Over the past century, few foods have overtaken our food supply quite like sugar and sweeteners. Yet despite decades of research, the relationship between added sugar and human health is still a controversial topic. Around the globe, public health guidelines—including the World Health Organization, the Dietary Guidelines for Americans, the Nordic Nutrition Recommendations, and Public Health England—recommend limiting added (or “free”) sugar intake to 10 percent of total calories or less, due to well-established effects on dental health, total caloric intake, obesity, inflammation, and related conditions. At the same time, some people question whether this upper limit is scientifically sound, leading to public confusion about how important it really is to avoid sugar.
Part of the confusion is due to aggressive action by the beverage industry (which relies on high-fructose corn syrup to sweeten drinks) to cast doubt on the validity of sugar research. For example, a systematic review published in December 2016 concluded that public health recommendations to reduce sugar intake were based on weak or inconclusive evidence, but this review was funded by the food and drink industry, which has a vested interest in neutralizing consumers’ negative perception of sugar and sugary foods.
In reality, the evidence for greatly reducing or avoiding consumption of processed sweeteners is compelling. Beginning with sucrose (table sugar) and continuing with high-fructose corn syrup and artificial and noncaloric sweeteners (like sucralose, aspartame, and stevia), these ingredients can be large contributors to chronic disease. Even natural sweeteners like honey and maple syrup aren’t off the hook; although since they contain some redeeming nutritional features, they earn middle-ground status.
What are Carbohydrates?
Carbohydrates are made up of sugar molecules, or saccharides, the most important and prevalent of which is glucose, the primary metabolic fuel for the human body and indeed most forms of life on Earth.
Chemically, carbohydrates are classified based on the number of saccharides they contain: monosaccharides are made up of a single sugar molecule, disaccharides contain two sugar molecules, oligosaccharides are medium-length chains of three to ten sugar molecules, and polysaccharides are long chains of sugar molecules that can be hundreds long. From a dietary perspective however, it’s more relevant to classify carbohydrates based on how they’re digested and absorbed:
- Sugars, also called simple carbohydrates or simple sugars, include monosaccharides like glucose, fructose and galactose, and disaccharides like sucrose (one fructose and one glucose), lactose (one glucose and one galactose) and maltose (two glucoses). Sugars give food a sweet taste and are naturally found in fruit, dairy products and natural sweeteners like honey. They are digested and absorbed quickly and the glucose they contain has a rapid impact on blood sugar levels and insulin secretion. (Note that the presence of fiber in whole fruit helps to slow down the digestion of the sugars in fruit, see Why Fruit is a Good Source of Carbohydrates).
- Starches are complex carbohydrates, polysaccharides composed predominantly of glucose. Starch is produced by most plants as an energy storage molecule and is commonly found in grains, legumes, and root vegetables such as potatoes, sweet potatoes, and cassava. Starch takes longer to break down during digestion and has a more gradual impact on blood sugar levels. See also What is a Safe Starch?
- Fiber is also a complex carbohydrate, oligosaccharides and polysaccharides that don’t get fully broken down by our digestive enzymes and instead are fermented by the bacteria and other microorganisms that live in our digestive tracts. Fiber is discussed in detail starting in The Fiber Manifesto-Part 1 of 5: What Is Fiber and Why Is it Good?, The Fiber Manifesto-Part 2 of 5: The Many Types of Fiber, and The Fiber Manifesto-Part 3 of 5: Soluble vs. Insoluble Fiber.
Whole-food carbohydrates, like fruits and vegetables, contain a mix of simple and complex carbohydrates, including fiber which slows own digestion and blunts the blood sugar response. Blood sugar regulation is further improved by ingesting fruits and vegetables as part of a complete meal that also includes protein and fats.
Refined carbohydrates refer to carbohydrates that have been processed. For example, when the bran and germ are milled away to make refined grain products, most of the fiber is removed. The resultant starches are digested and absorbed rapidly, sometimes raising blood glucose levels as quickly as simple sugars.
