Sports NutritionSports Nutrition

Digestion and AbsorptionDigestion and Absorption

OverviewOverview Functions and overview of structure of GI tractFunctions and overview of structure of GI tract

Provide fuel for energy, material for growth/repairProvide fuel for energy, material for growth/repair

Digestion of macronutrientsDigestion of macronutrients Absorption of macronutrients inc waterAbsorption of macronutrients inc water Absorption of vitamins and mineralsAbsorption of vitamins and minerals Factors regulating gastric emptyingFactors regulating gastric emptying GI problems and exerciseGI problems and exercise

Anatomy of GI tractAnatomy of GI tract

Steps in the digestive processSteps in the digestive process Ingestion. (Active process)Ingestion. (Active process) Mechanical processing.Mechanical processing.

Tearing, chewing, Tearing, chewing, ↑SA↑SA Digestion.Digestion.

Chemical breakdown into monosaccharides, amino acids. Chemical breakdown into monosaccharides, amino acids. Secretion.Secretion.

Water, acids, enzymes, buffers, salts by epithelium & glands.Water, acids, enzymes, buffers, salts by epithelium & glands. Absorption.Absorption.

Organic substrates, vitamins, electrolytes, water.Organic substrates, vitamins, electrolytes, water. Excretion.Excretion.

Waste and indigestible residue, egestion (defecation).Waste and indigestible residue, egestion (defecation).

Anatomy GI tractAnatomy GI tract MouthMouth

Makes food particles smaller Makes food particles smaller ↑ ↑ gastric emptying & ↑ SAgastric emptying & ↑ SA Mixes with salivaMixes with saliva

OesophagusOesophagus Oesophageal sphincter at end releases food Oesophageal sphincter at end releases food

into stomach (malfunction common in into stomach (malfunction common in runners/cyclists runners/cyclists heartburn) heartburn)

Anatomy GI tractAnatomy GI tract StomachStomach

Stores food and regulates is release (via Stores food and regulates is release (via pyloric sphincter) into intestinepyloric sphincter) into intestine

Mixes with gastric secretions Mixes with gastric secretions chyme chyme Little absorption (some water and alcohol).Little absorption (some water and alcohol).

Small intestine (duodenum Small intestine (duodenum jejenum jejenum ileum)ileum) 95% absorption in d and j95% absorption in d and j Surface is folded, and covered in fingerlike Surface is folded, and covered in fingerlike

structures (villi) which covered themselves in structures (villi) which covered themselves in microvilli microvilli

VilliVilli Highly vascularHighly vascular Water, water soluble Water, water soluble

particles and electrolytes particles and electrolytes are transported/diffuse are transported/diffuse across epithelium into across epithelium into blood vesselblood vessel

Transported via hepatic Transported via hepatic portal vein to liverportal vein to liver

Also contains lacteal - Also contains lacteal - transports non-water sol transports non-water sol substances eg. long-substances eg. long-chain FA’s via lymphatic chain FA’s via lymphatic vessels which drain into vessels which drain into large veins near heart.large veins near heart.

Gall bladderGall bladder Stores, concentrates and releases bile Stores, concentrates and releases bile

via hepatic duct following a meal (esp if via hepatic duct following a meal (esp if rich in fat)rich in fat)

Bile made in liver – water, electrolytes, Bile made in liver – water, electrolytes, bile salts, cholesterol, lecithin and bile salts, cholesterol, lecithin and bilirubinbilirubin

Bile reabsorbed in ileum Bile reabsorbed in ileum Facilitates digestion/absorption of fat Facilitates digestion/absorption of fat

PancreasPancreas

Secretes sodium bicarbonate to buffer Secretes sodium bicarbonate to buffer HCl released in stomach, and digestive HCl released in stomach, and digestive enzymes to digest CHO, protein and fatenzymes to digest CHO, protein and fat Pancreatic lipasePancreatic lipase Pancreatic Pancreatic αα-amylase-amylase ProteasesProteases

Dependent on type and amount of food Dependent on type and amount of food ingestedingested

Lg intestineLg intestine

Now chyme called faecesNow chyme called faeces Colon (ascending, transverse, Colon (ascending, transverse,

descending and sigmoid) and rectumdescending and sigmoid) and rectum ColonColon

Absorption water and electrolytesAbsorption water and electrolytes Absorption of vitamins (K, biotin)Absorption of vitamins (K, biotin) Colonic bacteria Colonic bacteria flatus and Vit K flatus and Vit K Storage of faeces until expelledStorage of faeces until expelled

