Highly processed carbs wreak havoc on our bodies
To fully understand the damage these highly processed foods do to our bodies, we need to review the basics of carbohydrate digestion. First, a disclaimer: our bodies’ digestive and hormonal systems are incredibly complex and even now are not fully understood. What I describe here represents a simplified version of what happens when you eat and digest food. Some of the complexities of this process are the subject of intense debate among scientists, and much of it is the focus of ongoing research.
For example, there is a growing awareness that our microbiome—the tens of trillions of bacteria that live in our bodies and with which we have a symbiotic relationship—plays a much more important role in digestion, metabolism, and overall health than previously appreciated. The full impact of these microscopic tenants is only beginning to be understood, but we are learning more each day about how a healthy and diverse microbiome can aid in digestion, protect us from disease, and possibly help regulate our moods.
But generally speaking, here’s what happens after we bite into a slice of bread or a hamburger roll, a donut or a slice of pizza, a cupcake or a cinnamon bun. Any of these foods will easily absorb your saliva, forming a nice, soft paste in your mouth. By itself, this bolus is not especially pleasurable, as Gail Civille pointed out. But as we’ve seen, that wad of carbohydrate plays an essential role in delivering the fat, sugar, and salt that have been designed to make us overeat.
From the mouth, the bolus travels down the digestive tract, through the esophagus, and into the stomach. Normally, the grinding motion of the stomach and its acidic environment break foods apart, facilitating subsequent digestion. The feeling of a full stomach is one of the vital cues that tells our brains we are satisfied and can stop eating.
But fast carbs arrive in the stomach as a soft, porous paste. The stomach doesn’t have to do any work to ready them for absorption. Instead, the wad of predigested carbs is passed on quickly to the small intestine.
The small intestine has three sections: the duodenum, which receives partially digested food from the stomach; the jejunum, which absorbs most of the nutrients into the bloodstream; and the ileum, which absorbs any nutrients that were not taken up earlier. As nutrients are absorbed, the small intestine allows them to cross the intestinal lining and travel into the bloodstream.
In evolutionary terms, the job of the duodenum is to continue breaking down food through the release of enzymes. But the processing that strips a carbohydrate away from its protective shell turns a very long-chain starch into a very short-chain molecule. There is no need for the food to travel through the whole digestive tract. Instead, the duodenum easily breaks down the short-chain molecules into monosaccharide glucose, which is absorbed through the walls of the small intestine and released directly into the bloodstream.
By doing so, highly processed carbs short-circuit our innate biology. 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.
What happens next is that this large infusion of glucose in the blood quickly triggers the release of insulin. There’s also an earlier place where taste or conditioning can trigger a “cephalic” phase of insulin production. Cells in the duodenum release a hormone called GIP, which increases insulin production. Insulin signals the cells in your muscles and liver either to use the glucose as energy or to store it as fat. Whether you’ve eaten a tablespoon of sugar or a slice of bread made with processed carbs, the effect is similar: your insulin levels spike. In fact, when scientists study the glycemic index, they use pure glucose or white bread interchangeably for testing purposes.
Fast carbs flood our system with spikes of glucose and insulin. And because we are eating fast carbs all day long, these spikes happen one after the other.