How ‘fast carbs’ can undermine your health

In recent weeks, foods of all kinds have flown off the shop shelves as people stocked up to weather the coronavirus pandemic. But sales of “comfort foods” such as crisps, pretzels and biscuits have seen a particularly dramatic surge.

That may not be surprising: they are cheap, satisfying and shelf stable.

Unfortunately, for the many sheltered at home, avoiding the urge to make frequent trips to the kitchen throughout the day to snack on these foods can be tricky.

Dr David Kessler, a former commissioner of the Food and Drug Administration in the United States, has a simple message for people who want to keep their metabolic health and weight in check when temptation is just a few steps away: try to avoid eating foods that contain what he calls “fast carbs,” such as refined grains, starches, corn and sugar.

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These foods, such as bagels, bread, breakfast cereals, juices, tortilla chips and anything made with processed flour, tend to be highly processed and devoid of fibre. They are rapidly absorbed and converted to glucose in the body, causing blood sugar and insulin levels to spike and preventing the release of hormones that quench hunger. Over time, researchers have found, this pattern of eating can wreak havoc on metabolic health, leading to weight gain and increasing the risk of type two diabetes and cardiovascular disease, conditions that can increase the risk of complications from Covid-19.

In his new book Fast Carbs, Slow Carbs, Kessler explores the science behind highly processed carbohydrates and how they affect our physiology.

Yet even he is not immune from the problems plaguing many of us. For years he yo-yo dieted and fought to control his weight. To this day he struggles to resist the pull of fast carbs – bagels are his biggest weakness – especially when stress and tensions are high. “We’re all stressed and anxious right now, and people need comfort,” he said. “Despite the fact that I know what’s good for me and I’ve written this book, I still find myself reaching for things that are fast carbs, and two minutes later I say to myself: ‘Why did I do that?’”

Obesity and metabolic disease are complex conditions, driven by a variety of factors, including genetics, environment, diet and lifestyle. But after poring over decades of research and interviewing leading nutrition researchers, Kessler found that one thing most successful diets have in common is that they limit highly processed carbs.

Industrial processing

Yet, foods that contain these fast carbs have become a mainstay for many people. Humans have been processing foods in various ways for thousands of years, whether cooking, boiling, grinding or milling them. But Dr Kessler argues that the industrial processing of carbs that occurs today has a far more pronounced effect on food than the techniques used by our ancestors. “If cooking and milling were early forms of processing,” he writes, “today’s food manufacturing strategies are more aptly called ultraprocessing”.

Most of the grains that are used in foods such as breakfast cereals, corn chips and crackers are milled by high-speed steel rollers. Then they are further pulverised through a variety of high-pressure techniques. One of these is extrusion cooking, a thermal and mechanical process that dramatically alters the chemical structures of grains, breaking down their long chains of glucose into smaller starch molecules that can be rapidly digested.

“The physical properties of the original starch molecule are no longer the same,” Dr Kessler writes. “The granule structure has been destroyed, the glucose polymer chains have been reduced in size, and their surface area has expanded, which increases how fast we absorb these foods from our digestive tract into our bloodstream.”

Our intestines average about 25 feet in length, an evolutionary adaptation that allows us to gradually extract glucose from relatively intact starches as they move through our systems. But processes such as extrusion essentially predigest starches for us: they arrive in our stomachs as a soft, porous paste, and the glucose they contain is largely absorbed in the first part of the small intestine beyond the stomach, the duodenum, circumventing the need to travel through the whole digestive tract.

“Highly processed carbs short-circuit our innate biology,” Dr Kessler writes. “The laborious series of steps we developed over millennia to digest whole fruits, grains and vegetables through the entire length of the digestive system is undermined.”

This creates a number of metabolic problems.

Slow carbs such as broccoli, beans and brown rice slowly release glucose as they travel through our systems, eventually reaching the lower parts of the gastrointestinal tract. There they trigger a hormone called GLP-1 that tells our bodies we are being fed, resulting in feelings of satiety. But because fast carbs are rapidly absorbed in the upper parts of the digestive tract, they flood our systems with glucose and insulin, the fat-storage hormone, while failing to stimulate GLP-1. As a result, Dr Kessler said, they fail to turn off our hunger switch.

Neurological response

At the same time, studies suggest, they elicit a potent neurological response, lighting up the reward centre in the brain in a way that compels people to eat more even when they are not hungry. Processing also affects the amount of calories that we absorb from our food. When we eat a starchy carb that is minimally processed, much of it passes through the small intestine undigested. Then it is either used by bacteria in the colon or excreted. Industrial processing makes more of those calories available to our bodies, which can accelerate weight gain.

Dr Kessler stressed that he is not telling people they should never eat these foods – just to be mindful about what they are and how they affect their health. The less often you eat them, he said, the less you will crave them.

He encourages people to follow three steps to improve their health. Limit fast carbs and prioritise slow carbs such as beans, legumes, whole grains, nuts, fruits and vegetables. Watch your LDL cholesterol, a strong driver of heart disease, and eat a largely plant-based diet to help lower it if necessary. And lastly, engage in daily exercise to help control your weight and improve your overall metabolic health.

“When we get through this current epidemic, we are all going to want to be healthy,” he said. “We know what it takes. But doing it is really hard, and we have to work on it.” – The New York Times