Simple sugars can also be refined. A prominent example of a processed sugar is high fructose corn syrup. In this case, corn syrup is treated with enzymes to turn a proportion of the syrup’s glucose into fructose. High fructose corn syrup is discussed further in Why is High Fructose Corn Syrup Bad For Us? , Is Fructose a Key Player in the Rise of Chronic Health Problems? and Fructose and Vitamin D Deficiency: The Perfect Storm?.
The Krebs Cycle
Most of the digestible carbohydrates that we consume break down into glucose, which is absorbed into our blood stream and shuttled into our cells by insulin. Once in our cells, glucose is converted into adenosine triphosphate (ATP), the energy currency for all cells, in a series of chemical reactions, collectively referred to as cellular respiration (since the process uses oxygen and produces carbon dioxide). Many ATP molecules can be formed from a single glucose molecule. Glucose molecules are first converted into pyruvate via glycolysis which yields some ATP. Pyruvate then enters the mitochondria where it is oxidized into acetyl-CoA, which can also yield some ATP. Acetyl-CoA is then converted into more ATP in what is called the Krebs or citric acid cycle, an 8-step process involving 18 different enzymes and co-enzymes. Other high-energy products of the Krebs cycle (NADH and FADH2) are converted into yet more ATP in the last step of cellular respiration, oxidative phosphorylation in the electron transport chain. This is complex biochemistry; the important part here is that there’s a whole lot of chemical reactions required to make sugar into a useable energy source for our cells!
Glucose isn’t the only molecule that can be converted into ATP via cellular respiration. Protein (amino acids), fats (fatty acids and glycerol), and other carbohydrates (like fructose) can be converted to various intermediates of glycolysis, pyruvate oxidation and the Krebs cycle, allowing them to slip into the cellular respiration pathway at multiple points. However, glucose is the easiest to convert into ATP (it requires the least amount of oxygen and can even produce some ATP anaerobically) so it is the preferred fuel for cells. In between meals, once the glucose that enters the bloodstream has been used up, cells metabolize stored fat and glycogen (stored carbohydrates) for energy. A flexible metabolism is one that can easily switch between carbohydrates and fats, depending on what’s available. Although protein is not a preferred source of energy, it can be used if needed—this is why people lose muscle mass in addition to fat stores when they are too severely calorically restricted, fasting, or starving.
Sugar, Insulin and Inflammation
Where does inflammation fit into this picture? A byproduct of cellular respiration is the production of reactive oxygen species (ROS), aka oxidants or free radicals. Reactive oxygen species (ROS) are a group of chemically reactive molecules that contain oxygen. T ROS have important roles in cell signaling (the complex communication between and within cells) and in homeostasis (the maintenance of a stable environment inside and outside the cell). But ROS are also potent signals for inflammation and can damage cells and tissue. In fact, they are produced and secreted by the cells of the immune system as one weapon in its arsenal to defend us from pathogens.
In general, the more energy (food) consumed, the more ROS produced. This production of ROS after meals is called postprandial oxidative stress or postprandial inflammation, and it continues to be a topic of intense study. In this sense, all foods are inflammatory—it is the price we pay for being aerobic organisms, of course our use of oxygen in our metabolism is also what allows us to have such wonderfully complex biological structure (anaerobic organisms are almost all single cell). However, some eating patterns cause more oxidative stress and inflammation than others. Overeating in general is the biggest culprit, stimulating the production of ROS by flooding the body with energy, but so does high carbohydrate (especially refined carbohydrate) intake, even in the context of judicious caloric intake. High-carbohydrate diets cause more postprandial inflammation than low-carbohydrate diets do, everything else being equal. Put simply, the more glucose we eat, the more inflammation!
