Carbohydrates (including sugar, starch and fiber) are a class of organic molecules with the basic structural components being sugar molecules, or saccharides, which have a general molecular formula of Cm(H2O)n. Their two main roles in supporting health is as an energy source and as fermentable substrate for the gut microbiome.
Chemically, carbohydrates are classified based on the number of saccharides they contain: monosaccharides are made up of a single sugar molecule (examples are glucose and fructose), disaccharides contain two sugar molecules (examples are sucrose and lactose), oligosaccharides are medium-length chains of three to ten sugar molecules, and polysaccharides are long chains of sugar molecules that can be hundreds long (think of polysaccharides of long chains of monosaccharide units therefore they can be broken down in our digestive system into simple sugar molecules, either by our own digestive processes or by our gut bacteria).
Types of Carbohydrates
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 and cane sugar. They are digested and absorbed quickly and the glucose they contain has a rapid impact on blood sugar levels and insulin secretion.
- 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.
- Fiber is also a complex carbohydrate, oligosaccharides and polysaccharides from plant cell walls 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 can be further divided into a few major classes based on molecular structure, including: cellulose, hemicellulose, pectin, lignin, chitin, chitosan, gums, glucans, mucilages, fructans and resistant starch. It can also be classified based on its solubility, viscosity and how readily it is fermented by our gut bacteria.
Whole-food sources of carbohydrates, like fruits, legumes, whole grains 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.
Health Benefits of Fiber
Fiber is an important nutrient; it regulates peristalsis (the coordinated contraction and relaxation of the intestinal muscular tissue to propel food down the gut) and some gastric hormones, in addition to supplying our gut bacteria with fermentable substrate, i.e., food! High fiber intake reduces the risk of cardiovascular disease and of many forms of cancer (especially colorectal cancer, but also liver cancer, pancreatic cancer, and others), and promote overall lower inflammation. High-fiber diets reduce the risk of mortality in cases of kidney disease and diabetes, and can even reduce your risk of dying from an infection!
Carbohydrates As Energy
When we consume non-fiber carbohydrates, our digestive system first breaks complex carbohydrates down into monosaccharides (mainly glucose molecules), which are absorbed into our blood stream, causing a resulting rise in our blood sugar levels, (a.k.a. blood glucose levels). In response to that rise in blood sugar, the pancreas releases the hormone insulin, which facilitates the transport of glucose into the cells of the body and signals to the liver to convert glucose into glycogen for short-term energy storage in liver and muscle tissues and into triglycerides for long-term energy storage in adipose tissues. Insulin sensitivity can be maintained with adequate sleep, activity and stress management along with avoiding frequent excessive consumption of simple carbohydrates that can cause maladaptation to chronically elevated blood glucose levels.
Once inside our cells, glucose in an energy source, being rapidly converted into ATP, the energy currency for all cells, via the Kreb’s cycle (a process that also uses oxygen and produces carbon dioxide, also called the Citric Acid Cycle or Cellular Respiration). 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.
How Much Carbohydrates Do We Need?
The Accepted Macronutrient Distribution Ranges (AMDR) were established by the Food and Nutrition Board of the Institute of Medicine using evidence from interventional trials with support of epidemiological evidence that suggest a role in the prevention or increased risk of chronic diseases, and based on ensuring sufficient intake of essential nutrients.
The AMDR for carbohydrates is between 45% to 65% of total energy (and below 25% from total sugars, and below 10% from added sugars). On 2000-calorie per day diet, this translates to 225 to 325 grams of carbohydrates.