New scoring system aims to set the record straight on ‘bad carbs’

Carbohydrate-rich foods are a ubiquitous and vital source of energy, with many providing high levels of fibre, vitamins and minerals. Despite this, carbs are often misunderstood. There is a very vast array of carbohydrate-containing foods — from grains to vegetables, fruits and sugary snacks. Long-held assumptions tend to separate carbs into simplistic ‘good’ and ‘bad’ categories, a dichotomy that commonly places starchy vegetables — chief among them, the potato — at the bottom of the nutritional pile, along with processed snack foods. But scientific evidence continues to challenge these faulty stereotypes.

“Starchy vegetables really belong with other vegetables and beans, not with candy and soda,” says epidemiologist Adam Drewnowski, who directs the Center for Public Health Nutrition at the University of Washington in Seattle. “The a priori classification of potatoes as ‘bad carbs’ in research analysis is wrong.”

The main challenge for researchers and policymakers has been finding a metric that captures what carb-rich foods truly offer. The glycemic index, a measure of how quickly the body converts carbohydrates into simple sugars, is a tool for diabetes management but has become a shorthand for carbohydrate quality more broadly. However, studies¹ have called into question the validity of this approach, and the latest draft nutritional guidance from the World Health Organization (WHO) recommends against its use as a measure of carb quality.

Now, scientists have developed a classification scheme that corresponds more closely with recommendations from the US government’s Dietary Guidelines for Americans (DGA). The Carbohydrate Food Quality Score (CFQS) is split into two models — one for non-grains such as fruits and vegetables (CFQS-4) and one for grain products like breads, rice and pasta (CFQS-5). Taken together, the system’s developers argue it captures nutritional nuance from across the heterogenous world of carbs. The CFQS could help inform policy, as well as the design of studies on dietary intake and its impact on human health.

“We tried to develop an algorithm that considers the things that are already looked at, but goes beyond to align with the latest dietary guidelines,” says Judith Rodriguez, a dietician and nutritional anthropologist at the University of North Florida in Jacksonville, who, along with Drewnowski and others, developed the new system. “It’s a better measure of carbohydrate quality.”

Settling the score

Rodriguez and Drewnowski are two of six experts who comprise the Quality Carbohydrate Coalition’s Scientific Advisory Council (QCC-SAC), an initiative spearheaded by Potatoes USA to develop quantitative tools for classifying carbohydrate-containing foods. In 2021, the team surveyed the scientific literature and found a handful of previously proposed indicators of carbohydrate quality².

Most were an improvement over glycemic index or glycemic load, which only reflect metabolic responses to food. But by focusing primarily on fibre and sugar, they failed to account for whole grains, as well as other dietary components of public health concern listed by the DGA — namely potassium and sodium.

So the researchers set out to create a new metric.

The CFQS-4 model measures fibre, sugar, sodium and potassium content — the four nutrients referenced in the model’s name — while CFQS-5 adds whole grains as a fifth metric for grain products. The models thus encompass two dietary components that tend to be underconsumed (fibre and potassium), two that are overconsumed (free sugars and sodium) and an assessment, where relevant, of whole grains, which provide essential nutrients and offer a range of health benefits compared to their refined counterparts.

All in the preparation

The researchers applied the metric to more than 2,500 different carb-rich products. The CFQS system placed a large proportion of beans, vegetables and fruits — all foods high in potassium and low in sodium — among the top-scoring carbohydrates, along with cereals, oatmeal, whole-grain breads and crackers³.

Drewnowski then took this one step further. With colleagues, he compared several different proposed models for capturing carbohydrate quality, including the CFQS. While these indices commonly challenged popular assumptions about carb quality, the additional metrics considered by the CFQS models did have a marked effect on the distribution of ‘high-quality’ carbs.

Because of their high carbohydrate-to-fibre ratio, starchy vegetables such as potatoes, corn and peas have tended to score low in rankings of carbohydrate-rich products. But factor in sodium and potassium content, as the CFQS does, and the picture begins to look different. Under this model, a boiled potato or a baked yam scores well above sweetened beverages and closer to leafy greens, whole fruits and other foods previously acknowledged as ‘high-quality’ carbohydrates⁴.

According to Drewnowski, the maligning of many carbohydrate-rich foods, including potatoes, likely has less to do with their intrinsic dietary worth and more to do with background diet and lifestyle factors. Serve your carbs as a fast-food burger and fries, with apple pie for dessert, and it changes the nutritional calculation.

“French fries, contributing potassium and vitamins C and B6, could be the most nutritious part of a teenager's meal,” says Drewnowski. “Potatoes have an intrinsic nutritional value and ought to stay with vegetables where they belong.”

The carb universe

To date, most of the analysis of carbohydrate quality comes from nutrient profile data compiled by the US Department of Agriculture, which maintains a repository of foods for researchers to study. That database includes information on carbs prepared thousands of ways. In the potato category alone, there are descriptions of nutrient levels when they are boiled, baked, fried, mashed, roasted or made into salad.

But there’s no listing for Mexican potato tacos. No Filipino cassava cake. No Puerto Rican yuca empanadas. Rodriguez is leading an effort to fill gaps on some of the most-eaten dishes across the United States — cultural staples that may have otherwise been unfairly lumped into the ‘bad carb’ category. “Everybody eats all kinds of cultural foods now,” she says. “These foods are important, and in the right context, they can be excellent, high-quality carb sources.”

The designers of the CFQS hope the new metric will reach researchers and policymakers. The score aligns with the aims of many public health bodies to promote nutrient-dense foods that are low in free sugars and sodium, and high in potassium, fibre and whole grains. It could therefore simplify the task of advisory groups within the WHO or the US government as they update guidelines for carbohydrate intake within a balanced diet.

Confusing and outdated messaging around carbohydrates has done a disservice to the general understanding of what they offer nutritionally. Beyond crude groupings of ‘good’ and ‘bad’ carbs, new approaches like the CFQS serve as a reminder of both their importance and their heterogeneity.

“You really cannot have full nutrient density without carbs,” says Drewnowski. “What we have shown is just how broad the carbohydrate universe is.”