To conduct her study, Mozaffarian identified 545 grain products from two grocery store chains in the Boston area (Walmart and Stop & Shop). The products fell into eight categories of frequently consumed grains, including bread, bagels, English muffins, cereals, crackers, cereal bars, granola bars, and chips. Looking at the list of ingredients and nutrition information, she then asked which of five common ways of defining whole grain foods fared best in terms of identifying the most healthful products.

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When all the results were in, Mozaffarian made some surprising discoveries. She found, for example, that the Whole Grain Stamp was indeed useful for identifying foods with whole grains. However, she found that many foods with the stamp were also very high in added sugar, making them less healthful overall. This is because added sugars contribute energy (kcal) to foods but not micronutrients or phytochemicals; thus added sugars reduce the nutrient density of foods, even those that are made with whole grains.

Likewise, looking for whole grains (for example, whole corn meal, whole wheat flour, bulgar) as the first ingredient on food labels identified foods with ample amounts of whole grains, but also tended to identify foods with added sugar. Looking for the word “whole” before grain ingredients further down the list was even less helpful.

It may seem confusing to condemn carbohydrate-rich foods for their added sugar content. After all, haven’t we just learned that all complex carbohydrates are, in essence, chains of linked sugar molecules? The problem is that sugar—whether naturally found in a product or added to it—is a source of energy. And when it comes to energy, it is indeed possible to have too much of a good thing. When foods rich in refined carbohydrates and added sugars are plentiful in our diet, our ability to meet our nutrient needs within our energy budget is reduced. Over time, ingesting more calories than we expend in activity will cause weight gain.

As we will see, different carbohydrate-rich foods also vary in the amount of time it takes to digest them into simple sugars, which affects how many calories we are prone to consume.