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Breaking Down How Carbs Break Down In Your Body — Two Diabetes Experts Explain

Cyrus Khambatta, Ph.D. & Robby Barbaro MPH
Nutritional Biochemist & Public Health Expert By Cyrus Khambatta, Ph.D. & Robby Barbaro MPH
Nutritional Biochemist & Public Health Expert
Khambatta earned a doctorate in Nutritional Biochemistry from the University of California Berkeley. Barbaro has a master's degree in public health from American Public University.
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Since the Atkins diet first gained popularity in the early 2000s, carbohydrates have been blamed for a host of metabolic disorders including diabetes, obesity, heart disease, cancer, and insulin resistance. Because of this, low carbohydrate diets have taken the world by storm, resulting in millions of people who actively avoid carbohydrate energy, whether from refined sources like bread, pasta, cookies, and crackers or from whole sources like fruits, starchy vegetables, legumes, and whole grains.

But if you spend time going over the scientific evidence with a fine-tooth comb, you'll find that blaming carbohydrates for insulin resistance and diabetes is shortsighted and lacks scientific rigor.

But before we exonerate all carbohydrate-rich foods as "healthy," let's break down the details of carbohydrate biology so you can see that like fatty acids, all carbohydrates are not created equal and that subtle differences in the way carbohydrates exist in food can make a dramatic difference in your overall health. 

Step 1: Your liver absorbs glucose slowly.

Foods containing intact whole-carbohydrate chains require considerably more time to break down into single monosaccharide units than do foods containing refined carbohydrates, and this limits the rate and amount of glucose that your liver is exposed to during and after a meal. The presence of fiber molecules—which remain fully or partially intact in the absence of a refining process—slows down the digestive process altogether, resulting in a delayed rise in blood glucose following a meal.

Ample large-scale research studies show that simply increasing your intake of intact fiber from whole foods can dramatically reduce the frequency and magnitude of high blood glucose following a meal and reduces your risk for insulin resistance, type 2 diabetes, and cardiovascular disease. Controlling the rate at which your liver is exposed to glucose following a meal by choosing slower-digesting carbohydrate-rich whole foods can dramatically improve the health of your liver, blood vessels, muscle, and adipose tissue while decreasing your level of insulin resistance.

In contrast, foods containing refined carbohydrates and refined sweeteners may lead to large increases in glucose in your blood immediately following a meal. When your liver is exposed to large concentrations of glucose in a short period of time, your brain responds by increasing your total energy expenditure to "waste" excess calories.

Plagued by a rapid and large influx of glucose, your liver does its best to absorb as much glucose as possible to protect other tissues from experiencing the same thing, and your muscles and liver slow the rate at which they burn fatty acids. In effect, multiple organs communicate with one another and say, "Help! Let's all do our part to get rid of this excess glucose as efficiently as possible. Liver—you absorb as much as you can. Brain—increase energy output. Muscles and liver—temporarily reduce your dependence on fatty acids and use this glucose as fuel."

In an effort to dispose of this excess glucose, your liver seeks alternative biochemical pathways, too. One of those pathways results in the conversion of glucose to fat, known as de novo lipogenesis (DNL). De novo lipogenesis is a very well-studied (and unnecessarily controversial) phenomenon in animals and humans and occurs to a small extent either when you increase your intake of refined carbohydrate foods or when you massively overeat calories. In reality, DNL occurs to a very small extent in humans and is a biologically expensive process that is considered a "pathway of last resort" when no other options exist.

Many low-carbohydrate nutrition experts claim that your liver converts large amounts of carbohydrate energy into fat every time you eat carbohydrate-rich foods and therefore suggest that you avoid eating carbohydrate-rich foods to prevent this unwanted carbohydrate-to-fat conversion. Even though this may sound plausible, the truth is that DNL is an active pathway in pigs, rats, mice, cows, dogs, cats, and birds and a largely inactive pathway in humans. Extremely rigorous scientific experiments using state-of-the-art methods (stable isotope tracers) consistently demonstrate that DNL does not occur in humans nearly as much as scientists once believed.

But what about in the setting of insulin resistance—does eating carbohydrate-rich foods increase DNL? While it is true that excess insulin can certainly stimulate DNL, a study performed in insulin-resistant obese men demonstrated that the amount of newly synthesized fatty acids is so small that it does not account for even a minor part of excess body fat present in obesity. In other words, even in the insulin-resistant state when DNL rates are increased, the amount of newly synthesized fatty acids created from DNL in response to eating carbohydrate-rich foods is so small that it is biologically irrelevant.