Major functions of GI Major functions of GI tracttractORGANORGAN FUNCTIONFUNCTION

MouthMouth Mech digestionMech digestion

Salivary glandsSalivary glands Secretion of fluid and digestive enzymesSecretion of fluid and digestive enzymes

StomachStomach Secretion HCl and proteasesSecretion HCl and proteases

PancreasPancreas Secretion of NaHCO3 and digestive Secretion of NaHCO3 and digestive enzymesenzymes

LiverLiver Secretion bileSecretion bile

GallbladderGallbladder Storage and concentration of bileStorage and concentration of bile

Sm intestineSm intestine Digestion of food, absorption of water, Digestion of food, absorption of water, nutrients and electrolytesnutrients and electrolytes

Lg intestineLg intestine Absorption of water and electrolytesAbsorption of water and electrolytes

CHO digestionCHO digestion Saliva containsSaliva contains

αα-amylase – breaks down starch into -amylase – breaks down starch into maltose, trisaccharides and small maltose, trisaccharides and small oligosaccharidesoligosaccharides

Amylase less active due to acid in Amylase less active due to acid in stomachstomach

Pancreatic juice containsPancreatic juice contains αα-amylase-amylase

Disaccharides further digested by Disaccharides further digested by lactase, sucrase and maltase in brush lactase, sucrase and maltase in brush borderborder

Lipid digestionLipid digestion Lingual and pancreatic lipase digests Lingual and pancreatic lipase digests

TG’s into fa’s and glycerolTG’s into fa’s and glycerol Duodenum – fat in lg lipid globulesDuodenum – fat in lg lipid globules Bile acid emusify fats into smaller Bile acid emusify fats into smaller

droplets droplets micelles micelles Micelles absorbed through villiMicelles absorbed through villi

Protein digestionProtein digestion Hydrolysis into amino acids, di and tripeptides Hydrolysis into amino acids, di and tripeptides

in stomach and small intestinein stomach and small intestine HCl in stomach activates pepsinogen HCl in stomach activates pepsinogen

pepsin, and denatures the protein pepsin, and denatures the protein Pepsin degrades collagenous connective Pepsin degrades collagenous connective

tissue in meat allowing other proteases to acttissue in meat allowing other proteases to act Trypsin + others in sm intestine digest any Trypsin + others in sm intestine digest any

remaining proteins/polypeptides remaining proteins/polypeptides amino amino acids, di and tripeptidesacids, di and tripeptides

CHO AbsorptionCHO Absorption Monosaccharides absorbed by carrier-Monosaccharides absorbed by carrier-

mediated transportmediated transport Glucose and galactose taken into Glucose and galactose taken into

epithelial cell with 2 x Na+ (SGLT)epithelial cell with 2 x Na+ (SGLT) Na is then actively pumped back into lumenNa is then actively pumped back into lumen

Fructose – Na-independent facilitated Fructose – Na-independent facilitated diffusion transporter (GLUT 5)diffusion transporter (GLUT 5)

GLUT 2 transporter on contra-luminal GLUT 2 transporter on contra-luminal side accepts all 3 monosaccharides side accepts all 3 monosaccharides hepatic portal vein.hepatic portal vein.

Fatty acid absorptionFatty acid absorption FAs diffuse across membrane into FAs diffuse across membrane into

epithelial cell where reform into TGsepithelial cell where reform into TGs TG’s combine with cholesterol or TG’s combine with cholesterol or

phopholipid phopholipid chylomicrons chylomicrons lacteal lacteal Short and medium chain fa’s are more Short and medium chain fa’s are more

water sol – diffuse into blood, bind to water sol – diffuse into blood, bind to albumin albumin liver in hpv liver in hpv

Amino acid absorptionAmino acid absorption Amino acids, di and tripeptides absorbed Amino acids, di and tripeptides absorbed

by active transport in small intestine by active transport in small intestine liver via hpv where converted to glucose, liver via hpv where converted to glucose, fat or protein or released as free amino fat or protein or released as free amino acidsacids

Amino acids transported against Amino acids transported against concentration gradient with at least 7 concentration gradient with at least 7 usually Na-dependent brush border usually Na-dependent brush border transport proteinstransport proteins

Di and tripeptides further broken down in Di and tripeptides further broken down in epithelial cell to amino acidsepithelial cell to amino acids

