carbohydrates and sugars

see also:

• lactose
• fructose
• irritable bowel syndrome (IBS)
• obesity and weight management

• carbohydrates are biologic molecules consisting of carbon, hydrogen and oxygen, usually with the empirical formula of C_m(H₂O)_n (except for deoxyribose which is in DNA) and form a substantive part of animal diets and energy supply
• they can be divided into:
  • monosaccharides eg. fructose, glucose, galactose, xylose, ribose, glyceraldehydes
  • disaccharides those that consist of two monosaccharides such as sucrose (fructose + glucose), lactose (galactose + glucose), maltose, trehalose
  • polyol sugars such as sorbitol, mannitol
  • oligosaccharides such as maltodextrins, raffinose, stchyose, fructo-oligosaccharides
  • polysaccharides serve as a storage of energy eg. starch (amylose, amylopectin), glycogen, or as structural elements (eg. cellulose, arabinoxylans, chitin), and play a number of important other biologic roles
    • some form dietary fibre as they are insoluble and undigestible in the human GIT such as:
      • cellulose, chitin, resistant starch, inulin - some of which can be broken down by colinic bacteria to form short chain fatty acids and gases
• maple syrup is mainly sucrose, with some glucose and fructose
• honey is approximately half fructose and half glucose with some sucrose

• for nearly all life forms including bacteria, glucose is the preferred sugar substrate for energy
• all carbohydrates must be digested in the gut into glucose or galactose before they can be absorbed into the blood stream
• any that cannot be metabolized to these sugars either become dietary fibre if insoluble, or, are metabolized by colonic bacteria with production of gases which can contribute to flatulence and irritable bowel syndrome (IBS)
• some Japanese people are able to digest CHOs from seaweed which is their staple diet thanks to a gut bacteria, Bacteroides plebeius, having acquired the gene and thus enzyme required, possibly from the marine bacterium Zobellia galactanivorans
• many people have trouble digesting lactose, fructose and other sugars and this recognition has formed the basis for the FODMAP diet developed by gastroenterologists at Melbourne's Monash University which is designed to minimise Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols

• simple sugars such as glucose yield 3.87 kilocalories/g
• glucose is trapped within cells via either:
  • hexokinase enzyme converting it to glucose-6-phosphate with a negative feedback loop on the enzyme
  • hepatic glucokinase converting it to glucose-6-phosphate without a negative feedback loop on the enzyme, but this primarily only occurs when blood glucose levels are high
• any excess glucose is converted to, and stored as glycogen or converted to fatty acids and cholesterol
• glucose can be made within the liver by gluconeogenesis
• glucose is metabolized to create ATP which is used within cells as the primary intracellular energy storage via:
  • glycolysis
    • glucose is converted to pyruvate and in the process converting 2 NAD to 2 NADH and 2 ADP to 2 ATP
    • this requires the NAD to be replenished and this can be done via:
      • anaerobic NAD production via conversion of pyruvate to lactic acid
        • for short periods in hypoxic cells, or in bacteria, pyruvate can be converted to lactic acid to convert NADH back to NAD so the glycolysis pathway still has NAD to rapidly utilise otherwise it would cease, however build up of lactic acid inhibits the glycolysis pathway (mammalian livers remove lactic acid by converting it back to pyruvate via the Cori cycle)
      • aerobic NAD replenishment via mitochondria and the Kreb's cycle¹⁾
        • mitochondria increase the energy production from pyruvate 15-fold compare to cellular energy production from pyruvate
          • mitochondria are surrounded by two membranes. The outer one is porous, and pyruvate can easily pass through, but the inner membrane is impermeable to pyruvate.
          • similar to a lock on a canal for boats, a newly discovered mitochondrial pyruvate carrier (MPC) transports pyruvate into the mitochondrion, first an outer 'gate' of the carrier opens, allowing pyruvate to enter the carrier. This gate then closes, and the inner gate opens, allowing the molecule to pass through into the mitochondrion ²⁾
          • MPC activity can influence several metabolic pathways, including the tricarboxylic acid cycle, gluconeogenesis, the malate-aspartate shuttle, the urea cycle, and lipid metabolism
          • MPC activity is important for embryonic development, hence its mutations are rare but can result in severe disease phenotypes, including lactic acidosis, brain dysfunction, and developmental abnormalities
          • MPC inhibitors are being sought as potential therapeutic agents
        • pyruvate (plus NAD⁺) is then converted to acetyl-CoA, CO₂ and NADH + H⁺ within the mitochondria
        • NAD⁺ is transferred into mitochondria via the malate-aspartate shuttle and the glycerol phosphate shuttle
        • acetyl-CoA enters the citric acid cycle (Krebs Cycle), where the acetyl group of the acetyl-CoA is converted into carbon dioxide by two decarboxylation reactions with the formation of yet more intra-mitochondrial NADH + H⁺
        • the NADH + H⁺ is oxidized to NAD⁺ by the electron transport chain, using oxygen
        • oxidative phosphorylation then produces 2.5 ATP for every NADH + H⁺
        • the citric acid cycle is also utilised for amino acid synthesis, nucleotide synthesis and tetrapyrrole synthesis

• see fructose