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Human Physiology · Digestion and metabolism

Secretion, Digestion and Absorption

Every day the gut breaks down food into molecules small enough to absorb, using about seven litres of secretions. This page follows saliva, gastric juice, bile and pancreatic juice, explains how carbohydrate, protein and fat are digested and absorbed, and links this to nutrition for training and recovery.

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Part 7 · Digestion and Metabolism · Chapter 31 of 47

Learning objectives

By the end of this page you should be able to:

  • Describe the main digestive secretions and their control.
  • Explain the digestion and absorption of carbohydrate, protein and fat.
  • Explain why the small intestine has a large surface area.
  • Describe absorption of water, iron, calcium and vitamin B12.
  • Relate carbohydrate absorption to endurance exercise.
  • Summarise evidence on protein intake and resistance training.

Why it matters

Rehabilitation builds tissue, and tissue needs nutrients. Physiotherapists advise on fuelling for exercise, recognise malnutrition, and see the effects of malabsorption, such as anaemia, weight loss and bone weakness.

Understanding digestion also explains why some patients cannot tolerate exercise soon after meals and why gut problems are common in endurance events.

Digestive secretions

Figure 1 · The gut handles far more fluid than we drink

Daily fluid entering and leaving the gut About nine litres enter the gut each day: two from food and drink and seven from secretions including saliva, gastric juice, bile, pancreatic juice and intestinal secretions. The small intestine absorbs about 7.5 litres, the colon about 1.3, and only 0.1 to 0.2 litres leave in stool. 2.0Food and drink1.5Saliva2.0Gastric juice0.5Bile1.5Pancreatic juice1.5Intestinal secretionsInAbout 9 litres enter the gut each day (litres)Outsmall intestine absorbs about 7.5colon about 1.3stool about 0.1–0.2
Diarrhoea can dehydrate fast. Losing even part of the daily secretions causes fluid and electrolyte loss. Typical values.
SecretionMain contentsFunctionControl
SalivaAmylase, mucus, waterLubricates; starts starch digestionAutonomic reflexes; sight and smell of food
Gastric juiceAcid, pepsin, intrinsic factorKills microbes; starts protein digestion; B12 absorptionVagus, gastrin, histamine
BileBile salts, bilirubinEmulsifies fatCholecystokinin releases it from the gallbladder
Pancreatic juiceBicarbonate, enzymes for all nutrientsNeutralises acid; main digestionSecretin (bicarbonate), cholecystokinin (enzymes)
Intestinal secretionsMucus, water, brush-border enzymesFinal digestionLocal reflexes

Secretion runs in phases. The cephalic phase begins with the sight, smell and thought of food through the vagus nerve. The gastric phase follows as food stretches the stomach, and the intestinal phase as chyme enters the duodenum.

Digestion of the main nutrients

Figure 2 · From food to absorbable units

Digestion of carbohydrate, protein and fat Three rows. Carbohydrate is digested by amylase and brush-border enzymes to simple sugars. Protein is digested by pepsin, trypsin and peptidases to amino acids and small peptides. Fat is emulsified by bile and digested by lipase to fatty acids and monoglycerides, which are packaged into chylomicrons and enter lymph. Carbohydratestarch, sugarsDigested byamylase (saliva, pancreas);brush-border enzymesAbsorbed asglucose, fructose,galactoseProteinmeat, dairy, legumesDigested bypepsin (stomach); trypsin,peptidases (pancreas, gut)Absorbed asamino acids,small peptidesFattriglyceridesDigested bybile emulsifies;pancreatic lipaseAbsorbed asfatty acids, monoglycerides→ chylomicrons → lymph
Most digestion and absorption happen in the small intestine. Fat absorption depends on bile, so liver and gallbladder disease impair it.

Digestion converts large molecules into small ones. Most of the work is done by pancreatic enzymes in the small intestine, finished by enzymes on the surface of intestinal cells. Fat is insoluble in water, so bile salts break it into tiny droplets and then micelles that carry fatty acids to the cell surface.

Absorption

Figure 3 · Folding builds a huge surface

Successive folding of the small intestine lining increases surface area Four panels: a smooth tube, circular folds that roughly triple the area, finger-like villi that multiply it about tenfold more, and microvilli on each cell that multiply it about twentyfold more. Smooth tubebaselineCircular folds× 3Villi× 30Microvilli× 600Each level of folding multiplies the absorptive surface
Surface is everything for absorption. Diseases that flatten villi, such as coeliac disease, cause malabsorption, weight loss and anaemia. Approximate multipliers.
NutrientWhereHow
Glucose, galactoseSmall intestineCo-transport with sodium (SGLT1)
FructoseSmall intestineFacilitated diffusion (GLUT5)
Amino acidsSmall intestineSodium-linked carriers
FatSmall intestineMicelles; repackaged into chylomicrons; enter lymph
IronDuodenumRegulated by hepcidin (Chapter 26)
CalciumDuodenum and small intestineNeeds active vitamin D (Chapter 38)
Vitamin B12Terminal ileumNeeds intrinsic factor from the stomach
Water and saltSmall intestine and colonFollows solute absorption

Watch out

Surgery to the stomach or terminal ileum, coeliac disease, pancreatic disease and some diabetes and weight-loss drugs affect absorption. Fatigue, anaemia, neuropathy or fragile bones in such patients may have a nutritional cause.