When you eat carbohydrate-rich foods, your liver does not convert carbohydrate into fatty acids in substantial quantities unless you massively overeat carbohydrate energy to the tune of 2,000 grams per day (or 4,500 excess calories per day) for a minimum of 7 to 10 days at a time!

So, the next time you hear someone tell you to avoid carbohydrates in food because "carbs turn into fat," recognize that this statement is a gross misstatement of the truth, is biologically inaccurate, and demonstrates a fundamental lack of understanding of human biochemistry and human bioenergetics.

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Step 2: Your liver burns glucose.

When you eat whole carbohydrate-rich foods, glucose enters your liver at a physiologically acceptable rate, slowed mainly by the presence of intact fiber molecules. As soon as glucose enters your liver, cells in your liver choose to either burn glucose for energy or store it for later use as glycogen. Glucose that is burned for energy immediately is sent through a series of complex reactions that eventually creates ATP, the cellular equivalent of energy.

A simple way to understand ATP is to think of it as the biological equivalent of money. Money is used to purchase items, pay for services, conduct trade, and power economies. In the same way, ATP is used in biological systems to power chemical reactions, allow pathways to function, manufacture hormones, stimulate muscle contraction, generate electrical impulses in nerves, and much, much more.

Think of your liver as a metabolic command center—a supercomputer that oversees thousands of chemical reactions every second of your life. Given that your liver is involved in a wider range of metabolic activities than any other tissue in your body, cells in your liver have a high demand for ATP, which means they also have a high demand for vitamins, minerals, antioxidants, and phytochemicals. Whole carbohydrate-rich foods are an excellent source of glucose for ATP as well as a diverse collection of micronutrients to facilitate an organized and perpetual biochemical orchestra.

Step 3: Your liver stores glucose as glycogen.

The glucose that is stored for later use is stored as glycogen, a densely packed treelike structure containing thousands of glucose molecules. Think of liver glycogen as a glucose fuel tank that can be accessed whenever liver cells need it. Only your liver and muscle can store large quantities of glycogen, and the amount of glycogen stored in both tissues directly relates to the amount of carbohydrate in your diet. People who eat diets containing more carbohydrate energy increase the size of their liver and muscle glycogen stores over time, whereas people who eat diets low in carbohydrate energy have limited glycogen stores in both their liver and muscle.

The main reason your liver stores glycogen is to maintain a backup supply of glucose for your brain because your brain is incapable of storing glucose for energy and requires a steady supply of glucose in order to function as designed. Your brain is unique in that it has an extremely strong preference for glucose as fuel, as evidenced by a high reliance on enzymatic machinery that is perfectly designed to extract ATP from glucose at all times.

When you restrict the amount of carbohydrate energy in your diet, not only do you shrink the amount of glycogen in your liver and muscle, but your brain is forced to adapt to a new fuel called ketone bodies, which your liver manufactures as an emergency backup fuel to maintain mental function. It's important to understand that maintaining a large glycogen reserve in your liver is an excellent way to "drip feed" your brain with a stable supply of glucose 24 hours a day for optimal function.

In addition to feeding your brain, liver glycogen is used to power itself during times of limited glucose availability—namely, when you're sleeping, when you're exercising, or when you're fasting. Think of liver glycogen as a protective glucose storage tank designed to provide glucose to tissues in need to cover a wide range of daily activities.

Step 4: Your muscles store glucose as glycogen.

When you eat a carbohydrate-rich diet, your liver absorbs some of the glucose that enters your blood but also allows other tissues like your brain and muscle to uptake their own supply. In a given day, your brain will burn up to 60% of all the glucose in circulation because it is the largest consumer of glucose in your body and does not possess the ability to burn amino acids from protein or fatty acids from lipids.

Your muscles are the second-largest consumer of glucose and will absorb 20 to 30% of the glucose in your blood with the help of insulin. Just like your liver, your muscle can either burn glucose immediately for ATP or store glucose as glycogen for later use. Some fraction of all the glucose your muscle absorbs will be burned for energy immediately, and the remainder will be stored as glycogen for later use, during rest and while performing exercise.

Reprinted from Mastering Diabetes by arrangement with Avery, an imprint of Penguin Publishing Group, a division of Penguin Random House LLC. Copyright © 2020, Cyrus Khambatta and Robby Barbaro.

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