1. Sports Nutrition Digestion and Absorption

2. Overview  Functions and overview of structure of GI tract  Provide fuel for energy, material for growth/repair  Digestion of macronutrients  Absorption of macronutrients inc water  Absorption of vitamins and minerals  Factors regulating gastric emptying  GI problems and exercise

3. Anatomy of GI tract

4. Steps in the digestive process  Ingestion. (Active process)  Mechanical processing.  Tearing, chewing, ↑SA  Digestion.  Chemical breakdown into monosaccharides, amino acids.  Secretion.  Water, acids, enzymes, buffers, salts by epithelium & glands.  Absorption.  Organic substrates, vitamins, electrolytes, water.  Excretion.  Waste and indigestible residue, egestion (defecation).

5. Anatomy GI tract  Mouth  Makes food particles smaller  ↑ gastric emptying & ↑ SA  Mixes with saliva  Oesophagus  Oesophageal sphincter at end releases food into stomach (malfunction common in runners/cyclists  heartburn)

7. Anatomy GI tract  Stomach  Stores food and regulates is release (via pyloric sphincter) into intestine  Mixes with gastric secretions  chyme  Little absorption (some water and alcohol).  Small intestine (duodenum  jejenum  ileum)  95% absorption in d and j  Surface is folded, and covered in fingerlike structures (villi) which covered themselves in microvilli

9. Villi  Highly vascular  Water, water soluble particles and electrolytes are transported/diffuse across epithelium into blood vessel  Transported via hepatic portal vein to liver  Also contains lacteal - transports non-water sol substances eg. long- chain FA’s via lymphatic vessels which drain into large veins near heart.

10. Gall bladder  Stores, concentrates and releases bile via hepatic duct following a meal (esp if rich in fat)  Bile made in liver – water, electrolytes, bile salts, cholesterol, lecithin and bilirubin  Bile reabsorbed in ileum  Facilitates digestion/absorption of fat

11. Pancreas  Secretes sodium bicarbonate to buffer HCl released in stomach, and digestive enzymes to digest CHO, protein and fat  Pancreatic lipase  Pancreatic α-amylase  Proteases  Dependent on type and amount of food ingested

12. Lg intestine  Now chyme called faeces  Colon (ascending, transverse, descending and sigmoid) and rectum  Colon  Absorption water and electrolytes  Absorption of vitamins (K, biotin)  Colonic bacteria  flatus and Vit K  Storage of faeces until expelled

13. Major functions of GI tract ORGANFUNCTION Mouth Mech digestion Salivary glands Secretion of fluid and digestive enzymes Stomach Secretion HCl and proteases Pancreas Secretion of NaHCO3 and digestive enzymes Liver Secretion bile Gallbladder Storage and concentration of bile Sm intestine Digestion of food, absorption of water, nutrients and electrolytes Lg intestine Absorption of water and electrolytes

14. CHO digestion  Saliva contains  α-amylase – breaks down starch into maltose, trisaccharides and small oligosaccharides  Amylase less active due to acid in stomach  Pancreatic juice contains  α-amylase  Disaccharides further digested by lactase, sucrase and maltase in brush border

15. Lipid digestion  Lingual and pancreatic lipase digests TG’s into fa’s and glycerol  Duodenum – fat in lg lipid globules  Bile acid emusify fats into smaller droplets  micelles  Micelles absorbed through villi

17. Protein digestion  Hydrolysis into amino acids, di and tripeptides in stomach and small intestine  HCl in stomach activates pepsinogen  pepsin, and denatures the protein  Pepsin degrades collagenous connective tissue in meat allowing other proteases to act  Trypsin + others in sm intestine digest any remaining proteins/polypeptides  amino acids, di and tripeptides

18. CHO Absorption  Monosaccharides absorbed by carrier- mediated transport  Glucose and galactose taken into epithelial cell with 2 x Na+ (SGLT)  Na is then actively pumped back into lumen  Fructose – Na-independent facilitated diffusion transporter (GLUT 5)  GLUT 2 transporter on contra-luminal side accepts all 3 monosaccharides  hepatic portal vein.

19. Fatty acid absorption  FAs diffuse across membrane into epithelial cell where reform into TGs  TG’s combine with cholesterol or phopholipid  chylomicrons  lacteal  Short and medium chain fa’s are more water sol – diffuse into blood, bind to albumin  liver in hpv

20. Amino acid absorption  Amino acids, di and tripeptides absorbed by active transport in small intestine  liver via hpv where converted to glucose, fat or protein or released as free amino acids  Amino acids transported against concentration gradient with at least 7 usually Na-dependent brush border transport proteins  Di and tripeptides further broken down in epithelial cell to amino acids

21. Water absorption  Mostly in duodenum (72%) by osmosis  Water can move in both directions depending on the osmolarity of chyme SolutionmOsm/L Water10-20 Sweat170-220 Gastric fluid 280-303 Lucozade sport 280 Isostar296 Gatorade349 Coke650 Fruit juice 450-690

22. Vitamin and mineral absorption  Vitamins  Mostly by diffusion in jejenum and lieum;  Fat sol vits absorbed with fa’s  Minerals  Not well absorbed and depends on form  Eg. Heme iron in meat vs nonheme iron in plants  15%heme iron absorbed in small intestine vs. only 2-10% of non-heme  Na actively transported into chyme and reabsorbed each day along with daily ingestion (5-8g)

23. Gastric emptying  Volume of fluid  Rate of gastric emptying is exponential – so when volume is reduced rate of gastric emptying is reduced  Exercise intensity  Unaffected up to ~80% VO2max  >80% may see reduction in delivery to small intestine (Sole and Noakes, 1989; Costill and Saltin, 1974)  But VO2max cannot normally be sustained for long at >80%VO2max and so CHO/fluid delivery not a problem  Short bouts of high intensity exericse (as in football match) can slow gastric emptying of liquids (Leiper et al., 2001)

24. Gastric emptying  Osmolarity  Higher osmolarity may reduce gastric emptying and decrease water absorption  No effect in range 200 – 400 mOsm/L (Brouns et al., 1995)  Energy density  Fat strong inhibitor of gastric emptying  Increasing CHO or protein content of beverage also slows rate  1-7% slower than water, 8%+ significantly slower  Energy density more important than osmolarity (Vist and Maughan, 1995)

25. Gastric emptying  Food/fluid temperature  No real effect unless very cold or very hot drinks  Psychological stress  Usually slows due to stress hormones eg. adrenaline reducing blood flow to GI tract  Other  Dehydration and hyperthermia slows gastric emptying  Women slower than men – more prone to GI complaints after prolonged endurance exercise  Inter-individual difference – possible adaptation

26. GI problems  High prevalence in endurance athletes (Brouns, 1987, 1991)  Abdominal pain, abnormal defecation, nausea, heartburn, stitch and vomiting  Avoid:  concentrated CHO solutions during the race;  High fibre or fat intake before race (24 hrs)  Dehydration  Dairy products?