Fuel absorption during exercise

In long events, ingested carbohydrate can supply fuel, but only as fast as the gut absorbs it. Jeukendrup describes how glucose absorption saturates the SGLT1 transporter at about 1 g/min [2]. Adding fructose, which uses GLUT5, raised oxidation of ingested carbohydrate to about 1.75 g/min [2].

Figure 4 · Two transporters, more fuel

Maximum oxidation of ingested carbohydrate with glucose alone or glucose plus fructose Two bars. With glucose alone, oxidation of ingested carbohydrate reaches about 1 gram per minute. With glucose and fructose together it reaches about 1.75 grams per minute, because fructose uses a separate intestinal transporter. 1.00 g/minGlucose only1.75 g/minGlucose + fructoseExogenous carbohydrate oxidationWhy?glucose uses SGLT1,which saturatesfructose uses GLUT5,a separate routetwo doors absorb more
Absorption limits fuel supply in long events. Mixed sugars raise delivery and may reduce gut upset in exercise of 3 hours or more [2].

Mixed sugars also improved fluid delivery and reduced fatigue in events lasting three hours or more [2]. During hard exercise, blood flow to the gut falls, which is one reason high-intensity effort after a large meal causes discomfort.

Protein and resistance training

Absorbed amino acids supply the building blocks for muscle repair and growth. Morton and colleagues pooled 49 trials with 1,863 adults doing resistance training for at least six weeks [1].

Figure 5 · Protein supports resistance training

Effect of protein supplementation on gains from resistance training Left: extra lean mass gain rises with total protein intake up to about 1.6 grams per kilogram per day and then levels off. Right: protein supplementation added about 2.49 kilograms to one-repetition maximum strength and 0.30 kilograms of lean mass, with smaller effects in older people. about 1.6 g/kg/dayno further gain [1]0.81.21.62.02.4Total protein intake (g/kg/day)Extra lean mass gainStrength (1RM)+2.49 kgLean mass+0.30 kgEffect with older agesmaller
Enough protein, not unlimited protein. Pooled from 49 trials with 1,863 participants [1]. Curve is a schematic of the reported breakpoint.

Protein supplementation added modest gains in strength and lean mass [1]. Benefits levelled off above a total intake of about 1.6 g/kg/day, were smaller with increasing age, and were larger in trained people [1].

Practical skill · nutrition in rehabilitation

Ask about appetite, weight change and protein intake in older and post-surgical patients. Encourage protein-containing meals across the day alongside strength training, and refer to a dietitian when intake is poor or weight is falling [1].

Clinical relevance for the physiotherapist

  • Strength training. Adequate protein supports gains; very high intakes add little [1].
  • Older adults. Responses are smaller; ensure intake and training are both adequate [1].
  • Endurance events. Mixed carbohydrates help in events of three hours or more [2].
  • Timing. Avoid intense exercise soon after large meals.
  • Malabsorption. Consider nutritional causes of fatigue, neuropathy and fragility.

Assessment and application

SituationInterpretationAction
Older adult losing weight during a strength programmeInadequate intake [1]Dietitian referral; protein across meals
Ultramarathon runner with gut upset using glucose-only gelsTransporter saturation [2]Discuss mixed glucose–fructose sources
Patient after gastrectomy with numb feetPossible B12 deficiencyReport; assess sensation and balance
Diarrhoea for several days, dizzy on standingFluid and salt lossPostural BP; hydration; inform team

Common mistakes

  • Thinking more protein always means more muscle [1].
  • Assuming the stomach absorbs most nutrients.
  • Recommending glucose-only drinks at very high rates in long events [2].
  • Forgetting B12 needs intrinsic factor from the stomach.
  • Ignoring nutrition as a barrier to rehabilitation.

Exam preparation

Long answer

Describe the digestion and absorption of fat.

  • Lingual and gastric lipase; emulsification by bile.
  • Pancreatic lipase and micelles (Figure 2).
  • Chylomicrons and lymphatic absorption.
  • Control by cholecystokinin.
  • Clinical: malabsorption and steatorrhoea.

Short notes

  • Gastric juice.
  • Pancreatic secretion and its control.
  • Absorption of vitamin B12.
  • Carbohydrate absorption in exercise [2].

