Carbohydrates are essential nutrients that fuel our bodies. They're broken down into glucose, which provides energy for cells. The body regulates blood sugar levels through various processes, storing excess glucose as glycogen for later use.

Simple carbs are quickly digested, causing rapid blood sugar spikes. Complex carbs and fiber offer slower digestion and more stable energy. Fiber also supports digestive health, lowers cholesterol, and may reduce cancer risk. Understanding carb metabolism helps make informed dietary choices.

Carbohydrate Metabolism and Nutrition

Carbohydrate metabolism in the body

• Digestion begins in the mouth with salivary amylase (enzyme) breaking down carbohydrates into smaller units
• Pancreatic amylase continues digestion in the small intestine, further breaking down carbohydrates into monosaccharides (glucose, fructose, galactose)
• Monosaccharides are absorbed through the walls of the small intestine and transported to the liver via the portal vein
• In the liver, fructose and galactose are converted into glucose, which is then stored as glycogen (storage form of glucose) or released into the bloodstream to maintain blood sugar levels
• Glucose enters cells via insulin-mediated transport using glucose transporter proteins (GLUT4)
• Inside cells, glucose undergoes glycolysis, producing pyruvate (3-carbon molecule) and ATP (energy currency)
• Pyruvate enters the Krebs cycle and electron transport chain in the mitochondria, generating more ATP to power cellular functions

Glucose regulation and storage

• Glycogenesis: The process of converting excess glucose into glycogen for storage in the liver and muscles
• Glycogenolysis: The breakdown of stored glycogen into glucose when blood sugar levels are low
• Gluconeogenesis: The production of glucose from non-carbohydrate sources (e.g., amino acids, lactate) to maintain blood glucose levels during fasting or prolonged exercise
• Ketosis: A metabolic state where the body uses fat for energy instead of carbohydrates, often occurring during prolonged fasting or very low-carbohydrate diets

Simple vs complex carbohydrates

• Simple carbohydrates include monosaccharides (glucose, fructose, galactose) and disaccharides (sucrose, lactose, maltose)
  • Found in fruits, vegetables, milk, and refined sugars (table sugar, syrup)
  • Rapidly digested and absorbed, causing a quick rise in blood glucose levels
  • Have a high glycemic index, meaning they rapidly increase blood sugar
• Complex carbohydrates include polysaccharides (starch, fiber)
  • Found in whole grains (oats, brown rice), legumes (beans, lentils), and vegetables
  • Slowly digested and absorbed, leading to a gradual rise in blood glucose levels
  • Have a low to moderate glycemic index, promoting more stable blood sugar
• Blood glucose effects differ between simple and complex carbohydrates
  • Simple carbohydrates cause rapid spikes and drops in blood glucose, which can lead to energy crashes and cravings
  • Complex carbohydrates promote more stable blood glucose levels, providing sustained energy and reducing the risk of diabetes and other metabolic disorders
  • Fiber helps slow glucose absorption, reducing blood glucose spikes and improving insulin sensitivity
• Glycemic load: A measure that takes into account both the quantity and quality of carbohydrates consumed, providing a more accurate representation of a food's impact on blood sugar levels

Fiber's role in nutrition

• Soluble fiber (pectins, gums, mucilages) dissolves in water, forming a gel-like substance
  • Found in fruits (apples, citrus), vegetables (carrots, peas), oats, and legumes
  • Promotes digestive health by softening stool and preventing constipation
  • Feeds beneficial gut bacteria (probiotics), supporting a healthy microbiome and immune function
• Insoluble fiber (cellulose, hemicellulose, lignin) does not dissolve in water
  • Found in whole grains (wheat bran), nuts, and vegetables (cauliflower, green beans)
  • Adds bulk to stool and promotes regular bowel movements, preventing constipation and diverticular disease
• Fiber aids in blood glucose control by slowing glucose absorption, helping to maintain stable blood sugar levels, which is particularly beneficial for individuals with diabetes
• Soluble fiber binds to bile acids in the intestine, reducing cholesterol reabsorption and helping lower LDL (bad) cholesterol levels, thus decreasing the risk of cardiovascular disease
• Fiber increases feelings of fullness (satiety) and reduces overall calorie intake, aiding in weight management and obesity prevention
• Adequate fiber intake lowers the risk of certain cancers, particularly colorectal cancer, by promoting healthy bowel function and reducing exposure to carcinogens in the digestive tract