When it comes to sugar being inflammatory, dose matters. Today, the average American consumes almost 152 pounds of sugar each year, a staggering amount of refined simple carbohydrates equivalent to 6 cups of white sugar every week. This may be the single biggest dietary contributor to the rise in chronic disease. Consumption of glucose is associated with increased production of ROS and markers of inflammation, even in healthy people. However, it is exaggerated in people who are obese or have type 2 diabetes, high cholesterol, or metabolic syndrome. This is because postprandial inflammation is proportional to insulin sensitivity, or how effectively the body responds to insulin: the less insulin-sensitive (that is, the more insulin-resistant) someone is, the more inflammation is created every time he or she eats. And, because of this, simple sugars and refined carbohydrates that spike blood sugar levels cause more inflammation than whole food sources of complex carbohydrates.
A healthy body has the ability to control both the amount of and the damage caused by ROS. In normal circumstances, the deleterious effects of these highly reactive molecules are balanced out by antioxidants—certain vitamins, minerals, and most phytochemicals. However, when the production of ROS exceeds the availability of antioxidants, the resulting imbalance causes problems. Specifically, the overproduction of ROS stimulates inflammation and damages cells and tissue; this is called oxidative stress.
There is evidence that insulin itself is pro-inflammatory. A study of healthy subjects with controlled (and normal) blood glucose who were intravenously infused with insulin and glucose to achieve hyperinsulinemia (elevated blood insulin) showed that hyperinsulinemia caused an exaggerated inflammatory response to endotoxin (a toxin from the cell wall of Gram-negative bacteria like E. coli). An exaggerated stress response was also observed, meaning that hyperinsulinemia also contributes to increased cortisol. Another study measured the levels of fasting insulin (the levels recorded first thing in the morning) in volunteers with normal blood sugar levels and found that those with higher fasting insulin levels also had more markers of inflammation, like C-reactive protein, or CRP. In a person who is insulin-resistant, the pancreas secretes more and more insulin to handle elevated blood sugar, which contributes to inflammation and insulin resistance. See also The Hormones of Fat: Leptin and Insulin, The Hormones of Hunger and 3 Ways to Regulate Insulin that Have Nothing to Do With Food.
Non-Caloric Sweeteners Are Even Worse
Humans have a sweet tooth, which means that as we learn the importance of limiting sugar intake, we are instinctively drawn to sugar substitutes. Unfortunately, there just isn’t any way to cheat sweet and all sugar substitutes, even natural ones like stevia, are riddles with problems. These are discussed in more detail in: Is It Paleo? Fructose and Fructose-Based Sweeteners (I’m looking at you, Agave!), Is It Paleo? Splenda, Erythritol, Stevia and other low-calorie sweeteners, and Teaser Excerpt from The Paleo Approach: The Trouble with Stevia.
What About Natural Sugars?
The good news is that small amounts of natural sweeteners are unlikely to be harmful and can even contribute some valuable micronutrients to our diets. The bad news is that it’s still a case of “the dose makes the poison”: these foods become harmful when they start displacing more-nutritious items from the menu or when they increase our energy intake beyond what we need. Therefore, they’re conditionally allowed on a Paleo diet. Blackstrap molasses is king of natural sugars (see Blackstrap Molasses: The Sugar You Can Love!) but for more information on which are the best options, see Natural Sugars and their Place in Paleo.
How Many Carbohydrates Should We Eat?
The takeaway here is that blood sugar regulation is essential for controlling inflammation. The Western diet is typified by the excessive consumption of calorie-dense, nutrition-poor foods that cause abnormal surges in blood glucose with little in the way of dietary antioxidants to balance things out. This doesn’t mean that we should aim to eat a “low-carb diet” but that we should avoid eating a high-carb diet and instead focus on a variety of nutrient-dense whole foods. When we consider hunter-gatherer evidence, low-carb is not the way to go (see Carbs Vs. Protein Vs. Fat: Insight from Hunter-Gatherers). Somewhere around 150 to 200 grams of carbohydrates from whole vegetables and fruits is a good target for most peoples (discussed in detail in How many carbs should you eat?)
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