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Why This MD Doesn't Like The Term "Complex Carbs"

David Kessler, M.D.
Former FDA commissioner
By David Kessler, M.D.
Former FDA commissioner
David Kessler, M.D., is a pediatrician and has been the dean of the medical schools at Yale and the University of California, San Francisco. He served as commissioner of the US Food and Drug Administration under Presidents George H. W. Bush and Bill Clinton.
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March 31, 2020

Until the 19th century, sugar was expensive and rare in our diet, but today, this rapidly absorbable carb is available everywhere. Many people who try to lose weight already know to avoid sugar, but they may not realize that rapidly digestible starches, including bread, other baked goods, cereals, and many of our favorite snack foods have the same effect. 

The Dietary Goals issued by the government in the 1970s did not reflect any of this information. Instead, all carbs except sugar were grouped together under the misleading heading of "complex carbohydrates." This is a term that should be retired because it fails to distinguish between carbs that are rapidly absorbed and those that are not.

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That ambiguity has long hidden the true nature of carbohydrates

What neither our public servants nor the scientific experts who were guiding them took into account when formulating national nutrition guidelines were the crucial differences in the way our bodies respond to different types of carbohydrates.

They definitely did not understand the nature of the new fast carbs that were being introduced into the food system. Even now, government guidelines continue to lump together almost all carbohydrates, except sugar, with some distinction for "whole grains." To get back on track, we need to understand what carbohydrates are and how they are used by the body. 

Instead of "complex carbs," we should think of them as "slow carbs."

Long-chain carbohydrates are chains made of glucose molecules, which are essential for life. Glucose fuels essential metabolic functions that produce energy and power our brains. The vast majority of carbohydrates in our diet come from plants and can be categorized into three classes: starches, sugar, and dietary fiber. Virtually all starches and sugar are easily converted to glucose, while dietary fiber is not because human bodies don't make the enzymes to break the unique bonds that hold together the glucose molecules contained in fiber. 

Plants store energy as starch. In their natural state, the starch molecules in grains are extremely long. Before processing, a single starch molecule might consist of over 100,000 glucose molecules, bound together chemically in a twisted chain. Our bodies have evolved to digest these chains and use the energy inside. First, we grind the starch with our teeth, then the enzymes in our gut break down the chemical bonds of the starch, reducing them to simple glucose molecules that can be absorbed in our intestines. 

Unprocessed fiber is made by plants to give their cells rigid structures that can bear weight. It consists of very long chains of glucose, but our enzymes cannot break the bonds that hold the fiber molecules together, so they are absorbed much more slowly. Fiber can only be digested when it reaches our large intestine, where it is fermented by bacteria, producing short-chain fatty acids that can be absorbed. 

Sugars are a much simpler form of carbohydrate. Table sugar (sucrose) contains equal parts of two simple sugar molecules, glucose and fructose. They are quickly broken down in the mouth and absorbed as soon as they enter the small intestine—they never reach the large intestine. 

The bottom line? Timing matters.

As the distinctions among starch, fiber, and sugar demonstrate, the length of a carbohydrate chain and the type of chemical bonds that hold it together make a crucial difference in how quickly these carbs are absorbed by our bodies.

When we talk about the absorbability of starch, we are talking about the pace and degree to which it can be digested, and where in the body that digestion takes place. Absorbability is a crucial consideration when we classify carbohydrates. 

Rather than "refined" or "complex," perhaps the binary should be "fast" or "slow."

Adapted from the book Fast Carbs, Slow Carbs by David A. Kessler, M.D. Copyright © 2020 by David A. Kessler. Published on March 31, 2020, by Harper Wave, an imprint of HarperCollins Publishers. Reprinted by permission.
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David Kessler, M.D.
David Kessler, M.D.
Former FDA commissioner

David Kessler, M.D., is a pediatrician and has been the dean of the medical schools at Yale and the University of California, San Francisco. He served as commissioner of the US Food and Drug Administration under Presidents George H. W. Bush and Bill Clinton. From 1981 to 1984 he served as a consultant for the U.S. Senate Committee on Labor and Human Resources, and from 1982 to 1984 he was special assistant to the president of Montefiore Medical Center in New York. David then went on to serve as medical director of the Hospital of the Albert Einstein College of Medicine from 1984 until President Bush appointed him as FDA commissioner in December of 1990. A graduate of Amherst College, the University of Chicago Law School, and Harvard Medical School, David is the father of two and lives with his wife in California. He is also the author of the book, Fast Carbs, Slow Carbs: The Simple Truth about Food, Weight, and Disease.