Viva and practical

  • “Which hormone releases bile?” — cholecystokinin.
  • “Why combine glucose and fructose?” — separate transporters [2].
  • “How much protein is enough for training?” — about 1.6 g/kg/day total [1].
  • OSCE: screen a patient for nutritional risk.

Current evidence

Exam and traditional teaching

Digestion is taught enzyme by enzyme, with little link to exercise or rehabilitation.

What the evidence adds

Protein supplementation modestly boosts training gains, with a ceiling around 1.6 g/kg/day [1].

Intestinal transporters limit carbohydrate supply in exercise; mixed sugars raise it [2].

What the student should understand

Exercise builds tissue only if the gut supplies the materials. Nutrition and training work together.

Check yourself

10 questions on this page. Tap one to see the answer and the reasoning.

Q1. Protein supplementation during resistance training increased one-repetition maximum strength by about:
  1. (A) 0.2 kg
  2. (B) 2.5 kg
  3. (C) 25 kg
  4. (D) No change

Answer: (B) Strength gains were about 2.49 kg greater [1].

Q2. Beyond about what total protein intake did supplementation give no further lean mass gain?
  1. (A) 0.8 g/kg/day
  2. (B) 1.6 g/kg/day
  3. (C) 3.5 g/kg/day
  4. (D) 5 g/kg/day

Answer: (B) The breakpoint was about 1.62 g/kg/day [1].

Q3. The effect of protein supplementation on lean mass gain:
  1. (A) Increased with age
  2. (B) Decreased with increasing age
  3. (C) Was the same at all ages
  4. (D) Was negative in young people

Answer: (B) Efficacy fell with increasing age [1].

Q4. Protein supplementation was more effective in:
  1. (A) Untrained people only
  2. (B) Resistance-trained individuals
  3. (C) People not training
  4. (D) Children

Answer: (B) It was more effective in resistance-trained people [1].

Q5. The main limit to oxidising ingested carbohydrate during exercise appears to be:
  1. (A) Muscle uptake
  2. (B) Intestinal absorption via saturated transporters
  3. (C) Liver storage
  4. (D) Taste

Answer: (B) Transporter saturation in the gut limits absorption [2].

Q6. Combining glucose and fructose raised exogenous carbohydrate oxidation to about:
  1. (A) 0.5 g/min
  2. (B) 1.0 g/min
  3. (C) 1.75 g/min
  4. (D) 5 g/min

Answer: (C) Rates reached about 1.75 g/min, versus about 1 g/min with glucose alone [2].

Q7. Fructose is absorbed mainly by:
  1. (A) SGLT1
  2. (B) GLUT5
  3. (C) Na/K ATPase
  4. (D) Bile salts

Answer: (B) Using a different transporter allows extra carbohydrate absorption [2].

Q8. Multiple transportable carbohydrates are most useful in endurance events lasting about:
  1. (A) 10 minutes
  2. (B) 3 hours or more
  3. (C) 30 seconds
  4. (D) Any duration equally

Answer: (B) They benefit sports of 3 hours or more [2].

Q9. Most nutrient absorption occurs in the:
  1. (A) Stomach
  2. (B) Small intestine
  3. (C) Colon
  4. (D) Oesophagus

Answer: (B) The small intestine’s transporters set absorption limits, as shown for carbohydrate [2].

Q10. For older adults doing resistance training, a practical message is:
  1. (A) Protein does not matter
  2. (B) Ensure adequate total protein, recognising effects may be smaller with age
  3. (C) Avoid protein
  4. (D) Use only carbohydrate

Answer: (B) Age reduces the response, but adequate protein still supports training [1].

Quick review

Key takeaways

  • About 9 L/day enters the gut; about 7 L is secretions.
  • Pancreatic enzymes do most digestion; bile emulsifies fat.
  • Folds, villi and microvilli create a vast absorptive surface.
  • B12 needs intrinsic factor; absorbed in terminal ileum.
  • Glucose + fructose raise carbohydrate supply to about 1.75 g/min [2].
  • Protein boosts training gains up to about 1.6 g/kg/day [1].

Further reading

SourceWhy it earns its place
Morton RW et al. — Br J Sports Med, 2018Protein and resistance training [1]
Jeukendrup AE — Curr Opin Clin Nutr Metab Care, 2010Carbohydrate absorption in exercise [2]
Johnson LR — Gastrointestinal PhysiologySecretion and absorption

References

  1. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine. 2018;52(6):376–384. doi:10.1136/bjsports-2017-097608 PMID 28698222 Systematic review and meta-analysis
  2. Jeukendrup AE. Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care. 2010;13(4):452–457. doi:10.1097/MCO.0b013e328339de9f PMID 20574242 Review

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Reviewed by the Physiotherapist India Team. · Human Physiology contents