Carbohydrate

• 1. Introduction

  ›

  A carbohydrate is, at its simplest, a sugar or a sugar derivative, and yet it carries the backbone of life itself. In the simplest carbohydrates, carbon, hydrogen, and oxygen lock together in a 1:2:1 ratio. That tidy formula belongs to a family of molecules that stores energy, builds the walls of plants, and spells out the genetic code. Alongside amino acids, fats, and nucleic acids, carbohydrates rank as one of the major families of biomolecules. The same chemistry that sweetens honey also stiffens the shell of an insect and threads through every strand of DNA. How did one class of molecule come to do so many unrelated jobs? Why did a German chemist need to coin a name for it in 1844, and why did sugar travel from a single island to reshape the diets of entire continents? The answers run from a Roman conifer to the floor of the human gut.

• 2. A Molecule Of Many Trades

  ›

  Ribose, a 5-carbon monosaccharide, sits at the heart of the coenzymes ATP, FAD, and NAD, and forms the backbone of RNA. Its relative deoxyribose builds DNA. The same broad chemistry that runs the genetic machinery also handles bulk storage and construction. Starch and glycogen serve as energy stores, while cellulose stiffens plants and chitin armors arthropods and fungi.

  Saccharides reach well beyond fuel and scaffolding. Their derivatives play roles in the immune system, in fertilization, in preventing pathogenesis, in blood clotting, and in development. The OH groups along a carbohydrate can be swapped for other chemistry entirely. N-acetyl groups appear in chitin, sulfate groups in glycosaminoglycans, and deoxy modifications turn up in fucose and sialic acid.

  Glycosylation, the enzymatic linking of sugars, fuses carbohydrates to other molecules through the glycosidic bond, producing glycans. The most abundant of these glycoconjugates in mammalian cells are glycoproteins, proteoglycans, and glycolipids, found mostly on the outer cell membrane and in secreted fluids. There they help cells recognize one another, since the cell surface bristles with glycan binding receptors. The N-linked glycans of a protein can even modulate that protein's function, in some cases acting as an on-off switch.

• 3. From Saccharides To Glycans

  ›

  Monosaccharides are the floor of the whole system: they cannot be hydrolyzed into anything smaller, and they usually follow the formula Cm(H2O)n. Glucose, galactose, fructose, and xylose belong here. Chemically, saccharides are polyhydroxy aldehydes and ketones, along with the polymers derived from them through acetal-type linkages.

  Disaccharides such as sucrose pair two units together, while sucrose, lactose, maltose, isomaltulose, and trehalose round out that group. Above them sit the oligosaccharides, counted at three to nine units, including raffinose, stachyose, and the fructo-oligosaccharides. Beyond nine units lie the polysaccharides, where starch splits into amylose and amylopectin, and the non-starch family gathers glycogen, cellulose, hemicellulose, pectins, and hydrocolloids.

  Classification by degree of polymerization is one axis, but not the only one. Many polyols, such as sorbitol and mannitol, also count as carbohydrates. The word itself stretches across contexts. In food science, carbohydrate often just means a food rich in starch, like cereals, bread, and pasta, and it can also point to dietary fiber such as cellulose.

• 4. Sugar's Long Road

  ›

  Sugar cane was first grown in New Guinea, and from that single origin a global commodity spread. The mass cultivation came in India, where techniques were developed to isolate crystalline sugar. Cane sugar reached Europe around the 13th century, then crossed to the New World, where its production became industrialized.

  Constantin Kirchhoff opened the chemical era in 1811 when he found that grape sugar, or glucose, forms when starch is boiled with acid; the starch industry began the very next year. In 1819 Henri Braconnot discovered that sugar forms when sulfuric acid acts on cellulose. After Joseph Louis Gay-Lussac and Thenard analyzed sugar and starch, William Prout named the whole group "saccharine," and in 1844 the German chemist Carl Schmidt first proposed the term "carbohydrate."

  Discovery accelerated into a long run of honors. In 1856 the French physiologist Claude Bernard discovered glycogen, the storage form of carbohydrate in animal livers. Emil Fischer won the 1902 Nobel Prize in Chemistry for his work on sugars and purines, and Otto Meyerhof took the 1922 Nobel Prize in Physiology or Medicine for glucose metabolism. Hans von Euler-Chelpin and Arthur Harden shared the 1929 chemistry prize for research on sugar fermentation and the role of enzymes in it. In 1947 Bernardo Houssay was recognized for the pituitary gland's role in carbohydrate metabolism, while Carl and Gerty Cori were honored for the conversion of glycogen. Luis Leloir followed with the 1970 chemistry prize for sugar nucleotides in biosynthesis, and in 1988 Raymond Dwek coined the word glycobiology to mark the merger of carbohydrate chemistry with biochemistry.

• 5. Fuel For The Body

  ›

  Simple sugars yield 3.87 kilocalories of energy per gram, while complex carbohydrate in most other foods yields 3.57 to 4.12 kilocalories per gram. Starch, abundant in wheat, maize, and rice, in potatoes, and in flour-based foods like bread, pizza, and pasta, supplies much of that energy. Sugars enter the diet mainly as table sugar, drawn from sugarcane or sugar beets, along with lactose from milk and the glucose and fructose found in honey, many fruits, and some vegetables.