Water absorptionWater absorption

Mostly in duodenum Mostly in duodenum (72%) by osmosis(72%) by osmosis

Water can move in Water can move in both directions both directions depending on the depending on the osmolarity of chymeosmolarity of chyme

SolutionSolution mOsm/LmOsm/L

WaterWater 10-2010-20

SweatSweat 170-220170-220

Gastric fluidGastric fluid 280-303280-303

Lucozade Lucozade sportsport

280280

IsostarIsostar 296296

GatoradeGatorade 349349

CokeCoke 650650

Fruit juiceFruit juice 450-690450-690

Vitamin and mineral Vitamin and mineral absorptionabsorption

VitaminsVitamins Mostly by diffusion in jejenum and lieum;Mostly by diffusion in jejenum and lieum; Fat sol vits absorbed with fa’sFat sol vits absorbed with fa’s

MineralsMinerals Not well absorbed and depends on formNot well absorbed and depends on form Eg. Heme iron in meat vs nonheme iron in plantsEg. Heme iron in meat vs nonheme iron in plants

15%heme iron absorbed in small intestine vs. only 2-10% 15%heme iron absorbed in small intestine vs. only 2-10% of non-hemeof non-heme

Na actively transported into chyme and reabsorbed Na actively transported into chyme and reabsorbed each day along with daily ingestion (5-8g)each day along with daily ingestion (5-8g)

Gastric emptyingGastric emptying Volume of fluidVolume of fluid

Rate of gastric emptying is exponential – so when Rate of gastric emptying is exponential – so when volume is reduced rate of gastric emptying is volume is reduced rate of gastric emptying is reducedreduced

Exercise intensityExercise intensity Unaffected up to ~80% VO2maxUnaffected up to ~80% VO2max >80% may see reduction in delivery to small >80% may see reduction in delivery to small

intestine (Sole and Noakes, 1989; Costill and Saltin, intestine (Sole and Noakes, 1989; Costill and Saltin, 1974)1974)

But VO2max cannot normally be sustained for long But VO2max cannot normally be sustained for long at >80%VO2max and so CHO/fluid delivery not a at >80%VO2max and so CHO/fluid delivery not a problemproblem

Short bouts of high intensity exericse (as in football Short bouts of high intensity exericse (as in football match) can slow gastric emptying of liquids (Leiper match) can slow gastric emptying of liquids (Leiper et al., 2001)et al., 2001)

Gastric emptyingGastric emptying OsmolarityOsmolarity

Higher osmolarity may reduce gastric emptying and Higher osmolarity may reduce gastric emptying and decrease water absorptiondecrease water absorption

No effect in range 200 – 400 mOsm/L (Brouns et al., No effect in range 200 – 400 mOsm/L (Brouns et al., 1995)1995)

Energy densityEnergy density Fat strong inhibitor of gastric emptyingFat strong inhibitor of gastric emptying Increasing CHO or protein content of beverage also Increasing CHO or protein content of beverage also

slows rateslows rate 1-7% slower than water, 8%+ significantly slower1-7% slower than water, 8%+ significantly slower Energy density more important than osmolarity (Vist Energy density more important than osmolarity (Vist

and Maughan, 1995)and Maughan, 1995)

Gastric emptyingGastric emptying Food/fluid temperatureFood/fluid temperature

No real effect unless very cold or very hot drinksNo real effect unless very cold or very hot drinks

Psychological stressPsychological stress Usually slows due to stress hormones eg. Usually slows due to stress hormones eg.

adrenaline reducing blood flow to GI tractadrenaline reducing blood flow to GI tract

OtherOther Dehydration and hyperthermia slows gastric Dehydration and hyperthermia slows gastric

emptyingemptying Women slower than men – more prone to GI Women slower than men – more prone to GI

complaints after prolonged endurance exercisecomplaints after prolonged endurance exercise Inter-individual difference – possible adaptationInter-individual difference – possible adaptation

GI problemsGI problems High prevalence in endurance athletes High prevalence in endurance athletes

(Brouns, 1987, 1991)(Brouns, 1987, 1991) Abdominal pain, abnormal defecation, Abdominal pain, abnormal defecation,

nausea, heartburn, stitch and vomitingnausea, heartburn, stitch and vomiting Avoid:Avoid:

concentrated CHO solutions during the race;concentrated CHO solutions during the race; High fibre or fat intake before race (24 hrs)High fibre or fat intake before race (24 hrs) DehydrationDehydration Dairy products?Dairy products?