI as a marker of carbohydrate quality. In a research setting, including my own where we study acute hyperglycemia on vascular dysfunction, GI is a helpful tool to model responses to isolated foods or to induce a physiological effect. However, it is nearly impossible to predict the GI of a mixed food based on its macronutrient composition. There are too many variables in play including carbohydrate type, and interactions with other dietary constituents.”
Research and guidance must move beyond GI to consider other impacts on health – both glycemic and otherwise
While carbohydrate-containing foods have often been classified according to their GI value, this solitary and flawed focus needs to shift. According to Siddhartha Angadi, PhD, a cardiovascular exercise physiologist and assistant professor at the University of Virginia, carbohydrates may not even be the most concerning macronutrient for insulin resistance and impaired glucose tolerance. “Physiologists have known for close to a century that if you want to make someone insulin resistant or worsen glucose tolerance in a clinical laboratory setting, you feed them an excess amount of fat (especially saturated fat) – not carbohydrates,” explains Dr. Angadi. “The decades-long focus on defining carbohydrate foods based on GI therefore needs to shift to better health indicators, especially when considering outcomes related to chronic disease.”
Of course, it is important to note that the health impact of a carbohydrate-containing food is not just related to its nutritional components. Dr. Angadi explains that “factors such as genetics, physical activity, sex, age, and body mass index can have just as much of an impact on the body’s metabolic response as the food itself.” Meanwhile, individual choices such as food preparation techniques or specific meal combinations can also have a notable impact, as explained by Dr. Bellissimo.
Developing a new definition for quality carbohydrate-containing foods thus requires a shift away from the singular focus on GI towards a more holistic approach. While focusing on the nutritional value of a food is an important first step in defining quality, the approach must also recognize the need for individualistic adaptations based on personal lifestyle choice and genetic factors. Only then can a quality carbohydrate definition be a truly valuable tool in helping consumers choose the right foods that promote their health.