  Glucose is the nearly universal currency of energy, and organisms tend to burn it first. In Escherichia coli, the lac operon switches on enzymes to digest lactose when it appears, but when both lactose and glucose are present the operon is repressed and glucose goes first. Many organisms break starch into glucose with ease, yet most cannot handle cellulose, chitin, or arabinoxylans. Ruminants and termites get around this by enlisting microorganisms that ferment cellulose into caloric short-chain fatty acids.

  Humans lack the enzymes to digest fiber, but fiber still matters. It promotes healthy digestion, helps regulate postprandial glucose and insulin, lowers cholesterol, and encourages satiety. Resistant starch and inulin feed bacteria in the large intestine, which metabolize them into short-chain fatty acids.

  Guidelines try to set the right share. The Institute of Medicine recommends that American and Canadian adults draw between 45 and 65 percent of dietary energy from whole-grain carbohydrates. The Food and Agriculture Organization and the World Health Organization jointly suggest 55 to 75 percent of total energy from carbohydrates, but only 10 percent directly from sugars. A 2017 Cochrane Systematic Review found insufficient evidence that whole grain diets affect cardiovascular disease.

• 6. Speed Of Sugar

  ›

  A high-GI food scores more than 70 on the glycemic index, where pure glucose is set at 100. Moderate-GI foods fall between 56 and 69, and low-GI foods sit below 55. The index measures how quickly a carbohydrate raises blood glucose against that reference.

  Refined carbohydrates from white bread or rice, soft drinks, and desserts digest readily and often carry a high glycemic index, producing an abrupt rise in blood glucose that falls just as fast. Whole, unprocessed, fiber-rich foods like beans, peas, and whole grains release glucose more slowly and steadily. Glycemic load goes a step further, weighing the quality of a food's carbohydrates by the amount in a single serving.

  The label "complex carbohydrate" carries its own history. It first appeared in the U.S. Senate Select Committee on Nutrition and Human Needs publication Dietary Goals for the United States in 1977, meant to separate sugars from carbohydrates thought nutritionally superior. That report placed fruit, vegetables, and whole grains in the complex column even though they may contain sugars too. The standard chemical usage is cleaner: simple for sugars, complex for polysaccharides and oligosaccharides.

• 7. The Diet Wars

  ›

  The ketogenic diet, an extreme low-carbohydrate regimen, is established as a medical treatment for epilepsy. Through celebrity endorsement in the early 21st century it became a fad for weight loss, carrying risks like low energy, increased hunger, insomnia, nausea, and gastrointestinal discomfort. The British Dietetic Association named it one of the top 5 worst celeb diets to avoid in 2018.

  Carbohydrate restriction can match low-fat diets for short-term weight loss when overall calories drop, but the advantage stops there. An Endocrine Society scientific statement noted that when calorie intake is held constant, body-fat accumulation does not appear to be affected by even very pronounced shifts between fat and carbohydrate. In the long term, low-carbohydrate diets show no metabolic advantage, and weight outcomes hinge on calorie restriction rather than macronutrient ratios.

  For specific conditions the picture sharpens. Carbohydrate-restricted diets are no more effective than a conventional healthy diet at preventing type 2 diabetes, yet for people who already have it they offer a viable route to weight loss or glycemic control. Evidence for routine use in type 1 diabetes is limited. The American Diabetes Association recommends a generally healthy diet rather than one fixated on carbohydrate or any single macronutrient. Low-carbohydrate eating can also miss the fiber and phytochemicals of legumes, pulses, whole grains, fruits, and vegetables, and one meta-analysis of moderate quality listed halitosis, headache, and constipation among its adverse effects.

• 8. Reading The Glycome

  ›

  Porous graphitic carbon, used as a stationary phase, lets even non-derivatized glycans be analyzed by liquid chromatography. That trick points to a wider problem: glycans are hard to read, and the field has assembled a toolkit to do it. NMR spectroscopy is common, though its major challenge is spectral overlap.

  Mass spectrometry and high-performance liquid chromatography do much of the heavy lifting. N-glycans from glycoproteins are routinely run through reversed phase, normal phase, and ion exchange HPLC after the reducing end is tagged with a fluorescent label. The choices of label have multiplied, with 2-aminobenzamide, anthranilic acid, 2-aminopyridin, 2-aminoacridone, and 3-(acetylamino)-6-aminoacridine among them, matched to different ESI modes and MS systems. O-glycans are usually analyzed without any tags.

  Fractionated glycans can be pushed further by MALDI-TOF-MS to probe structure and purity, and whole glycan pools can sometimes be read directly, though telling isobaric structures apart grows difficult. For targeted work, multiple reaction monitoring runs on a triple quadrupole instrument that detects a precursor ion, fragments it, then detects a chosen fragment, all in concurrent duty cycles. Its sensitivity and wide linear range suit glycan biomarker discovery, and it is now being used to characterize the immune glycome.