What it is

Cereal grains consist mostly of starch, made of glucose chains; their outer layers contain most of the fibre 123.

Per 100 g, whole-grain wheat flour contains 57.8 g starch and 10.7 g fibre, white flour 2.7 g fibre 1. Dietary starch is about 25% amylose, a linear α-1,4-glucan, and 75% amylopectin, branched by α-1,6 links 2. Fibre comprises lignin and non-digestible carbohydrates, such as oat β-glucan and retrograded amylose 3.

In the body

Digestive enzymes split starch into glucose; colonic bacteria ferment fibre into short-chain fatty acids 3.

α-Amylase hydrolyses internal α-1,4 links 2. Brush-border maltase-glucoamylase and sucrase-isomaltase release glucose; sucrase-isomaltase supplies almost all α-1,6 activity 2.

Digestion of starch and absorption of glucoseMouth and small-intestinal lumen; brush border, cytosol and basolateral membrane of enterocytes
  1. Starch (amylose and amylopectin)
  2. α-Amylase (salivary AMY1, pancreatic AMY2A)Hydrolyses internal α-1,4 links and bypasses α-1,6 branch points
  3. Maltose, maltotriose and α-limit dextrins
  4. Maltase-glucoamylase (MGAM) and sucrase-isomaltase (SI)Brush-border exohydrolases; SI supplies almost all α-1,6 activity
  5. Glucose (intestinal lumen)
  6. SGLT1 (SLC5A1)2 Na⁺Secondary active transport across the apical membrane
  7. Glucose (enterocyte)
  8. GLUT2 (SLC2A2)Facilitated diffusion across the basolateral membrane
  9. Glucose (portal blood)

Glycaemic index varies with the amylose:amylopectin ratio and with gelatinisation, which makes starch more accessible to digestive enzymes 3. Intact grains, parboiled rice and the protein network of pasta lower it 3.

Colonic bacteria make butyrate from undigested polysaccharides; the acetyl-CoA pathway predominates in stool metagenomes 4.

Butyrate synthesis by colonic bacteria (acetyl-CoA pathway)Colonic lumen; cytosol of butyrate-producing bacteria
  1. Fermentable polysaccharides
  2. several stepsStarch, xylan and other polysaccharides that escape digestion; via pyruvate
  3. Acetyl-CoA
  4. Thiolase (Thl)Acetyl-CoACondenses two acetyl-CoA and releases CoA
  5. Acetoacetyl-CoA
  6. β-Hydroxybutyryl-CoA dehydrogenase (Hbd)
  7. 3-Hydroxybutyryl-CoA
  8. Crotonase (Cro)
  9. Crotonyl-CoA
  10. Butyryl-CoA dehydrogenase–electron-transferring flavoprotein complex (Bcd-Etfαβ)FerredoxinShared by all four butyrate pathways; postulated energy conservation
  11. Butyryl-CoA
  12. Butyryl-CoA:acetate CoA-transferase (But)Acetate → acetyl-CoAAlternative: phosphate butyryltransferase (Ptb) and butyrate kinase (Buk)
  13. Butyrate

Acetate and propionate are absorbed as fuels; butyrate is a main fuel of the colonic mucosa 3.

In coeliac disease, an immune response to gluten inflames the small intestine of genetically susceptible people 56.

Most patients carry HLA-DQ2, encoded by DQA1*0501 and DQB1*02; most others carry HLA-DQ8 6. Of 94 Norwegian children with the disease, 93 carried DQA1 and DQB1 genes that may encode DQ2, against 14 of 56 controls 56. Tissue transglutaminase, the main coeliac autoantigen, converts selected gliadin glutamines to glutamic acid 6. Two immunodominant α-gliadin peptides stimulated intestinal T cells only when deamidated, which raised their affinity for DQ2 6.

What studies show

In observational studies, people eating the most fibre had less heart disease, diabetes, colorectal cancer and early death 7.

Highest against lowest fibre intake was associated with an all-cause mortality risk ratio of 0.85 (95% CI 0.79 to 0.91), or 13 fewer deaths per 1000 participants during the studies 7. For coronary heart disease incidence, the risk ratio was 0.76 (0.69 to 0.83), 6 fewer cases per 1000 7. For whole grain, the all-cause mortality risk ratio was 0.81 (0.72 to 0.90), 26 fewer deaths per 1000 7. In trials of at least 4 weeks, more fibre lowered body weight by 0.37 kg and total cholesterol by 0.15 mmol/L 7. WHO commissioned the work and funded it with five New Zealand bodies; a WHO expert group set its questions and confirmed or modified its certainty ratings 7.